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+The Project Gutenberg EBook of A Treatise on Meteorological Instruments, by 
+Henry Negretti and Joseph Zambra
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: A Treatise on Meteorological Instruments
+       Explanatory of Their Scientific Principles, Method of
+       Construction, and Practical Utility
+
+Author: Henry Negretti
+        Joseph Zambra
+
+Release Date: June 18, 2011 [EBook #36457]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK TREATISE ON METEOROLOGICAL INSTRUMENTS ***
+
+
+
+
+Produced by The Online Distributed Proofreading Team at
+https://www.pgdp.net (This file was produced from images
+generously made available by The Internet Archive.)
+
+
+
+
+
+
+
+
+
+A TREATISE ON METEOROLOGICAL INSTRUMENTS.
+
+
+
+
+  LONDON:
+  PRINTED BY WILLIAMS AND STRAHAN,
+  7 LAWRENCE LANE, CHEAPSIDE, E.C.
+
+
+
+
+  A TREATISE ON METEOROLOGICAL INSTRUMENTS:
+
+  EXPLANATORY OF
+
+  THEIR SCIENTIFIC PRINCIPLES,
+  METHOD OF CONSTRUCTION, AND PRACTICAL UTILITY.
+
+
+  BY NEGRETTI & ZAMBRA,
+
+  METEOROLOGICAL INSTRUMENT MAKERS TO THE QUEEN, THE ROYAL OBSERVATORY,
+  GREENWICH, THE BRITISH METEOROLOGICAL SOCIETY, THE BRITISH AND FOREIGN
+  GOVERNMENTS, ETC. ETC. ETC.
+
+
+  LONDON:
+  PUBLISHED AND SOLD AT NEGRETTI & ZAMBRA'S ESTABLISHMENTS:
+  1 HATTON GARDEN, E.C., 59 CORNHILL, E.C., 122 REGENT STREET W.,
+  AND 153 FLEET STREET, E.C.
+
+  1864.
+
+  _Price Five Shillings._
+
+
+
+
+PREFACE.
+
+
+The national utilisation of Meteorology in forewarning of storms, and the
+increasing employment of instruments as weather indicators, render a
+knowledge of their construction, principles, and practical uses necessary
+to every well-informed person. Impressed with the idea that we shall be
+supplying an existing want, and aiding materially the cause of
+Meteorological Science, in giving a plain description of the various
+instruments now in use, we have endeavoured, in the present volume, to
+condense such information as is generally required regarding the
+instruments used in Meteorology; the description of many of which could
+only be found in elaborate scientific works, and then only briefly touched
+upon. Every Meteorological Instrument now in use being fully described,
+with adequate directions for using, the uninitiated will be enabled to
+select those which seem to them best adapted to their requirements. With
+accounts of old or obsolete instruments we have avoided troubling the
+reader; on the other hand, we were unwilling to neglect those which,
+though of no great practical importance, are still deserving of notice
+from their being either novel or ingenious, or which, without being
+strictly scientific, are in great demand as simple weather-glasses and
+articles of trade.
+
+We trust, therefore, that the work (however imperfect), bearing in mind
+the importance of the subject, will be acceptable to general readers, as
+well as to those for whose requirements it has been prepared.
+
+The rapid progress made in the introduction of new apparatus of
+acknowledged superiority has rendered the publication of some description
+absolutely necessary. The Report of the Jurors for Class XIII. of the
+International Exhibition, 1862, on Meteorological Instruments, fully bears
+out our assertion, as shown by the following extract:--
+
+"The progress in the English department has been very great;--in
+barometers, thermometers, anemometers, and in every class of instruments.
+At the close of the Exhibition of 1851, there seemed to have arisen a
+general anxiety among the majority of makers to pay every attention to all
+the essentials necessary for philosophical instruments, not only in their
+old forms, but also with the view of obtaining other and better forms.
+This desire has never ceased; and no better idea can be given of the
+continued activity in these respects, than the number of patents taken out
+for improvements in meteorological instruments in the interval between the
+recent and preceding exhibitions, which amount to no less than forty-two."
+* * * "In addition to numerous improvements patented by Messrs. Negretti
+and Zambra, there is another of great importance, which they did not
+patent, viz. enamelling the tubes of thermometers, enabling the makers to
+use finer threads of mercury in the construction of all thermometers; for
+the contrast between the opaque mercury and the enamel back of the tubes
+is so great, that the finest bore or thread of mercury, which at one time
+could not be seen without the greatest difficulty, is now seen with
+facility; and throughout the British and Foreign departments, the makers
+have availed themselves of this invention, the tubes of all being made
+with enamelled backs. It is to be hoped that the recent exhibition will
+give a fresh stimulus to the desire of improvement, and that the same rate
+of progress will be continued."
+
+To fulfil the desire of the International Jury in the latter portion of
+the above extract will be the constant study of
+
+NEGRETTI & ZAMBRA.
+
+_1st January, 1864._
+
+
+
+
+TABLE OF CONTENTS.
+
+
+  CHAPTER I. INSTRUMENTS FOR ASCERTAINING THE ATMOSPHERIC PRESSURE.
+
+  SECTION
+
+  1. Principle of the Barometer.
+
+  2. Construction of Barometers.
+
+  3. Fortin's Barometer Cistern.
+
+  4. STANDARD BAROMETER.
+
+  5. Correction due to Capillarity.
+
+  6.      "        "   Temperature.
+
+  7.      "        "   Height.
+
+  8. The Barometer Vernier.
+
+  9. SELF-COMPENSATING STANDARD BAROMETER.
+
+  10. BAROMETER WITH ELECTRICAL ADJUSTMENT.
+
+  11. PEDIMENT BAROMETERS.
+
+  12. The Words on the Scale.
+
+  13. Correction due to Capacity of Cistern.
+
+  14. PUBLIC BAROMETERS.
+
+  15. FISHERY OR SEA-COAST BAROMETERS.
+
+  16. Admiral FitzRoy's Words for the Scale.
+
+  17. Instructions for Sea-coast Barometer.
+
+  18. French Sea-coast Barometer.
+
+  19. COMMON MARINE BAROMETER.
+
+  20. THE KEW MARINE BAROMETER.
+
+  21. Method of verifying Barometers.
+
+  22. FITZROY'S MARINE BAROMETER.
+
+  23. Words for its Scale.
+
+  24. Trials of this Barometer under Gun-fire.
+
+  25. NEGRETTI AND ZAMBRA'S FARMER'S BAROMETER AND DOMESTIC WEATHER-GLASS.
+
+  26. Rules for Foretelling the Weather.
+
+  27. Causes which may bring about a Fall or a Rise in the Barometer.
+
+  28. Use of the Barometer in the Management of Mines.
+
+  29. Use of the Barometer in estimating the Height of Tides.
+
+
+  CHAPTER II. SYPHON TUBE BAROMETERS.
+
+  30. Principle of.
+
+  31. DIAL, OR WHEEL, BAROMETERS.
+
+  32. STANDARD SYPHON BAROMETER.
+
+
+  CHAPTER III. BAROGRAPHS, OR SELF-REGISTERING BAROMETERS.
+
+  SECTION
+
+  33. MILNE'S SELF-REGISTERING BAROMETER.
+
+  34. MODIFICATION OF MILNE'S BAROMETER.
+
+  35. KING'S SELF-REGISTERING BAROMETER.
+
+  36. SYPHON, WITH PHOTOGRAPHIC REGISTRATION.
+
+
+  CHAPTER IV. MOUNTAIN BAROMETERS.
+
+  37. GAY LUSSAC'S MOUNTAIN BAROMETER.
+
+  38. FORTIN'S MOUNTAIN BAROMETER.
+
+  39. NEWMAN'S MOUNTAIN BAROMETER.
+
+  40. NEGRETTI AND ZAMBRA'S PATENT MOUNTAIN AND OTHER BAROMETERS.
+
+  41. Short Tube Barometer.
+
+  42. Method of Calculating Heights by the Barometer; Tables and Examples.
+
+
+  CHAPTER V. SECONDARY BAROMETERS.
+
+  43. Desirability of Magnifying the Barometer Range.
+
+  44. HOWSON'S LONG-RANGE BAROMETER.
+
+  45. MCNEIL'S LONG-RANGE BAROMETER.
+
+  46. The Water-glass Barometer.
+
+  47. SYMPIESOMETERS.
+
+  48. ANEROIDS.
+
+  49. SMALL SIZE ANEROIDS.
+
+  50. WATCH ANEROID.
+
+  51. Measurement of Heights by the Aneroid; Example.
+
+  52. METALLIC BAROMETER.
+
+
+  CHAPTER VI. INSTRUMENTS FOR ASCERTAINING TEMPERATURE.
+
+  53. Temperature.
+
+  54. Thermometric Substances.
+
+  55. Description of the Thermometer.
+
+  56. STANDARD THERMOMETER.
+
+  57. Method of ascertaining the exact Boiling Temperature; Tables, &c.
+
+  58. Displacement of the Freezing Point.
+
+  59. The Scale.
+
+  60. The method of testing Thermometers.
+
+  61. Porcelain Scale-Plates.
+
+  62. Enamelled Tubes.
+
+  63. THERMOMETERS OF EXTREME SENSITIVENESS.
+
+  64. VARIETIES OF THERMOMETERS.
+
+  65. SUPERHEATED STEAM THERMOMETER.
+
+  66. THERMOMETER FOR SUGAR BOILING.
+
+  67. EARTH THERMOMETER.
+
+  68. MARINE THERMOMETER.
+
+
+  CHAPTER VII. SELF-REGISTERING THERMOMETERS.
+
+  69. Importance of.
+
+  70. RUTHERFORD'S MAXIMUM THERMOMETER.
+
+  71. PHILLIPS'S    DITTO    DITTO.
+
+  72. NEGRETTI AND ZAMBRA'S PATENT MAXIMUM THERMOMETER.
+
+  73. RUTHERFORD'S ALCOHOL MINIMUM THERMOMETER.
+
+  74. HORTICULTURAL MINIMUM THERMOMETER.
+
+  75. BAUDIN'S ALCOHOL MINIMUM THERMOMETER.
+
+  76. Mercurial Minima Thermometers desirable.
+
+  77. NEGRETTI AND ZAMBRA'S PATENT MERCURIAL MINIMUM THERMOMETER.
+
+  78. NEGRETTI AND ZAMBRA'S SECOND PATENT MERCURIAL MINIMUM THERMOMETER.
+
+  79. CASELLA'S PATENT MERCURIAL MINIMUM THERMOMETER.
+
+  80. Day and Night Thermometer.
+
+  81. SIXE'S SELF-REGISTERING THERMOMETER.
+
+
+  CHAPTER VIII. RADIATION THERMOMETERS.
+
+  82. Solar and Terrestrial Radiation considered.
+
+  83. SOLAR RADIATION THERMOMETER.
+
+  84. VACUUM SOLAR RADIATION THERMOMETER.
+
+  85. TERRESTRIAL RADIATION THERMOMETER.
+
+  86. �THRIOSCOPE.
+
+  87. PYRHELIOMETER.
+
+  88. ACTINOMETER.
+
+
+  CHAPTER IX. DEEP-SEA THERMOMETERS.
+
+  89. ON SIXE'S PRINCIPLE.
+
+  90. JOHNSON'S METALLIC THERMOMETER.
+
+
+  CHAPTER X. BOILING-POINT THERMOMETERS.
+
+  91. Ebullition.
+
+  92. Relation between Boiling-Point and Elevation.
+
+  93. HYPSOMETRIC APPARATUS.
+
+  94. Precautions to ensure Correct Graduation.
+
+  95. Method of Calculating Heights from Observations with the Mountain
+      Thermometer; Example.
+
+  96. THERMOMETERS FOR ENGINEERS.
+
+
+  CHAPTER XI. INSTRUMENTS FOR ASCERTAINING THE HUMIDITY OF THE AIR.
+
+  97. Hygrometric Substances.
+
+  98. SAUSSURE'S HYGROMETER.
+
+  99. Dew-Point.
+
+  100. DROSOMETER.
+
+  101. Humidity.
+
+  102. LESLIE'S HYGROMETER.
+
+  103. DANIEL'S HYGROMETER.
+
+  104. REGNAULT'S CONDENSER HYGROMETER.
+
+  105. Temperature of Evaporation.
+
+  106. MASON'S HYGROMETER.
+
+  107. SELF-REGISTERING HYGROMETER.
+
+  108. Causes of Dew.
+
+  109. Plan of Exposing Thermometers.
+
+
+  CHAPTER XII. INSTRUMENTS USED FOR MEASURING THE RAINFALL.
+
+  110. HOWARD'S RAIN-GAUGE.
+
+  111. GLAISHER'S RAIN-GAUGE.
+
+  112. RAIN-GAUGE WITH FLOAT.
+
+  113. RAIN-GAUGE WITH SIDE TUBE.
+
+  114. FITZROY'S RAIN-GAUGE.
+
+  115. SELF-REGISTERING RAIN-GAUGE.
+
+  116. The principle of Measurement.
+
+  117. Position for Rain-gauge, &c.
+
+  118. Cause of Rain.
+
+  119. Laws of Rainfall.
+
+  120. Utility of Statistics of Rainfall.
+
+  121. NEW FORM OF RAIN-GAUGE.
+
+
+  CHAPTER XIII. APPARATUS EMPLOYED FOR REGISTERING THE DIRECTION,
+  PRESSURE, AND VELOCITY OF THE WIND.
+
+  122. THE VANE.
+
+  123. LIND'S WIND-GAUGE.
+
+  124. HARRIS'S WIND-GAUGE.
+
+  125. ROBINSON'S ANEMOMETER.
+
+  126. WHEWELL'S ANEMOMETER.
+
+  127. OSLER'S ANEMOMETER AND PLUVIOMETER.
+
+  128. BECKLEY'S ANEMOMETER.
+
+  129. SELF-REGISTERING WIND-GAUGE.
+
+  130. Anemometric Observations.
+
+
+  CHAPTER XIV. INSTRUMENTS FOR INVESTIGATING ATMOSPHERIC ELECTRICITY.
+
+  131. ATMOSPHERIC ELECTROSCOPE.
+
+  132. VOLTA'S ELECTROMETER.
+
+  133. PELTIER'S ELECTROMETER.
+
+  134. BOHNENBERGER'S ELECTROSCOPE.
+
+  135. THOMSON'S ELECTROMETER.
+
+  136. Fundamental Facts.
+
+  137. Lightning Conductors.
+
+  138. Precautions against Lightning.
+
+
+  CHAPTER XV. OZONE AND ITS INDICATORS.
+
+  139. Nature of Ozone.
+
+  140. SCHONBEIN'S OZONOMETER.
+
+  141. MOFFAT'S OZONOMETER.
+
+  142. CLARK'S OZONE CAGE.
+
+  143. Distribution and Effects of Ozone.
+
+  144. LANCASTER'S REGISTERING OZONOMETER.
+
+
+  CHAPTER XVI. MISCELLANEOUS INSTRUMENTS.
+
+  145. CHEMICAL WEATHER GLASS.
+
+  146. LESLIE'S DIFFERENTIAL THERMOMETER.
+
+  147. ROMFORD'S DIFFERENTIAL THERMOMETER.
+
+  148. GLAISHER'S THERMOMETER STAND.
+
+  149. THERMOMETER SCREEN, FOR USE AT SEA.
+
+  150. ANEMOSCOPE.
+
+  151. EVAPORATING DISH, OR GAUGE.
+
+  152. ADMIDOMETER.
+
+  153. CLOUD REFLECTOR.
+
+  154. SUNSHINE RECORDER.
+
+  155. SET OF PORTABLE INSTRUMENTS.
+
+  156. IMPLEMENTS.
+
+  157. HYDROMETER.
+
+  158. NEWMAN'S SELF-REGISTERING TIDE-GAUGE.
+
+
+
+
+TABLES.
+
+
+                                                                      PAGE
+
+  Table of Corrections, for Capillary Depression of the Mercury
+  in Boiled and in Unboiled Barometer-Tubes                              6
+
+  Tables for Deducing Heights by means of the Barometer:--
+
+    No. 1. Approximate Height due to Barometric Pressure                42
+
+    No. 2. Correction for Mean Temperature of Air                       44
+
+    No. 3. Correction due to Latitude                                   44
+
+    No. 4. Correction due to Approximate Elevation                      45
+
+  Tables for Determining the Temperature of the Vapour of Boiling
+  Water at any Place:--
+
+    No. 5. Factor due to Latitude                                       62
+
+    No. 6. Temperature and Tension                                      62
+
+  Table of Temperature of the Soil                                      69
+
+  Table of Difference of Elevation corresponding to a fall of 1�
+  in the Boiling-point of Water                                         98
+
+  Table showing Proportion of Salt for various Boiling Temperatures
+  of Sea-Water                                                         100
+
+  Table for finding the Degree of Humidity from Observations with
+  Mason's Hygrometer                                                   108
+
+  Table showing Amount and Duration of Rain at London, in 1862         112
+
+  Table of Average British Rainfall in Westerly, Central, and
+  Easterly districts                                                   114
+
+  Table showing Force of Wind, for use with Lind's Wind-Gauge          118
+
+  Tables for Correcting Observations made with--
+
+    Brass Hydrometers                                                  142
+
+    Glass Hydrometers                                                  143
+
+
+
+
+ADDENDA.
+
+
+                                                                      PAGE
+
+  1. Rule for converting Millimetres into Inches, et vice versa        146
+
+  2. Old French Lineal Measure, with English Equivalents               146
+
+  3. Rule for finding Diameter of Bore of Barometer Tube               146
+
+  4. Wind Scales                                                       147
+
+  5. Letters to denote the State of the Weather                        147
+
+  6. Table of Expansion of Bodies                                      148
+
+  7. Table of Specific Gravity of Bodies                               148
+
+  8. Important Temperatures                                            148
+
+  9. Table of Meteorological Elements, forming Exponents of the
+  Climate of London                                                    149
+
+  10. List of Works on Meteorology                                     151
+
+
+
+
+METEOROLOGICAL INSTRUMENTS.
+
+
+In the pursuits and investigations of the science of Meteorology, which is
+essentially a science of observation and experiment, instruments are
+required for ascertaining, 1. the pressure of the atmosphere at any time
+or place; 2. the temperature of the air; 3. the absorption and radiation
+of the sun's heat by the earth's surface; 4. the humidity of the air; 5.
+the amount and duration of rainfall; 6. the direction, the horizontal
+pressure, and the velocity of winds; 7. the electric condition of the
+atmosphere, and the prevalence and activity of ozone.
+
+
+
+
+CHAPTER I.
+
+INSTRUMENTS FOR ASCERTAINING THE ATMOSPHERIC PRESSURE.
+
+
+[Illustration: Fig. 1.]
+
+=1. Principle of the Barometer.=--The first instrument which gave the
+exact measure of the pressure of the atmosphere was invented by
+Torricelli, in 1643. It is constructed as follows:--A glass tube, CD (fig.
+1), about 34 inches long, and from two to four-tenths of an inch in
+diameter of bore, having one end closed, is filled with mercury. In a cup,
+B, a quantity of mercury is also poured. Then, placing a finger securely
+over the open end, C, invert the tube vertically over the cup, and remove
+the finger when the end of the tube dips into the mercury. The mercury in
+the tube then partly falls out, but a column, AB, about 30 inches in
+height, remains supported. This column is a weight of mercury, the
+pressure of which upon the surface of that in the cup is precisely
+equivalent to the corresponding pressure of the atmosphere which would be
+exerted in its place if the tube were removed. As the atmospheric pressure
+varies, the length of this mercurial column also changes. It is by no
+means constant in its height; in fact, it is very seldom stationary, but
+is constantly rising or falling through a certain extent of the tube, at
+the level of the sea, near which the above experiment is supposed to be
+performed. It is, therefore, an instrument by which the fluctuations
+taking place in the pressure of the atmosphere, arising from changes in
+its weight and elasticity, can be shown and measured. It has obtained the
+name _Barometer_, or measurer of heaviness,--a word certainly not happily
+expressive of the utility of the invention. If the bore of the barometer
+tube be uniform throughout its length, and have its sectional area equal
+to a square inch, it is evident that the length of the column, which is
+supported by the pressure of the air, expresses the number of cubic inches
+of mercury which compose it. The weight of this mercury, therefore,
+represents the statical pressure of the atmosphere upon a square inch of
+surface. In England the annual mean height of the barometric column,
+reduced to the sea-level and to the temperature of 32� Fahrenheit, is
+about 29�95 inches. A cubic inch of mercury at this temperature has been
+ascertained to weigh 0�48967 lbs. avoirdupois. Hence, 29�95 � 0�48967=
+14�67 lbs., is the mean value of the pressure of the atmosphere on each
+square inch of surface, near the sea-level, about the latitude of 50
+degrees. Nearer the equator this mean pressure is somewhat greater; nearer
+the poles, somewhat less. For common practical calculations it is assumed
+to be 15 lbs. on the square inch. When it became apparent that the
+movements of the barometric column furnished indications of the probable
+coming changes in the weather, an attempt was made to deduce from recorded
+observations the barometric height corresponding to the most notable
+characteristics of weather. It was found that for fine dry weather the
+mercury in the barometer at the sea-level generally stood above 30 inches;
+changeable weather happened when it ranged from 30 to 29 inches, and when
+rainy or stormy weather occurred it was even lower. Hence, it became the
+practice to place upon barometer scales words indicatory of the weather
+likely to accompany, or follow, the movements of the mercury; whence the
+instruments bearing them obtained the name "Weather Glasses."
+
+
+=2. Construction of Barometers.=--In order that the instrument may be
+portable, it must be made a fixture and mounted on a support; and,
+further, to render it scientifically or even practically useful, many
+precautions are required in its construction. The following remarks apply
+to the construction of all barometers:--Mercury is universally employed,
+because it is the heaviest of fluids, and therefore measures the
+atmospheric pressure by the shortest column. Water barometers have been
+constructed, and they require to be at least 34 feet long. Oil, or other
+fluids, might be used. Mercury, however, has other advantages: it has
+feeble volatility, and does not adhere to glass, if pure. Oxidised, or
+otherwise impure mercury, may adhere to glass; moreover, such mercury
+would not have the density of the pure metal, and therefore the barometric
+column would be either greater or less than it should be. The mercury of
+commerce generally contains lead; sometimes traces of iron and sulphur. It
+is necessary, therefore, for the manufacturer to purify the mercury; and
+this is done by washing it with diluted acetic, or sulphuric acid, which
+dissolves the impurities. No better test can be found for ascertaining if
+the mercury be pure than that of filling a delicate thermometer tube; if,
+on exhausting the air from this thermometer, the mercury will freely run
+up and down the bore, which is probably one thousandth of an inch in
+diameter, the mercury from which this thermometer was made will be found
+fit for any purpose, and with it a tube may be filled and boiled, not only
+of one inch, but even of two inches diameter. In all barometers it is
+requisite that the space above the mercurial column should be completely
+void of air and aqueous vapour, because these gases, by virtue of their
+elasticity, would depress the column. To exclude these the mercury is
+introduced, and boiled in the tube, over a charcoal fire, kept up for the
+purpose. In this manner the air and vapour which adhere to the glass are
+expanded, and escape away. One can tell whether a barometer has been
+properly "boiled," as it is termed, by simply holding the tube in a
+slanting direction and allowing the mercury to strike the top. If the
+boiling has been well performed, the mercury will give a clear, metallic
+sound; if not, a dull, flat sound, showing some air to be present.
+
+When the mercury in a barometer tube rises or falls, the level of the
+mercury in the cup, or _cistern_, as it is generally termed, falls or
+rises by a proportionate quantity, which depends upon the relative areas
+of the interior of the tube and of the cistern. It is necessary that this
+should be taken into consideration in ascertaining the exact height of the
+column. If a fixed scale is applied to the tube, the correct height may be
+obtained by applying a correction for capacity. A certain height of the
+mercury is ascertained to be accurately measured by the scale, and should
+be marked on the instrument as the _neutral point_. Above this point the
+heights measured are all less, and below, all more, than they should be.
+The ratio between the internal diameters of the tube and cistern (which
+should also be stated on the instrument, as, for instance, capac. 1/50)
+supplies the data for finding the correction to be applied. This
+correction is obviated by constructing the cistern so as to allow of the
+surface of the mercury in it being adjustable to the commencement of the
+fixed scale, as by Fortin's or Negretti's plan. It is also unnecessary in
+barometers constructed on what is now called the "Kew method." These will
+all be detailed in their proper place. The tube, being fixed to the
+cistern, may have a moveable scale applied to it. But such an arrangement
+requires the utmost care and skill in observing, and is seldom seen except
+in first-class Observatories.
+
+
+[Illustration: Fig. 2.]
+
+=3. Fortin's Barometer.=--Fortin's plan of constructing a barometer
+cistern is shown in fig. 2. The cistern is formed of a glass cylinder,
+which allows of the level of the mercury within being seen. The bottom of
+the cylinder is made of sheep-skin or leather, like a bag, so as to allow
+of being pushed up or lowered by means of a screw, D B, worked from
+beneath. This screw moves through the bottom of a brass cylinder, C C,
+which is fixed outside, and protects the glass cylinder containing the
+mercury. At the top of the interior of the cistern is fixed a small piece
+of ivory, A, the point of which exactly coincides with the zero of the
+scale. This screw and moveable cistern-bottom serve also to render the
+barometer portable, by confining the mercury in the tube, and preventing
+its coming into the cistern, which is thus made too small to receive it.
+
+
+4. STANDARD BAROMETER.
+
+Fig. 3 represents a Standard Barometer on Fortin's principle. The
+barometer tube is enclosed and protected by a tube of brass extending
+throughout its whole length; the upper portion of the brass tube has two
+longitudinal openings opposite each other; on one side of the front
+opening is the barometrical scale of English inches, divided to show, by
+means of a vernier, 1/500th of an inch; on the opposite side is sometimes
+divided a scale of French millimetres, reading also by a vernier to 1/10th
+of a millimetre (see directions for reading the vernier, page 7). A
+thermometer, C, is attached to the frame, and divided to degrees, which
+can be read to tenths; it is necessary for ascertaining the temperature of
+the instrument, in order to correct the observed height of the barometer.
+
+[Illustration: Fig. 3.]
+
+As received by the observer, the barometer will consist of two parts,
+packed separately for safety in carriage,--1st, the barometer tube and
+cistern, filled with mercury, the brass tube, with its divided scale and
+thermometer; and 2nd, a mahogany board, with bracket at top, and brass
+ring with three adjusting screws at bottom.
+
+_Directions for fixing the Barometer._--In selecting a position for a
+barometer, care should be taken to place it so that the sun cannot shine
+upon it, and that it is not affected by direct heat from a fire. The
+cistern should be from two to three feet above the ground, which will give
+a height for observing convenient to most persons. A standard barometer
+should be compared with an observatory standard of acknowledged accuracy,
+to determine its index error; which, as such instruments are graduated by
+micrometrical apparatus of great exactitude, will be constant for all
+parts of the scale. It should be capable of turning on its axis by a
+movement of the hand, so that little difficulty can ever be experienced in
+obtaining a good light for observation. Having determined upon the
+position in which to place the instrument, fix the mahogany board as
+nearly vertical as possible, and ascertain if the barometer is perfect and
+free from air, in the following manner:--lower the screw at the bottom of
+the cistern several turns, so that the mercury in the tube, when held
+upright, may fall two or three inches from the top; then slightly incline
+the instrument from the vertical position, and if the mercury in striking
+the top elicit a sharp tap, the instrument is perfect. Supposing the
+barometer to be in perfect condition, as it is almost sure to be, it is
+next suspended on the brass bracket, its cistern passing through the ring
+at bottom, and allowed to find its vertical position, after which it is
+firmly clamped by means of the three thumb-screws.
+
+_To Remove the Instrument when fixed to another Position._--If it should
+be necessary to remove the barometer,--first, by means of the adjusting
+screw, drive the mercury to the top of the tube, turning it gently when it
+is approaching the top, and cease directly any resistance is experienced;
+next, remove from the upper bracket or socket; lift the instrument and
+invert it, carrying it with its lower end upwards.
+
+_Directions for taking an Observation._--Before making an observation, the
+mercury in the cistern must be raised or lowered by means of the
+thumb-screw, F, until the ivory point, E, and its reflected image in the
+mercury, D, are just in contact; the vernier is then moved by means of the
+milled head, until its lower termination just excludes the light from the
+top of the mercurial column; the reading is then taken by means of the
+scale on the limb and the vernier. The vernier should be made to read
+upward in all barometers, unless for a special object, as this arrangement
+admits of the most exact setting. In observing, the eye should be placed
+in a right line with the fore and back edges of the lower termination of
+the vernier; and this line should be made to form a tangent to the apex of
+the mercurial column. A small reflector placed behind the vernier and
+moving with it, so as to assist in throwing the light through the back
+slit of the brass frame on to the glass tube, is advantageous; and the
+observer's vision may be further assisted by the aid of a reading lens.
+The object is, in these Standard Barometers, to obtain an exact reading,
+which can only be done by having the eye, the fore part of the zero edge
+of the vernier, the top of the mercurial column, and the back of the
+vernier, in the same horizontal plane.
+
+_Uniformity of Calibre._--The diameter of that part of the tube through
+which the oscillations of the mercury will take place is very carefully
+examined to insure uniformity of calibre, and only those tubes are used
+which are as nearly as possible of the same diameter throughout. The size
+of the bore should be marked on the frame of the barometer in tenths and
+hundredths of an inch. A correction due to capillary action, and depending
+on the size of the tube, must be applied to the readings.
+
+
+=5. Correction due to Capillarity.=--When an open tube of small bore is
+plunged into mercury, the fluid will not rise to the same level inside as
+it has outside. Hence, the effect of capillary action is to depress the
+mercurial column; and the more so the smaller the tube. The following
+table gives the correction for tubes in ordinary use:--
+
+  Diameter     Depression, in     Depression, in
+  of tube.     boiled tubes.      unboiled tubes.
+   INCH.          INCH.               INCH.
+
+   0�60           0�002               0�004
+   0�55           0�003               0�005
+   0�50           0�003               0�007
+   0�45           0�005               0�010
+   0�40           0�007               0�015
+   0�35           0�010               0�021
+   0�15           0�044               0�029
+   0�10           0�070               0�041
+   0�30           0�014               0�058
+   0�25           0�020               0�086
+   0�20           0�029               0�140
+
+This correction is always additive to the observed reading of the
+barometer.
+
+
+=6. Correction due to Temperature.=--In all kinds of mercurial barometers
+attention must be given to the temperature of the mercury. As this metal
+expands and contracts very much for variations of temperature, its density
+alters correspondingly, and in consequence the height of the barometric
+column also varies. To ascertain the temperature of the mercury, a
+thermometer is placed near the tube, and is sometimes made to dip into the
+mercury in the cistern. The freezing point of water, 32�F., is the
+temperature to which all readings of barometers must be reduced, in order
+to make them fairly comparable. The reduction may be effected by
+calculation, but the practical method is by tables for the purpose; and
+for these tables we refer the reader to the works mentioned at the end of
+this book.
+
+
+=7. Correction due to Height above the Half-tide Level.=--Further, in
+order that barometrical observations generally may be made under similar
+circumstances, the readings, corrected for capacity, capillarity, and
+temperature, should be reduced to what they would be at the sea-level, by
+adding a correction corresponding to the height above the mean level of
+the sea, or of half-tide. For practical purposes of comparison with
+barometric pressure at other localities, add one-tenth of an inch to the
+reading for each hundred feet of elevation above the sea. For scientific
+accuracy this will not suffice, but a correction must be obtained by means
+of Schuckburg's formula, or tables computed therefrom.
+
+
+=8. The Barometer Vernier.=--The _vernier_, an invaluable contrivance for
+measuring small spaces, was invented by Peter Vernier, about the year
+1630. The barometer scale is divided into inches and tenths. The vernier
+enables us to accurately subdivide the tenths into hundredths, and, in
+first-class instruments, even to thousandths of an inch. It consists of a
+short scale made to pass along the graduated fixed scale by a sliding
+motion, or preferably by a rack-and-pinion motion, the vernier being fixed
+on the rack, which is moved by turning the milled head of the pinion. The
+principle of the vernier, to whatever instrumental scale applied, is that
+the divisions of the moveable scale are to those in an equal length of the
+fixed scale in the proportion of two numbers which differ from each other
+by unity.
+
+[Illustration: Fig. 4.]
+
+[Illustration: Fig. 5.]
+
+The scales of standard barometers are usually divided into half-tenths, or
+�05, of an inch, as represented, in fig. 5, by AB. The vernier, CD, is
+made equal in length to twenty-four of these divisions, and divided into
+twenty-five equal parts; consequently one space on the scale is larger
+than one on the vernier by the twenty-fifth part of �05, which is �002
+inch, so that such a vernier shows differences of �002 inch. The vernier
+of the figure reading upwards, the lower edge, D, will denote the top of
+the barometer column; and is the zero of the vernier scale. In fig. 4, the
+zero being in line exactly with 29 inches and five-tenths of the fixed
+scale, the barometer reading would be 29�500 inches. It will be seen that
+the vernier line, _a_, falls short of a division of the scale by, as we
+have explained, �002 inch; _b_, by �004; _c_, by �006; _d_, by �008; and
+the next line by one hundredth. If, then, the vernier be moved so as to
+make _a_ coincide with _z_, on the scale, it will have moved through �002
+inch; and if 1 on the vernier be moved into line with _y_ on the scale,
+the space measured will be �010. Hence, the figures 1, 2, 3, 4, 5 on the
+vernier measure hundredths, and the intermediate lines even thousandths of
+an inch. In fig. 5, the zero of the vernier is intermediate 29�65 and
+29�70 on the scale. Passing the eye up the vernier and scale, the second
+line above 3 is perceived to lie evenly with a line of the scale. This
+gives �03 and �004 to add to 29�65, so that the actual reading is 29�684
+inches. It may happen that no line on the vernier _accurately_ lies in the
+same straight line with one on the scale; in such a case a doubt will
+arise as to the selection of one from two equally coincident, and the
+intermediate thousandth of an inch should be taken.
+
+For the ordinary purposes of the barometer as a "weather-glass," such
+minute measurement is not required. Hence, in household and marine
+barometers the scale need only be divided to tenths, and the vernier
+constructed to measure hundredths of an inch. This is done by making the
+vernier either 9 or 11-10ths of an inch long, and dividing it into ten
+equal parts. The lines above the zero line are then numbered from 1 to 10;
+sometimes the alternate divisions only are numbered, the intermediate
+numbers being very readily inferred. Hence, if the first line of the
+vernier agrees with one on the scale, the next must be out one-tenth of a
+tenth, or �01 of an inch from agreement with the next _scale_ line; the
+following vernier line must be �02 out, and so on. Consequently, when the
+vernier is set to the mercurial column, the difference shown by the
+vernier from the tenth on the scale is the hundredths to be added to the
+inches and tenths of the scale.
+
+A little practice will accustom a person to set and read any barometer
+quickly; an important matter where accuracy is required, as the heat of
+the body, or the hand, is very rapidly communicated to the instrument, and
+may vitiate, to some extent, the observation.
+
+
+9. SELF-COMPENSATING STANDARD BAROMETER.
+
+This barometer has been suggested to Messrs. Negretti and Zambra by
+Wentworth Erk, Esq. It consists of a regular barometer; but attached to
+the vernier is a double rack worked with one pinion, so that in setting or
+adjusting the vernier in one position, the second rack moves in directly
+the opposite direction, carrying along with it a plug or plunger the exact
+size of the internal diameter of the tube dipping in the cistern, so that
+whatever the displacement that has taken place in the cistern, owing to
+the rise or fall of the mercury, it is exactly compensated by the plug
+being more or less immersed in the mercury, so that no capacity correction
+is required.
+
+A barometer on this principle is, however, no novelty, for at the Royal
+Society's room a very old instrument may be seen reading somewhat after
+the same manner.
+
+[Illustration: Fig. 6.]
+
+Fig. 6 is an illustration of the appearance of this instrument. The
+cistern is so constructed that the greatest amount of light is admitted to
+the surface of the mercury.
+
+
+10. BAROMETER WITH ELECTRICAL ADJUSTMENT.
+
+This barometer is useful to persons whose eyesight may be defective; and
+is capable of being read off to greater accuracy than ordinary barometers,
+as will be seen by the following description:--The barometer consists of
+an upright tube dipping into a cistern, so contrived, that an up-and-down
+movement, by means of a screw, can be imparted to it. In the top of the
+tube a piece of platina wire is hermetically sealed. The cistern also has
+a metallic connection, so that by means of covered copper wires (in the
+back of the frame) a circuit is established; another connection also
+exists by means of a metallic point dipping into the cistern. The circuit,
+however, can be cut off from this by means of a switch placed about midway
+up the frame; on one side of the tube is placed a scale of inches; a small
+circular vernier, divided into 100 parts, is connected with the dipping
+point, and works at right angles with this scale.
+
+To set the instrument in action for taking an observation, a small battery
+is connected by means of two small binding screws at the bottom of the
+frame. The switch is turned upwards, thereby disconnecting the dipping
+point; the cistern is then screwed up, so that the mercury in the tube is
+brought into contact with the platina wire at the top; the instant this is
+effected the magnetic needle seen on the barometer will be deflected. The
+switch is now turned down; by so doing the connection with the upper wire
+or platina is cut off, and established instead only between the dipping
+point carrying the circular vernier and the bottom of the cistern; the
+point is now screwed by means of the milled head until the needle is again
+deflected. We may now be sure that the line on the circular vernier that
+cuts the division on the scale is the exact height of the barometer.
+Although the description here given may seem somewhat lengthy, the
+operation itself is performed in less time than would be taken in reading
+off an ordinary instrument.
+
+
+11. PEDIMENT BAROMETERS.
+
+[Illustration: Fig. 7.]
+
+[Illustration: Fig. 8.]
+
+[Illustration: Fig. 9.]
+
+[Illustration: Fig. 10.]
+
+[Illustration: Fig. 11.]
+
+These Barometers, generally for household purposes, are illustrated by
+figs. 7 to 11. They are intended chiefly for "weather glasses," and are
+manufactured to serve not only a useful, but an ornamental purpose as
+well. They are usually framed in wood, such as mahogany, rosewood, ebony,
+oak or walnut, and can be obtained either plain or handsomely and
+elaborately carved and embellished, in a variety of designs, so as to be
+suitable for private rooms, large halls, or public buildings. The scales
+to the barometer and its attached thermometer may be ivory, porcelain, or
+silvered metal. It is not desirable that the vernier should read nearer
+than one-hundredth of an inch. Two verniers and scales may be fitted one
+on either side of the mercurial column, so that one can denote the last
+reading, and thus show at a glance the extent of rise or fall in the
+interval. The scale and thermometer should be covered with plate glass. A
+cheap instrument has an open face and plain frame, with sliding vernier
+instead of rack-and-pinion motion. The barometer may or may not have a
+moveable bottom to the cistern, with screw for the purpose of securing the
+mercury for portability. The cistern should not, however, require
+adjustment to a zero or fiducial point. It should be large enough to
+contain the mercury, which falls from 31 to 27 inches, without any
+appreciable error on the height read off on the scale.
+
+
+=12. The Words on the Scale.=--The following words are usually engraved on
+the scales of these barometers, although they are not now considered of so
+much importance as formerly:--
+
+  At 31  inches         Very dry.
+  "  30�5  "            Settled fair.
+  "  30    "            Fair.
+  "  29�5  "            Changeable.
+  "  29    "            Rain.
+  "  28�5  "            Much rain.
+  "  28    "            Stormy.
+
+The French place upon their barometers a similar formula:--
+
+  At 785 millim�tres    Tr�s-sec.
+  "  776      "         Beau-fixe.
+  "  767      "         Beau temps.
+  "  758      "         Variable.
+  "  749      "         Pluie ou vent.
+  "  740      "         Grande pluie.
+  "  731      "         Temp�te.
+
+Manufacturers of barometers have uniformly adopted these indications for
+all countries, without regard to the elevation above the sea, or the
+different geographical conditions; and as it can readily be shown that the
+height and variations of the barometer are dependent on these, it follows
+that barometers have furnished indications which, under many
+circumstances, have been completely false. Even in this country, and near
+the sea-level, storms are frequent with the barometer not below 29; rain
+is not uncommon with the glass at 30; even fine weather sometimes occurs
+with a low pressure; while it is evident that at an elevation of a few
+thousand feet the mercury would never rise to 30 inches; hence, according
+to the scale, there should never be fair weather there. If tempests
+happened as seldom in our latitude as the barometer gets down to 28
+inches, the maritime portion of the community at least would be happy
+indeed. These words have long been ridiculed by persons acquainted with
+the causes of the barometric fluctuations; nevertheless opticians continue
+to place them on the scales, evidently only because they appear to add to
+the importance of the instrument in the eyes of those who have not learned
+their general inutility. In different regions of the world, the
+indications of the barometer are modified by the conditions peculiar to
+the geographical position and elevation above the sea, and it is necessary
+to take account of these in any attempt to found rules of general utility
+in connection with the barometer as a weather guide. All that can be said
+in favour of these words is, that within a few hundred feet of the
+sea-level, when the column rises or falls gradually during two or three
+days towards "Fair" or "Rain," the indications they afford of the coming
+weather are generally extremely probable; but when the variations are
+quick, upward or downward, they presage unsettled or stormy weather.
+
+Admiral FitzRoy writes:--"The words on the scales of barometers should not
+be so much regarded, for weather indications, as the rising or falling of
+the mercury; for if it stands at _Changeable_, and then rises a little
+towards _Fair_, it presages a change of wind or weather, though not so
+great as if the mercury had risen higher; and, on the contrary, if the
+mercury stands above _Fair_ and falls, it presages a change, though not to
+so great a degree as if it had stood lower; besides which, the direction
+and force of wind are not in any way noticed. It is not from the point at
+which the mercury stands that we are alone to form a judgment of the state
+of the weather, but from its _rising_ or _falling_; and from the movements
+of immediately preceding days as well as hours, keeping in mind effects of
+change of _direction_ and dryness, or moisture, as well as alteration of
+force or strength of wind."[1]
+
+
+=13. Correction due to Capacity of Cistern.=--These barometers, having no
+adjustment for the zero of the scale, require a correction for the varying
+level of the mercury in the cistern, when the observations are required
+for strict comparison with other barometric observations, or when they are
+registered for scientific purposes; but for the common purpose of
+predicting the weather, this correction is unnecessary. The neutral point,
+and the ratio of the bore of the tube to the diameter of the cistern, must
+be known (see p. 3). Then the capacity correction, as it is termed, is
+found as follows:--Take the fractional part, expressed by the capacity
+ratio, of the difference between the observed reading and the height of
+the neutral point; then, if the mercury stand _below_ the neutral point,
+_subtract_ this result from the reading; if it stand _above_, _add_ it to
+the reading.
+
+For example, suppose the neutral point to be 29�95 inches, and the
+capacity ratio 1/50, required the correction when the barometer reads
+30�78.
+
+  Here                  30�78 - 29�95 = 0�83
+  Correction                = 0�83/50 = +0�02 nearly.
+  Scale reading                         30�78
+                                        -----
+  Correct reading                       30�80
+                                        =====
+
+Of course the correction could as easily be found to three decimal places,
+if desirable. It is evident that the correction is more important the
+greater the distance of the top of the mercury from the neutral point.
+
+
+14. PUBLIC BAROMETERS.
+
+Since the increased attention paid to the signs of forthcoming weather of
+late years, and the good which has resulted therefrom to farmers,
+gardeners, civil engineers, miners, fishermen, and mariners generally, by
+forewarning of impending wet or stormy weather, the desirability of having
+good barometers exposed in public localities has become evident.
+
+Barometers may now be seen attached to drinking fountains, properly
+protected, and are frequently consulted by the passers-by. But it is among
+those whose lives are endangered by sudden changes in the weather,
+fishermen especially, that the warning monitor is most urgently required.
+Many poor fishing villages and towns have therefore been provided by the
+Board of Trade, at the public expense, and through the humane effort of
+Admiral FitzRoy, with first-class barometers, each fixed in a conspicuous
+position, so as to be easily accessible to all who desire to consult it.
+Following this example, the Royal National Life Boat Institution has
+supplied each of its stations with a similar storm warner; the Duke of
+Northumberland and the British Meteorological Society have erected several
+on the coast of Northumberland; and many other individuals have presented
+barometers to maritime places with which they are connected.
+
+These barometers have all been manufactured by Messrs. Negretti and
+Zambra. The form given to the instrument seems well adapted for public
+purposes.
+
+
+[Illustration: Fig. 12.]
+
+=15. Fishery or Sea-coast Barometers.=--Fig. 12 gives a representation of
+these coast and fishery barometers. The frame is of solid oak, firmly
+screwed together. The scales are very legibly engraved on porcelain by
+Negretti and Zambra's patent process. The thermometer is large, and easily
+read; and as this instrument is exposed, it will indicate the actual
+temperature sufficiently for practical purposes. The barometer tube is
+three-tenths of an inch in diameter of bore, exhibiting a good column of
+mercury; and the cistern is of such capacity, in relation to the tube,
+that the change of height in the surface of the mercury in the cistern
+corresponding to a change of height of three inches of mercury in the
+tube, is less than one-hundredth of an inch, and therefore, as the
+readings are only to be made to this degree of accuracy, this small error
+is of no importance. The cistern is made of boxwood, which is sufficiently
+porous to allow the atmosphere to influence the mercurial column; but the
+top is plugged with porous cane, to admit of free and certain play.
+
+
+=16. Admiral FitzRoy's Scale Words.=--The directions given on the scales
+of these barometers were drawn up by Admiral FitzRoy, F.R.S. They appear
+to be founded on the following considerations:--
+
+Supposing a compass diagram, with the principal points laid down, the N.E.
+is the wind for which the barometer stands highest; for the S.W. wind it
+is lowest. This is found to be so in the great majority of cases; but
+there are exceptions to this, as to all rules. The N.E. and S.W. may
+therefore be regarded as the poles of the winds, being opposite each
+other. When the wind veers from the S.W. through W. and N. to N.E., the
+barometer gradually rises; on the contrary, when the wind veers from N.E.
+and E. to S.E., S. and S.W., the mercury falls. A similar curious law
+exists in relation to the veering of the wind, and the action of the
+thermometer. As the wind veers from the S.W. to W. and N., the thermometer
+falls; as it veers from N.E. to E. and S., it rises, because the wind gets
+from a colder to a warmer quarter. The polar winds are cold, dry, and
+heavy. Those from the equatorial regions are warm, moist, and
+comparatively light.
+
+These laws have been clearly developed and expressed by Professor Dov� in
+his work on the "Law of Storms." The warm winds of Europe are those which
+bring the greatest quantity of rain, as they blow from the ocean, and come
+heavily laden with moisture. The cold winds, besides containing less
+moisture, blow more from the land. The weight of the vapour of the warm
+winds tends to raise the barometric column; but, at the same time, the
+increased dilatation of the air tends to lower it. This latter influence
+being the stronger, the barometer always falls for these winds; and in
+regions where they traverse a large extent of land, retain their heat, and
+become necessarily very dry, the fall in the barometer will be greater.
+Admiral FitzRoy's words for the scales of barometers for use in northern
+latitudes, then, are as follows:--
+
+    _RISE._                      _FALL._
+      FOR                          FOR
+    N. ELY.                      S. WLY.
+  NW.--N.--E.                  SE.--S.--W.
+      DRY                          WET
+      OR                           OR
+     LESS                         MORE
+     WIND.                        WIND.
+   -------                      -------
+    EXCEPT                       EXCEPT
+   WET FROM                     WET FROM
+    N. ED.                       N. ED.
+   -------                      -------
+  Long foretold, long last;    First rise after low,
+  Short notice, soon past.     Foretells stronger blow.
+
+It will be perceived that the exception in each case applies to N.E.
+winds. The barometer may fall with north-easterly winds, but they will be
+violent and accompanied with rain, hail, or snow; again, it will rise with
+these winds accompanied with rain, when they are light, and bring only
+little rain. It rises, however, highest with the dry and light N.E. winds.
+
+These directions are very practically useful; they provide for
+geographical position--also for elevation above the sea--since they are
+not appended to any particular height of the column. They are suited to
+the northern hemisphere generally, as well as around the British Isles.
+The same directions are adapted to the southern hemisphere, by simply
+substituting for the letter N the letter S, reading south for north, and
+_vice versa_. South of the equator the cold winds come from the south; the
+warm, from the north. The S.E. wind in the southern hemisphere corresponds
+to the N.E. in the northern. The laws there are, while the wind veers from
+S.E. through E. to N. and N.W., the barometer falls and the thermometer
+rises. As the wind veers from N.W. through W. and S. to S.E., the
+barometer rises and the thermometer falls.
+
+
+=17. Instructions for the Sea-coast Barometer.=--The directions for fixing
+the barometer, and making it portable when it has to be removed, should be
+attended to carefully. The barometer should be suspended against a frame
+or piece of wood, so that light may be seen _through_ the tube. Otherwise
+a piece of paper, or a _white place_, should be behind the upper or _scale
+part_ of the _tube_.
+
+When suspended on a hook, or stout nail, apply the milled-head key (which
+will be found just below the scales) to the square brass pin at the lower
+end of the instrument, and turn _gently_ toward the left hand till the
+screw stops; then take off the key and replace it for use, near the scale,
+as it was before. The cistern bottom being thus _let down_, the mercury
+will sink to its proper level quickly.
+
+In removing this barometer it is necessary to _slope it gradually_, till
+the mercury is at the top of the tube, and then, with the instrument
+reversed, to screw up the cistern bottom, or bag, by the key, used
+_gently_, till it stops. It will then be portable, and may be carried with
+the _cistern_ end _upwards_, or lying flat; but it must not be jarred, or
+receive a concussion.
+
+
+=18. French Sea-coast Barometer.=--The French have imitated this form of
+barometer for coast service, and have translated Admiral FitzRoy's
+indications for the scale as follows:--
+
+          LA                            LA
+        HAUSSE                        BAISSE
+       INDIQUE.                      INDIQUE.
+       ---------                     ---------
+    DES VENTS DE LA               DES VENTS DE LA
+       PARTIE DU                     PARTIE DU
+          N.E.                          S.O.
+    (DU N.O. � l'E)               (DU S.E. � l'O.)
+    (PAR LE NORD. )               (PAR LE SUD.   )
+        DE LA                            DE
+      S�CHERESSE.                   L'HUMIDIT�.
+       ---------                     ---------
+        UN VENT                       UN VENT
+      PLUS FAIBLE                    PLUS FORT
+   EXCEPT� S'IL PLEUT            EXCEPT� S'IL PLEUT
+  AVEC DE FORTES BRISES        AVEC DE PETITES BRISES
+        DU N.E.                       DU N.E.
+       ---------                     ---------
+    Mouvements lents,             Le commencement
+     Temps durable.                de la hausse,
+       ---------                  apr�s une grande
+   Mouvements rapides,             baisse pr�sage
+    Temps variable.               un Vent violent.
+
+
+MARINE BAROMETERS.
+
+=19. The Common Form.=--The barometer is of great use to the mariner, who,
+by using it as a "weather glass," is enabled to foresee and prepare for
+sudden changes in the weather. For marine purposes, the lower portion of
+the glass tube of the barometer must be contracted to a fine bore, to
+prevent oscillation in the mercurial column, which would otherwise be
+occasioned by the movements of the ship. This tube is cemented to the
+cistern, which is made of boxwood, and has a moveable leathern bottom, for
+the purpose of rendering the instrument portable, by screwing up the
+mercury compactly in the tube. The tube is enclosed in a mahogany frame,
+which admits of a variety of style in shape, finish, and display, to meet
+the different fancies and means of purchasers. The frame is generally
+enlarged at the upper part to receive the scales and the attached
+thermometer, which are covered by plate glass. The cistern is encased in
+brass for protection, the bottom portion unscrewing to give access to the
+portable screw beneath the cistern. Figs. 13 and 14 illustrate this form
+of barometer. Marine barometers require to be suspended, so that they may
+remain in a vertical position under the changeable positions of a vessel
+at sea. To effect this they are suspended in gimbals by a brass arm. The
+gimbals consist of a loose ring fastened by thumb-screws to the middle
+part of the frame of the barometer, in front and back. The forked end of
+the arm supports this ring at the sides, also by the aid of thumb-screws.
+Hence the superior weight of the cistern end is always sufficient to cause
+the instrument to move on its bearing screws, so as always to maintain a
+perpendicular position; in fact, it is so delicately held that it yields
+to the slightest disturbance in any direction. The other end of the arm is
+attached to a stout plate, having holes for screws, or fitted to slip into
+a staple or bracket, by which it may be fixed to any part of the cabin of
+a ship; the arm is hinged to the plate, for the purpose of turning the arm
+and barometer up whenever it is desirable.
+
+[Illustration: Fig. 13.]
+
+[Illustration: Fig. 14.]
+
+Other forms of barometer (to be immediately described) have superseded
+this in the British Marine, but the French still give the preference to
+the wooden frames. They think the barometer can be more securely mounted
+in wood, is more portable, and less liable to be broken by a sudden
+concussion than if fitted in a metal frame. The English deem the ordinary
+wooden barometers not sufficiently accurate, owing to the irregular
+expansion of wood, arising from its hygrometric properties. Some of the
+English opticians have shown that very portable, and really accurate
+barometers can be made in brass frames, and therefore the preference is
+now given to this latter material.
+
+
+=20. The Kew Marine Barometer.=--The form of barometer so-called, is that
+recommended by the Congress of Brussels, held in 1853, for the purpose of
+devising a systematic plan of promoting meteorological observations at
+sea.
+
+The materials employed in its construction are mercury, glass, iron, and
+brass. The upper part of the tube is carefully calibrated to ensure
+uniformity of bore, as this is a point upon which the accuracy of the
+instrument to some extent depends. At sea, the barometer has never been
+known to stand above 31 inches, nor below 27. These extremes have been
+attained with instruments of undoubted accuracy, but they are quite
+exceptional. It is not necessary, therefore, to carry the scales of marine
+barometers beyond these limits, but they should not be made shorter. If
+the vernier is adjusted to read upward, the scale should extend to 32
+inches, to allow room for the vernier to be set to 31 inches at least.
+Cases have occurred in which this could not be done, and rare, but
+valuable observations have been lost in consequence. If the scale part of
+the tube be not uniform in bore, the index error will be irregular
+throughout the scale. Whether the bore of the rest of the tube varies in
+diameter, is of no moment. From two to three inches below the measured
+part, the bore is contracted very much, to prevent the pulsations in the
+mercurial column--called "pumping"--which, otherwise, would occur at sea
+from the motion of the ship. In ordinary marine barometers, this
+contraction extends to the end of the tube. Below the contracted part is
+inserted a pipette--or Gay Lussac air-trap--which is a little elongated
+funnel with the point downwards. Its object is to arrest any air that may
+work in between the glass and the mercury. The bubble of air lodges at the
+shoulder, and can go up no farther. It is one of those simple contrivances
+which turn out remarkably useful. If any air gets into the tube, it does
+not get to the top, and therefore does not vitiate the performance of the
+barometer; for the mercury itself works up and down through the funnel.
+Below this, the tube should not be unnecessarily contracted.
+
+[Illustration: Fig. 15.]
+
+The open end of the tube is fixed into an iron cylinder, which forms the
+cistern of the barometer. Iron has no action upon mercury, and is
+therefore used instead of any other metal. One or two holes are made in
+the top of the cistern, which are covered on the inside with strong
+sheep-skin leather, so as to be impervious to mercury, but sufficiently
+porous for the outer air to act upon the column. The cistern is of
+capacity sufficient to receive the mercury which falls out of the tube
+until the column stands lower than the scale reads; and when the tube is
+completely full, there is enough mercury to cover the extremity so as to
+prevent access of air. There is no screw required for screwing up the
+mercury.
+
+The glass tube thus secured to the cistern is protected by a brass tubular
+frame, into which the iron cistern fits and screws compactly. Cork is used
+to form bearings for the tube. A few inches above the cistern is placed
+the attached thermometer. Its bulb is enclosed in the frame, so as to be
+equally affected by heat with the barometric column. The upper end of the
+frame is fitted with a cap which screws on, and embraces a glass shield
+which rests in a gallery formed on the frame below the scale, and serves
+to protect the silvered scale, as well as the inner tube, from dust and
+damp. A ring, moveable in a collar fixed on the frame above the centre of
+gravity of the instrument, is attached to gimbals, and the whole is
+supported by a brass arm in the usual manner; so that the instrument can
+be moved round its axis to bring any source of light upon it, and will
+remain vertical in all positions of the ship. The vernier reads to
+five-hundredths of an inch. No words are placed upon the scale, as the old
+formulary was deemed misleading. The vernier can be set with great
+exactness, as light is admitted to the top of the mercury by a front and
+a back slit in the frame. The lower edge of the vernier should be brought
+to the top of the mercury, so as just to shut out the light.
+
+It is evident that this form of barometer must be more reliable in its
+indications than those in wooden frames. The graduations can be accurately
+made, and they will be affected only by well-known alterations due to
+temperature. Some think the tube is too firmly held, and therefore liable
+to be broken by concussion more readily than that of an inferior
+instrument. This, however, appears a necessary consequence of greater
+exactness. It is an exceedingly good portable instrument, and can be put
+up and taken down very readily. These barometers are preferred to marine
+barometers in wood, wherever they have been used. In merchant ships, and
+under careful treatment, they have been found very durable. They may be
+sent with safety by railway, packed carefully in a wooden box.
+
+_Directions for Packing._--In removing this barometer it is necessary to
+slope it gradually till the mercury reaches the top of the tube. It is
+then portable, if carried cistern end upwards or lying flat. If carried
+otherwise, it will very probably be broken by the jerking motion of the
+heavy mercury in the glass tube. Of course it must not be jarred, or
+receive concussion.
+
+_Position for Marine Barometer._--Admiral FitzRoy, to whose valuable
+papers we are much indebted, writes in his "Barometer Manual":--"It is
+desirable to place the barometer in such a position as not to be in danger
+of a side blow, and also sufficiently far from the deck above to allow for
+the spring of the metal arm in cases of sudden movements of the ship.
+
+"If there is risk of the instrument striking anywhere when the vessel is
+much heeled, it will be desirable to put some soft padding on that place,
+or to check movement in that direction by a light elastic cord; in fixing
+which, attention must be paid to have it acting only where risk of a blow
+begins, not interfering otherwise with the free swing of the instrument: a
+very light cord attached above, when possible, will be least likely to
+interfere injuriously."
+
+
+=21. Method of verifying Marine and other Barometers.=--"In nearly all the
+barometers which had been employed at sea till recently the index
+correction varied through the range of scale readings, in proportion to
+the difference of capacity between the cistern and the tube. To find the
+index correction for a land barometer, comparison with a standard, at any
+part of the scale at which the mercury may happen to be, is generally
+considered sufficient. To test the marine barometer is a work of much more
+time, since it is necessary to find the correction for scale readings at
+about each half inch throughout the range of atmospheric pressure to which
+it may be exposed; and it becomes necessary to have recourse to artificial
+means of changing the pressure of the atmosphere on the surface of the
+mercury in the cistern.
+
+"The barometers to be thus tested are placed, together with a standard, in
+an air-tight chamber, to which an air-pump is applied, so that, by
+partially exhausting the air, the standard can be made to read much lower
+than the lowest pressure to which marine barometers are likely to be
+exposed; and by compressing the air it can be made to read higher than the
+mercury ever stands at the level of the sea. The tube of the standard is
+contracted similarly to that of the marine barometer, but a provision is
+made for adjusting the mercury in its cistern to the zero point. Glass
+windows are inserted in the upper part of the iron air-chamber, through
+which the scales of the barometers may be seen; but as the verniers cannot
+be moved in the usual way from outside the chamber, a provision is made
+for reading the height of the mercury independent of the verniers attached
+to the scales of the respective barometers. At a distance of some five or
+six feet from the air-tight chamber a vertical scale is fixed. The
+divisions on this scale correspond exactly with those on the tube of the
+standard barometer. A vernier and telescope are made to slide on the scale
+by means of a rack and pinion. The telescope has two horizontal wires, one
+fixed and the other moveable by a micrometer screw, so that the difference
+between the height of the column of mercury and the nearest division on
+the scale of the standard, and also of all the other barometers placed by
+the side of it for comparison, can be measured either with the vertical
+scale and vernier or the micrometer wire. The means are thus possessed of
+testing barometers for index error in any part of the scale, through the
+whole range of atmospheric pressure to which they are likely to be
+exposed; and the usual practice is to test them at every half inch from
+27�5 to 31 inches.
+
+"In this way barometers of various other descriptions have been tested,
+and some errors found to be so large that a few barometers read half an
+inch and upwards too high, while others read as much too low. In some
+cases those which were correct in one part of the scale were found to be
+from half an inch to an inch wrong in other parts. These barometers were
+of an old and ordinary, not to say inferior, construction. In some the
+mercury would not descend lower than about 29 inches, owing to a fault
+very general in the construction of many common barometers till lately in
+frequent use:--the _cistern was not large enough_ to hold the mercury
+which descended from the tube in a _low atmospheric pressure_.
+
+"When used on shore, this contraction of the tube causes the marine
+barometer to be _sometimes_ a little behind an ordinary land barometer,
+the tube of which is not contracted. The amount varies according to the
+rate at which the mercury is rising or falling, and ranges from 0�00 to
+0�02 of an inch. As the motion of the ship at sea causes the mercury to
+pass more rapidly through the contracted tube, the readings are almost the
+same there as they would be if the tube were not contracted, and in no
+case do they differ enough to be of importance in maritime use."
+
+The cistern of this marine barometer is generally made an inch and a
+quarter in diameter, and the scale part of the tube a quarter of an inch
+in bore. The inches on the scale, instead of being true, are shortened by
+�04 of an inch, in order to avoid the necessity of applying a correction
+due to the difference of capacity of the tube and cistern. This is done
+with much perfection, and the errors of the instruments, when compared
+with a standard by the apparatus used at Kew and Liverpool Observatories,
+are determined to the thousandth of an inch, and are invariably very
+uniform and small. The error so determined includes the correction due to
+capillarity, capacity, and error of graduation, and forms a constant
+correction, so that only one variable correction, that due to temperature,
+need be applied, when the barometer is suspended near the water line of
+the ship, to make the observations comparable with others. With all the
+advantages of this barometer, however, it has recently been superseded, to
+some extent, because it was found to require more care than could
+ordinarily be expected to be given to it by the commander of a ship.
+Seamen do not exactly understand the value of such nice accuracy as the
+thousandth part of an inch, but prefer an instrument that reads only to a
+hundredth part.
+
+
+22. THE FITZROY MARINE BAROMETER.
+
+Admiral FitzRoy deemed it desirable to construct a form of barometer as
+practically useful as possible for marine purposes. One that should be
+less delicate in structure than the Kew barometer, and not so finely
+graduated. One that could be set at a glance and read easily; that would
+be more likely to bear the common shocks unavoidable in a ship of war.
+Accordingly, the Admiral has devised a barometer, which he has thus
+described:--
+
+"This marine barometer, for Her Majesty's service, is adapted to _general_
+purposes.
+
+"It differs from barometers hitherto made in points of detail, rather than
+principle:--1. The glass tube is packed with vulcanised india-rubber,
+which checks vibration from concussion; but does not hold it rigidly, or
+prevent expansion. 2. It does not oscillate (or pump), though extremely
+sensitive. 3. The scale is porcelain, _very legible_, and not liable to
+change. 4. There is no iron anywhere (_to rust_). 5. Every part can be
+unscrewed, examined, or cleaned, by any careful person. 6. There is a
+_spare_ tube, fixed in a cistern, filled with boiled mercury, and _marked_
+for adjustment in this, or _any similar_ instrument.
+
+"These barometers are graduated to hundredths, and they will be found
+accurate to _that_ degree, namely the second decimal of an inch.
+
+"They are packed with vulcanised caoutchouc, in order that (by this, and
+by a peculiar strength of glass tube) guns may be fired near these
+instruments without causing injury to them by ordinary concussion.
+
+"It is hoped that all such instruments, for the public service at sea,
+will be quite similar, so that any spare tube will fit _any_ barometer.
+
+"_To Shift a Tube._--Incline the barometer slowly, and then take it down,
+after allowing the mercury to fill the upper part. Lay the instrument on a
+table, unscrew the outer cap at the joining just below the cistern swell,
+then unscrew the tube _and_ cistern, by turning the cistern gently,
+against the sun, or to _the left_, and draw out the tube very carefully
+_without bending it in the least_, _turning_ it a little, if required, as
+moved. Then insert the new tube very cautiously, screw in, and adjust to
+the diamond-cut mark for 27 inches. Attach the cap, and suspend the
+barometer for use.
+
+"If the mercury does not immediately quit the top of the tube, tap the
+cistern end rather sharply. In a well-boiled tube, with a good vacuum, the
+mercury hangs, at times, so adhesively as to deceive, by causing a
+supposition of some defect.
+
+"In about ten minutes the mercurial column should be nearly right; but as
+local temperature affects the brass, as well as the mercury, slowly and
+unequally, it may be well to defer any _exact comparisons with other
+instruments_ for some few hours."
+
+Messrs. Negretti and Zambra are the makers of these barometers for the
+Royal Navy. Fig. 16 is an illustration.
+
+[Illustration: Fig. 16.]
+
+The tube is fixed to a boxwood cistern, which is plugged with very porous
+cane at the top, to allow of the ready influence of a variation in
+atmospheric pressure upon the mercury. Round the neck of the cistern is
+formed a brass ring, with a screw thread on its circumference. This screws
+into the frame, and a mark on the tube is to be adjusted to 27 inches on
+the scale, the cistern covering screwed on, and the instrument is ready to
+suspend. The frame and all the fittings are brass, without any iron
+whatever; because the contact of the two metals produces a galvanic
+action, which is objectionable. The spare tube is fitted with
+india-rubber, and ready at any time to replace the one in the frame. The
+ease with which a tube can be replaced when broken is an excellent feature
+of the instrument. The spare tube is carefully stowed in a box, which can
+also receive the complete instrument when not in use. All the parts are
+made to a definite gauge; the frames are, therefore, all as nearly as
+possible similar to each other, and the tubes--like rifle bullets--are
+adjustible to any frame. If, then, the tube in use gets broken, the
+captain can replace it by the other; but, as it is securely packed with
+india-rubber, there is very little liability of its being broken by fair
+usage. Every person who knows the importance of the barometer on board
+ship, will acknowledge that the supplementary tube is a decided
+improvement. Many instruments of this description are afloat in the Royal
+Navy, and in a short time it may be expected that all the frames and tubes
+of barometers in the public service at sea will be similar in size and
+character; so that should a captain have the misfortune to get both his
+tubes broken, he would be able to borrow another from any ship he fell in
+with that had one to spare, which would be perfectly accurate, because it
+would have been verified before it was sent out.
+
+
+=23. Admiral FitzRoy's Words for the Scale.=--The graduation of inches and
+decimals are placed in this barometer on the right-hand side of the tube;
+and on a similar piece of porcelain, on the left-hand, are engraved, as
+legibly as they are expressed succinctly, the following words, of
+universal application in the interpretation of the barometer movements:--
+
+     _RISE_            _FALL_
+      FOR               FOR
+      COLD              WARM
+      DRY               WET
+       OR                OR
+      LESS              MORE
+      WIND.             WIND.
+    --------          --------
+     EXCEPT            EXCEPT
+    WET FROM          WET FROM
+   COOLER SIDE.      COOLER SIDE.
+
+Reverting to the explanation of the words on the "Coast" barometers (at
+page 14), and comparing and considering them as given for northern
+latitudes, and as they must be altered for southern latitudes, it will be
+perceived, that for all _cold_ winds the barometer rises; and falls for
+_warm_ winds. The mercury also falls for _increased_ strength of wind; and
+rises as the wind _lulls_. Likewise before or with rain the column of
+mercury falls; but it rises with fine dry weather. Putting these facts
+together, and substituting for the points of the compass the terms "cold"
+and "warm," the appropriateness of the words on the scale of this
+barometer is readily perceived. These concise and practical indications of
+the movements in the barometer are applicable for instruments intended for
+use in any region of the world, and are in perfect accordance with the
+laws of winds and weather deduced by Dov� and other meteorologists. There
+is nothing objectionable in them, and being founded upon experience and
+the deductions made from numerous recorded observations of the weather in
+all parts of the world, as well as confirmed by the theories of science,
+they may consequently be considered as generally reliable. They involve no
+conjecture, but express succinctly scientific principles.
+
+
+=24. Trials of the FitzRoy Marine Barometer under Fire of Guns.=--Some of
+the first barometers made by Messrs. Negretti and Zambra on Admiral
+FitzRoy's principle were severely tried under the heaviest naval gun
+firing, on board H.M.S. _Excellent_; and under all the circumstances, they
+withstood the concussion. The purpose of the trials was "to ascertain
+whether the _vulcanized india-rubber packing_ round the glass tube of a
+_new marine barometer_ did check the vibration caused by firing, and
+whether guns might be fired close to these instruments without causing
+injury to them." In the first and second series of experiments, a marine
+barometer on Admiral FitzRoy's plan was tried against a marine barometer
+on the Kew principle, both instruments being new, and treated in all
+respects similarly. They were "hung over the gun, under the gun, and by
+the side of the gun, the latter both inside and outside a bulkhead,--in
+fact, in all ways that they would be tried in action with the bulkheads
+cleared away." The result was that the Kew barometer was broken and
+rendered useless, while the new pattern barometer was not injured in the
+least. In a third series of experiments, Mr. Negretti being present, five
+of the new pattern barometers were subjected to the concussion produced by
+firing a 68-pounder gun with shot, and 16 lbs. charge of powder. They were
+suspended from a beam immediately under the gun, then from a beam
+immediately over the gun, and finally they were suspended by the arm to a
+bulkhead, at a distance of only 3 ft. 6 in. from the axis of the gun; and
+the result was, according to the official report, "that all these
+barometers, however suspended, would stand, without the slightest injury,
+the most severe concussion that they would ever be likely to experience in
+any sea-going man-of-war." These trials were conducted under the
+superintendence of Captain Hewlett, C.B., and the guns were fired in the
+course of his _usual_ instructions. His reports to Admiral FitzRoy, giving
+all the particulars of the trials, are published in the "Ninth Number of
+Meteorological Papers," issued by the Board of Trade.[2]
+
+
+25. NEGRETTI AND ZAMBRA'S FARMER'S BAROMETER AND DOMESTIC WEATHER-GLASS.
+
+It is a well-known fact that the barometer is as much, or even more
+affected by a change of wind as it is by rain; and the objection raised
+against a simple barometer reading, as leaving the observer in doubt
+whether to expect wind or rain, is removed by the addition of the
+Hygrometer, an instrument indicating the comparative degree of dryness or
+dampness of the air;--a most important item in the determination of the
+coming weather.
+
+The farmer should not be content to let his crops lie at the mercy, so to
+speak, of the weather, when he has within his command instruments which
+may be the means of preventing damage to, and in cases total loss of, his
+crops.
+
+The farmer hitherto has had to depend for his prognostication of the
+weather on his own unassisted "Weather Wisdom;" and it is perfectly
+marvellous how expert he has become in its use. Science now steps in, not
+to ignore this experience, but on the contrary, to give it most valuable
+assistance by extending it, and enabling it to predict, with an accuracy
+hitherto unknown, the various changes that take place in this most
+variable of climates.
+
+To the invalid, the importance of predicting with tolerable accuracy the
+changes that are likely to occur in the weather, cannot be over-rated.
+Many colds would be prevented, if we could know that the morning so balmy
+and bright, would subside into a cold and cheerless afternoon. Even to the
+robust, much inconvenience may be prevented by a due respect to the
+indications of the hygrometer and the barometer, and the delicate in
+health will do well to regard its warnings.
+
+[Illustration: Fig. 17.]
+
+_Description of the Instrument._--The farmer's barometer, as figured in
+the margin, consists of an upright tube of mercury inverted in a cistern
+of the same fluid; this is secured against a strong frame of wood, at the
+upper end of which is fixed the scale, divided into inches and tenths of
+an inch. On either side of the barometer, or centre tube, are two
+thermometers--that on the left hand has its bulb uncovered and freely
+exposed, and indicates the temperature of the air at the place of
+observation; that on the right hand has its bulb covered with a piece of
+muslin, from which depend a few threads of soft lamp cotton; this cotton
+is immersed in the small cup situated just under the thermometer, this
+vessel being full of water; the water rises by capillary attraction to the
+muslin-covered bulb, and keeps it in a constantly moist state.
+
+These two thermometers, which we distinguish by the names "Wet Bulb" and
+"Dry Bulb," form the Hygrometer; and it is by the simultaneous reading of
+these two thermometers, and noting the difference that exists between
+their indications, that the humidity in the atmosphere is determined.
+
+Admiral FitzRoy's words (see p. 22) are placed upon the scale of the
+barometer, as the value of a reading depends, not so much on the actual
+height of the mercury in the tube, as it does on whether the column is
+rising, steady, or falling.
+
+The moveable screw at the bottom of the cistern is for the purpose of
+forcing the mercury to the top of the tube when the instrument is being
+carried from place to place, and it must always be unscrewed to its utmost
+limit when the barometer is hung in its proper place. After this it should
+never be touched.
+
+The manner in which the Hygrometer acts is as follows: It is a pretty
+well-known fact that water or wine is often cooled by a wet cloth being
+tied round the bottle, and then being placed in a current of air. The
+evaporation that takes place in the progressive drying of the cloth causes
+the temperature to fall considerably below that of the surrounding
+atmosphere, and the contents of the bottle are thus cooled. In the same
+manner, then, the covered wet bulb thermometer will be found _invariably_
+to read lower than the uncovered one; and the greater the dryness of the
+air, the greater will be the difference between the indications of the
+two thermometers; and the more moisture that exists in the air, the more
+nearly they will read alike.
+
+The cup must be kept filled with pure water, and occasionally cleaned out,
+to remove any dirt. The muslin, or cotton-wick, should also be renewed
+every few weeks. The hygrometer may be had separate from the barometer, if
+the combined instruments cannot be sufficiently exposed to the external
+air, this being essential for the successful use of the hygrometer.
+
+This farmer's weather-glass, then, consists of three distinct instruments:
+the barometer, the thermometer, and the hygrometer. He has thus at command
+the three instrumental data necessary for the prediction of the weather.
+And now to describe--
+
+_How to Use the Instrument._--The observations should be taken twice a
+day, say at 9 A.M. and 3 P.M.; and should be entered on a slip of paper,
+or a slate hung up by the barometer. The observer will then be able to see
+the different values of the readings from time to time, and to draw his
+conclusions therefrom.
+
+The thermometer on the left hand should first be read, and a note made of
+its indication, which is the temperature of the air. The wet bulb
+thermometer should now be read, and also noted; and the difference should
+be taken of these two readings. Next read the barometer by moving the
+small index at the side of the tube until it is on a level with the top of
+the mercury. Having noted the number of inches at which the column stands,
+compare with the last observation, and see immediately whether the
+barometer is rising, steady, or falling.
+
+Now, having taken the observations as above, we naturally ask the
+question, _What are we to predict from them?_
+
+And, probably, the best way of answering this query will be by giving an
+example. We will suppose that our readings yesterday were as
+follows:--Temperature, 70�; Wet Bulb, 69�; Difference, 1�; =very moist
+air. Barometer, 29�5, and that rain has fallen.
+
+To-day, we read:--Temperature, 60�; Wet Bulb, 55�; Difference, 5�; =dryer
+air. Barometer, 30. We may safely predict that the rain will cease, and
+probably we may have wind from the northward.
+
+In spring or autumn, if the barometric height be steady any where between
+29�5 and 30 inches, with the temperature about 60�, fresh to moderate
+south-westerly winds, with cloudy sky, will probably characterize the
+weather; the indications of the hygrometer being then specially
+serviceable in enabling us to foretell rain; but if the mercury become
+steady at about 30�5 inches, with temperature about 40�, north-easterly
+winds, dry air, and clear sky, may be confidently expected.
+
+Many cases will doubtless suggest themselves to the observer where these
+figures do not occur, and where he might find a difficulty in interpreting
+the indications of his instruments. We have, therefore, drawn up some
+concise rules for his guidance; and although they will not prove
+absolutely infallible guides to this acknowledged most difficult problem,
+still, they will be found of much service in foretelling the weather, when
+added to an intelligent observation of ordinary atmospheric phenomena, as
+force and direction of wind, nature of any particular season, and the time
+of year.
+
+
+26. RULES FOR FORETELLING THE WEATHER.
+
+A RISING BAROMETER.
+
+    A "Rapid" rise indicates unsettled weather.
+
+    A "Gradual" rise indicates settled weather.
+
+    A "Rise," with dry air, and cold increasing in summer, indicates wind
+    from northward; and if rain has fallen, better weather is to be
+    expected.
+
+    A "Rise," with moist air and a low temperature, indicates wind and
+    rain from northward.
+
+    A "Rise," with southerly wind, indicates fine weather.
+
+A STEADY BAROMETER,
+
+    With dry air and a seasonable temperature, indicates a continuance of
+    very fine weather.
+
+A FALLING BAROMETER.
+
+    A "Rapid" fall indicates stormy weather.
+
+    A "Rapid" fall, with westerly wind, indicates stormy weather from
+    northward.
+
+    A "Fall," with a northerly wind, indicates storm, with rain and hail
+    in summer, and snow in winter.
+
+    A "Fall," with increased moisture in the air, and the heat increasing,
+    indicates wind and rain from southward.
+
+    A "Fall," with dry air, and cold increasing (in winter), indicates
+    snow.
+
+    A "Fall," after very calm and warm weather, indicates rain with
+    squally weather.
+
+
+=27. Causes which may bring about a Fall or a Rise in the
+Barometer.=[3]--As heat produces rarefaction, a sudden rise of temperature
+in a distant quarter may affect the weight of the atmosphere over our
+heads, by producing an aerial current outwards, to supply the place of the
+lighter air which has moved from its former position; in which case the
+barometer will fall. Now such a movement in the atmosphere is likely to
+bring about an intermixture of currents of air of different temperatures,
+and from this intermixture rain is likely to result.
+
+On the other hand, as cold produces condensation, any sudden fall of
+temperature causes the column of air over the locality to contract and
+sink to a lower level, whilst other air rushes in from above to supply the
+void; and, accordingly, the barometer rises. Should this air, as often
+happens, proceed from the north, it will contain in general but little
+moisture; and hence, on reaching a warmer latitude, will take up the
+vapour of the air, so that dry weather will result.
+
+It is generally observed, that wind causes a fall in the instrument; and,
+indeed, in those greater movements of the atmosphere which we denominate
+storms or hurricanes, the depression is so considerable as to forewarn
+the navigator of his impending danger. It is evident, that a draught of
+air in any direction must diminish the weight of the column overhead, and
+consequently cause the mercury in the barometer to sink.
+
+The connection, therefore, of a sinking of the barometric column with rain
+is frequently owing to the wind causing an intermixture of the aerial
+currents which, by their motion, diminish the weight of the atmosphere
+over our heads; whilst a steady rise in the column indicates the absence
+of any great atmospheric changes in the neighbourhood, and a general
+exemption from those causes which are apt to bring about a precipitation
+of vapour.
+
+
+=28. Use of the Barometer in the management of Mines.=--The inflammable
+and suffocating gases, known to coal-miners as fire-damp and choke-damp,
+are specifically heavier than air; and as they issue from the fissures of
+the mine, or are released from the coal, the atmospheric pressure tends to
+drive them into the lowest and least ventilated galleries. Consequently a
+greatly reduced atmospheric pressure will favour a sudden outflow or
+advance of gas; whence may result cases of explosion or suffocation. It
+has been found that these accidents occur for the most part about the time
+of a low barometric column. A reliable barometer should, therefore, be
+systematically consulted by those entrusted with the management or control
+of coal-mines, so that greater vigilance and caution may be enjoined on
+the miners whenever the mercury falls low, especially after it has been
+unusually high for some days.
+
+
+=29. Use of the Barometer in estimating the Height of Tides.=--The
+pressure of the atmosphere affects the height of the tide, the water being
+in general higher as the barometer is lower. The expressions of seamen,
+that "frost nips the tide," and "fog nips the tide," are explained by the
+high barometer which usually accompanies frost and fog. M. Daussy, Sir J.
+C. Ross, and others, have established that a rise of one inch in the
+barometer will have a corresponding fall in the tide of about one foot.
+Therefore navigators and pilots will appreciate the following suggestion
+of Admiral FitzRoy:--
+
+"Vessels sometimes enter docks, or even harbours, where they have scarcely
+a foot of water more than their draught; and as docking, as well as
+launching large ships, requires a close calculation of height of water,
+the state of the barometer becomes of additional importance on such
+occasions."
+
+
+
+
+CHAPTER II.
+
+SYPHON TUBE BAROMETERS.
+
+
+=30. Principle of.=--If some mercury, or any other fluid, be poured into a
+tube of glass, bent in the form of =U=, and open at both ends, it will
+rise to the same height in both limbs, the tube being held vertically. If
+mercury be poured in first, and then water upon it at one end, these
+liquids will not come to the same level; the water will stand much higher
+than the mercury. If the height of the mercury, above the line of meeting
+of the fluids, be one inch, that of the water will be about
+thirteen-and-a-half inches. The explanation of this is, that the two
+columns balance each other. The pressure of the atmosphere in each limb is
+precisely similar; but the one column stands so much higher than the
+other, because the fluid of which it is composed is so much lighter, bulk
+for bulk, than the other. If one end of the tube be hermetically closed,
+the other limb be cut off within a few inches of the bend, and the tube
+carefully filled with mercury; by placing it in a vertical position, the
+mercury will fall, if the closed limb be long enough, until it is about
+thirty inches higher than that in the exposed limb, where it will remain.
+Here the atmosphere presses upon the short column; but not upon the long
+one. It is this pressure, therefore, which maintains the difference of
+level. In fact, it forms a barometer without a cistern, the short limb
+answering the purpose of a cistern. The first barometers on this principle
+were devised by the celebrated philosopher, Dr. Hook, as described in the
+next section.
+
+
+31. DIAL, OR WHEEL BAROMETERS.
+
+The familiar household "Weather Glasses" are barometers on the syphon
+principle. The portions of the two limbs through which the mercury will
+rise and fall with the varying pressure of the atmosphere are made of
+precisely the same diameter; while the part between them is contracted. On
+the mercury, in the exposed limb, rests a round float of ivory or glass;
+to this a string is attached and passed over and around a brass pulley,
+the other end carrying another lighter weight. The weight resting on the
+mercury rises and falls with it. On the spindle of the pulley, which
+passes through the frame and centre of the dial-plate, is fixed a light
+steel hand, which revolves as the pulley turns round. When the mercury
+falls for a decrease of atmospheric pressure, it rises by the same
+quantity in the short tube, and pushes up the float, the counterpoise
+falls, and thus moves the hand or pointer to the left. When the pressure
+increases, the pointer is drawn in a similar manner to the right.
+
+[Illustration: Fig. 18.]
+
+[Illustration: Fig. 19.]
+
+[Illustration: Fig. 20.]
+
+The dials are generally made of metal silvered over or enamelled, but
+porcelain may be used. If the circumference of the pulley, or "wheel," be
+two inches, it will revolve once for an alteration of level amounting to
+two inches in each tube, or four inches in the height of the barometric
+column; and as the dial may be from twenty to thirty-six inches in
+circumference, five to nine inches on the graduated scale corresponds to
+one inch of the column; and hence the sub-divisions are distinctly
+perceptible, and a vernier is not necessary.
+
+The motion of the pointer alone is visible; and a mahogany, or rosewood,
+frame, supports, covers, and renders the instrument ornamental and
+portable. In the back of the frame is a hinged door, which covers the
+cavity containing the tube and fixtures. The dial is covered by a glass in
+a brass rim, similar to a clock face. A brass index, working over the
+dial, moveable by a key or button, may be applied, and will serve to
+register the position of the hand when last observed. These instruments
+are usually fitted with a thermometer, and a spirit level; the latter for
+the purpose of getting the instrument perfectly vertical. They sometimes
+have, in addition, a hygrometer, a sympiesometer, an aneroid, a mirror, or
+a clock, &c., singly or combined. The frame admits of much variety of
+style and decoration. It may be carved or inlaid. The usual adjustment of
+scale is suited for localities at no considerable elevation above the
+sea. Accordingly, being commercial articles, they have been found
+frequently quite out of place. When intended for use at high elevations,
+they should have a special adjustment of scale. As household instruments
+they are serviceable, and ornamental. But the supply-and-demand principle
+upon which they are sold, has entailed upon those issued by inferior
+makers a generally bad adjustment of scale. The illustrations are those of
+ordinary designs.
+
+[Illustration: Fig. 21.]
+
+[Illustration: Fig. 22.]
+
+[Illustration: Fig. 23.]
+
+Dial barometers required for transmission to distant parts, as India and
+the Colonies, are furnished with a steel stop-cock, to render them
+portable more effectually than can be done by the method of _plugging_ the
+tube.
+
+
+32. STANDARD SYPHON BAROMETER.
+
+Fig. 24 represents the most accurate form of the Gay Lussac barometer. The
+short limb is closed at the top, after the mercury is introduced, and a
+small lateral puncture is made at _a_, which is covered over with a
+substance which permits the access of air, but prevents the escape of any
+mercury when the instrument is packed for travelling. The bent part of the
+tube is contracted to a capillary bore; and just above this, in the long
+limb, is placed the air-trap, already described (see p. 17), and here
+illustrated (fig. 25). When reversed, as it must be for portability, the
+capillary attraction keeps the mercury in the long branch. Should the
+mercury of the short column get detached, some small quantity of air _may_
+pass; but it will be arrested at the pipette, and will not vitiate the
+length of the barometric column. It can be easily expelled by gently
+shaking or tapping the instrument before suspending it for observation. In
+the illustration, the zero of the scale is placed at Z, near the middle of
+the tube; and the graduations extend above and below. In making an
+observation, it is necessary to take the reading ZA on the long branch,
+and ZB on the short one. The sum of the two gives the height of the
+barometer. The zero of the scale in some instruments is placed low down,
+so as to require the difference of the two readings to be taken. A
+thermometer is attached to the frame as usual.
+
+[Illustration: Fig. 24.]
+
+[Illustration: Fig. 25.]
+
+These instruments can be very accurately graduated, and are very exact in
+their indications, provided great care has been exercised in selecting the
+tubes, which must be of the same calibre throughout the parts destined to
+measure the variations of atmospheric pressure. They should be suspended
+so as to insure their hanging vertically.
+
+The syphon barometer does not require correction for capillarity nor for
+capacity, as each surface of the mercury is equally depressed by capillary
+attraction, and the quantity of mercury which falls from the long limb of
+the tube occupies the same length in the short one. The barometric height
+must, however, be corrected for temperature, as in the cistern barometer.
+Tables containing the temperature corrections to be applied to barometer
+readings for scales engraved on the glass tube, or on brass or wood
+frames, are published.
+
+
+
+
+CHAPTER III.
+
+BAROGRAPHS, OR SELF-REGISTERING BAROMETERS.
+
+
+=33. Milne's Self-Registering Barometer.=--For a long time a good and
+accurate self-recording barometer was much desired. This want is now
+satisfactorily supplied, not by one, but by several descriptions of
+apparatus. The one first to be described was the design of Admiral Sir A.
+Milne, who himself constructed, in 1857, we believe, the original
+instrument, which he used with much success. Since that time several of
+these instruments have been made, and have performed satisfactorily. The
+barometer tube is a syphon of large calibre, provided with a Gay Lussac
+pipette, or air-trap; and fitted with a float, a wheel, and a pointer, as
+in the "Dial" barometer. The float is attached to a delicate watch-chain,
+which passes over the wheel and is adequately counterpoised. Behind the
+indicating extremity of the pointer or hand is a projecting point, which
+faces the frame of the instrument, and is just within contact with the
+registering paper. A clock is applied, and fitted with auxiliary
+mechanism, so as to be able to move the mounted paper with regularity
+behind the pointer, and at designed equal intervals of time to release a
+system of levers and springs, so as to cause the marker to impress a dot
+on the paper, either by puncture or pencil-mark. The paper is ruled with
+horizontal lines for the range of the mercurial column, and parallel arcs
+of circles for the hours. Thus the barometer is rendered self-recording,
+by night or day, for a week or more; hence the great value of the
+instrument. The clock, index, and registering mechanism are protected from
+dust and interference by a glass front, hinged on and locked. As the
+temperature of the mercury is not registered, there is fixed to the frame
+a Sixe's thermometer to record the maximum and minimum temperatures, which
+should be noted at least every twenty-four hours.
+
+Admiral FitzRoy has suggested the name "Atmoscope" for Admiral Milne's
+barometer; and he has also termed it a "Barograph." This latter word
+appears to be applicable to all kinds of self-registering barometers
+hitherto designed. Of the arrangement under consideration Admiral FitzRoy
+writes:--"It shows the alterations in tension, or the pulsations, so to
+speak, of atmosphere, on a large scale, by hourly marks; and the diagram
+expresses, to a practised observer, what the 'indicator card' of a
+steam-cylinder shows to a skilful engineer, or a stethescope to a
+physician."
+
+
+[Illustration: Fig. 26.]
+
+=34. Modification of Milne's Barometer.=--The great difficulty to be
+overcome in Milne's barometer, is to adjust the mechanism for obtaining
+registration so that the action of the striker upon the pointer should not
+in the slightest degree move it from its true position. A different mode
+of registration, capable of recording accurately the least appreciable
+movement of the mercurial column, has been effected. In this instrument
+the registering paper is carried upon a cylinder or drum. By reference to
+the illustration, Fig. 26, the details of construction will be readily
+understood. It should, however, be mentioned, that it is not a picture of
+the outward appearance of the instrument. The position of the barometer
+should be behind the clock; it is represented on one side merely for the
+purpose of clearly illustrating the arrangement and principles. The
+instrument has a large syphon barometer tube, in which the mercurial
+column is represented. On the mercury at _A_, in its open end, rests a
+glass float, attached to a watch-chain, or suitable silken cord, the other
+end of which is connected to the top of the arched head on the short arm
+of a lever-beam. The long arm of the beam is twice the length of the short
+arm, for the following reason. As the mercury falls in the long limb, it
+rises through an equal space in the short limb of the tube, and _vice
+versa_. But the barometric column is the difference of height of the
+mercury in the two limbs; hence the rise or fall of the float through
+half-an-inch will correspond to a decrease or an increase of the
+barometric column of one inch. In order, then, to record the movements of
+the barometric column, and not those of the float, the arm of the beam
+connected with the float is only half the radius of the other arm. Both
+arms of the beam carry circular-arched heads, which are similar portions
+of the complete circles, the centre of curvature being the fulcrum, or
+axis. This contrivance maintains the leverage on each extremity of the
+beam always at the same distance from the fulcrum. From the top of the
+large arched head a piece of watch-chain descends, and is attached to the
+marker, _B_, which properly counterpoises the float, _A_, and is capable
+of easy movement along a groove in a brass bar, so as to indicate the
+barometric height on an ivory scale, _C_, fixed on the same vertical
+framing. On the opposite side of the marker, _B_, is formed a metallic
+point, which faces the registration sheet and is nearly in contact with
+it. The framing, which carries the scale and marker, is an arrangement of
+brass bars, delicately adjusted and controlled by springs, so as to permit
+of a quick horizontal motion, in a small arc, being communicated to it by
+the action of the hammer, _E_, of the clock, whereby the point of the
+marker is caused to impress a dot upon the paper. The same clock gives
+rotation to the hollow wooden cylinder, _D_, upon which is mounted the
+registering paper. The clock must be rewound when a fresh paper is
+attached to the cylinder, which may be daily, weekly, or monthly,
+according to construction; and the series of dots impressed upon the paper
+shows the height of the barometric column every hour by day and night. The
+space traversed by the marker is precisely equal to the range of the
+barometric column.
+
+
+=35. King's Self-Registering Barometer.=--Mr. Alfred King, Engineer of the
+Liverpool Gas-light Company, designed, so long ago as 1854, a barometer to
+register, by a continuous pencil-tracing, the variations in the weight of
+the atmosphere; and a highly-satisfactory self-recording barometer, on his
+principle and constructed under his immediate superintendence, has quite
+recently been erected at the Liverpool Observatory.
+
+[Illustration: Fig. 27.]
+
+Fig. 27 is the front elevation of this instrument. _A_, the barometer
+tube, is three inches in internal diameter, and it floats freely (not
+being fixed as usual) in the fixed cistern, _B_, guided by
+friction-wheels, _W_. The top end of the tube is fastened to a peculiar
+chain, which passes over a grooved wheel turning on finely-adjusted
+friction rollers. The other end of the chain supports the frame, _D_,
+which carries the tracing pencil. The frame is suitably weighted and
+guided, and faces the cylinder, _C_, around which the tracing paper is
+wrapped, and which rotates once in twenty-four hours by the movement of a
+clock. Mr. Hartnup, Director of the Liverpool Observatory, in his Annual
+Report, 1868, says:--"For one inch change in the mercurial column the
+pencil is moved through five inches, so that the horizontal lines on the
+tracing, which are half an inch apart, represent one-tenth of an inch
+change in the barometer. The vertical lines are hour lines, and being
+nearly three-quarters of an inch apart, it will be seen that the smallest
+appreciable change in the barometer, and the time of its occurrence, are
+recorded."
+
+"It has been remarked by persons in the habit of reading barometers with
+large tubes, that, in squally weather, sudden and frequent oscillations
+of the mercurial column are sometimes seen. Now, to register these small
+oscillations must be a very delicate test of the sensitiveness of a
+self-registering barometer, as the time occupied by the rise and fall of
+the mercury in the tube in some cases does not exceed one minute." Mr.
+Hartnup affirms that the tracing of this instrument exhibits such
+oscillations whenever the wind blows strong and in squalls.
+
+As the barometer in this instrument is precisely similar to the "Long
+Range Barometer" invented by Mr. McNeild (and which will be found
+described at page 48), it may be desirable to quote the following, from
+Mr. Hartnup's Report:--"Mr. King constructed a small model instrument to
+illustrate the principle. This instrument was entrusted to my care for
+examination, and it was exhibited to the scientific gentlemen who visited
+the Observatory in 1854, during the meeting of the British Association for
+the Advancement of Science."
+
+
+=36. Syphon, with Photographic Registration.=--A continuous
+self-registering barometer has been constructed, in which photography is
+employed. Those who may wish to adopt a similar apparatus, or thoroughly
+to understand the arrangements and mode of observation, should consult the
+detailed description given in the _Greenwich Magnetical and Meteorological
+Observations_, 1847. As the principles are applicable to photographic
+registration of magnetic and electric as well as meteorologic variations
+in instrumental indications, it would be beside our purpose to describe
+fully the apparatus.
+
+The barometer is a large syphon tube; the bore of the upper and lower
+extremities, through which the surfaces of the mercury rise and fall, is
+1-1/10 inch in diameter. The glass float in the open limb is attached to a
+wire, which moves a delicately-supported light lever as it alters its
+elevation. The fulcrum of the lever is on one side of the wire; the
+extremity on the other side, at four times this distance from the fulcrum,
+carries a vertical plate of opaque mica, having a small aperture. Through
+this hole the light of a gas-jet shines upon photographic paper wrapped
+round a cylinder placed vertically, and moved round its axis by a clock
+fixed with its face horizontal. The cylinder is delicately supported, and
+revolves in friction rollers. A bent wire on the axis is embraced by a
+prong on the hour hand of the time-piece; therefore the cylinder is
+carried round once in twelve hours. It might be arranged for a different
+period of rotation.
+
+As the cylinder rotates, the paper receives the action of the light, and a
+photographic trace is left of the movements of the barometer four times
+the extent of the oscillations of the float, or twice the length of the
+variations in the barometric column. Certain chemical processes are
+required in the preparation of the paper, and in developing the trace. The
+diagram which we give on the next page, with the explanation, taken from
+Drew's _Practical Meteorology_, will enable the above description to be
+better understood:
+
+[Illustration: Fig. 28.]
+
+"_Q e_ is a lever whose fulcrum is _e_, the counterpoise _f_ nearly
+supporting it; _s_ is an opaque plate of mica, with a small aperture at
+_p_, through which the light passes, having before been refracted by a
+cylindrical lens into a long ray, the portion only of which opposite the
+aperture _p_ impinges on the paper; _d_ is a wire supported by a float on
+the surface of the mercury; _G H_ is the barometer; _p_, the vertical
+cylinder charged with photographic paper; _r_, the photographic trace;
+_I_, the timepiece, carrying round the cylinder by the projecting arm _t_.
+It is evident that the respective distances of the float and the aperture
+_p_ from the fulcrum may be regulated so that the rise and fall of the
+float may be multiplied to any extent required." When _only_ the lower
+surface of the mercury in a syphon barometer is read, as in the instrument
+just described, a correction for temperature is strictly due to the height
+of the quicksilver in the _short_ tube; but this in so short a column will
+rarely be sensible.
+
+
+
+
+CHAPTER IV.
+
+MOUNTAIN BAROMETERS.
+
+
+=37. The Syphon Tube Mountain Barometer, on Gay Lussac's principle=,
+constructed as described at page 31, and fixed in a metallic tubular
+frame, forms a simple and light travelling instrument. The graduations are
+made upon the frame, and it is suspended for reading by a ring at the top,
+from beneath an iron tripod stand, which is usually supplied with it.
+Considerable care is requisite in adjusting the verniers, so as to keep
+the instrument steady and vertical. A drawback to the convenience of this
+barometer is the movement of the mercury in the short limb, which is
+generally not confined, and hence has every facility for becoming quickly
+oxidised in travelling. To remedy this, Messrs. Negretti and Zambra so
+construct the Mountain Syphon Barometer that by a simple half turn of a
+screw the mercury can be confined for portability, while the lower limb
+can be taken out for cleaning whenever found requisite.
+
+
+=38. Mountain Barometer on Fortin's principle.=--This barometer, with
+Fortin's cistern, as arranged by Messrs. Negretti and Zambra, is an
+elegant, manageable, and very accurate instrument for travelling purposes,
+and well adapted for careful measurement of heights. The cistern is made
+large enough to receive all the mercury that will fall from the tube at
+the highest attainable elevation. The screw at the bottom confines the
+mercury securely for carriage, and serves to adjust the surface of the
+mercury to the zero of the scale when making an observation. The vernier
+reads to �002 of an inch, and slides easily on the brass frame, which is
+made as small in diameter as is compatible with the size of the tube. The
+tube in this barometer should be altogether without contractions, so that
+the mercury will readily fall when it is set up for observation. It must
+be carefully calibrated, and its internal diameter ascertained, in order
+that correction may be made for capillarity. This correction, however,
+should be combined with the error of graduation, and form a permanent
+index error, ascertainable at any time by comparison with an acknowledged
+standard barometer.
+
+The barometer is supported in the tripod stand (furnished as part of the
+instrument) when used for observation. It is suspended by placing two
+studs, in the ring on the frame, in slots formed on the top of the stand,
+so that it hangs freely and vertically in gimbals. To the metal top of
+the stand, mahogany legs are hinged. To make the barometer portable, it
+must be lifted out of the stand, sloped gently until the mercury reaches
+the top, turning the screw at the bottom meanwhile; then invert and screw
+until the mercury is made tight. The inverted instrument packs in the
+stand, the legs being formed to fit round the frame; and receptacles are
+scooped out for the cistern, thermometer, gimbals, and vernier; so that
+the instrument is firmly surrounded by the wooden legs, which are held
+fast together by brass rings passed over them.
+
+
+[Illustration: Fig. 29.]
+
+=39. Newman's Mountain Barometer.=--Fig. 29 is an illustration of the
+mountain barometer known as Newman's. The cistern consists of two separate
+compartments;--the top of the lower and the bottom of the upper, being
+perfectly flat, are pivoted closely together at the centres, so that the
+lower can move through a small arc, when turned by the hand. This movement
+is limited by two stops. The top of the lower compartment and the bottom
+of the upper have each a circular hole, through which the mercury
+communicates. When the instrument is required for observation, the cistern
+is turned close up to the stop marked "_open_" or "_not portable_." When
+it is necessary to pack it for travelling, the mercurial column must be
+allowed to fill the tube by sloping the barometer gently; then invert it,
+and move the cistern to the stop marked "_shut_" or "_portable_." In this
+condition, the upper compartment is completely filled with mercury, and
+consequently that in the tube cannot move about, so as to admit air or
+endanger the tube. Nor can the mercury pass back to the lower compartment,
+as the holes are not now coincident, and the contact is made too perfect
+to allow the mercury to creep between the surfaces. The tube does not
+enter the lower compartment, which is completely full of mercury when the
+instrument is arranged for observation. The spare capacity of the upper
+cistern is sufficient to receive the mercury which descends from the tube
+to the limit of the engraved scale, which in these barometers generally
+extends only to about 20 inches. A lower limit could of course be given by
+increasing the size of the cisterns, which it is not advisable to do
+unless for a special purpose. This barometer may be had mounted in wood,
+or in brass frame. If in wood, it has a brass shield, which slides round
+the scale part of the frame, so as to be easily brought in front of the
+tube and scale as a protection in travelling; the vernier screw, in this
+case, being placed at the top of the instrument. When the scale is
+graduated with true inches, the neutral point, the capacity and
+capillarity corrections should be marked on the frame. The graduated
+scales, however, placed on these barometers in brass frames, are usually
+artificial inches, like the Kew plan of graduation; the advantage being
+that one simple correction only is required, viz. one for index error and
+capillarity combined, which can always be readily determined by comparison
+with a standard barometer; moreover, as no adjustment of cistern is
+required in reading, the instrument can be verified by artificial pressure
+throughout the scale, by the plan practised at Kew, Liverpool, &c., and
+already described (see p. 18).
+
+
+40. NEGRETTI & ZAMBRA'S PATENT MOUNTAIN AND OTHER BAROMETERS.
+
+This invention is intended to make mountain and other barometers of
+standard accuracy stronger, more portable, and less liable to derangement,
+when being carried about, than heretofore, by dispensing with the ordinary
+flexible cistern containing the mercury at the bottom of the instrument,
+and adapting in lieu thereof a rigid cistern constructed of glass and
+iron. The cistern is composed of a glass cylinder, which is secured in a
+metallic tube or frame. In order to render the cistern mercury-tight at
+top and bottom, metal caps are screwed into the tube or frame, and bear
+against leather washers placed between them and the edges of the glass
+cylinder. The upper cap of the cistern is tapped with a fine threaded
+screw to receive the iron plug or socket, into which the barometer tube is
+securely fixed. The whole length of this plug has a fine screw cut upon it
+by which the cistern can be screwed up or down. At the side of this plug
+or socket, extending from the lower end to within half an inch of the top,
+is cut a groove for admitting the air to the surface of the mercury within
+the cistern when the barometer is in use. An ivory point is screwed into
+the under surface of the plug, carrying the barometer tube. This ivory
+point is very carefully adjusted by measurement to be the zero point of
+the instrument, from which the barometer scale of inches is divided. The
+surface of the mercury in the cistern is adjusted to the zero point by
+screwing the cistern up or down until the ivory point and its reflected
+image are in contact.
+
+[Illustration: Fig. 30.]
+
+The instrument (fig. 30) is shown in a state of adjustment, ready to take
+an observation; but _when it is desired to render it portable, it must be
+inclined, until mercury from the cistern fills the tube; the cistern must
+then be screwed up on the socket_, so as to bring the face of the upper
+cap against the under side of the shoulder of the cover immediately above
+it; the instrument may then be carried without being liable to
+derangement.
+
+_Precautions necessary in using the Mountain Barometer._--On removing the
+barometer from its case after a journey, allow it to remain with its scale
+end downward, whilst the cistern is unscrewed to the extent of _one turn
+of the screw_, after which slightly shake the cistern; the mercury in it
+will then completely fill the end of the barometer tube, should any
+portion of it have escaped therefrom.
+
+The barometer is then inverted, and if it be desired to make an
+observation, suspend it vertically from its stand by the ring at top. The
+cistern must then be unscrewed, until the surface of the mercury is
+brought just level with the extreme end of the ivory or zero point fixed
+to the iron plug on which the glass cistern moves up and down.
+
+Should the elevation of the place where the barometer is to be used be
+considerably above the sea level, it will be well--after suspending it
+from the stand--to unscrew the cistern several turns, _holding the
+barometer in an oblique position_, as at great heights the mercury will
+fall considerably quicker than the cistern can be unscrewed, thereby
+filling it to overflowing; but by partly unscrewing the cistern first,
+room is given for the reception of a fall of mercury to the extent of
+several inches.
+
+The cistern must not be unscrewed when the _Instrument is_ INVERTED _more
+than_ two turns of the screw, otherwise the mercury will flow out through
+the groove.
+
+It is found safer when travelling to carry the barometer in a horizontal
+position, or with its cistern end uppermost.
+
+_To clean the Barometer._--Should at any time the mercury in the cistern
+become oxidised, and reading from its surface be difficult, it can be
+readily cleaned by removing the cistern and its contained mercury from the
+barometer frame by unscrewing it _when in a horizontal position_; this
+precaution is necessary that the mercury in the tube may not escape, and
+thereby allow air to enter. The cistern must then be emptied, and with a
+dry clean leather, or silk handkerchief, well cleaned.
+
+The operation of cleaning being performed, return the cistern to the
+frame, and screw it until the face is brought up against the under side of
+the shoulder, still keeping the instrument _horizontal_. The cistern is
+now ready for re-filling, to do which stand the barometer on end _head
+downwards_, and remove the small screw at bottom; through the aperture
+thus opened, pour in mercury, passing it through a paper funnel with a
+very small aperture. It is well to pass the mercury through a very small
+funnel two or three times before returning it to the barometer cistern, as
+by this process all particles of dust or oxide adhere to the paper, and
+are effectually removed.
+
+Should any small quantity of the mercury be lost during the operation of
+cleaning, it is of no importance so long as sufficient remains to allow of
+adjustment to the zero point. This latter constitutes one of the great
+advantages of this new instrument over the ordinary barometer; for, in the
+majority of cases, after an instrument has been compared carefully with a
+standard, should mercury be lost, there is no means of correcting the
+error unless a standard barometer be at hand; the new barometer is, in
+this respect, independent, a little mercury more or less being
+unimportant.
+
+
+=41. Short Tube Barometer.=--This is simply a tube shorter, as may be
+required, than that necessary to show the atmospheric pressure at the sea
+level. It is convenient for balloon purposes, and for use at mountain
+stations, being of course a special construction.
+
+
+=42. Method of Calculating Heights by the Barometer.=--The pressure of the
+atmosphere being measured by the barometer, it is evident that as the
+instrument is carried up a high mountain or elevated in a balloon, the
+length of the column must decrease as the atmospheric pressure decreases,
+in consequence of a stratum of air being left below. The pressure of air
+arises from its weight, or the attraction of gravitation upon it, and
+therefore the quantity of air below the barometer cistern cannot influence
+the height of the column. Hence it follows that a certain relation must
+exist between the difference of the barometric pressure at the foot and at
+the top of a hill or other elevation, and the difference of the absolute
+heights above the sea. Theoretical investigation, abundantly confirmed by
+practical results, has determined that the strata of air decrease in
+density in a geometrical proportion, while the elevations increase in an
+arithmetical one. Hence we have a method of determining differences of
+level, by observations made on the density of the air by means of the
+barometer. It is beyond our purpose to explain in detail the principles
+upon which this method is founded, or to give its mathematical
+investigation. We append Tables, which will be useful to practical
+persons,--surveyors, engineers, travellers, tourists, &c.,--who may carry
+a barometer as a travelling companion.
+
+Table I. is calculated from the formula, height in feet = 60,200 (log.
+29�922 - log. B) + 925; where 29�922 is the mean atmospheric pressure at
+32� F., and the mean sea-level in latitude 45�; and B is any other
+barometric pressure; the 925 being added to avoid minus signs in the
+Table.
+
+Table II. contains the correction necessary for the mean temperature of
+the stratum of air between the stations of observation; and is computed
+from Regnault's co-efficient for the expansion of air, which is �002036 of
+its volume at 32� for each degree above that temperature.
+
+Table III. is the correction due to the difference of gravitation in any
+other latitude, and is found from the formula, _x_ = 1 + �00265 cos. 2
+lat.
+
+Table IV. is to correct for the diminution of gravity in ascending from
+the sea-level.
+
+To use these Tables: The barometer readings at the upper and lower
+stations having been corrected and reduced to temperature 32� F., take out
+from Table I. the numbers opposite the corrected readings, and subtract
+the lower from the upper. Multiply this difference successively by the
+factors found in Tables II. and III. The factor from Table III. may be
+neglected unless precision is desired. Finally, add the correction taken
+from Table IV.
+
+TABLE I.
+
+_Approximate Height due to Barometric Pressure._
+
+  +----------------------------------------------+
+  |Inches.| Feet.||Inches.| Feet.||Inches.| Feet.|
+  |-------+------++-------+------++-------+------|
+  |  31�0 |    0 ||  28�2 | 2475 ||  25�4 | 5209 |
+  |  30�9 |   84 ||    �1 | 2568 ||    �3 | 5312 |
+  |    �8 |  169 ||  28�0 | 2661 ||    �2 | 5415 |
+  |    �7 |  254 ||  27�9 | 2754 ||    �1 | 5519 |
+  |    �6 |  339 ||    �8 | 2848 ||  25�0 | 5623 |
+  |    �5 |  425 ||    �7 | 2942 ||  24�9 | 5728 |
+  |    �4 |  511 ||    �6 | 3037 ||    �8 | 5833 |
+  |    �3 |  597 ||    �5 | 3132 ||    �7 | 5939 |
+  |    �2 |  683 ||    �4 | 3227 ||    �6 | 6045 |
+  |    �1 |  770 ||    �3 | 3323 ||    �5 | 6152 |
+  |  30�0 |  857 ||    �2 | 3419 ||    �4 | 6259 |
+  |  29�9 |  944 ||    �1 | 3515 ||    �3 | 6366 |
+  |    �8 | 1032 ||  27�0 | 3612 ||    �2 | 6474 |
+  |    �7 | 1120 ||  26�9 | 3709 ||    �1 | 6582 |
+  |    �6 | 1208 ||    �8 | 3806 ||  24�0 | 6691 |
+  |    �5 | 1296 ||    �7 | 3904 ||  23�9 | 6800 |
+  |    �4 | 1385 ||    �6 | 4002 ||    �8 | 6910 |
+  |    �3 | 1474 ||    �5 | 4100 ||    �7 | 7020 |
+  |    �2 | 1563 ||    �4 | 4199 ||    �6 | 7131 |
+  |    �1 | 1653 ||    �3 | 4298 ||    �5 | 7242 |
+  |  29�0 | 1743 ||    �2 | 4398 ||    �4 | 7353 |
+  |  28�9 | 1833 ||    �1 | 4498 ||    �3 | 7465 |
+  |    �8 | 1924 ||  26�0 | 4598 ||    �2 | 7577 |
+  |    �7 | 2015 ||  25�9 | 4699 ||    �1 | 7690 |
+  |    �6 | 2106 ||    �8 | 4800 ||  23�0 | 7803 |
+  |    �5 | 2198 ||    �7 | 4902 ||  22�9 | 7917 |
+  |    �4 | 2290 ||    �6 | 5004 ||    �8 | 8032 |
+  |    �3 | 2382 ||    �5 | 5106 ||    �7 | 8147 |
+  +----------------------------------------------+
+
+TABLE I.--_continued_.
+
+_Approximate Height due to Barometric Pressure._
+
+  +-------------------------------------------------+
+  |Inches.| Feet. ||Inches.| Feet. ||Inches.| Feet. |
+  |-------+-------++-------+-------++-------+-------|
+  | 22�6  |  8262 ||  18�9 | 12937 ||  15�2 | 18632 |
+  |   �5  |  8378 ||    �8 | 13076 ||    �1 | 18805 |
+  |   �4  |  8495 ||    �7 | 13215 ||  15�0 | 18979 |
+  |   �3  |  8612 ||    �6 | 13355 ||  14�9 | 19154 |
+  |   �2  |  8729 ||    �5 | 13496 ||    �8 | 19330 |
+  |   �1  |  8847 ||    �4 | 13638 ||    �7 | 19507 |
+  | 22�0  |  8966 ||    �3 | 13780 ||    �6 | 19685 |
+  | 21�9  |  9085 ||    �2 | 13923 ||    �5 | 19865 |
+  |   �8  |  9205 ||    �1 | 14067 ||    �4 | 20046 |
+  |   �7  |  9325 ||  18�0 | 14212 ||    �3 | 20228 |
+  |   �6  |  9446 ||  17�9 | 14358 ||    �2 | 20412 |
+  |   �5  |  9567 ||    �8 | 14505 ||    �1 | 20597 |
+  |   �4  |  9689 ||    �7 | 14652 ||  14�0 | 20783 |
+  |   �3  |  9811 ||    �6 | 14800 ||  13�9 | 20970 |
+  |   �2  |  9934 ||    �5 | 14949 ||    �8 | 21159 |
+  |   �1  | 10058 ||    �4 | 15099 ||    �7 | 21349 |
+  | 21�0  | 10182 ||    �3 | 15250 ||    �6 | 21541 |
+  | 20�9  | 10307 ||    �2 | 15402 ||    �5 | 21734 |
+  |   �8  | 10432 ||    �1 | 15554 ||    �4 | 21928 |
+  |   �7  | 10558 ||  17�0 | 15707 ||    �3 | 22124 |
+  |   �6  | 10684 ||  16�9 | 15861 ||    �2 | 22321 |
+  |   �5  | 10812 ||    �8 | 16016 ||    �1 | 22520 |
+  |   �4  | 10940 ||    �7 | 16172 ||  13�0 | 22720 |
+  |   �3  | 11069 ||    �6 | 16329 ||  12�9 | 22922 |
+  |   �2  | 11198 ||    �5 | 16487 ||    �8 | 23126 |
+  |   �1  | 11328 ||    �4 | 16646 ||    �7 | 23331 |
+  | 20�0  | 11458 ||    �3 | 16806 ||    �6 | 23538 |
+  | 19�9  | 11589 ||    �2 | 16967 ||    �5 | 23746 |
+  |   �8  | 11721 ||    �1 | 17129 ||    �4 | 23956 |
+  |   �7  | 11853 ||  16�0 | 17292 ||    �3 | 24168 |
+  |   �6  | 11986 ||  15�9 | 17456 ||    �2 | 24381 |
+  |   �5  | 12120 ||    �8 | 17621 ||    �1 | 24596 |
+  |   �4  | 12254 ||    �7 | 17787 ||  12�0 | 24813 |
+  |   �3  | 12389 ||    �6 | 17954 ||  11�9 | 25032 |
+  |   �2  | 12525 ||    �5 | 18122 ||    �8 | 25253 |
+  |   �1  | 12662 ||    �4 | 18291 ||    �7 | 25476 |
+  | 19�0  | 12799 ||    �3 | 18461 ||    �6 | 25700 |
+  +-------------------------------------------------+
+
+TABLE II.
+
+_Correction due to Mean Temperature of the Air._
+
+  +-------------------------------------------+
+  |Mean |Factor.||Mean |Factor.||Mean |Factor.|
+  |Temp.|       ||Temp.|       ||Temp.|       |
+  |-----+-------++-----+-------++-----+-------|
+  | 10� | 0�955 || 35� | 1�006 || 60� | 1�057 |
+  | 11  |  �957 || 36  | 1�008 || 61  | 1�059 |
+  | 12  |  �959 || 37  | 1�010 || 62  | 1�061 |
+  | 13  |  �961 || 38  | 1�012 || 63  | 1�063 |
+  | 14  |  �963 || 39  | 1�014 || 64  | 1�065 |
+  | 15  |  �965 || 40  | 1�016 || 65  | 1�067 |
+  | 16  |  �967 || 41  | 1�018 || 66  | 1�069 |
+  | 17  |  �969 || 42  | 1�020 || 67  | 1�071 |
+  | 18  |  �971 || 43  | 1�022 || 68  | 1�073 |
+  | 19  |  �974 || 44  | 1�024 || 69  | 1�075 |
+  | 20  |  �976 || 45  | 1�026 || 70  | 1�077 |
+  | 21  |  �978 || 46  | 1�029 || 71  | 1�079 |
+  | 22  |  �980 || 47  | 1�031 || 72  | 1�081 |
+  | 23  |  �982 || 48  | 1�033 || 73  | 1�083 |
+  | 24  |  �984 || 49  | 1�035 || 74  | 1�086 |
+  | 25  |  �986 || 50  | 1�037 || 75  | 1�088 |
+  | 26  |  �988 || 51  | 1�039 || 76  | 1�090 |
+  | 27  |  �990 || 52  | 1�041 || 77  | 1�092 |
+  | 28  |  �992 || 53  | 1�043 || 78  | 1�094 |
+  | 29  |  �994 || 54  | 1�045 || 79  | 1�096 |
+  | 30  |  �996 || 55  | 1�047 || 80  | 1�098 |
+  | 31  | 0�998 || 56  | 1�049 || 81  | 1�100 |
+  | 32  | 1�000 || 57  | 1�051 || 82  | 1�102 |
+  | 33  | 1�002 || 58  | 1�053 || 83  | 1�104 |
+  | 34  | 1�004 || 59  | 1�055 || 84  | 1�106 |
+  +-------------------------------------------+
+
+TABLE III.
+
+  +-------------------------------------------------------+
+  |Latitude.|Factor.||Latitude.|Factor.||Latitude.|Factor.|
+  |---------+-------++---------+-------++---------+-------|
+  |   80�   |0�99751||   50    |0�99954||   20    |1�00203|
+  |   75    |0�99770||   45    |1�00000||   15    |1�00230|
+  |   70    |0�99797||   40    |1�00046||   10    |1�00249|
+  |   65    |0�99830||   35    |1�00090||    5    |1�00261|
+  |   60    |0�99868||   30    |1�00132||    0    |1�00265|
+  |   55    |0�99910||   25    |1�00170||         |       |
+  +-------------------------------------------------------+
+
+TABLE IV.
+
+  +----------------------------------------------------+
+  |  Height in   |Correction||  Height in   |Correction|
+  |Thousand Feet.| Additive.||Thousand Feet.| Additive.|
+  |--------------+----------++--------------+----------|
+  |       1      |     3    ||      14      |    44    |
+  |       2      |     5    ||      15      |    48    |
+  |       3      |     8    ||      16      |    52    |
+  |       4      |    11    ||      17      |    56    |
+  |       5      |    14    ||      18      |    60    |
+  |       6      |    17    ||      19      |    65    |
+  |       7      |    20    ||      20      |    69    |
+  |       8      |    23    ||      21      |    74    |
+  |       9      |    26    ||      22      |    78    |
+  |      10      |    30    ||      23      |    83    |
+  |      11      |    33    ||      24      |    88    |
+  |      12      |    37    ||      25      |    93    |
+  |      13      |    41    ||      26      |    98    |
+  +----------------------------------------------------+
+
+EXAMPLE 1. On October 21st, 1852, when Mr. Welsh ascended in a balloon, at
+3h. 30m. p.m., the barometer, corrected and reduced, was 18�85, the air
+temperature 27�, while at Greenwich, 159 feet above the sea, the barometer
+at the same time was 29�97 inches, air temperature 49�, the balloon not
+being more than 5 miles S.W. from over Greenwich; required its elevation.
+
+                                            Feet.
+
+  Barometer in Balloon    18�85, Table I. = 13007
+      "     at Greenwich  29�97     "         883
+                                            -----
+                                            12124
+  Mean Temperature, 38�, Table II. Factor   1�012
+                                            -----
+                                            12269�
+                                            -----
+  Latitude 51-1/2�, Factor from Table III. �99941
+                                            -----
+                                            12262
+  Correction from Table IV.                    38
+                                            -----
+                                            12300
+  Elevation of Greenwich                      159
+                                            -----
+      "        Balloon                      12459 feet.
+                                            =====
+
+The following examples, from the balloon ascents of J. Glashier, Esq.,
+F.R.S., will serve for practice.[4]
+
+2. Ascended from Wolverhampton, 18th August, 1862, at 2h. 38m. p.m.;
+barometer (in all cases corrected and reduced to 32� F) was 14�868, the
+temperature of the air 26�; at the same time, at Wrottesley Hall, 531 feet
+above the sea, in latitude 52-1/2� N, the barometer was 29�46, and the
+temperature of the air 65��4; find the elevation of the balloon above the
+sea.
+
+  Height, 18,959 feet.
+
+3. From the same place an ascent was made 5th September, 1862, when at 1h.
+48m. p.m. barometer was 11�954, air O�; at Wrottesley Hall 29�38, air 56�.
+
+  Height, 23,923 feet.
+
+4. From the Crystal Palace a balloon ascent was made 20th August, 1862. At
+6h. 47m. p.m. barometer was 25�55, air 50��5; and at the same time at
+Greenwich Observatory, at 159 feet above the sea, the barometer was 29�81,
+air 63�.
+
+  Height, 4,406 feet.
+
+5. From the same place an ascent was made 8th September, 1862. At 5 p.m.,
+the balloon being over Blackheath, barometer was 25�60, and the air 49��5,
+while at Greenwich, barometer was 29�92, air 66��4.
+
+  Height, 4,461 feet.
+
+
+
+
+CHAPTER V.
+
+SECONDARY BAROMETERS.
+
+
+=43. Desirability of Magnifying the Barometer Range.=--The limits within
+which the ordinary barometric column oscillates, do not exceed four inches
+for extreme range, while the ordinary range is confined to about two
+inches; hence it has often been felt that the public utility of the
+instrument would be greatly enhanced if by any means the scale indications
+could be increased in length. This object was sought to be obtained by
+bending the upper part of the tube from the vertical, so that the inches
+on the scale could be increased in length in proportion to the secant of
+the angle it made with the vertical. This was called "the diagonal
+barometer." The upper part of the tube has also been formed into a spiral,
+and the scale, placed along it, is thus greatly enlarged.
+
+But these methods of enlarging the indications cannot be so successfully
+accomplished, nor so cheaply nor so elegantly, as is done by the principle
+employed in the dial barometer. Hence they are not in use.
+
+
+[Illustration: Fig. 31.]
+
+=44. Howson's Long Range Barometer.=--Very recently quite a novel design
+has been patented by Mr. Howson, for a long range barometer. The
+construction requires neither distortion of the tube, nor mechanism for
+converting a short scale into a long one; but the mercury itself rises and
+falls, through an extended range, naturally, and in simple obedience to
+the varying pressure of the atmosphere. The tube is fixed, but its cistern
+is sustained by the mere pressure of the atmosphere. Looking at the
+instrument, it seems a perfect marvel. It appears as though the cistern
+with the mercury in it must fall to the ground. The bore of the tube is
+wide, about an inch across. A long glass rod is fixed to the bottom of the
+glass cistern, where a piece of cork or some elastic substance is also
+placed. The tube is filled with mercury; the glass rod is plunged into the
+tube as it is held top downwards, until the cork gets close up to the tube
+and fits tightly against it. The pressure against the cork simply prevents
+the mercury from coming out while the instrument is being inverted. When
+it is inverted, the mercury partly falls, and forms an ordinary barometric
+column. When the top is held, the cistern and glass rod, instead of
+falling away, remain perfectly suspended. There is no material support to
+the cistern; the tube only is fixed, the cistern hangs to it. Glass is
+many times lighter than mercury. When the glass rod is introduced, it
+displaces an equal volume of mercury. The glass rod, being so much lighter
+than mercury, floats and sustains the additional weight of the cistern by
+its buoyancy. In the mean time, the atmosphere is acting upon the mercury,
+keeping up the ordinary barometric column. Supposing there is a rise in
+the ordinary barometer, the atmosphere presses some more mercury up the
+tube. This mercury is taken out of the cistern, which of course becomes
+lighter, and therefore the rod and cistern float up a little higher, which
+thus causes the column of mercury to rise still more. The increased
+pressure and buoyancy thus acting together, increase the ascent in the
+barometric column, as shown by the fixed scale. One inch in the barometer
+might be represented by two or more inches in this instrument, according
+to construction. Supposing there was a decrease of pressure, the mercury
+would fall, come into the cistern, make it heavier, and increase the fall
+somewhat. Friction guides, at the top of the rod, prevent it coming into
+contact with the side of the tube when vertically suspended. The
+illustration, Fig. 31, shows the appearance of the instrument as framed in
+wood by the makers, Messrs. Negretti and Zambra.
+
+
+=45. McNeild's Long Range Barometer.=--A barometer designed by a gentleman
+named McNeild is on a directly opposite principle to the one just
+described. The tube is made to float on the mercury in the cistern. It is
+filled with mercury, inverted in the usual manner, then allowed to float,
+being held vertically by glass friction points or guides. By this
+contrivance, the ordinary range of the barometer is greatly increased. One
+inch rise or fall in the standard barometer may be represented by four or
+five inches in this instrument, so that it shows small variations in
+atmospheric pressure very distinctly. As the mercury falls in the tube
+with a decrease of pressure, the surface of the mercury in the cistern
+rises, and the floating tube rises also, which causes an additional
+descent in the column, as shown by fixed graduations on the tube. With an
+increase of pressure, some mercury will leave the cistern and rise in the
+tube, while the tube itself will fall, and so cause an additional ascent
+of mercury. This barometer is identical in principle with King's Barograph
+(see p. 34).
+
+The construction of Howson's and McNeild's Barometers has been assigned to
+Messrs. Negretti and Zambra. These instruments are usually made for
+domestic purposes with a scale of from three to five, and for public use
+from five to eight times the scale of the ordinary standard. Their
+sensitiveness is consequently increased in an equal proportion, and they
+have the additional advantage of not being affected by differences of
+level in the cistern. However, these novelties have not been sufficiently
+tried to determine their practical value for strictly scientific
+purposes; but as weather-glasses, for showing minute changes, they are
+superior to the common barometer.
+
+
+=46. The Water-glass Barometer.=--If a Florence flask, having a long neck,
+have a small quantity of water poured into it, and then be inverted and so
+supported that the open end dips into a vessel containing water, a small
+column of water will be confined in the neck of the bottle, the pressure
+of which, upon the surface of the exposed water, will be equal to the
+difference between the atmospheric pressure and the elasticity of the
+confined air in the body of the bottle. As the pressure of the atmosphere
+varies, this column will alter in height. But the elasticity of the
+confined air is also subject to variations, owing to changes of
+temperature. It follows, then, that the oscillations of the column are
+dependent on alterations of temperature and atmospheric pressure. Such an
+arrangement has been called "the Water-glass Barometer," and bears about
+the same relative value to the mercurial barometer, as an exponent of
+weather changes, that a cat-gut hygrometer bears to a thermometric
+hygrometer, as an indicator of relative moisture.
+
+
+47. SYMPIESOMETER.
+
+Nevertheless the instrument now about to be described, depending upon
+similar principles, but scientifically constructed and graduated, is a
+very useful and valuable substitute for the mercurial barometer. It
+consists of a glass tube, varying, according to the purposes for which the
+instrument is required, from six to twenty-four inches in length. The
+upper end is closed, and formed into a bulb; the lower is turned up,
+formed into a cistern, and open at top, through a pipette, or cone. A
+plug, moveable by a catch from below, can be made to close this opening,
+so as to render the instrument portable.
+
+[Illustration: Fig. 32.]
+
+The upper portion of the tube is filled with air; the lower portion, and
+part of the cistern, with sulphuric acid, coloured so as to render it
+plainly visible. Formerly, hydrogen and oil were used. It was found,
+however, that, by the process known to chemists as _osmosis_, this light
+gas in time partially escaped, and the remainder became mixed with air,
+the consequence being that the graduations were no longer correct. They
+are more durable as at present constructed. The liquid rises and falls in
+the tube with the variations of atmospheric pressure and temperature
+acting together. If the pressure were constant, the confined air would
+expand and contract for temperature only, and the instrument would act as
+a thermometer. In fact, the instrument is regarded as such in the
+manufacture; and the thermometric scales are ascertained and engraved on
+the scale. A good mercurial thermometer is also mounted on the same frame.
+If, therefore, at any time the mercurial and the air thermometers do not
+read alike, it must evidently be due to the atmospheric pressure acting
+upon the air in the tube; and it is further evident that, under these
+circumstances, the position of the top of the liquid may be marked to
+represent the barometric pressure at the time. In this manner a scale of
+pressure is ascertained by comparison with a standard barometer, extending
+generally from 27 to 31 inches.
+
+When made correctly, these instruments agree well with the mercurial
+barometer for a number of years, and their subsequent adjustment is not a
+matter of much expense.
+
+For use at sea, the liquid column is contracted at the bend. The
+sympiesometer is very sensitive, and feels the alterations in the
+atmospheric pressure sooner than the ordinary marine barometer.
+
+The scale is usually on silvered brass, mounted on a mahogany or rosewood
+frame, protected in front by plate glass. It is generally furnished with a
+revolving register, to record the observation, in order that it may be
+known whether the pressure has increased or decreased in the interval of
+observation.
+
+Small pocket sympiesometers are sometimes fitted with ivory scales, and
+protected by a neat velvet-lined pasteboard or morocco case.
+
+_How to take an Observation._--In practice, the indications of the
+atmospheric pressure are obtained from the sympiesometer by noting, first,
+the temperature of the mercurial thermometer; secondly, adjusting the
+pointer of the pressure scale to the same degree of temperature on the
+scale of the air column; thirdly, reading the height of the liquid on the
+sliding scale.
+
+_Directions for Use._--The sympiesometer should be carried and handled so
+as to keep the top always upwards, to prevent the air mechanically mixing
+with the liquid. Care should also be taken to screen it from casual rays
+of the sun or cabin fire.
+
+
+48. ANEROIDS.
+
+The beautiful and highly ingenious instrument called by the name
+_Aneroid_, is no less remarkable for the scientific principles of its
+construction and action, than for the nicety of its mechanism. It is a
+substitute, and perhaps the best of all substitutes, for the mercurial
+barometer. As its name implies, it is constructed "without fluid." It was
+invented by M. Vidi of Paris. In the general form in which it is made it
+consists of a brass cylindrical case about four inches in diameter and one
+and a half inch deep, faced with a dial graduated and marked similarly to
+the dial-plate of a "wheel-barometer," upon which the index or pointer
+shows the atmospheric pressure in inches and decimals of an inch in
+accordance with the mercurial barometer. Within the case, for ordinary
+sizes, is placed a flat metal box, generally not more than half an inch
+thick and about two inches or a little more in diameter, from which nearly
+all the air is exhausted. The top and bottom of this box is corrugated in
+concentric circles, so as to yield inwardly to external pressure, and
+return when the pressure is removed. The pressure of the atmosphere,
+acting externally, continually changes, while the elastic pressure of the
+small quantity of air within can only vary by its volume being increased
+or decreased, or by change of temperature. Leaving out of consideration,
+for the moment, the effect of temperature, we can readily perceive that as
+the pressure is lessened upon the outside of the box, the elastic force of
+the air within will force out the top and bottom of the box; and when the
+outer pressure is increased they will be forced in. Thus with the varying
+pressure of the atmosphere, the top and bottom of the box approach to and
+recede from each other by a small quantity; but the bottom being fixed,
+nearly all this motion takes place on the top. Thus the top of the box is
+like an elastic cushion, which rises and falls according as the
+compressing force lessens or increases. To the eye these expansions and
+contractions would not be perceptible, so small is the motion. But they
+are rendered very evident by a nice mechanical arrangement. To the box is
+attached a strong piece of iron, kept pressed upon it by a spring at one
+extremity; so that as the top of the box rises, the motion is made
+sensible at the point held by the spring, and when the top descends the
+spring draws the piece of iron into close contact with it. This piece of
+iron acts as a lever, having its fulcrum at one extremity, the power at
+the centre of the box-top, and the other extremity controlled by the
+spring. Thus it is evident that the small motion of the centre of the
+box-top is much increased at the spring extremity. The motion thus
+obtained is communicated to a system of levers; and, by the intervention
+of a piece of watch-chain and a fine spring passing round the arbour,
+turns the index to the right or left, according as the external pressure
+increases or decreases. Thus, when by increase of pressure the vacuum box
+is compressed, the mechanism transfers the movement to the index, and it
+moves to the right; when the vacuum box bulges out under diminished
+pressure, the mechanical motion is reversed, and the index moves to the
+left. As the index traverses the dial, it shows upon the scale the
+pressure corresponding with that which a good mercurial barometer would at
+the same time and place indicate; that is, supposing it correctly
+adjusted.
+
+A different and more elegant arrangement has since been adopted. A broad
+curved spring is connected to the top of the vacuum box, so as to be
+compressed by the top of the box yielding inward to increased pressure,
+and to relax itself and the box as the pressure is lessened. The system of
+levers is connected to this spring, which augments and transfers the
+motion to the index, in the manner already described. Increase of pressure
+causes the levers to slacken the piece of watch-chain connected with them
+and the arbour of the index. The spring now uncoils, winds the chain upon
+the arbour, and turns the index to the right. Decrease of pressure winds
+the chain off the barrel, tightens the spiral spring, which thus turns the
+index to the left. The graduations of the aneroid scale are obtained by
+comparisons with the correct standard reading of a mercurial barometer,
+under the normal and reduced atmospheric pressure. Reduced pressure is
+obtained by placing both instruments under the receiver of an air pump.
+
+[Illustration: Fig. 33.]
+
+Fig. 33 represents the latest improved mechanism of an aneroid. The outer
+case and the face of the instrument are removed, but the hand is attached
+by its collet to the arbour. _A_ is the corrugated box, which has been
+exhausted of air through the tube, _J_, and hermetically sealed by
+soldering. _B_ is a powerful curved spring, resting in gudgeons fixed on
+the frame-plate, and attached to a socket behind, _F_, in the top of the
+box. A lever, _C_, joined to the stout edge of the spring, is connected,
+by the bent lever at _D_, with the chain, _E_, the other end of which is
+coiled round, and fastened to the arbour, _F_. As the box, _A_, is
+compressed by the weight of the atmosphere increasing, the spring, _B_, is
+tightened, the lever, _C_, depressed, and the chain, _E_, uncoiled from
+_F_, which is thereby turned so that the hand, _H_, moves to the right. In
+the mean while the spiral spring, _G_, coiled round _F_, and fixed at one
+extremity to the frame-work and by the other to _F_, is compressed. When,
+therefore, the pressure decreases, _A_ and _B_ relax, by virtue of their
+elasticity; _E_ slackens, _G_ unwinds, turning _F_, which carries _H_ to
+the left. Near _J_ is shown an iron pillar, cast as part of the stock of
+the spring, _B_. A screw works in this pillar through the bottom of the
+plate, by means of which the spring, _B_, may be so adjusted to the box,
+_A_, as to set the hand, _H_, to read on the scale according to the
+indications of a mercurial barometer. The lever, _C_, is composed of brass
+and steel, soldered together, and adjusted by repeated trials to correct
+for the effects of temperature.
+
+A thermometer is sometimes attached to the aneroid, as it is convenient
+for indicating the temperature of the air. As regards the instrument
+itself, no correction for temperature can be applied with certainty. It
+should be set to read with the mercurial barometer at 32� F. Then the
+readings from it are supposed to require no correction.
+
+In considering the effects of temperature upon the aneroid, they are found
+to be somewhat complex. There is the effect of expansion and contraction
+of the various metals of which the mechanism is composed; and there is the
+effect on the elasticity of the small portion of air in the box. An
+increase of temperature produces greater, a diminution less elasticity in
+this air. The compensation for effects of temperature is adjusted by the
+process of "trial and error," and only a few makers do it well. It is very
+often a mere sham. Admiral FitzRoy writes, in his _Barometer Manual_, "The
+known expansion and contraction of metals under varying temperatures,
+caused doubts as to the accuracy of the aneroid under such changes; but
+they were partly removed by introducing into the vacuum box a small
+portion of gas, as a compensation for the effects of heat or cold. The gas
+in the box, changing its bulk on a change of temperature, was intended to
+compensate for the effect on the metals of which the aneroid is made.
+Besides which, a further and more reliable compensation has lately been
+effected by a combination of brass and steel bars."
+
+"Aneroid barometers, if often compared with good mercurial columns, are
+similar in their indications, and valuable; but it must be remembered that
+they are not independent instruments, that they are set originally by a
+barometer, require adjustment occasionally, and may deteriorate in time,
+though slowly."
+
+"The aneroid is quick in showing the variation of atmospheric pressure;
+and to the navigator who knows the difficulty, at times, of using
+barometers, this instrument is a great boon, for it can be placed
+anywhere, quite out of harm's way, and is not affected by the ship's
+motion, although faithfully giving indication of increased or diminished
+pressure of air. In ascending or descending elevations, the hand of the
+aneroid may be seen to move (like the hand of a watch), showing the height
+above the level of the sea, or the difference of level between places of
+comparison."
+
+In the admiral's _Notes on Meteorology_, he says, "The aneroid is an
+excellent _weather glass_, if well made. Compensation for heat or cold has
+lately been introduced by efficient mechanism. In its _improved_
+condition, when the cost may be about �5, it is fit for measuring heights
+as far as 5,000 feet with approximate accuracy; but even at the price of
+�3, as a _weather-glass_ only, it is exceedingly valuable, because it can
+be carried anywhere; and if now and then compared with a good barometer,
+it may be relied on sufficiently. I have had one in constant use for ten
+years, and it appears to be as good now as at first. For a ship of war
+(considering concussion by the fire of guns), for boats, or to put in a
+drawer, or on a table, I believe there is nothing better than it for use
+as a common weather-glass."
+
+Colonel Sir H. James, R.E., in his _Instructions for taking Meteorological
+Observations_, says of the aneroid, "This is a most valuable instrument;
+it is extremely portable. I have had one in use for upwards of ten years,
+and find it to be the best form of barometer, as a "weather-glass," that
+has been made."
+
+One of the objects of Mr. Glaisher's experiments in balloons was "to
+compare the readings of an aneroid barometer with those of a mercurial
+barometer up to five miles." In the comparisons the readings of the
+mercurial barometer were corrected for index-error and temperature. The
+aneroid readings, says Mr. Glaisher, "prove all the observations made in
+the several ascents may be safely depended upon, and also that an aneroid
+barometer can be made to read correctly to pressures below twelve inches."
+As one of the general conclusions derived from his experiments he states,
+"that an aneroid barometer read correctly to the first place, and probably
+to the second place of decimals, to a pressure as low as seven inches."
+The two aneroids used by Mr. Glaisher were by Messrs. Negretti and Zambra.
+
+Aneroids are now manufactured almost perfectly compensated for
+temperature. Such an instrument therefore ought to show the same pressure
+in the external air at a temperature say of 40�, as it would in a room
+where the temperature at the same time may be 60�; provided there is no
+difference of elevation. To test it thoroughly would require an
+examination and a comparison with barometer readings reduced to 32� F.,
+conducted through a long range of temperature and under artificially
+reduced pressure. A practical method appears to be to compare the aneroid
+daily, or more often, for a few weeks with the readings of a mercurial
+barometer reduced to 32�; and if the error so found be constant, the
+object of the compensation may be assumed to be attained, particularly if
+the temperature during the period has varied greatly.
+
+_Directions for using the Aneroid._--Aneroids are generally suspended with
+the dial vertical; but if they be placed with the dial horizontal, the
+indications differ a few hundredths of an inch in the two positions.
+Hence, if their indications are registered, they should be kept in the
+same position.
+
+The aneroid will not answer for exact scientific purposes, as it cannot be
+relied upon for a length of time. Its error of indication changes slowly,
+and hence the necessity of its being set from time to time with the
+reading of a good barometer. To allow of this being done, at the back of
+the outer case is the head of a screw in connection with the spring
+attached to the vacuum box. By applying a small turnscrew to this screw,
+the spring of the vacuum box may be tightened or relaxed, and the index
+made to move correspondingly to the right or left on the dial. By this
+means, besides being enabled to correct the aneroid at any time, "if the
+measure of a height rather greater than the aneroid will commonly show be
+required, it may be _re-set_ thus: When at the upper station (_within its
+range_), and having noted the reading carefully, touch the screw behind so
+as to bring back the hand a few inches (if the instrument will admit),
+then read off and start again. _Reverse the operation when descending._
+This may add some inches of measure _approximately_."--_FitzRoy._
+
+
+[Illustration: Fig. 34.]
+
+=49. Small Size Aneroids.=--The patent for the Aneroid having expired,
+Admiral FitzRoy urged upon Messrs. Negretti & Zambra the desirability of
+reducing the size at which it had hitherto been made, as well as of
+improving its mechanical arrangement, and compensation for temperature.
+They accordingly engaged skilful workmen, who, under their directions, and
+at their expense, by a great amount of labour and experiment, succeeded
+in reducing its dimensions to two inches in diameter, and an inch and a
+quarter thick. The exact size and appearance of this aneroid are shown in
+fig. 34. The compensation is carefully adjusted, and the graduations of
+the dial ascertained under reduced pressure, so that they are not quite
+equal, but more accurate.
+
+
+=50. Watch Aneroid.=--Subsequently the aneroid has been further reduced in
+size and it can now be had from an inch and a quarter to six inches in
+diameter. The smallest size can be enclosed in watch cases, fig. 35, or
+otherwise, so as to be adapted to the pocket. By a beautifully simple
+contrivance, a milled rim is adjusted to move round with hand pressure,
+and carry a fine index or pointer, outside and around the scale engraved
+on the dial, or face, for the purpose of marking the reading, so that the
+subsequent increase or decrease of pressure may be readily seen. These
+very small instruments are found to act quite as correctly as the largest,
+and are much more serviceable. Besides serving the purpose of a
+weather-glass in the house or away from home, if carried in the pocket,
+they are admirably suited to the exigencies of tourists and travellers.
+They may be had with scale sufficient to measure heights not exceeding
+8,000 feet; with a scale of elevation in feet, as well as of pressure in
+inches, engraved on the dial. The scale of elevation, which is for the
+temperature of 50�, was computed by Professor Airy, the Astronomer Royal,
+who kindly presented it to Messrs. Negretti and Zambra, at the same time
+suggesting its application. Moderate-sized aneroids, fitted in leathern
+sling cases, are also good travelling instruments, and will be found
+serviceable to pilots, fishermen, and for use in coasting and small
+vessels, where a mercurial barometer cannot be employed, because requiring
+too much space.
+
+[Illustration: Fig. 35.]
+
+Admiral FitzRoy, in a communication to the _Mercantile Marine Magazine_,
+December, 1860, says:--"Aneroids are now made more portable, so that a
+pilot or chief boatman may carry one in his pocket, as a railway guard
+carries his timekeeper; and, thus provided, pilots cruising for expected
+ships would be able to caution strangers arriving, if bad weather were
+impending, or give warning to coasters or fishing boats. Harbours of
+Refuge, however excellent and important, are not always accessible, even
+when most wanted, as in snow, rain, or darkness, when neither land, nor
+buoy, nor even a lighthouse-light can be seen."
+
+
+=51. Measurement of Heights by the Aneroid.=--For measuring heights not
+exceeding many hundred feet above the sea-level by means of the aneroid,
+the following simple method will suffice:--
+
+Divide the difference between the aneroid readings at the lower and upper
+stations by �0011; the quotient will give the approximate height in feet.
+
+Thus, supposing the aneroid to read at the
+
+  Lower Station                30�385 inches.
+  Upper Station                30�025
+                               ------
+                Difference       �360
+                               ======
+
+  Divided gives �360/�0011 = 327 feet.
+
+As an illustration of the mode in which the aneroid should be used in
+measuring heights, the following example is given:--
+
+A gentleman who ascended Helvellyn, August 12th, 1862, recorded the
+following observations with a pocket aneroid by Negretti and Zambra:--
+
+Near 10 a.m., at the first milestone from Ambleside, found by survey to be
+188 feet above the sea, the aneroid read 29�89 inches; about 1 p.m., at
+the summit of Helvellyn, 26�81; and at 5 p.m., at the milestone again,
+29�76. The temperature of the lower air was 57�, of the upper, 54�. Hence
+the height of the mountain is deduced as follows:--
+
+                                          Inches.
+
+  Reading at 10 a.m.                       29�89
+        "     5 p.m.                       29�76
+                                           ------
+              Mean                         29�825 Table I.[5]   1010
+  Upper Reading                            26�81     "          3796
+                                                               -----
+                   Difference                                   2786
+  Mean Temperature 55��5, gives in Table II.                   1�048
+                                                               -----
+                                                                2920
+  Lat. 55� N., gives in Table III.                             �9991
+                                                               -----
+                                                                2917
+                         Table IV.                                 5
+                                                               -----
+                   Difference of height                         2922
+  Height of lower station                                        188
+                                                               -----
+        "   Helvellyn                                           3110
+
+  In Sir J. Herschell's _Physical Geography_ it is given as     3115 ft.
+
+So near an agreement is attributable to the excellence of the aneroid, and
+the careful accuracy of the observer.
+
+
+52. METALLIC BAROMETER.
+
+This instrument, the invention of M. Bourdon, has a great resemblance to
+the aneroid, but is much simpler in arrangement. The inventor has applied
+the same principle to the construction of metallic steam-pressure gauges.
+We are here, however, only concerned with it as constructed to indicate
+atmospheric pressure. It consists of a long slender flattened metallic
+tube, partially exhausted of air, and hermetically closed at each end,
+then fixed upon its centre, and bent round so as to make the ends face
+each other. The transverse section of this tube is an elongated ellipse.
+The principle of action is this: interior pressure tends to straighten the
+tube, external pressure causes it to coil more. Hence as the atmospheric
+pressure decreases, the ends of the tube become more apart.
+
+This movement is augmented and transferred by a mechanical arrangement of
+small metallic levers to a radius bar, which carries a rack formed on the
+arc of its circle. This moves a pinion, upon the arbour of which a light
+pointer, or "hand," is poised, which indicates the pressure upon a dial.
+When the pressure increases, the ends of the tube approach each other, and
+the pointer moves from left to right over the dial. The whole mechanism is
+fixed in a brass case, having a hole at the back for adjusting the
+instrument to the mercurial barometer by means of a key, which sets the
+pointer without affecting the levers. The dial is generally open to show
+the mechanism, and is protected by a glass, to which is fitted a moveable
+index.
+
+This barometer is very sensitive, and has the advantage of occupying
+little space, although it has not yet been made so small as the aneroid.
+Both these instruments admit of a great variety of mounts to render them
+ornamental. The metallic barometer can be constructed with a small clock
+in its centre, so as to form a novel and beautiful drawing-room ornament.
+
+Admiral FitzRoy writes, "Metallic barometers, by Bourdon, have not yet
+been tested in very moist, hot, or cold air for a sufficient time. They
+are dependent, or secondary instruments, and liable to deterioration. For
+limited employment, when sufficiently compared, they may be very useful,
+especially in a few cases of electrical changes, _not foretold or shown by
+mercury_, which these seem to indicate remarkably."
+
+They are not so well adapted for travellers, nor for measurements of
+considerable elevations, as aneroids.
+
+
+
+
+CHAPTER VI.
+
+INSTRUMENTS FOR ASCERTAINING TEMPERATURE.
+
+
+=53. Temperature= is the energy with which heat affects our sensation of
+feeling.
+
+Bodies are said to possess the same temperature, when the amounts of heat
+which they respectively contain act outwardly with the same intensity of
+transfer or absorption, producing in the one case the sensation of warmth,
+in the other that of coldness. Instruments used for the determination and
+estimation of temperatures are called _Thermometers_.
+
+Experience proves that the same body always occupies the same space at the
+same temperature; and that for every increase or decrease of its
+temperature, it undergoes a definite dilatation or contraction of its
+volume. Provided, then, a body suffers no loss of substance or peculiar
+change of its constituent elements or atoms, while manifesting changes of
+temperature it will likewise exhibit alterations in volume; the latter
+may, therefore, be taken as exponents of the former. The expansion and
+contraction of bodies are adopted as arbitrary measures of changes of
+temperature; and any substance will serve for a thermometer in which these
+changes of volume are sensible, and can be rendered measureable.
+
+
+=54. Thermometric Substances.=--Thermometers for meteorological and
+domestic purposes are constructed with liquids, and generally either
+mercury or alcohol, because their alterations of volume for the same
+change of temperature are greater than those of solids; while being more
+manageable, they are preferred to gases. Mercury is of all substances the
+best adapted for thermometric purposes, as it maintains the liquid state
+through a great alteration of heat, has a more equable co-efficient of
+expansion than any other fluid, and is peculiarly sensitive to changes of
+temperature. The temperature of solidification of mercury, according to
+Fahrenheit's scale of temperature, is -40�; and its temperature of
+ebullition is about 600�. Sulphuric ether, nitric acid, oil of sassafras,
+and other limpid fluids, have been employed for thermometers.
+
+
+=55. Description of the Thermometer.=--The ordinary thermometer consists
+of a glass tube of very fine bore, having a bulb of thin glass at one
+extremity, and closed at the other. The bulb and part of the tube contains
+mercury; the rest of the tube is a vacuum, and affords space for the
+expansion of the liquid. This arrangement renders very perceptible the
+alterations in volume of the mercury due to changes of temperature. It is
+true, the glass expands and contracts also; but only by about
+one-twentieth of the extent of the mercury. Regarding the bulb, then, as
+unalterable in size, all the changes in the bulk of the fluid must take
+place in the tube, and be exhibited by the expansion and contraction of
+the column, which variations are made to measure changes of temperature.
+
+
+56. STANDARD THERMOMETER.
+
+The peculiarities in the construction of thermometers will be best
+understood by describing the manufacture of a _Standard Thermometer_,
+which is one of the most accurate make, and the scale of which is divided
+independently of any comparison with another thermometer. Fig. 36 is an
+illustration of such an instrument, on a silvered brass scale.
+
+[Illustration: Fig. 36]
+
+_Selection of Tube._--In selecting the glass tube, much care is requisite
+to ascertain that its bore is perfectly uniform throughout. As received
+from the glass-house, the tubes are generally, in their interior, portions
+of very elongated cones, so that the bore is wider at one end than at the
+other. With due care, however, a proper length of tube can be selected, in
+which there is no appreciable difference of bore. This is ascertained by
+introducing into the tube a length of mercury of about a half or a third
+of an inch, and accurately measuring it in various positions in the tube.
+To accomplish this, the workman blows a bulb at one end of the tube, and
+heats the bulb a little to drive out some of the air. Then, placing the
+open end in mercury, upon cooling the elasticity of the enclosed air
+diminishes, and the superior pressure of the atmosphere drives in some
+mercury. The workman stops the process so soon as he judges sufficient
+mercury has entered. By cooling or heating the bulb, as necessary, the
+mercury is made to pass from one end of the tube to the other. Should the
+length of this portion of mercury alter in various parts of the bore, the
+tube must be rejected. If it is, as nearly as possible, one uniform
+length, the tube is set aside for filling.
+
+The _bulb_ is never blown by the breath, but by an elastic caoutchouc ball
+containing air, so that the introduction of moisture is avoided. The
+spherical form is to be preferred; for it is best adapted to resist the
+varying pressure of the atmosphere. The bulbs should not be too large, or
+the mercury will take some time to indicate sudden changes of temperature.
+Cylindrical bulbs are sometimes desirable, as they offer larger surfaces
+to the mercury, and enable thermometers to be made more sensitive.
+
+The _mercury_, with which the bulb is to be filled, should be quite pure,
+and freed from moisture and air by recent boiling.
+
+_Filling the Tube._--The filling is effected by heating the bulb with the
+flame of a spirit-lamp, while the open end is embedded in mercury. Upon
+allowing the bulb to cool, the atmospheric pressure drives some mercury
+into it; and the process of heating and cooling is thus continued until
+sufficient mercury is introduced. The mercury is next boiled in the tube,
+to expel any air or moisture that may be present. In order to close the
+tube and exclude all air, the artist ascertains that the tube contains
+the requisite quantity of mercury; then, by holding the bulb over the
+spirit flame, he causes the mercury to fill the whole of the tube, and
+dexterously removing it from the source of heat, he, at the same instant,
+closes it with the flame of a blow-pipe. If any air remain in the tube, it
+is easily detected; for if the instrument be inverted, the mercury will
+fall to the extremity of the tube, if there is a perfect vacuum, unless
+the tube be so finely capillary that its attraction for the mercury is
+sufficient to overcome the force of gravity, in which case the mercury
+will retain its position in every situation of the instrument. If,
+however, the mercury fall and does not reach quite to the extremity of the
+bore, some air is present, which must be removed.
+
+_The Graduation._--The thermometer is now prepared for graduation, the
+first part of which process is the determination of two fixed points.
+These are given by the temperatures of melting ice and of the vapour of
+boiling water. Melting ice has always the same temperature in every place
+and under all circumstances; provided only that the water from which the
+ice is congealed is free from salts. The temperature of the vapour of
+boiling water depends upon the pressure of the atmosphere, but is always
+constant for the same pressure.
+
+The fixed point corresponding to the temperature of melting ice is called
+the _freezing point_. It is obtained by keeping the bulb and the part of
+the tube occupied by mercury immersed in melting ice, until the mercury
+contracts to a certain point, where it remains stationary. This position
+of the end of the mercury is then marked upon the tube.
+
+The _boiling point_ is not so easily determined, for the barometer must be
+consulted about the same time. The boiling apparatus is generally
+constructed of copper. It consists of a cylindrical boiler, heated from
+the base by a spirit lamp or charcoal fire. An open tube two or three
+inches in diameter and of suitable length enters the top of the boiler.
+This tube is enveloped by another fixed to the top of the boiler but not
+opening into it, and so that the two tubes are about an inch apart. The
+object of the outer tube is to protect the inner tube from the cold
+temperature of the air. The outer tube has an opening at the top for the
+admission of the thermometer, and a hole near the bottom for the escape of
+steam through a spout. When the water is made to boil, the steam rises in
+the inner tube, fills the space between the tubes, and escapes at the
+spout. The thermometer is then passed down into the inner cylinder, and
+held securely from the top by means of a piece of caoutchouc. The tubes or
+cylinders should be of sufficient length to prevent the thermometer
+entering the water. This is necessary because the temperature of boiling
+water is influenced by any substance which it holds in chemical solution;
+and, moreover, its temperature increases with the depth, owing to the
+pressure of the upper stratum. The thermometer being thus surrounded with
+steam, the mercury rises in the tube. As it does so, the tube should be
+depressed so as always to keep the top of the mercury just perceptible.
+When the temperature of the vapour is attained, the mercury ceases to
+rise, and remains stationary. The position of the end of the mercury is
+now marked upon the tube, and the "_boiling-point_" is obtained.
+
+
+=57. Methods of ascertaining the exact Boiling Temperature.=--The normal
+boiling temperature of water all nations have tacitly agreed to fix under
+a normal barometric pressure of 29�922 inches of mercury, having the
+temperature of melting ice, in the latitude of 45�, and at the sea-level.
+If the atmospheric pressure at the time or place of graduating a
+thermometer does not equal this, the boiling temperature will be higher or
+lower according as the pressure is greater or less. Hence a reading must
+be taken from a reliable barometer, which must also be corrected for
+errors and temperature, and reduced for latitude, in order to compare the
+actual atmospheric pressure at the time with the assumed normal pressure.
+Tables of vapour tension, as they are termed, have been computed from
+accurate experimental investigations and theory,--giving the temperatures
+of the vapour of water for all probable pressures; Regnault's, the most
+recent, is considered the most accurate; and his investigations are based
+upon the standard pressure given above, and are for the same latitude. His
+Table, therefore, will give the temperature on the thermometric scale
+corresponding to the pressure.
+
+The Commissioners appointed by the British Government to construct
+standard weights and measures, decided that the normal boiling-point,
+212�, on the thermometer should represent the temperature of steam
+generated under an atmospheric pressure equal in inches of mercury, at the
+temperature of freezing water, to 29�922 + (cos. 2 latitude � �0766) +
+(�00000179 � height in feet above the sea-level). Hence, at London, lat.
+51�30� N., we deduce 29�905 as the barometric pressure representing the
+normal boiling point of water,--the trifling correction due to height
+being neglected. If then, in the latitude of London, the barometric
+pressure, at the time of fixing the boiling point, be not 29�905 inches,
+that point will be higher or lower, according to the difference of the
+pressure from the normal. Near the sea-level about 0�59 inch of such
+difference is equivalent to 1� Fahrenheit in the boiling point.
+
+Suppose, then, the atmospheric pressure at London to be 30�785 inches, the
+following calculation gives the corresponding boiling temperature for
+Fahrenheit's scale:--
+
+  Observed pressure    30�785
+  Normal       "       29�905
+                       ------
+  Difference             �880
+                      =======
+
+As 0�59 is to 0�88, so is 1� to 1��5.
+
+That is, the water boils at 1��5 above its normal temperature; so that, in
+this case, the normal temperature to be placed on the scale, viz. 212�,
+must be 1��5 lower than the mark made on the tube at the height at which
+the mercury stood under the influence of the boiling water.
+
+The temperature of the vapour of boiling water may be found, at any time
+and place, as follows:--Multiply the atmospheric pressure by the factor
+due to the latitude, given in the annexed Table V., and with the result
+seek the temperature in Table VI.
+
+        TABLE V.                         TABLE VI.
+
+  +----------------------------------------------------------------+
+  |Latitude.| Factor. |||Temperature|Tension.||Temperature|Tension.|
+  |         |         ||| of Vapour.|        || of Vapour.|        |
+  |---------+---------+++-----------+--------++-----------+--------|
+  |Degrees. |         ||| Degrees.  |Inches. || Degrees.  |Inches. |
+  |    0    | 0�99735 |||    179    | 14�934 ||    197    | 22�036 |
+  |    5    | 0�99739 |||    180    | 15�271 ||    198    | 22�501 |
+  |   10    | 0�99751 |||    181    | 15�614 ||    199    | 22�974 |
+  |   15    | 0�99770 |||    182    | 15�963 ||    200    | 23�456 |
+  |   20    | 0�99797 |||    183    | 16�318 ||    201    | 23�946 |
+  |   25    | 0�99830 |||    184    | 16�680 ||    202    | 24�445 |
+  |   30    | 0�99868 |||    185    | 17�049 ||    203    | 24�952 |
+  |   35    | 0�99910 |||    186    | 17�425 ||    204    | 25�468 |
+  |   40    | 0�99954 |||    187    | 17�808 ||    205    | 25�993 |
+  |   45    | 1�00000 |||    188    | 18�197 ||    206    | 26�527 |
+  |   50    | 1�00046 |||    189    | 18�594 ||    207    | 27�070 |
+  |   55    | 1�00090 |||    190    | 18�998 ||    208    | 27�623 |
+  |   60    | 1�00132 |||    191    | 19�409 ||    209    | 28�185 |
+  |   65    | 1�00170 |||    192    | 19�828 ||    210    | 28�756 |
+  |   70    | 1�00203 |||    193    | 20�254 ||    211    | 29�335 |
+  |   75    | 1�00230 |||    194    | 20�688 ||    212    | 29�922 |
+  |   80    | 1�00249 |||    195    | 21�129 ||    213    | 30�515 |
+  |         |         |||    196    | 21�578 ||    214    | 31�115 |
+  +----------------------------------------------------------------+
+
+_How to use the Tables._--When the _temperature_ is known to decimals of a
+degree, take out the tension for the degree, and multiply the difference
+between it and the next tension by the decimals of the temperature, and
+add the product to the tension, for the degree.
+
+Required the tension corresponding to 197��84.
+
+   �
+  197           = 22�036        �465 � �84 =   �391
+  198           = 22�501         197�      = 22�036
+                  ------                     ------
+    Difference      �465         197�84    = 22�427
+                  ======                     ======
+
+When the _tension_ is given, take the difference between it and the next
+less tension in the Table, and divide this difference by the difference
+between the next less and next greater tensions. The quotient will be the
+decimals to add to the degree opposite the next less tension.
+
+Thus, for 23�214 inches, required the temperature.
+
+  Given 23�214             Next greater         23�456
+        22�974             Next less            22�974
+        ------                                  ------
+          �240                   Difference       �482
+                     �240
+                 And ----                    =      �5
+                     �482
+      Temperature opposite next less             199�0
+                                                 -----
+      Temperature required                       199�5
+                                                 =====
+
+A similar method of interpolation in taking out numerical quantities is
+applicable to almost all tables; and should be practised with all those
+given in this work.
+
+_Example._--Thus, in Liverpool, lat. 53� 30� N., the barometer reading
+29�876 inches, its attached thermometer 55�, and the correction of the
+instrument being + �015 (including index error, capillarity and capacity),
+what temperature should be assigned for the boiling point marked on the
+thermometer?
+
+  Observed barometer               29�876
+  Correction                      +  �015
+                                   ------
+                                   29�891
+  Correction for temperature      -  �074
+                                   ------
+  Reduced reading                  29�817
+  Factor from Table V.            1�00077
+                                  -------
+                                   208719
+                                  208719
+                               29817
+                               -----------
+  Equivalent for lat. 45�      29�83995909
+                               ===========
+
+In Table VI., 29�84 gives temperature 211��86.
+
+
+=58. Displacement of the Freezing Point.=--Either the prolonged effect of
+the atmospheric pressure upon the thin glass of the bulbs of thermometers,
+or the gradual restoration of the equilibrium of the particles of the
+glass after having been greatly disturbed by the operation of boiling the
+mercury, seems to be the cause of the freezing points of standard
+thermometers reading from a few tenths to a degree higher in the course of
+some years, as has been repeatedly observed. To obviate this small error,
+it is our practice to place the tubes aside for about six months before
+fixing the freezing point, in order to give time for the glass to regain
+its former state of aggregation. The making of accurate thermometers is a
+task attended with many difficulties, the principal one being the
+liability of the zero or freezing point varying constantly, so much so,
+that a thermometer that is perfectly correct to-day, if immersed in
+boiling water, will be no longer accurate; at least, it will take some
+time before it again settles into its normal state. Then, again, if a
+thermometer is recently blown, filled, and graduated immediately, or, at
+least, before some months have elapsed, though every care may have been
+taken with the production of the instrument, it will require some
+correction; so that the instrument, however carefully made, should from
+time to time be plunged into finely-pounded ice, in order to verify the
+freezing point.
+
+
+=59. The Scale.=--The two fixed points having been determined, it is
+necessary to apply the scale. The thermometers in general use in the
+United Kingdom, the British Colonies, and North America are constructed
+with Fahrenheit's scale. Fahrenheit was a philosophical instrument maker
+of Amsterdam, who, about the year 1724, invented the scale which has given
+his name to the thermometer. The freezing point is marked 32�, the boiling
+point 212�, so that the intermediate space is divided into 180 equal
+parts, called degrees. "The principle which dictated this _peculiar
+division_ of the scale is as follows:--When the instrument stood at the
+greatest cold of Iceland, or 0 degree, it was computed to contain 11124
+equal parts of quicksilver, which, when plunged in melting snow, expanded
+to 11156 parts; hence the intermediate space was divided into 32 equal
+portions, and 32 was taken as the freezing point of water: when the
+thermometer was plunged in boiling water, the quicksilver was expanded to
+11336; and therefore 212� was marked as the boiling point of that fluid.
+In _practice_, Fahrenheit determined the divisions of his scale from two
+fixed points, the freezing and boiling of water. _The theory_ of the
+division, if we may so speak, was derived from the lowest cold observed in
+Iceland, and the expansions of a given portion of mercury" (_Professor
+Trail_).
+
+The divisions of the scale can be carried beyond the fixed points, if
+requisite, by equal graduations. Fahrenheit's scale is very convenient in
+some respects. The meteorological observer is seldom troubled with
+negative signs, as the zero of the scale is much below freezing. Again,
+the divisions are more numerous, and consequently smaller, than on other
+scales in use; and the further subdivision into tenths of degrees, seems
+to give all the minuteness usually required.
+
+_Celcius_, a Swede, in 1742, proposed zero for the freezing point, and 100
+for the boiling point, all temperatures below zero being distinguishable
+by the sign (--) minus. This scale is known as the _centigrade_, and is in
+use in France, Sweden, and the southern part of Europe. It has the
+advantage of the decimal notation, with the embarrassment of the negative
+sign.
+
+_Reaumur_, a Frenchman, proposed zero for the freezing point, and 80� for
+the boiling point, an arrangement inferior to the centigrade. It is,
+however, in use in Spain, Switzerland, and Germany.
+
+It is merely a simple arithmetical operation to change the indications of
+any one of these scales into the equivalents on the others. To facilitate
+such conversions, tables are convenient, when a large number of
+observations are under discussion; and they can be easily formed or
+obtained.
+
+In the absence of such tables, the following formul� will insure accuracy
+of method, and save thinking, when occasional conversions are wanted to be
+made:--F. stands for Fahrenheit, C. for Centigrade, and R. for Reaumur.
+
+  Given.   Required.     Solution.
+    F.        C.     =  (F.-32) 5/9
+    F.        R.     =  (F.-32) 4/9
+    C.        F.     =  9/5 C. + 32
+    C.        R.     =  4/5 C.
+    R.        F.     =  9/5 R. + 32
+    R.        C.     =  5/4 R.
+
+_Example._--Convert 25� of Fahrenheit's scale into the corresponding
+temperature on the Centigrade scale.
+
+  Here C. = (25 - 32) 5/9
+       C. = -35/9 = -3�9
+
+or nearly 4� _below_ zero of the Centigrade scale. The algebraical sign
+must be carefully attended-to in the calculations.
+
+
+=60. The method of testing Thermometers= for meteorological purposes is
+very simple. Such thermometers are seldom required to read above 120�. In
+these the freezing point having been determined, the divisions of the
+scale are ascertained by careful comparisons, with a standard thermometer,
+in water of the requisite temperature. "For the freezing point, the bulbs,
+and a considerable portion of the tubes of the thermometers, are immersed
+in pounded ice. For the higher temperatures, the thermometers are placed
+in a cylindrical glass vessel containing water of the required heat: the
+scales of the thermometers intended to be tested, together with the
+Standard with which they are to be compared, are read through the glass.
+In this way the scale readings may be tested at any required degree of
+temperature, and the usual practice is to test them at every ten degrees
+from 32� to 92� of Fahrenheit."--_FitzRoy._
+
+
+=61. Porcelain Scale Plates.=--Thermometer scales of brass, wood, or
+ivory, either by atmospheric influence or dipping in sea-water, are very
+liable to become soiled and discoloured, so much so that after a very
+little time the divisions are rendered nearly invisible. To obviate this
+inconvenience, Messrs. Negretti and Zambra were the first to introduce
+into extensive use thermometer and barometer scale-plates made of
+porcelain, having the divisions and figures engraved thereon by means of
+fluoric acid, and permanently burnt-in and blackened, so as always to
+present a clear legible scale. That these scales have been found superior
+to all others, may be inferred from the fact that all the thermometers now
+supplied to the various government departments are provided with such
+scales.
+
+They can be adapted to replace any of the old forms of brass or zinc
+scales, the divisions and figures of which have become obliterated or
+indistinct.
+
+
+=62. Enamelled Tubes.=--Nearly all thermometer tubes are now made with
+enamelled backs. This contrivance of enamelling the backs of the tubes
+enables the makers to use finer threads of mercury than had before been
+found practicable; for were it not for the great contrast between the dark
+thread of mercury and the white enamel on the glass, many of the
+thermometers now in use would be positively illegible. The enamelling of
+thermometers is an invention of Messrs. Negretti and Zambra. It is
+necessary to state this, as many persons, from interested motives, are
+anxious to ignore to whom the credit of the invention is due.
+
+
+=63. Thermometers of extreme Sensitiveness.=--Thermometers for delicate
+experiments are no novelty. Thermometers have been made with very delicate
+bulbs to contain a very small quantity of mercury. Such instruments have
+also been made with spiral or coiled tubular bulbs, but the thickness of
+glass required to keep these coils or spirals in shape, and in fact to
+prevent their falling to pieces, served to nullify the effect sought to be
+produced, viz. instantaneous action; and where a small thin bulb was
+employed, the indicating column was generally so fine that it was
+positively invisible except by the aid of a powerful lens. Messrs.
+Negretti and Zambra have now introduced a new form of thermometer, which
+combines sensitiveness and quickness of action, together with a good
+visible column. The bulb of this thermometer is of the gridiron form. Care
+has been taken in constructing the bulb, so that the objections attending
+spirals and other forms have been overcome; for whilst the reservoir or
+bulb is made of glass so thin that it is only by a spirit lamp and not a
+glass blower's blowpipe that it can be formed, yet it is still so rigid
+(owing to its peculiar configuration) that no variations in its
+indications can be detected, whether it be held in a horizontal, vertical,
+or oblique position, nor will any error be detected if it be stood on its
+own bulb. They have made thermometers with bulbs or reservoirs formed of
+about nine inches of excessively thin cylindrical glass, whose outer
+diameter is not more than a twentieth of an inch; so that, owing to the
+large surface presented, the indications are positively instantaneous.
+This form of thermometer was constructed expressly to meet the
+requirements of scientific balloon ascents, to enable thermometrical
+readings to be taken at the precise elevation. It was contemplated to
+procure a metallic thermometer, but on the production of this perfect
+instrument the idea was abandoned.
+
+
+64. VARIETIES OF THERMOMETERS.
+
+Fig. 37 is an illustration of boxwood scale thermometers for general use
+and common purposes.
+
+Fig. 38, Negretti and Zambra's Travelling Thermometer; it is fixed in a
+plated metal (silver or otherwise) case, similar to a pencil-case, and has
+the scale divided upon its stem.
+
+Fig. 39, Thermometer mounted on a slab of glass, upon which the scale is
+etched, the back being either oak, mahogany, or ebony.
+
+Fig. 40, Portable Thermometer, in a bronzed brass or German silver
+revolving case.
+
+Fig. 41, Pocket Thermometer, on ivory or metallic scale, in morocco or
+papier-m�ch� case.
+
+[Illustration: Fig. 37.]
+
+[Illustration: Fig. 38.]
+
+[Illustration: Fig. 39.]
+
+[Illustration: Fig. 40.]
+
+[Illustration: Fig. 41.]
+
+Fig. 42, an Ornamental Drawing-room Thermometer, on ebony or ivory stand,
+with glass shade.
+
+Fig. 43, representation of highly carved or engine-turned design for
+thermometer mounts, in ivory or wood, for the drawing-room. Some have the
+addition of a sundial or compass at the top; they may also be formed for a
+watch-stand.
+
+Fig. 44, =Bath Thermometer=, having a float to admit of its being kept in
+the water.
+
+[Illustration: Fig. 42.]
+
+[Illustration: Fig. 43.]
+
+[Illustration: Fig. 44.]
+
+Fig. 45, Thermometer with ivory scale in glass cylinder, mounted on oak
+bracket with metal top, for out-door use; as at a window.
+
+Fig. 46, Thermometer for the window, on patent porcelain or glass scale,
+with oak bracket and convenient brass supports, for placing the instrument
+at any angle.
+
+Fig. 47, =Chemical Thermometer=, on boxwood scale, jointed near the bulb
+on a brass hinge, ranging from 300� to 600�.
+
+Fig. 48, =Chemical Thermometer=, for acids, graduated on its own stem,
+suitable for insertion in the tubulure of retorts; they are also made
+insulated in glass cylinder to protect the graduated stem; ranging from 0�
+to 600�.
+
+[Illustration: Fig. 45.]
+
+[Illustration: Fig. 46.]
+
+[Illustration: Fig. 47.]
+
+[Illustration: Fig. 48.]
+
+[Illustration: Fig. 49.]
+
+[Illustration: Fig. 50.]
+
+
+=65. Superheated Steam Thermometer.=--The great advantage gained by the
+use of superheated steam in marine and other steam-engines being now
+generally admitted by engineers, reliable thermometers, reading to 600� at
+least, are of the utmost importance. To meet this want, Messrs. Negretti
+and Zambra have constructed for the purpose a substantial form of
+thermometer, on their patent porcelain scales, in strong and convenient
+metal mountings, with perforated protection to the bulb. The scales cannot
+be deteriorated by steam, heat, oil, or dirt; and an occasional wiping
+will be all that is necessary to keep the divisions and figures clean and
+visible for any length of time; while careful calibration of the
+thermometer tubes ensures the most accurate indications attainable. These
+thermometers are illustrated by figs. 49 & 50. A similar, but cheaper,
+construction is given to thermometers to be used with hot air, or hot
+water, apparatus.
+
+
+=66. Thermometer for Sugar Boiling= is protected by a metallic frame; and
+is usually from three to four feet long, the graduations being confined to
+a space of about twelve inches at the upper part of the instrument,
+allowing the bulb and greater part of the tube to be immersed in the
+boiling sugar. The graduations extend to 270� or further. An index is
+sometimes attached to the scale, which may be set to any degree of heat
+required to be maintained.
+
+
+67. EARTH THERMOMETER.
+
+The Earth Thermometer is for ascertaining the temperature of the soil at
+various depths. It is protected by a brass frame, pointed and strengthened
+at the end to facilitate insertion into the ground, as in fig. 51.
+
+[Illustration: Fig. 51.]
+
+_Utility of a Knowledge of the Temperature of the Soil._--The temperature
+of the soil is an important element in the consideration of climate, as it
+concerns the vegetable kingdom.
+
+Dr. Daubeny, in his _Lectures on Climate_, gives the following statement
+with respect to some temperatures which have been observed just beneath
+the earth's surface, in different parts of the globe:--
+
+  --------------------------------------------------------------------
+     Country.      |          Temperature.          |   Authority.
+  -----------------+--------------------------------------------------
+  Tropics, often   |162-184�                        | Humboldt.
+                   |                                |
+  Egypt            |133-144                         | Edwards & Colin.
+                   |                                |
+  Orinoco          |In white sand, 140              | Humboldt.
+                   |                                |
+  Chili            |113-118, among dry grass        | Boussingault.
+                   |                                |
+  Cape of Good Hope|150, under the soil of a bulb   | Herschell.
+                      garden                        |
+                   |                                |
+  Bermuda          |142, thermometer barely covered | Emmet.
+                   |  in earth                      |
+                   |                                |
+  China            |Water of the fields, 113;       | Meyer.
+                   |  adjacent sand, much higher;   |
+                   |  blackened sides of the boat   |
+                   |  at midday, 142-150            |
+                   |                                |
+  France           |118-122, and in one instance 127| Arago.
+  --------------------------------------------------------------------
+
+"The importance of this to vegetation may be estimated by the following
+considerations:--
+
+"It is known that every plant requires a certain amount of heat, varying
+in the case of each species, for the renewal of its growth, at the
+commencement of the season.
+
+"Now when this degree of heat has spurred into activity those parts that
+are above ground, and caused them to elaborate the sap, it is necessary
+that the subterranean portions should at the same time be excited by the
+heat of the ground to absorb the materials which are to supply the plant
+with nourishment. Unless the latter function is provided for, the aerial
+portions of the plant will languish from want of food to assimilate.
+Indeed, it is even advisable that the roots should take the start of the
+leaves, in order to have in readiness a store of food for the latter to
+draw upon." In another place the professor remarks:--"It has been
+calculated by Mr. Raikes, from experiments made at Chat Moss, that the
+temperature of the soil when drained averages 10� more than it does when
+undrained; and this is not surprising, when we find that 1 lb. of water
+evaporated from 1,000 lbs. of soil will depress the whole by 10�, owing to
+the latent heat which it absorbs in its conversion into vapour."
+
+
+68. MARINE THERMOMETER.
+
+This instrument is a special construction to meet the requirements of
+navigation. It consists of a carefully constructed thermometer divided on
+its stem to degrees, which are sufficiently large to admit of subdivision
+into tenths of degrees by estimation, and ranging from 0� to 130�. The
+scale is porcelain, having the degrees etched upon it, and burnt-in a
+permanent black. The instrument is made to slide into a japanned metallic
+case, for handy use and protection. It is therefore adapted for almost any
+ordinary purpose; and cannot be injuriously affected by any chemical
+action arising from air or sea-water. A set of these thermometers consists
+of six, carefully packed in a neat box; two having japanned metallic cases
+(fig. 52), the others being designed for use without the case, or to
+replace a breakage.
+
+[Illustration: Fig. 52.]
+
+This thermometer is employed in the Royal Navy, and for the observations
+made at sea for the Board of Trade.
+
+The thermometer is now considered a necessary instrument on board ship.
+Not only is it of invaluable utility in connection with the barometer as a
+guide to the weather, but its indications are of service in showing the
+presence of a warm or cold current in the sea; many of the great oceanic
+currents being characterised by the warmth or coldness of their waters. In
+seas visited by icebergs, the habitual use of the thermometer would
+indicate their proximity, as the water is rendered colder for some
+distance around by the thawing of huge masses of ice. The water over a
+shoal in the sea is generally colder than the surface-water of the
+surrounding ocean; which may result from the cold water being brought to
+the surface by the current of water encountering the shoal. With this fact
+navigators are well acquainted; and therefore a fall in the sea-water
+thermometer may forebode that shallow water is at hand. It has been
+ascertained that fish inhabit regions of the oceans and seas having the
+peculiar temperature suitable to their habits. The better and firmer sort
+of fish are found where cold waters exist. Those taken in warmer belts or
+streams of water, even in the same latitude, are far inferior in
+condition, and less approved by the palate. The fish of the Mediterranean,
+a warm sea, are generally poor and scarce. Fish taken in the cold waters
+between the American shore and the Gulf Stream are much esteemed; while in
+and on the other side of the stream they are said to be tasteless, and of
+no flavour. Between the coasts of China and the warm waters of the
+Japanese current, the seas abound with excellent fish; but in the warm
+waters of the current and beyond, they are never seen in such shoals.
+
+In fact, it is clearly ascertained that fishes are adapted to climates,
+like birds and beasts. It has been even affirmed, after careful
+investigation, that herrings, which abound in the British Seas, and form a
+most important branch of our fisheries, can only be found in a temperature
+varying from 54� to 58�. Hence the thermometer, if brought into use by the
+fishermen, would guide them to the spots where they may with the best
+chance cast their nets on dark nights, when other indications are not
+perceptible.
+
+This thermometer in its metallic case is perfectly suited for dipping
+overboard, or placing in a bucket of water just taken from the sea, to
+ascertain its temperature.
+
+
+
+
+CHAPTER VII.
+
+SELF-REGISTERING THERMOMETERS.
+
+
+=69. Importance of Self-Registering Thermometers.=--Heat being apparently
+the most effective agent in producing meteorological phenomena, the
+determination of the highest temperature of the day, and the lowest during
+the night, is a prime essential to enable an estimate of the climate of
+any place to be formed. To observe these extremes by means of the ordinary
+thermometer would be impracticable, from the constant watchfulness which
+would be necessary. Hence, the utility and importance of self-recording
+thermometers are evident. A thermometer constructed to _register_ the
+highest temperature is usually called a _maximum thermometer_; one to show
+the lowest temperature is termed a _minimum thermometer_; and if made to
+record both extremes of temperature, it is designated a
+_maximum-and-minimum thermometer_. We will, for the sake of method,
+describe the instruments in use in this order.
+
+It would carry us beyond our scope to explain in detail the methods of
+dealing with temperature observations; but we may remark that half the sum
+of the maximum and minimum temperature of each day of twenty-four hours,
+is not what meteorologists designate the _mean daily temperature_,
+although it very frequently approximates to it. The mean temperature of
+the day is understood to be the average of twenty-four consecutive hourly
+readings of a thermometer; and meteorology now supplies formul� whereby
+this result can be deduced from two or three observations only in a day.
+But we would observe that the actual mean temperature of any place has not
+such an important influence upon life, either animal or vegetable, as the
+abruptness and magnitude of the variations of temperature. Climate,
+therefore, should be estimated more by the range of the thermometer than
+by the average of its indications. The Registrar General's returns prove
+that with a wide range of the thermometer, the mortality greatly
+increases; and it is now becoming apparent to meteorologists that the
+daily range of the thermometer marks the effects of temperature on the
+health of men, and the success of crops, better than any other
+meteorological fact of which we take cognizance. Now that self-registering
+thermometers are constructed with mercury, the most appropriate of all
+thermometric substances, not only for maxima, but likewise for minima
+temperatures, the determination of the diurnal range of temperature is
+rendered more certain, and observations at different places are more
+strictly comparable.
+
+
+MAXIMA THERMOMETERS.
+
+=70. Rutherford's Maximum Thermometer.=--The maximum thermometer, invented
+by Dr. John Rutherford, differs from an ordinary thermometer in having a
+small cylinder of steel, porcelain, or aluminium, moving freely in the
+tube beyond the mercury, so as to form an index. The stem of the
+thermometer is fixed horizontally on the frame, which must be suspended in
+the same position, as represented in fig. 53. The instrument is set by
+holding it bulb downward, so as to allow the index to fall by its own
+gravity into contact with the mercury. Increase of heat produces expansion
+of the mercury, which consequently pushes forward the index. When the
+temperature decreases, the mercury recedes from the index, leaving it so
+that the extremity which was in contact with the mercury indicates upon
+the scale the highest temperature since the instrument was last set.
+
+[Illustration: Fig. 53.]
+
+As it is easily constructed and is comparatively cheap, it is still
+employed for ordinary purposes. Its disadvantages are, firstly, its
+liability of soon getting out of order by the index becoming embedded in
+the mercury, or fixed by oxidation, thus rendering it altogether useless;
+secondly, the ease with which the index can be displaced by the wind
+moving the instrument, or other accidental disturbance, so as to cause it
+to give erroneous indications occasionally; and thirdly, its consequent
+total unfitness for use at sea.
+
+In the part of the tube beyond the mercury, a small quantity of air is
+enclosed for the purpose of preventing the metal flowing freely in the
+tube. This necessitates the construction of a larger bulb, which renders
+the thermometer less sensitive. Moreover, as it frequently happens that
+some mercury passes the index, particles of air insinuate themselves in
+the metal, and cause separations in the column, which very often can be
+removed only by a maker. To facilitate this re-adjustment, a small chamber
+is left at the end of the tube, and the mercury being expanded into it by
+heat until the index and air bubbles are forced into it, if possible, upon
+the cooling down again, by a little management, the mercury will contract,
+leaving the air and index behind. Yet sometimes the index cannot be moved
+in the least from its place of fixture, so that the instrument must be
+virtually reconstructed.
+
+
+=71. Phillip's Maximum Thermometer.=--A maximum thermometer, better
+perhaps in its action than Rutherford's, has been suggested by Professor
+John Phillips, of Oxford. A small portion of air is introduced into an
+ordinary thermometer, so as to cut off about half an inch of the mercurial
+thread near its end in the tube. This forms a maximum thermometer, when
+the stem is arranged horizontally. The isolated portion is pushed forward
+by expansion, and is left in this position when the mercury contracts.
+The end remote from the bulb shows on the scale the maximum temperature.
+
+When made with a capillary tube so fine that the attraction arising from
+capillarity overcomes the force of gravity, and prevents the mercury
+falling to the end of the tube when the instrument is inverted, it forms a
+very serviceable thermometer, quite portable and suitable for use on board
+ship. In such a tube a smart shake from a swing of the hand is required to
+bring the detached portion back to the column, so as to set the instrument
+for future observation; no ordinary motion will move it. When the
+thermometer has not this peculiarity, the mercury will flow to the end, if
+held bulb downward; and in this state it is not at all a satisfactory
+instrument, as the air is likely to be displaced, and a great deal of tact
+is requisite to again get it to divide the column suitably. It has been
+found in practice that the air bubble at different temperatures assumes
+different lengths, and if very small it disappears in a few years by
+oxidation and by diffusion with the mercury, so that the instrument
+becomes defective and uncertain in action,--results which led to the
+construction of the self-registering mercurial maximum thermometer,
+invented and patented by Messrs. Negretti and Zambra. It has been before
+the public about twelve years; we may therefore, now, safely speak of its
+merits.
+
+
+=72. Negretti and Zambra's Patent Maximum Thermometer= consists of a glass
+tube containing mercury fitted on an engraved scale, as shown in fig. 54.
+The part of the thermometer tube above the mercury is entirely free from
+air; and at the point A in the bend above the bulb, is inserted and fixed
+with the blow-pipe a small piece of solid glass, or enamel, which acts as
+a valve, allowing mercury to pass on one side of it when heat is applied,
+but not allowing it to return when the thermometer cools. When mercury has
+been once made to pass the contraction, which nothing but the expansive
+force of heat can effect, and has risen in the tube, the upper end of the
+column registers the maximum temperature. To return the mercury to the
+bulb, we must apply a force equal to that which raised it in the tube; the
+force employed is gravity, assisted when necessary by a little agitation
+of the instrument.
+
+[Illustration: Fig. 54.]
+
+The degrees are generally divided on the stems of these thermometers, but
+their frames of course bear a scale as well. The makers have various
+styles of framing in wood, metal, porcelain, and even glass. Each material
+is eligible according to requirements. Porcelain scales, having the marks
+_etched_ upon them by acid and permanently blackened and baked in,--by a
+process for which the inventors have a separate patent,--will be found
+very serviceable, as they do not corrode or tarnish by exposure to any
+kind of weather; while any amount of dust and dirt can readily be cleaned
+off.
+
+The chief recommendation of this thermometer is its simplicity of
+construction, enabling it to be used with confidence and safety. Of no
+other maximum thermometer can it be said that it is impossible to derange
+or put it out of order; hence, as regards durability, it surpasses all
+others. Nothing short of actual breakage can cause it to fail. Hence it is
+the most easily portable of all self-registering thermometers, an
+advantage which renders it suitable for travellers, and for transmission
+abroad. In the year 1852, the British Meteorological Society reported this
+thermometer to be "the best which has yet been constructed for maximum
+temperature, and particularly for sun observations." Since then eleven
+years have elapsed, and it is still without a rival.
+
+_Directions for use._ In using this thermometer for meteorological
+observations, it should be suspended by means of two brass plates B, C,
+attached for that purpose, in such manner that it hangs raised up a little
+at C, and so placed that it is in the shade, with the air passing freely
+to it from all sides; then, on an increase of heat, the mercury will pass
+up the tube as in an ordinary thermometer, and continue doing so as long
+as the heat increases. On a decrease of heat, the contraction of mercury
+will take place _below_ the _bend_ in the tube, leaving the whole column
+of mercury in the tube, thus registering the highest temperature, and
+showing such till the instrument is disturbed.
+
+To prepare the instrument for future observations, remove and hold it
+perpendicularly, with the bulb downward, and then shake it. The mercury
+will then descend in the tube, and indicate the temperature of the air at
+that time; and, when again suspended, is prepared for future observation.
+
+After the temperature has attained a maximum, there will be, with a
+decrease of heat, a slight contraction of mercury in the tube--as well as
+of that in the bulb--and hence doubts have arisen as to the accuracy of
+the registration; but calculation shows, and critical trial has proved,
+that the greatest daily range of temperature will not produce an error
+large enough to be appreciable on the scale.
+
+A very great advantage of this thermometer is that the mercury may be
+allowed to flow to the end of the tube without the maximum temperature
+attained during an experiment being lost. It can be employed with the bulb
+uppermost. All that is necessary for reading the maximum temperature is to
+slope the instrument so that the mercury flows gently towards the bulb. It
+will then stop at the contraction so as to show the maximum temperature on
+the scale. Afterwards the mercury is driven into the bulb by agitating the
+instrument while held in the hand. Hence the instrument is invaluable as a
+registering thermometer on board ship, as its indications are in no way
+affected by the motions and tremors of the vessel.
+
+For physiological experiments, such as taking the temperature of the
+mouth in fever, this thermometer is the only one that can be used with
+certainty, as it can be held in any position, without losing the maximum
+temperature attained.
+
+
+MINIMA THERMOMETERS.
+
+=73. Rutherford's Alcohol Minimum Thermometer=, fig. 55, consists of a
+glass tube, the bulb and part of the bore of which is filled with
+perfectly pure spirits of wine, in which moves freely a black glass index.
+A slight elevation of the thermometer, bulb uppermost, will cause the
+glass index to flow to the surface of the liquid, where it will remain,
+unless violently shaken. On a _decrease_ of temperature the alcohol
+recedes, taking with it the glass index; on an _increase_ of temperature
+the alcohol alone ascends in the tube, leaving the end of the index
+_farthest_ from the bulb indicating the minimum temperature.
+
+[Illustration: Fig. 55.]
+
+_Directions for using, &c._--Having caused the glass index to flow to the
+end of the column of spirit, by slightly tilting the thermometer, bulb
+uppermost, suspend the instrument (in the shade with the air passing
+freely to it on all sides) by the two brass plates attached for that
+purpose,--in such manner that the bulb is about half an inch lower than
+the upper, or the end of the thermometer farthest from the bulb; then, on
+a decrease of temperature, the spirits of wine will descend, carrying with
+it the glass index; on an increase of temperature, however, the spirits of
+wine will ascend in the tube, leaving that end of the small glass index
+farthest from the bulb indicating the minimum temperature. To reset the
+instrument, simply raise the bulb end of the thermometer a little, as
+before observed, and the index will again descend to the end of the
+column, ready for future observation.
+
+_Precautions._--1. By no means jerk or shake an alcohol minimum
+thermometer _when resetting_ it, for by so doing it is liable to
+disarrange the instrument, either by causing the index to leave the
+spirit, or by separating a portion of the spirit from the main column.
+
+2. As alcohol thermometers have a tendency to read lower by age, owing to
+the volatile nature of the fluid allowing particles in the form of vapour
+to rise and lodge in the tube, it becomes necessary to compare them
+occasionally with a mercurial thermometer whose index error is known; and
+if the difference be more than a few tenths of a degree, examine well the
+upper part of the tube to see if any alcohol is hanging in the bore
+thereof; if so, the detached portion of it can be joined to the main
+column by swinging the thermometer with a pendulous motion, _bulb
+downwards_.
+
+3. The spirit column is sometimes much separated by jolting in travelling.
+If the instrument is in such a condition when received, it should be held
+by the right hand, bulb downward, and the frame tapped smartly, but
+cautiously, against the palm of the left hand. The broken thread of spirit
+will soon begin to join, and by continuing the operation a sufficient time
+all the bubbles will disappear, and the thermometer become as good as
+ever.
+
+
+=74. Horticultural Minimum Thermometer.=--This instrument, represented in
+fig. 56, is a special construction of Rutherford's minimum thermometer to
+meet the requirements of horticulturists. It is desirable, if not
+essential, that gardeners should have the means of ascertaining to what
+temperature stoves and greenhouses descend on cold nights, especially in
+winter. This thermometer is mounted on a strong cast zinc frame, with the
+divisions and figures of the scale raised.
+
+[Illustration: Fig. 56.]
+
+The sunk surface of the frame is painted dark; the figures and division a
+bright colour, so that observations can be made without a close inspection
+of the instrument.
+
+The directions for using are the same as those given in the preceding
+section. It may be used as an ordinary thermometer, by simply hanging it
+from the top loop, in which position, the coloured liquid will always
+indicate the present temperature.
+
+It was a source of annoyance with the ordinary boxwood and flat metal
+scales, that after a time, exposure to a damp warm atmosphere favoured the
+growth of conferv� upon them, and obliterated the divisions; the plan of
+raising the figures and divisions of the scale has been found to prevent
+the destruction of the instrument in this way.
+
+
+=75. Baudin's Alcohol Minimum Thermometer.=--This instrument resembles
+Rutherford's thermometer in appearance; its indications are given by the
+expansion and contraction of alcohol, and its minimum temperature is
+likewise registered by a glass index being pulled back and left behind by
+the alcohol, as in Rutherford's instrument. There is, however, a great
+improvement in Baudin's instrument; for whilst Rutherford's thermometer
+can only register in a horizontal position, Baudin's can be used either
+horizontally or vertically, as necessity may require. This important
+change is effected in the following manner:--Instead of the index in the
+thermometer being loose and free to run up and down according to the
+position in which the instrument is held, as in Rutherford's, the index in
+the new instrument is made to fit the bore of the tube as nearly tight as
+possible, so much so that in holding the thermometer even upside down, or
+shaking it, the index will not shift from its position; but, inasmuch as a
+minimum thermometer with an immoveable index could not be set when
+required for observation, and would consequently be useless, the inventor
+has introduced behind the index a piece of solid glass, about
+one-and-a-half inch in length, which moves freely in the alcohol. The
+addition of the weight of this piece of glass on the top of the index,
+when turned upside down, forces the index down to the edge of the alcohol;
+and it is there left, as in the case of the ordinary Rutherford's
+thermometer. It is, therefore, by turning the thermometer upside down, and
+letting the moveable piece of glass fall on the index, that the index is
+driven to the end of the alcohol; after this operation the thermometer is
+hung up either horizontally or vertically, and will then be ready for use.
+
+The index, although immoveable _per se_, is by the alcohol drawn back, as
+in the ordinary minimum, and its indications are read off on the scale
+from the top of the index.
+
+
+=76. Mercurial Minima Thermometers desirable.=--Alcohol does not expand
+equally for equal increments of heat, consequently errors are likely to
+exist in the scale indications unless the graduations are very
+accurately--not necessarily equally--made. On this account, as well as
+from the volatility of alcohol, and the intervention of gaseous partitions
+in the tube, a good and thoroughly reliable minimum thermometer was for a
+long time a desideratum. It was desirable to obtain a thermometer which
+should register the lowest temperature by mercury, the fluid in general
+use for meteorological thermometers. Several instruments have recently
+been invented to meet this requirement, which are suitable and
+satisfactory for land purposes, but one well adapted for use on board ship
+is still very much wanted.
+
+For very low temperatures, alcohol thermometers will always be required;
+as mercury freezes at -40� F, and contracts very irregularly much before
+this point, while alcohol has never yet been frozen.
+
+
+=77. Negretti and Zambra's Patent Mercurial Minimum Thermometer=,
+represented by fig. 57, has a cylindrical bulb of large size, which, at
+first sight, might induce the idea that the instrument would not be
+sufficiently sensitive; but as length is given to the cylinder instead of
+increasing its diameter, it will be found as sensitive as a globular bulb
+of the same diameter, and much more so than an ordinary alcohol
+thermometer.
+
+[Illustration: Fig. 57.]
+
+The reason for having the bulb large is to allow the internal diameter of
+the thermometer tube to be larger than that generally used for
+thermometrical purposes, so that a steel index, pointed at both ends, may
+move freely within when required.
+
+The tube is blown, filled and regulated in the usual way, 60� of
+temperature being about half-way up the tube. A small cylindrical bulb is
+then formed at the upper end of the tube, and then is introduced a steel
+needle pointed at both ends, that in contact with the mercury being
+abrupt, the other more prolonged. The open extremity of the tube is now
+drawn out into a fine capillary tube, and the bulb of the instrument
+warmed so as to cause the mercury to fill the tube completely. When the
+mercury reaches the capillary tube, the flame of a blow-pipe is applied;
+the glass is dexterously melted, the superfluous part taken away, and the
+tube left hermetically closed. During this operation, the steel index has
+been embedded in the heated mercury. As the instrument cools, if held
+upright, the mercury will recede and expose the needle, which will then
+follow the descending column simply by its own gravity. In this condition
+the thermometer resembles Rutherford's maximum, being a tube of mercury
+with a steel index floating on its surface; but it possesses these
+important advantages: it is quite free from air, so that the mercury can
+move with perfect freedom; and the index is pointed at both ends, to allow
+the mercury to pass, instead of being ground flat to prevent it.
+
+_To use the Thermometer_, it is suspended perpendicularly (figure 57) with
+the steel index resting on the surface of the mercurial column. As the
+mercury in the cylinder contracts, that in the tube descends, and the
+index, of its own gravity, follows it; on the contrary, as the mercury
+expands and rises in the tube, it passes the index on one side, and in
+rising, exerts a lateral pressure on the needle, and jams it to one side
+of the tube, where it remains firmly fixed, leaving the upper point of the
+needle indicating the minimum temperature. In this thermometer, the
+reading is always from the upper point of the needle, and not from the
+mercury itself.
+
+_To extricate the Needle_ from the mercury, a magnet is used, when, if the
+needle is embedded only a few degrees, it can readily be withdrawn without
+altering the position of the instrument. Should the magnet not be
+sufficient for the purpose, we simply turn the thermometer on its support
+from the upright position, slightly elevating the bulb (fig. 58 (=2=)).
+The mercury and index will then flow into the small reservoir. Should the
+index not freely leave the tube with the mercury, assist it with a magnet,
+and when the mercury and index are in the upper bulb (figure =2=), apply a
+magnet outside, which will attract and hold fast the index; and whilst
+thus holding it, again bring the thermometer to the upright position, when
+the mercury will immediately fall back into the tube, leaving the index
+attached to the magnet (figure =4=), with which it is guided down to the
+surface of the mercury, ready for another observation.
+
+[Illustration: Fig. 58.]
+
+Care must be taken not to withdraw the magnet until the index is in
+contact with mercury; for, if released before touching, it might plunge
+too deeply, and give a false indication. The rule for re-setting it will
+be to bring the needle-point in contact with the mercury, and then
+withdraw the magnet, having previously ascertained that no particles of
+mercury are attached to the index.
+
+It may sometimes, though rarely, happen, that from the time a minimum
+temperature is registered by the index, and by the time an observation is
+made, the mercury may have risen so high in the tube as to completely pass
+the index, as shown (figure =3=). Should it so happen, the space which the
+index occupies will readily be observed, as it will be pressed to one side
+of the tube, causing a different appearance in that part, although the
+point of the needle may not be seen. If such be the case, apply a magnet
+to the spot where you see the index is fixed: this will hold the needle
+firmly. Then, by slightly tilting the thermometer bulb uppermost, the
+mercury will flow into the top bulb, leaving the index attached to the
+magnet, and quite uncovered. Having taken the reading, draw the needle
+into the top bulb, and hold it there whilst you adjust the thermometer by
+again bringing it to the upright position.
+
+By contracting the bore of this thermometer, at the bend of the tube,
+sufficiently to keep the mercury from flowing out of its bulb with too
+much freedom by motion, the instrument becomes perfectly safe for
+transmission abroad.
+
+
+=78. Negretti & Zambra's Second Patent Mercurial Minimum Thermometer.=--In
+this thermometer a principle is used that has been long known to
+scientific men, viz. the affinity of mercury for platinum. If mercury be
+placed in contact with platinum under ordinary circumstances, no effect
+will take place; but if the mercury is once made to attack the platinum,
+the amalgamation is permanent and the contact perfect, so much so, that
+the principle was made use of in constructing standard barometers. A ring
+of platinum was fused round the end of the tube, dipping into the mercury;
+and the contact between the platinum and mercury became so perfect that
+air could not creep down the tube and up the bore, as in ordinary
+barometer tubes. This principle of adhesion or affinity of mercury for
+platinum has been brought into play for the purpose of arresting the
+mercury after it has reached the minimum temperature in a thermometer.
+This thermometer is made as follows:--behind the bulb is placed a
+supplementary chamber; in the space or neck between the bulb of the
+thermometer and the chamber, is placed a small piece of platinum; this may
+be of any shape or size, but the smaller the better. This is not to fit in
+the neck; it must, on the contrary, be rather loose; it may be fastened in
+position or not. The instrument is represented by fig. 59.
+
+[Illustration: Fig. 59.]
+
+_Directions for using._--Having suspended the thermometer in a horizontal
+position, the mercury is made to stand in exact contact with the platinum
+plug by slightly elevating the bulb end of the instrument. The thermometer
+is now ready for observation. On a decrease of temperature, the mercury
+will endeavour to contract first from the easier passage, viz. behind the
+bulb; but in consequence of the adhesion of the mercury to the platinum,
+it cannot recede from here, it is therefore forced to contract from the
+indicating tube, and will continue to do so as long as the temperature
+decreases; and as no indices are employed in this thermometer, the extreme
+end of the mercurial column will show "how cold it has been." On an
+increase of temperature the mercury will glide over the platinum plug and
+expand by the easier passage into the supplementary chamber, and there
+remain until a decrease of temperature again takes place, when the mercury
+that had gone into the supplementary chamber will be the first to recede,
+until it reaches the platinum plug, its further progress being arrested;
+it will then fall in the indicating tube, and there remain until re-set.
+
+
+=79. Casella's Mercurial Minimum Thermometer.=--The general form and
+arrangement of this instrument is shown in fig. 60. A tube with large
+bore, _a_, has at the end a _flat glass diaphragm_ formed by the abrupt
+junction of a small chamber, _b c_, the inlet to which at _b_ is larger
+than the bore of the indicating tube. The result of this is that on
+setting the thermometer, as described below, the contracting force of the
+mercury in cooling withdraws the fluid in the indicating stem only; whilst
+on its expanding with heat, the long column does not move, the increased
+bulk of mercury finding an easier passage into the small pear-shaped
+chamber attached.
+
+[Illustration: Fig. 60.]
+
+We believe that a small speck of air must be confined in the chamber, _b
+c_, to act as a spring to start the mercury from the chamber in the act of
+setting the thermometer. Were this air not present, the mercury would so
+adhere to the glass that no amount of shaking could induce it to flow from
+the chamber.
+
+_To set the Instrument_, place it in a horizontal position, with the back
+plate, _d_, suspended on a nail, and the lower part supported on a hook,
+_e_. The bulb end may now be gently raised or lowered, causing the mercury
+to flow slowly until the bent part, _a_, _is full_ and the chamber, _b c_,
+_quite empty_. At this point the flow of mercury in the long stem of the
+tube is arrested, _and indicates the exact temperature_ of the bulb or air
+at the time. On an increase of temperature the mercury will expand into
+the small chamber, _b c_; and a return of cold will cause its recession
+from this chamber only, until it reaches the diaphragm, _b_. Any further
+diminution of heat withdraws the mercury down the bore to whatever degree
+the cold may attain, where it remains until farther withdrawn by increased
+cold, or till re-set for future observation.
+
+
+MAXIMA AND MINIMA THERMOMETERS.
+
+=80. Rutherford's= arrangement for obtaining a complete instrument for the
+registration of heat and cold was simply mounting a maximum thermometer
+and a minimum thermometer upon the same frame or slab. Thus constructed,
+they are often called "day and night" thermometers, though somewhat
+inappropriately; for in temperate climates the temperature of the night
+sometimes exceeds that of the day, notwithstanding the reverse is the
+general law of temperature. Fig. 61 will explain the arrangement of
+Rutherford's day and night thermometer.
+
+[Illustration: Fig. 61.]
+
+
+=81. Sixe's Self-Registering Thermometer.=--The very ingenious and
+certainly elegant instrument about to be described was invented by James
+Sixe, of Colchester. It consists of a long cylindrical bulb, united to a
+tube of more than twice its length, bent round each side of it in the form
+of a syphon, and terminated in a smaller, oval-shaped bulb. Figure 62
+gives a representation of this instrument. The lower portion of the syphon
+is filled with mercury; the long bulb, the other parts of the tube, and
+part of the small bulb, with highly rectified alcohol. A steel index moves
+in the spirit in each limb of the syphon. The two indices are terminated
+at top and bottom with a bead of glass, to enable them to move with the
+least possible friction, and without causing separation of the spirit, or
+allowing mercury to pass easily. They would, from their weight, always
+rest upon the mercury; but each has a fine hair tied to its upper
+extremity and bent against the interior of the tube, which acts as a
+spring with sufficient elasticity to keep the index supported in the
+spirit in opposition to gravity.
+
+[Illustration: Fig. 62.]
+
+The instrument acts as follows:--A rise of temperature causes the spirit
+in the long bulb to expand and press some of the mercury into the other
+limb of the syphon, into which it rises also from its own expansion, and
+carries the index with it, until the greatest temperature is attained. The
+lower end of this index then indicates upon the engraved scale the maximum
+temperature. As the temperature falls the spirit and the mercury contract,
+and in returning towards the bulb the second index is met and carried up
+by the mercury until the lowest temperature occurs, when it is left to
+indicate upon the scale the minimum temperature. The limb of the syphon
+adjoining the bulb requires, therefore, a descending scale of
+thermometric degrees; the other limb, an ascending scale. The graduations
+must be obtained by comparisons with a standard thermometer under
+artificial temperatures, which should be done in this way for every 5�, in
+order to correct for the inequality in the bore of the tube, and the
+irregular expansion of the spirit. The instrument is set for observation
+by bringing the indices into contact with the mercury, by means of a small
+magnet, which attracts the steel through the glass, so that it is readily
+drawn up or down. They should be drawn nearly to the top of the limbs when
+it is desired to remove the instrument, which should be carefully carried
+in the vertical position; for should it be inverted, or laid flat, the
+spirit may get among the mercury, and so break up the column as to require
+the skill of a maker to put it in order again. For transmission by
+ordinary conveyances, it requires that attention be given to keep it
+vertical. The entanglement of a small portion of mercury with the indices
+is sometimes a source of annoyance in this instrument, for the readings
+are thereby rendered somewhat incorrect. Small breakages in the mercury,
+either from intervening bubbles of spirit or adhesion to the indices, may
+generally be rectified by cautiously tapping the frame of the instrument,
+so as to cause the mercury to unite by the assistance thus given to its
+superior gravity.
+
+These thermometers, when carefully made and adjusted to a standard
+thermometer, are strongly recommended for ordinary purposes, where strict
+scientific accuracy is not required. This is also the only fluid
+thermometer applicable for determining the temperature of the sea at
+depths.
+
+
+
+
+CHAPTER VIII.
+
+RADIATION THERMOMETERS.
+
+
+=82. Solar and Terrestrial Radiation considered.=--The surface of the
+earth absorbs the heat of the sun during the day, and radiates heat into
+space during the night. The envelope of gases and vapour, which we call
+the atmosphere, exerts highly important functions upon these processes.
+Thanks to the researches of Professor Tyndall, we are now enabled to
+understand these functions much more clearly than heretofore. His
+elaborate, patient, and remarkably sagacious series of experiments upon
+radiant heat, have satisfactorily demonstrated that _dry_ air is as
+transparent to radiant heat as the vacuum itself; while air _perfectly
+saturated_ with aqueous vapour absorbs more than five per cent. of radiant
+heat, estimated by the thermal unit adopted for the galvanometer
+indications of the effect upon a thermo-electric pile.
+
+Aqueous vapour, in the form of fog or mist, as is well known, gives to our
+sensation a feeling of cold, and interferes with the healthy action of the
+skin and the lungs; the cause being its property of absorbing heat from
+our person.
+
+Air containing moisture in an invisible state likewise exerts a remarkable
+influence in radiating and absorbing heat. By reason of these properties,
+aqueous vapour acts as a kind of blanket upon the ground, and maintains
+upon it a higher temperature than it would otherwise have. "Regarding the
+earth as a source of heat, no doubt at least ten per cent. of its heat is
+intercepted within ten feet of the surface." Thus vapour--whether
+transparent and invisible, or visible, as cloud, fog, or mist--is
+intimately connected with the important operations of solar and
+terrestrial radiation. Cloudy, or humid days, diminish the effect upon the
+soil of solar radiation; similar nights retard the radiation from the
+earth. A dry atmosphere is the most favourable for the direct transmission
+of the sun's rays; and the withdrawal of the sun from any region over
+which the air is dry, must be followed by very rapid cooling of the soil.
+"The removal, for a single summer night, of the aqueous vapour from the
+atmosphere which covers England, would be attended by the destruction of
+every plant which a freezing temperature could kill. In Sahara, where 'the
+soil is fire and the wind is flame,' the refrigeration at night is often
+painful to bear. Ice has been formed in this region at night. In
+Australia, also, the _diurnal range_ of temperature is very great,
+amounting, commonly, to between 40 and 50 degrees. In short, it may be
+safely predicted, that wherever the air is _dry_, the daily thermometric
+range will be great. This, however, is quite different from saying that
+when the air is _clear_, the thermometric range will be great. Great
+clearness to light is perfectly compatible with great opacity to heat; the
+atmosphere may be charged with aqueous vapour while a deep blue sky is
+overhead; and on such occasions the terrestrial radiation would,
+notwithstanding the 'clearness,' be intercepted." The great range of the
+thermometer is attributable to the absence of that protection against gain
+or loss of heat which is afforded when aqueous vapour is present in the
+air; and during such weather the rapid abstraction of moisture from the
+surface of plants and animals is very deleterious to their healthy
+condition. "The nipping of tender plants by frost, even when the air of
+the garden is some degrees above the freezing temperature, is also to be
+referred to chilling by radiation." Hence the practice of gardeners of
+spreading thin mats, of bad radiating material, over tender plants, is
+often attended with great benefit.
+
+By means of the process of terrestrial radiation ice is artificially
+formed in Bengal, "where the substance is never formed naturally. Shallow
+pits are dug, which are partially filled with straw, and on the straw flat
+pans containing water which had been boiled is exposed to the clear
+firmament. The water is a very powerful radiant, and sends off its heat
+into space. The heat thus lost cannot be supplied from the earth--this
+source being cut off by the non-conducting straw. Before sunrise a cake of
+ice is formed in each vessel.... To produce the ice in abundance, the
+atmosphere must not only be clear, but it must be comparatively free from
+aqueous vapour."
+
+Considering, therefore, the important consequences attending both
+terrestrial and solar radiation, it appears to us that observations from
+radiation thermometers are of much more utility in judging of climate than
+is usually supposed. These observations are very scanty; and what few are
+upon record are not very reliable, principally from bad exposure of the
+instruments, while the want of uniformity in construction may be another
+cause. Herschell's actinometer and Pouillet's pyrheliometer, instruments
+for ascertaining the absolute heating effect of the sun's rays, should,
+however, be more generally employed by meteorologists. In comparing
+observations on radiation it should be kept in mind, that "the difference
+between a thermometer which, properly confined [or shaded], gives the true
+temperature of the night air, and one which is permitted to radiate freely
+towards space, must be greater at high elevations than at low ones;"[6]
+because the higher the place, the less the thickness of the vapour-screen
+to intercept the radiation.
+
+
+=83. Solar Radiation Thermometer.=--"As the interchange of heat between
+two bodies by radiation depends upon the relative temperature which they
+respectively possess, the earth, by the rays transmitted from the sun
+during the day, must be continually gaining an accession of heat, which
+would be far from being counterbalanced by the opposite effect of its own
+radiation into space. Hence, from sunrise till two or three hours after
+mid-day, the earth goes on gradually increasing in temperature, the
+augmentation being greatest where the surface consists of materials
+calculated, from their colour and texture, to absorb heat, and where it is
+deficient in moisture, which, by its evaporation, would have a tendency
+to diminish it."[7] It is, therefore, important to have instruments for
+measuring the efficacy of solar radiation, apart from those for exhibiting
+the temperature of the place in the shade.
+
+[Illustration: Fig. 63.]
+
+Fig. 63 shows the arrangement of Negretti & Zambra's maximum thermometer,
+for registering the greatest heat of the sun's direct rays, hence called a
+_solar radiation thermometer_. It has a blackened bulb, the scale divided
+on its own stem, and the divisions protected by a glass shield. In use it
+should be placed nearly horizontally, resting on Y supports of wood or
+metal, with its bulb in the full rays of the sun, resting on grass, and,
+if possible, so that lateral winds should not strike the bulb; and at a
+sufficient distance from any wall, so that it does not receive any
+_reflected_ heat from the sun. Some observers place the thermometer as
+much as two feet from the ground. It would be very desirable if one
+uniform plan could be recognized: that of placing the instrument as
+indicated in the figure appears to be most generally adopted, and the
+least objectionable.
+
+
+=84. Vacuum Solar Radiation Thermometer.=--In order that the heat absorbed
+by the blackened bulb of the solar radiation thermometer may not in part
+be carried off by the currents of air which would come into contact with
+it, the instrument has been improved by Messrs. Negretti and Zambra into
+the _vacuum solar radiation thermometer_, as illustrated by fig. 64.
+
+[Illustration: Fig. 64.]
+
+This consists of a blackened-bulb radiation thermometer, enclosed in a
+glass tube and globe, from which all air is exhausted. Thus protected from
+the loss of heat which would ensue if the bulb were exposed, its
+indications are from 20� to 30� higher than when placed side by side with
+a similar instrument with the bulb exposed to the passing air. At times
+when the air has been in rapid motion, the difference between the reading
+of a thermometer giving the true temperature of the air in the shade, and
+an ordinary solar radiation thermometer, has been 20� only, whilst the
+difference between the air temperature and the reading of a radiation
+thermometer in vacuo has been as large as 50�. It is also found that the
+readings are almost identical at distances from the earth varying from six
+inches to eighteen inches. By the use of this improvement, it is hoped
+that the amounts of solar radiation at different places may be rendered
+comparable; hitherto they have not been so; the results found at different
+places cannot be compared, as the bulbs of the thermometers are under very
+different circumstances as to exposure and currents of air. Important
+results are anticipated from this arrangement. The observations at
+different places are expected to present more agreement. Observers would
+do well to note carefully the effect of any remarkable degree of intensity
+in the solar heat upon particular plants, crops, fruit or other trees.
+
+
+=85. Terrestrial Radiation Thermometer= is an alcohol minimum thermometer,
+with the graduations etched upon the stem, and protected by a glass
+shield, as shown in figure 65, instead of being mounted on a frame. The
+bulb is transparent; that is to say, the spirit is not coloured.
+
+[Illustration: Fig. 65.]
+
+In use, it should be placed with its bulb fully exposed to the sky,
+resting on grass, the stem being supported by little forks of wood. The
+precautions required with this thermometer are similar to those for
+ordinary spirit thermometers, explained at page 76.
+
+
+[Illustration: Fig. 66.]
+
+=86. �thrioscope.=--The celebrated experimental philosopher, Sir John
+Leslie, was the inventor of this instrument, the purpose of which is to
+give a comparative idea of the radiation proceeding from the surface of
+the earth towards the sky. It consists, as represented in fig. 66, of two
+glass bulbs united by a vertical glass tube, of so fine a bore that a
+little coloured liquid is supported in it by its own adhesion, there being
+air confined in each of the bulbs. The bulb, _A_, is enclosed in a highly
+polished brass sphere, _D_, made in halves and screwed together. The bulb,
+_B_, is blackened and placed in the centre of a metallic cup, _C_, which
+is well gilt on the inside, and which may be covered by a top, _F_. The
+brass coverings defend both bulbs from solar radiation, or any
+adventitious source of heat. When the top is on, the liquid remains at
+zero of the scale. On removing the top and presenting the instrument to a
+clear sky, either by night or by day, the bulb, _B_, is cooled by
+terrestrial radiation, while the bulb, _A_, retains the temperature of the
+air. The air confined in _B_, therefore, contracts; and the elasticity of
+that within _A_ forces the liquid up the tube, to a height proportionate
+to the intensity of the radiation. Such is the sensitiveness of the
+instrument, that the smallest cloud passing over it checks the rise of the
+liquid. Sir John Leslie says:--"Under a clear blue sky, the _�thrioscope_
+will sometimes indicate a cold of fifty millesimal degrees; yet, on other
+days, _when the air seems equally bright_, the effect is hardly 30�." This
+anomaly, according to Dr. Tyndall, is simply due to the difference in the
+quantity of aqueous vapour present in the atmosphere. The presence of
+invisible vapour intercepts the radiation from the �thrioscope, while its
+absence opens a door for the escape of this radiation into space.
+
+
+=87. Pouillet's Pyrheliometer.=--"This instrument is composed of a shallow
+cylinder of steel, _A_, fig. 67, which is filled with mercury. Into the
+cylinder a thermometer, _D_, is introduced, the stem of which is protected
+by a piece of brass tubing. We thus obtain the temperature of the mercury.
+The flat end of the cylinder is to be turned towards the sun, and the
+surface, _B_, thus presented is coated with lamp black. There is a collar
+and screw, _C_, by means of which the instrument may be attached to a
+stake driven into the ground, or into the snow, if the observations are
+made at considerable heights. It is necessary that the surface which
+receives the sun's rays should be perpendicular to the rays; and this is
+secured by appending to the brass tube which shields the stem of the
+thermometer, a disk, _E_, of precisely the same diameter as the steel
+cylinder. When the shadow of the cylinder accurately covers the disk, we
+are sure that the rays fall, as perpendiculars, on the upturned surface of
+the cylinder.
+
+[Illustration: Fig. 67.]
+
+"The observations are made in the following manner:--First, the instrument
+is permitted, not to receive the sun's rays, but to radiate its own heat
+for five minutes against an unclouded part of the firmament; the decrease
+of the temperature of the mercury consequent on this radiation is then
+noted. Next, the instrument is turned towards the sun, so that the solar
+rays fall perpendicularly upon it for five minutes; the augmentation of
+heat is now noted. Finally, the instrument is turned again towards the
+firmament, away from the sun, and allowed to radiate for another five
+minutes, the sinking of the thermometer being noted as before. In order to
+obtain the whole heating power of the sun, we must add to his observed
+heating power the quantity lost during the time of exposure, and this
+quantity is the mean of the first and last observations. Supposing the
+letter _R_ to represent the augmentation of temperature by five minutes'
+exposure to the sun, and that _t_ and _t�_ represent the reductions of
+temperature observed before and after, then the whole force of the sun,
+which we may call _T_, would be thus expressed:--_T = R + 1/2(t + t�)_.
+
+"The surface on which the sun's rays here fall is known; the quantity of
+mercury within the cylinder is also known; hence we can express the effect
+of the sun's heat upon a given area, by stating that it is competent, in
+five minutes, to raise so much mercury so many degrees in
+temperature."--_Dr. Tyndall's "Heat considered as a Mode of Motion."_
+
+
+[Illustration: Fig. 68.]
+
+=88. Sir John Herschell's Actinometer=, for ascertaining the absolute
+heating effect of the solar rays, in which _time_ is considered one of the
+elements of observation, is illustrated by fig. 68. The actinometer
+consists of a large cylindrical thermometer bulb, with a scale
+considerably lengthened, so that minute changes may be easily seen. The
+bulb is of transparent glass filled with a deep blue liquid, which is
+expanded when the rays of the sun fall direct on the bulb. To take an
+observation, the actinometer is placed in the shade for one minute and
+read off; it is then exposed for one minute to sunshine, and its
+indication recorded; it is finally restored to the shade, and its reading
+noted. The mean of the two readings in the shade, subtracted from that in
+the sun, gives the actual amount of expansion of the liquid produced by
+the sun's rays in one minute of time. For further information, see _Report
+of the Royal Society on Physics and Meteorology_; or _K�mtz's
+Meteorology_, translated by C. V. Walker; or the _Admiralty Manual of
+Scientific Instructions_.
+
+
+
+
+CHAPTER IX.
+
+DEEP-SEA THERMOMETERS.
+
+
+=89. On Sixe's Principle.=--Thermometers for ascertaining the temperature
+of the sea at various depths are constructed to register either the
+maximum or minimum temperature, or both. The principle of each instrument
+is that of Sixe. There are very few parts of the ocean in which the
+temperature below is greater than at the surface, except in the Polar
+Seas, where it is generally found to be a few degrees warmer at
+considerable depths than at the surface. When the instrument is required
+to register only one temperature, it can be made narrower and more
+compact--a great advantage in sounding; and with less length of bulb and
+glass tube, so that the liability of error is diminished. Hence, the
+minimum is the most generally useful for deep-sea soundings. These
+thermometers must be sufficiently strong to withstand the pressure of the
+ocean at two or three miles of depth, where there may be a force exerted
+to compress them exceeding three or four hundred atmospheres (of 15 lbs.
+to the square inch).
+
+Many have been the contrivances for obtaining correct deep-sea
+indications. Thermometers and machines of various sorts have been
+suggested, adopted, and eventually abandoned as only approximate
+instruments. The principal reason for such instruments failing to give
+correct or reliable indications, has been that the weight or pressure on
+the bulbs at great depths has interfered with the correct reading of the
+instruments. Thermometers have been enclosed in strong water-tight cases
+to resist the pressure; but this contrivance has only had the tendency to
+retard the action, so much so as to throw a doubt on the indications
+obtained by the instrument so constructed.
+
+The thermometers constructed by Messrs. Negretti and Zambra for this
+purpose do not differ materially from those usually made under the
+denomination of Sixe's thermometers, except in the following most
+important particular:--The usual Sixe's thermometers have a central
+reservoir or cylinder containing alcohol; this reservoir, which is the
+only portion of the instrument likely to be affected by pressure, has
+been, in Negretti and Zambra's new instrument, superseded by a strong
+outer cylinder of glass, containing mercury and rarefied air; by this
+means the portion of the instrument susceptible of compression, has been
+so strengthened that no amount of pressure can possibly make the
+instrument vary. This instrument has been tested in every possible manner,
+and the results have been highly satisfactory, so much so as to place
+their reliability beyond any possible doubt.
+
+The scales are made of porcelain, and are firmly secured to a back of oak,
+which holds in a recess the bulb with its protecting shield, and is
+rounded off so as to fit easily and firmly in a stout cylindrical copper
+case, in which the thermometer is sent down when sounding (see fig. 69).
+The lid of the case is made to fit down closely, and water-tight. At the
+bottom of the case is a valve opening upward; and the lid has a similar
+valve. These allow the water to pass through the case as the instrument
+sinks, so that the least amount of obstruction is offered to the descent.
+At the lower end of the case is a stout brass spring, to protect the
+instrument from a sudden jar if it should touch the bottom while
+descending rapidly. As the instrument is drawn up, the valves close with
+the weight of water upon them, and it arrives at the surface filled with
+water brought up from its lowest position. The deep-sea thermometers used
+in the Royal Navy are of this pattern.
+
+[Illustration: Fig. 69.]
+
+
+=90. Johnson's Metallic Deep-Sea Thermometer.=--The objection to the
+employment of mercurial thermometers for ascertaining the temperature of
+the ocean at depths, arising from the compression of the bulbs, which was
+of such serious consequence previous to the modification made in the
+construction of the instrument by Messrs. Negretti and Zambra, led to the
+construction of a metallic thermometer altogether free from liability of
+disturbance from compression by the surrounding water; which, however, is
+certainly not so sensitive to changes of temperature as mercury. This
+instrument is the invention of Henry Johnson, Esq., F.R.A.S., and is thus
+described by him:--
+
+"During the year 1844 some experiments were made by James Glaisher, Esq.,
+F.R.S., on the temperature of the water of the Thames near Greenwich at
+the different seasons of the year; when that gentleman found that the
+indications of temperature were greatly affected by the pressure on the
+bulbs of the thermometers. At a depth of 25 feet this pressure would be
+nearly equal to the presence of three-fourths of an atmosphere. These
+observations demonstrate the importance of using in deep-sea soundings an
+instrument free from liability of disturbance from compression by the
+surrounding water, and have ultimately led to the construction of the
+thermometer now to be described.
+
+"The instrument is composed of solid metals of considerable specific
+gravity, viz. of brass and steel, the specific gravity of these metals
+being 8�39 and 7�81 respectively. They are therefore not liable to
+compression by the water, which under a pressure of 1,120 atmospheres, or
+at a depth of 5,000 fathoms in round numbers, acquires a density or
+specific gravity of 1�06. In the construction of this instrument,
+advantage has been taken of the well-known difference in the ratios of
+expansion and contraction by heat and cold of brass and steel, to form
+compound bars of thin bars of these metals riveted together; and which
+will be found to assume a slight curve in one direction when heat has
+expanded the brass more than the steel, and a slight one in the contrary
+direction when cold has contracted the brass more than the steel.
+
+"The indications of the instrument record the motions under changes of
+temperature of such compound bars; in which the proportion of brass, the
+more dilatable metal, is two-thirds, and of steel one-third.
+
+[Illustration: Fig. 70.]
+
+"Upon one end of a narrow plate of metal about a foot long, _a_, are fixed
+three scales of temperature, _h_, which ascend from 25� to 100� F., and
+which are shown more clearly in the drawing detached from the instrument.
+Upon one of these scales the present temperature is shown by the pointer,
+_e_, which turns upon a pivot in its centre. The register index, _g_, to
+the maximum temperature, and the index, _f_, to the minimum temperature,
+are moved along the other scales by the pin upon the moving pointer, at
+_e_, where they are retained by stiff friction. At equal distances from
+the centre of the pointer are two connecting pieces, _d d_, by which it is
+attached to the free ends of two compound bars, _b b_, and its movements
+correspond with the movements of the compound bars under variations of
+temperature. The other ends of the bars are fastened by the plate, _c_, to
+the plate, _a_, on which the scales of temperature are fixed. The
+connection of the bars with both sides of the centre of the pointer
+prevents disturbance of indication by lateral concussion. The case of the
+instrument has been improved at the suggestion of Admiral FitzRoy, and now
+presents to the water a smooth cylindrical surface, with rounded ends, and
+without projection of fastenings.
+
+"In surveying expeditions, this instrument would be found useful in giving
+notice of variation of depth of water, and of the necessity for taking
+soundings. A diminution of the temperature of water has been observed by
+scientific voyagers to accompany diminution of depth, as on nearing land,
+or approaching hidden rocks or shoals. Attention would also thus be
+attracted to the vicinity of icebergs."
+
+This thermometer might easily be modified to serve for several other
+important purposes, such as the determination of the temperature of
+intermittent hot springs, and mud volcanoes.
+
+[Illustration: Fig. 71.]
+
+The principle of this thermometer is not altogether new; but the duplicate
+arrangement of the bars, which effectually prevents the movement of the
+indices by any shaking, and the application are certainly novel. Professor
+Trail, in the _Library of Useful Knowledge_, writes:--"In 1803, Mr. James
+Crighton, of Glasgow, published a new 'metallic thermometer,' in which
+the unequal expansion of zinc and iron is the moving power. A bar is
+formed by uniting a plate of zinc (fig. 71), _c d_, 8 inches long, 1 inch
+broad, and 1/4 inch thick, to a plate of iron, _a b_, of the same length.
+The lower extremity of the compound bar is firmly attached to a mahogany
+board at _e e_; a pin, _f_, fixed to its upper end, plays in the forked
+opening in the short arm of the index, _g_. When the temperature is
+raised, the superior expansion of the zinc, _c d_, will bend the whole
+bar, as in the figure; and the index, _g_, will move along the graduated
+arc, from right to left, in proportion to the temperature. In order to
+convert it into a _register thermometer_, Crighton applied two slender
+hands, _h h_, on the axis of the index; these lie below the index, and are
+pushed in opposite directions by the stud, _i_,--a contrivance seemingly
+borrowed from the instrument of Fitzgerald," a complicated metallic
+thermometer, described by the Professor previously.
+
+
+
+
+CHAPTER X.
+
+BOILING-POINT THERMOMETERS.
+
+
+=91. Ebullition.=--The temperature at which a fluid _boils_ is called the
+_boiling-point_ of that particular fluid. It is different for different
+liquids; and, moreover, in the same liquid it varies with certain changes
+of circumstance. Thus the same liquid in various states of purity would
+have its boiling temperature altered in a slight degree. There is also an
+intimate connection with the pressure under which a fluid is boiled, and
+its temperature of ebullition. Liquids boiled in the open air are
+subjected to the atmospheric pressure, which is well known to vary at
+different times and places; and the boiling-point of the liquid exhibits
+corresponding changes. When the pressure is increased on the surface of
+any fluid, the temperature of ebullition rises; and with a decrease of
+pressure, the boiling goes on at a lower degree of heat.
+
+In the case of water, we commonly state the boiling-point to be 212� F.;
+but it is only so at the level of the sea, under the mean pressure of the
+atmosphere, represented, in the latitude of London, by a column of 29�905
+inches of mercury, at a temperature of 32� F., and when the water is fresh
+and does not contain any matter chemically dissolved in it. When steam is
+generated and confined in a boiler, the pressure upon the boiling water
+may be several times greater than that of the atmosphere. Experimentally
+it has been found, that if the pressure in the boiler be 25 lbs. on the
+square inch, the temperature of the boiling water, and of the steam
+likewise, is raised to 241�; while under the exhausted receiver of an
+air-pump, water will boil at 185�, when the pressure is reduced to 17
+inches of mercury.
+
+
+=92. Relation between the Boiling-Point and Elevation.=--Now, as the
+atmospheric pressure is diminished by ascent, as shown by the fall of
+mercury in the barometer, it follows that in elevated localities water, or
+any other fluid, heated in the open air, will boil at a temperature lower
+than at the sea-level. Therefore, there must be some relation between the
+height of a hill, or mountain, and the temperature at which a fluid will
+boil at that height. Hence, the thermometer, as used to determine the
+boiling-point of fluids, is also an indicator of the atmospheric pressure;
+and may be used as a substitute for the barometer in measuring elevations.
+
+If the atmospheric pressure were constant at the sea-level, and always the
+same for definite heights, we might expect the boiling-points of fluids
+also to be in exact accordance with height; and the relation once
+ascertained, we could readily, by means of the thermometer and boiling
+water, determine an unknown height, or for a known elevation assert the
+boiling temperature of a liquid. However, as the atmospheric pressure is
+perpetually varying at the same place, within certain limits, so there
+are, as it were, sympathetic changes in the boiling temperatures of
+fluids. It follows from this, that heights can never be accurately
+measured, either by the barometer or the boiling-point thermometer, by
+simply observing at the places whose elevations are required. To determine
+a height with any approach to accuracy, it is necessary that a similar
+observation should be made at the same time at a lower station, not very
+remote laterally from the upper, and that they should be many times
+repeated. When such observations have been very carefully conducted, the
+height of the upper station above the lower may be ascertained with great
+precision, as has been repeatedly verified by subsequent trigonometrical
+measurement of elevations so determined. If the lower station be at the
+sea-level, of course the absolute height of the upper is at once obtained.
+
+
+=93. Mountain Thermometer; sometimes called Hypsometric Apparatus.=--We
+have now to examine the construction of the boiling-point thermometer, and
+its necessary appendages, as adapted for the determination of heights.
+
+Messrs. Negretti and Zambra's arrangement of the instrument is shown in
+figures 72 and 73.
+
+[Illustration: Fig. 72.]
+
+[Illustration: Fig. 73.]
+
+The thermometer is made with an elongated bulb, so as to be as sensitive
+as possible. The scale, about a foot long, is graduated on the stem, and
+ranges from 180� to 214�, each degree being sufficiently large to show the
+divisions of tenths of a degree. A sliding metallic vernier might perhaps
+with advantage be attached to the stem, which would enable the observer to
+mark hundredths of a degree; which, however, he can pretty well do by
+estimation. The boiler is so contrived as to allow, not only the bulb, but
+the stem also of the thermometer, to be surrounded by the steam. The
+arrangement is readily understood by reference to the accompanying
+diagram, fig. 73.
+
+_C_, is a copper boiler, supported by a tripod stand so as to allow a
+spirit-lamp, _A_, made of metal to be placed underneath. The flame from
+the lamp may be surrounded by a fine wire gauze, _B_, which will prevent
+it being extinguished when experimenting in the external air. _E E E_, is
+a three-drawn telescope tube, proceeding from the boiler, and open also at
+top. Another tube, similarly constructed, envelops this, as shown by _D D
+D_. This tube is screwed to the top of the boiler, and has two openings,
+one at the top to admit the thermometer, the other low down, _G_, to give
+vent to the steam. As the steam is generated, it rises in the inner tube,
+passes down between the tubes, and flows away at _G_. The thermometer is
+passed down, supported by an india-rubber washer, fitting steam tight, so
+as to leave the top of the mercury, when the boiling-point is attained,
+sufficiently visible to make the observation. The telescopic movement, and
+the mode of supporting the thermometer, enable the observer always to keep
+the bulb near the water, and the double tube gives all the protection
+required to obtain a steady boiling-point. Some boiling-point thermometers
+are constructed with their scales altogether exposed to the air, which may
+be very cold, and consequently may contract to some extent the thread of
+mercury outside the boiler. The steam, having the same temperature as the
+boiling water, keeps the tube, throughout nearly its whole length, at the
+same degree of heat, in the apparatus described. The whole can be packed
+in a tin case very compactly and securely for travelling, as in fig. 72.
+
+_Directions for Using._--When the apparatus is required for practical use,
+sufficient water must be poured into the boiler to fill it about one
+third, through an opening, _F_, which must be afterwards closed by the
+screw plug. Then apply the lighted lamp. In a short time steam will issue
+from _G_; and the mercury in the thermometer, kept carefully immersed,
+will rise rapidly until it attains a stationary point, which is the
+boiling temperature. The observation should now be taken and recorded with
+as much accuracy as possible, and the temperature of the external air must
+be noted at the same time by an ordinary thermometer.
+
+The water employed should be pure. Distilled water would therefore be the
+best. If a substance is held mechanically suspended in water, it will not
+affect the boiling-point. Thus, muddy water would serve equally as well as
+distilled water. However, as it cannot be readily ascertained that nothing
+is dissolved chemically when water is dirty, we are only correct when we
+employ pure water.
+
+
+=94. Precautions to ensure correct Graduation.=--Those who possess a
+boiling-point thermometer should satisfy themselves that it has been
+correctly graduated. To do this, it is advisable to verify it with the
+reading of a standard barometer reduced to 32� F. The table of "Vapour
+Tension" (given at p. 62) will furnish the means of comparison. Thus, if
+the reduced reading of the barometer, corrected also for latitude, be
+29�922, the thermometer should show 212� as the boiling-point of water at
+the same time and place; if 29�745, the thermometer should read 211�7; and
+so on as per table. In this way the error of the chief point of the scale
+can be obtained. Other parts of the scale may be checked with a standard
+thermometer, by subjecting both to the same temperature, and comparing
+their indications. The graduations as fixed by some makers are not always
+to be trusted; and this essential test should be conducted with the utmost
+nicety and care.
+
+Admiral FitzRoy writes, in his _Notes on Meteorology_:--"Each degree of
+the boiling-point thermometer is equivalent to about 550 _feet of ascent_,
+or one-tenth to 55 feet; therefore, the smallest error in the graduation
+of the thermometer itself will affect the height deduced materially.
+
+"In the thermometer which is graduated from 212� (the boiling-point) to
+180�, similarly to those intended for the purpose of measuring heights,
+there must have been a starting point, or zero, from which to begin the
+graduation. I have asked an optician in London how he fixed that zero, the
+boiling-point. 'By boiling water at my house,' he replied. 'Where is your
+house?' In such a part of the town, he answered. I said: 'What height is
+it above the sea?' to which he replied, 'I do not know;' and when I asked
+the state of the barometer when he boiled the water, whether the mercury
+was high or low, he said that he had not looked at it! Now, as this
+instrument is intended to measure heights and to decide differences of
+some hundred, if not thousand feet upwards, at least one should endeavour
+to ascertain a reliable starting point. From inquiries made, I believe
+that the determination of the boiling-point of ordinary thermometers has
+been very vague, not only from the extreme difficulties of the process
+itself (which are well known to opticians), but from the radical errors of
+not allowing for the pressure of the atmosphere at the time of
+graduation--which may be much, even an inch higher or lower, than the
+mean, or any _given height_--while the elevation of the place above the
+level of the sea is also unnoticed. Then there is another source of error,
+a minor one, perhaps: the inner limit, the 180� point, is fixed only by
+comparison with another thermometer; it may be right, or it may be very
+much out, as may be the intermediate divisions; for the difficulty of
+ascertaining degree by degree is great: and it must be remembered that the
+measurement of a very high mountain depends upon those inner degrees from
+200� down to 180�, thereabouts. Hence, the difficulty of making a reliable
+observation by boiling water seems to be greater than has been generally
+admitted."
+
+
+=95. Method of Calculating Heights from Observations with the Mountain
+Thermometer.=--Having considered how to make observations with the proper
+care and accuracy, it becomes necessary to know how to deduce the height
+by calculation. That a constant intimate relation exists between the
+boiling temperature of water and the pressure of the air, we have already
+learned. This knowledge is the result of elaborate experiments made by
+several scientific experimentalists, who have likewise constructed formul�
+and tables for the conversion of the boiling temperatures into the
+corresponding pressures of vapour, or, which is equivalent, of the
+atmosphere, when the operation is performed in the open air. As might be
+expected, there is not a perfect accord in the results arrived at by
+different persons. Regnault is the most recent, and his experiments are
+considered the most reliable.
+
+From Regnault's table of vapour tension, we can obtain the pressure in
+inches of mercury at 32�, which corresponds to the observed boiling-point;
+or _vice versa_, if required. From the pressure, the height may be deduced
+by the method for finding heights by means of the barometer.
+
+The following table expresses very nearly the elevation in feet
+corresponding to a fall of 1� in the temperature of boiling water:--
+
+  Boiling Temperatures      Elevation in Feet
+      between.               for each Degree.
+
+  214� and 210--                  520
+  210  and 200--                  530
+  200  and 190                    550
+  190  and 180                    570
+
+These numbers agree very well with the results of theory and actual
+observation. The assumption is that the boiling-point will be diminished
+1� for each 520 feet of ascent until the temperature becomes 210�, then
+530 feet of elevation will lower it one degree until the water boils at
+200�, and so on; the air being at 32�.
+
+Let _H_ represent the vertical height in feet between two stations; _B_
+and _b_, the boiling-points of water at the lower and upper stations
+respectively; _f_, the factor found in the above table. Then
+
+  _H_ = _f_(_B_ - _b_)
+
+Further, let _m_ be the mean temperature of the stratum of air between the
+stations. Now, if the mean temperature is less than 32�, the column of air
+will be shorter; and if greater, longer than at 32�. According to
+Regnault, air expands 1/491�13 or �002036 of its volume at 32�, for each
+degree increase of heat. Calling the correction due to the mean
+temperature of air _C_, its value will be found from the equation,
+
+  _C_ = _H_ (_m_ - 32) �002036
+
+Calling the corrected height _H'_, it will be found from the formula,
+
+            _H'_ = _H_ + _H_ (_m_ - 32) �002036
+  that is,  _H'_ = _H_ { 1 + (_m_ - 32) �002036 }
+
+and substituting the value of _H_,
+
+  _H'_ = _f_(_B_ - _b_) { 1 + (_m_ - 32) �002036 }
+
+Strictly, according to theoretical considerations, there is a correction
+due to latitude, as in the determination of heights by the barometer; but
+its value is so small that it is practically of no importance.
+
+If a barometer be observed at one of the stations, the table of vapour
+tensions (p. 62) will be useful in converting the pressure into the
+corresponding boiling-point, or _vice versa_; so that the difference of
+height may be found either by the methods employed for the boiling-point
+thermometer or the barometer.
+
+In conclusion, it may be remarked that observers who have good instruments
+at considerable elevations, as sites on mountains or plateaus, would
+confer a benefit to science, by registering for a length of time the
+barometer along with the boiling temperature of water, as accurately as
+possible. Such observations would serve to verify the accuracy of
+theoretical deductions, and fix with certainty the theoretical scale with
+the barometer indications.
+
+_Example, in calculating Heights from the Observations of the
+Boiling-point of Water._--1. At Geneva the observed boiling-point of water
+was 209��335; on the Great St. Bernard it was 197��64; the mean
+temperature of the intermediate air was 63��5; required the height of the
+Great St. Bernard above Geneva.
+
+Method by formula:--
+
+  _H'_ = _f_ (_B_ - _b_) { 1 + (_m_ - 32�) �002036 }
+
+In this case _f_ is between 530 and 550, or 540.
+
+  _B_ = 209�335          _m_ = 63�5
+  _b_ = 197�64                 32
+        -------                -----
+         11�695                31�5
+  _f_  =    540                  �002036
+        -------                ---------
+         6315�3                 0�0641340
+          1�064                 1
+        -------                 -----
+  _H'_ = 6719�5 feet.           1�064
+         ======
+
+Method by Tables supplied with boiling-point apparatus made by Messrs.
+Negretti and Zambra:--
+
+  209�335 gives 1464 in Table I.
+  197�64      "     7736      "
+                    ----
+                    6272
+   63�5       "        1�07 in Table II.
+                    ----
+  Height            6711
+                    ====
+
+
+=96. Thermometers for Engineers.=--_1st. Salinometer._--Under the
+circumstances at which fresh water boils at 212�, sea water boils at
+213��2. The boiling temperature is raised by the chemical solution of any
+substance in the water, and the more with the increase of matter
+dissolved.
+
+From a knowledge of this principle, marine engineers make use of the
+thermometer to determine the amount of salts held in solution by the water
+in the boilers of sea-going steamers. Common sea-water contains 1/33 of
+its volume of salt and other earthy matters. As evaporation proceeds, the
+solution becomes proportionally stronger, and more heat is required to
+produce steam. The following table from the work of Messrs. Main and
+Brown, on the Marine Steam-Engine, shows the relation between the
+boiling-point under the mean pressure of the atmosphere, or 80 inches of
+mercury, and the proportion of matter dissolved in the water:--
+
+  Proportion of Salt in 100 parts of water   0    Boiling-point 212�
+            "                     "         1/33       "        213�2
+            "                     "         2/33       "        214�4
+            "                     "         3/33       "        215�5
+            "                     "         4/33       "        216�6
+            "                     "         5/33       "        217�9
+            "                     "         6/33       "        219�0
+            "                     "         7/33       "        220�2
+            "                     "         8/33       "        221�4
+            "                     "         9/33       "        222�5
+            "                     "        10/33       "        223�7
+            "                     "        11/33       "        224�9
+            "                     "        12/33       "        226�0
+
+When the salts in solution amount to 12/33, the water is saturated. It has
+also been ascertained that, when a solution of 4/33 is attained,
+incrustation of the substances commences on the boiler. Hence, it is a
+rule with engineers to expel some of the boiling water, when the
+thermometer indicates a temperature of 216�, and introduce some more cold
+water, in order to prevent incrustation, which not only injures the
+boiler, but opposes the passage of heat to the water. The thermometer used
+for this purpose should be very accurately graduated, and the scale must
+be considerably higher than, though it need not read much below 212�.
+
+_2nd. Pressure Gauge._--The elasticity of gases augments by increase of
+temperature, and _vice versa_; it follows, therefore, that when steam is
+generated in a closed boiler, its temperature rises beyond the boiling
+temperature of 212�, owing to the increased pressure upon the water. The
+law connecting the pressure and the corresponding temperature of steam is
+the same as that upon which the boiling of fluids under diminished
+atmospheric pressure takes place. Hence, the indications of the
+thermometer become exponents of steam pressure. Engineers are furnished,
+in works on the steam-engine, with tables, from which the pressure
+corresponding to a given temperature, or the converse, can be obtained by
+mere inspection.
+
+[Illustration: Fig. 74.]
+
+Fig. 74 represents the thermometer employed as a steam-pressure gauge. It
+is fitted in a brass case, with screw-plug and washers for closing the
+boiler when the thermometer is not in use. The scale shows the pressure
+corresponding to the temperature, from 15 to 120 lbs., above the
+atmospheric pressure, which is usually taken as 15 lbs. on the square
+inch.
+
+
+
+
+CHAPTER XI.
+
+INSTRUMENTS FOR ASCERTAINING THE HUMIDITY OF THE AIR.
+
+
+=97. Hygrometric Substances.=--The instruments devised for the purpose of
+ascertaining the humidity of the atmosphere are termed _hygrometers_. The
+earliest invented hygrometers were constructed of substances readily acted
+upon by the vapour in the air, such as hair, grass, seaweed, catgut, &c.,
+which all absorb moisture, and thereby increase in length, and when
+deprived of it by drying they contract. Toy-like hygrometers, upon the
+principle of absorption, are still common as ornaments for mantel-pieces.
+A useful little instrument of this class, formed from the beard of the
+wild oat, is made to resemble a watch in external appearance, and is
+designed to prove the dampness or dryness of beds: a moveable hand points
+out on the dial the hygrometric condition of the clothes upon which the
+instrument is laid.
+
+
+=98. Saussure's Hygrometer=, formerly used as a meteorologic instrument,
+but now regarded as an ornamental curiosity, is represented in fig. 75.
+Its action depends upon a prepared hair, fixed at one end to the frame of
+the instrument, and wound round a pulley at the other. The pulley carries
+a pointer which has a counterpoise sufficient to keep the hair stretched.
+By this means the shrinking and lengthening of the hair cause the pointer
+to traverse a graduated arc indicating the relative humidity.
+
+[Illustration: Fig. 75.]
+
+Such instruments, however ingenious, are not of scientific value; because
+they do not admit of rigid comparison, are liable to alter in their
+contractile and expansive properties, and cannot be made to indicate
+precisely alike.
+
+
+=99. Dew-Point.=--The amount of water which the air can sustain in an
+invisible form increases with the temperature; but for every definite
+temperature there is a limit to the amount of vapour which can be thus
+diffused. When the air is cooled, the vapour present may be more than it
+can sustain; part will then be condensed as dew, rain, hail or snow,
+according to the meteorologic circumstances. The temperature which the air
+has when it is so fully saturated with vapour that any excess will be
+deposited as dew, is called the _dew-point_.
+
+
+=100. Drosometer.=--"To measure the quantity of dew deposited each night,
+an instrument is used called a _Drosometer_. The most simple process
+consists in exposing to the open air bodies whose exact weight is known,
+and then weighing them afresh after they are covered with dew. According
+to Wells, locks of wool, weighing about eight grains, are to be preferred,
+which are to be divided [formed] into spherical masses of the diameter of
+about two inches."--_Koemtz._
+
+
+=101. Humidity.=--The proportion existing between the amount of vapour
+actually present in the air at any time, and the quantity necessary to
+completely saturate it, is called _the degree of humidity_. It is usually
+expressed in a centesimal scale, 0 being perfect dryness, and 100 complete
+saturation.
+
+The pressure, or tension, of vapour at the dew-point temperature, divided
+by the tension of vapour at the air temperature and the quotient
+multiplied by 100, gives the degree of humidity. (Regnault's Tables should
+be used.)
+
+Hence the utility of instruments for determining the dew-point.
+
+
+=102. Leslie's Hygrometer.=--This instrument consists of a glass syphon
+tube, terminated with a bulb or ball at each end, turned outwards from
+each other, as in fig. 76. The tube is partly filled with concentrated
+sulphuric acid, tinged by carmine. One of the balls is covered smoothly
+with fine muslin, and is kept continually moistened with pure water, drawn
+from a vase placed near it by the capillary attraction of a few strands of
+clean cotton-wick. The descent of the coloured liquid in the other stem
+will mark the diminution of temperature caused by the evaporation of the
+water from the humid surface. The drier the ambient air is, the more
+rapidly will the evaporation go on; and the cold produced will be greater.
+When the air is nearly saturated with moisture, the evaporation goes on
+slowly; the cold produced is moderate, because the ball regains a large
+portion of its lost heat from surrounding bodies; and the degree of
+refrigeration of the ball is an index of the dryness of the air.
+
+[Illustration: Fig. 76.]
+
+"Should the water become frozen on the ball, this hygrometer will still
+act; for evaporation goes on from the surface of ice in proportion to the
+dryness of the air. Leslie estimates, that when the ball is moist, air, at
+the temperature of the ball, will take up moisture equal to the
+sixteen-thousandth part of its weight, for each degree of his hygrometer;
+and as ice in melting requires one-seventh of the caloric consumed in
+converting water into vapour, when the ball is frozen, the hygrometer will
+sink more than when wet by 1� in 7�; and hence, in the frozen state, we
+must increase the value of the degrees one-seventh: so that each of them
+will correspond to an absorption of moisture equal to
+one-fourteen-thousandth part of the weight of the air.
+
+"When this hygrometer stands at 15�, the air feels damp; from 30� to 40�,
+we reckon it dry; from 50� to 60�, very dry; and from 70� upwards, we
+should call it intensely dry. A room would feel uncomfortable, and would
+probably be unwholesome, if the instrument in it did not reach 30�.[8] In
+thick fogs it keeps almost at the beginning of the scale. In winter, in
+our climate, it ranges from 5� to 15�; in summer often from 15� to 55�;
+and sometimes attains 80� or 90�. The greatest degree of dryness ever
+noticed by Leslie was at Paris, in the month of September, when the
+hygrometer indicated 120�."--_Professor Trail, in "Library of Useful
+Knowledge."_
+
+In estimating the value of the indications of this hygrometer, it should
+be borne in mind that the scale adopted by Leslie was _millesimal_, that
+is to say, from the freezing to the boiling-point of water was divided
+into a thousand parts; ten millesimal degrees are therefore equal to one
+of the scale of Celsius.
+
+
+103. DANIEL'S HYGROMETER.
+
+This instrument was invented about the year 1820, by Professor Daniel, the
+distinguished author of _Meteorological Essays_; and it entirely
+superseded all hygrometers depending upon the absorption of moisture. The
+form of the instrument is shown in fig. 77.
+
+[Illustration: Fig. 77.]
+
+It consists of a glass tube, about one-eighth of an inch in diameter of
+bore, bent twice at right angles, and terminated, at each end, in a bulb
+about one inch and a quarter in diameter. In one limb of the tube is
+enclosed a delicate thermometer, which descends to the centre of the
+adjoining bulb, which is about three-parts filled with sulphuric ether.
+All the other parts of the tube are carefully freed from air, so that they
+are occupied by the vapour of the ether. This bulb is generally made of
+black glass; the other is transparent, but covered with a piece of fine
+muslin. The support for the tube has a thermometer attached, which shows
+the temperature of the external air. The tube can be removed from the
+stand, and the parts are made to pack, with a necessary phial of ether, in
+a small box, which can easily be got into the pocket.
+
+_How to use the Hygrometer._--This instrument gives the dew-point by
+direct observation, which must be made in the following manner:--Having
+fixed the tube upon the stand, with the bulbs vertically downward, the
+ether is all caused to flow into the lower ball by inclining the tube. The
+temperature of the air is noted by the exposed thermometer. Then some
+ether is poured, from a dropping tube fitting into the neck of the phial,
+upon the muslin-covered bulb. The rapid evaporation of this ether cools
+the bulb and causes condensation of the ethereal vapour in its interior.
+This gives rise to rapid evaporation of the ether in the lower bulb,
+whereby its temperature is greatly reduced. The air in the vicinity is
+deprived of its warmth by the cold bulb, and is soon cooled to the
+temperature at which it is perfectly saturated with the vapour which it
+contains. Cooled ever so little below this temperature, some aqueous
+vapour will be condensed, and will form a dew upon the black-glass bulb.
+At the first indication of the deposit of dew the reading of the internal
+thermometer is taken: which is the dew-point.
+
+This hygrometer has undeniable disadvantages. The surface upon which the
+dew condenses is small, and requires a peculiar direction of light in
+which to see it well. The observer, having his attention on the bulb and
+the thermometer, cannot always fix with precision the dew-point; and hence
+he is recommended to note the temperature at the appearance and at the
+disappearance of the dew, in order that the chance of error may be
+diminished. Without doubt, the necessarily long continuance of the
+observer near the instrument influences, to some extent, the observed
+temperatures; and the difficulty of not being always able to procure pure
+ether for the experiments is not the least of the drawbacks to the use of
+the instrument. Some of these disadvantages are obviated in Regnault's
+hygrometer.
+
+
+104. REGNAULT'S CONDENSER HYGROMETER
+
+(Fig. 78) consists of a tube, _C_, made of silver, very thin, and
+perfectly polished; the tube is larger at one end than the other, the
+large part being 1�8 inches in depth, by 0�8 in diameter; this is fitted
+tightly to a brass stand, _B_, with a telescopic arrangement for adjusting
+when making an observation.
+
+[Illustration: Fig. 78.]
+
+The tube, _C_, has a small lateral tubulure, to which is attached an
+India-rubber tube, with ivory mouth-piece; this tubulure enters _C_ at
+right angles near the top, and traverses it to the bottom of the largest
+part.
+
+A delicate thermometer, _D_, is inserted through a cork, or India-rubber
+washer, at the open end of the tube, _C_, the bulb of which descends to
+the centre of its largest part.
+
+_G_ is an attached thermometer for taking the temperature of the air, and
+_F_ is a bottle containing ether.
+
+_To use the Condenser Hygrometer_, a sufficient quantity of ether is
+poured into the silver tube to cover the thermometer bulb: on allowing air
+to pass bubble by bubble through the ether, by breathing in the tube, _E_,
+an uniform temperature will be obtained; if the ether continues to be
+agitated, by breathing briskly through the tube a rapid reduction of
+temperature will be the result; at the moment the ether is cooled down to
+the dew-point temperature, the external surface of that portion of the
+silver tube containing ether will become covered with a coating of
+moisture, and the degree shown by the thermometer at that instant will be
+the temperature of the dew-point.
+
+This form of hygrometer, for ascertaining by direct observation the
+dew-point, is so superior to Daniell's, both from its being more certain
+in its indications and economical in use, that Messrs. Negretti and
+Zambra have been induced to modify it, and reduce its price to little more
+than that of a good Daniell's Hygrometer.
+
+
+=105. Temperature of Evaporation.=--When the air is not saturated with
+vapour, evaporation is going on with more or less activity, according as
+the temperature is high or low, rising or falling. Now vapour cannot be
+formed without an expenditure of heat; as we invariably find that the
+process of evaporation lowers the temperature of the liquid from which the
+vapour is produced, and, by communication, that of contiguous substances
+also. Thus the emigrant, crossing the line under the scorching influence
+of the vertical sun, wraps a wet towel round his can of water, swings it
+in the breeze, to evaporate the moisture of the towel, and obtains a glass
+of cool water. So also, European residents in India, during the hot
+season, spread out mats in their apartments, and keep them wet, in order
+that the evaporation may cool the air. This principle has been applied,
+for the purpose of ascertaining the hygrometric condition of the air, in
+the instrument known as Mason's hygrometer, or psychrometer, which is now
+in general use, from its simplicity, accuracy, and ease of observing.
+
+
+106. MASON'S HYGROMETER.
+
+=The Dry and Wet Bulb Hygrometer, or Psychrometer=, known also as Mason's
+hygrometer (fig. 79), consists of two parallel thermometers, as nearly
+identical as possible, mounted on a wooden bracket, one marked _dry_, the
+other _wet_. The bulb of the wet thermometer is covered with thin muslin,
+and round the neck is twisted a conducting thread of lamp-wick, which
+passes into a vessel of water, placed at such a distance as to allow a
+length of conducting thread, of about three inches; the cup or glass is
+placed on one side, and a little beneath, so that the water within may not
+affect the reading of the _dry bulb thermometer_. In observing, the eye
+should be placed on a level with the top of the mercury in the tube, and
+the observer should refrain from breathing whilst taking an observation.
+
+[Illustration: Fig. 79.]
+
+The _dry_ bulb thermometer indicates the temperature of the air itself;
+while the wet bulb, cooled by evaporation, shows a lower temperature
+according to the rapidity of evaporation.
+
+_To find the Dew-point._--From the readings of the two thermometers, the
+dew-point can be deduced by formul� (that known as Apjohn's is considered
+the most theoretically true), or from the valuable Hygrometric Tables by
+J. Glaisher, Esq., F.R.S.
+
+For practical purposes in estimating the comparative humidity, the annexed
+table, which is a reduction from Mr. Glaisher's elaborate work, will be
+sufficient; it will at least serve to assist in familiarising the
+inexperienced in the value of the psychrometer's indications:--
+
+  +------------------------------------------+
+  |            | Difference between Dry-bulb |
+  |            |    and Wet-bulb Readings.   |
+  |Temperature |-----------------------------|
+  |   by the   | 2� | 4� | 6� | 8� | 10�| 12�|
+  |  Dry Bulb  |-----------------------------|
+  |Thermometer.|     Degree of Humidity.     |
+  |------------------------------------------|
+  |     34�    | 79 | 63 | 50 | .. | .. | .. |
+  |     36     | 82 | 66 | 53 | .. | .. | .. |
+  |     38     | 83 | 68 | 56 | 45 | .. | .. |
+  |     40     | 84 | 70 | 58 | 47 | .. | .. |
+  |     42     | 84 | 71 | 59 | 49 | .. | .. |
+  |     44     | 85 | 72 | 60 | 50 | .. | .. |
+  |     46     | 86 | 73 | 61 | 51 | .. | .. |
+  |     48     | 86 | 73 | 62 | 52 | 44 | .. |
+  |     50     | 86 | 74 | 63 | 53 | 45 | .. |
+  |     52     | 86 | 74 | 64 | 54 | 46 | .. |
+  |     54     | 86 | 74 | 64 | 55 | 47 | .. |
+  |     56     | 87 | 75 | 65 | 56 | 48 | .. |
+  |     58     | 87 | 76 | 66 | 57 | 49 | .. |
+  |     60     | 88 | 76 | 66 | 58 | 50 | 43 |
+  |     62     | 88 | 77 | 67 | 58 | 50 | 44 |
+  |     64     | 88 | 77 | 67 | 59 | 51 | 45 |
+  |     66     | 88 | 78 | 68 | 60 | 52 | 45 |
+  |     68     | 88 | 78 | 68 | 60 | 52 | 46 |
+  |     70     | 88 | 78 | 69 | 61 | 53 | 47 |
+  |     72     | 89 | 79 | 69 | 61 | 54 | 48 |
+  |     74     | 89 | 79 | 70 | 62 | 55 | 48 |
+  |     76     | 89 | 79 | 71 | 63 | 55 | 49 |
+  |     78     | 89 | 79 | 71 | 63 | 56 | 50 |
+  |     80     | 90 | 80 | 71 | 63 | 56 | 50 |
+  |     82     | 90 | 80 | 72 | 64 | 57 | 51 |
+  |     84     | 90 | 80 | 72 | 64 | 57 | 51 |
+  |     86     | 90 | 80 | 72 | 64 | 58 | 52 |
+  +------------------------------------------+
+
+The total quantity of aqueous vapour which at any temperature can be
+diffused in the air being represented by 100, the per-centage of vapour
+actually present will be found in the table opposite the temperature of
+the dry thermometer, and under the difference between the dry-bulb and
+wet-bulb temperatures. The degree of humidity for intermediate
+temperatures and differences to those given in the table can be easily
+estimated sufficiently accurately for most practical purposes.
+
+The difference between the two thermometer readings taken from the reading
+of the wet bulb, gives the dew-point very nearly, when the air is at any
+temperature between freezing and 80�. This simple rule will be found
+serviceable to horticulturists, since it will enable them to estimate the
+chilling effect of dew or hoar-frost on tender plants.
+
+_Use as an Indicator of Weather._--In our climate, the usual difference
+between the thermometer readings,--in the open air, shaded from the sun,
+reflected heat, and currents of air,--ranges from one to twelve degrees.
+In hot and dry climates, as India and Australia, the range out of doors
+has been found as much as 30�, occasionally.
+
+When the moisture is frozen, the bulb should be wetted afresh, and the
+reading taken just before it again freezes; but the observation then is of
+little value, and for general purposes need not be taken, as the air is
+known to be dry in frosty weather.
+
+The muslin or cotton rag should be washed once or twice a week by pouring
+water over the bulb; and it should be replaced by a fresh piece at least
+once a month. Accuracy depends very much upon keeping the wet bulb clean,
+and not _too_ wet.
+
+In connection with the barometer, this hygrometer is very useful, not only
+on land, but especially at sea, where other kinds of hygrometers cannot be
+practically used. A fall in the barometer is indicative of coming wind or
+rain: if the hygrometer shows increasing dampness by the difference of the
+readings becoming smaller,--rain may therefore be anticipated. On the
+contrary, if the hygrometer shows continuing or increasing dryness, a
+stronger wind is probable, without rain.
+
+_Domestic Uses._--Mason's hygrometer is useful in regulating the moisture
+of the air of apartments; a difference in the thermometer readings of from
+5� to 8� being considered healthy. Many complaints require that the
+temperature and humidity of the air which the invalid breathes should be
+carefully regulated. Hence it is a valuable household instrument. In a
+room, it should be placed away from the fire as much as possible, but not
+exposed to draughts of air.
+
+Figs. 80 and 81 show cheap arrangements of the instrument for domestic
+purposes. Other arrangements are given to the instrument to make it
+suitable for exhibiting the hygrometrical state of the air in hot-houses,
+conservatories, malting-houses, warehouses, manufactories, &c.
+
+[Illustration: Fig. 80.]
+
+[Illustration: Fig. 81.]
+
+[Illustration: Fig. 82.]
+
+Fig. 82 shows the instrument arranged on brass tripod stand, with folding
+legs and metal cover, to render it portable.
+
+
+=107. Self-Registering Hygrometer.=--A maximum thermometer and a minimum
+thermometer, each fitted up as a wet-bulb thermometer, record the highest
+and lowest temperature of evaporation during the interval of observation.
+Negretti's mercurial maximum, and an alcohol minimum, answer best.
+
+
+=108. Causes of Dew.=--"The aqueous vapour of our atmosphere is a powerful
+radiant; but it is diffused through air which usually exceeds its own mass
+more than one hundred times. Not only, then, its own heat, but the heat of
+the large quantity of air which surrounds it, must be discharged by the
+vapour, before it can sink to its point of condensation. The retardation
+of chilling due to this cause enables good solid radiators, at the earth's
+surface, to outstrip the vapour in their speed of refrigeration; and
+hence, upon these bodies, aqueous vapour may be condensed to liquid, or
+even congealed to hoar-frost, while at a few feet above the surface it
+still maintains its gaseous state."[9] The amount of moisture so deposited
+will vary with different atmospheric conditions. If the sky be decidedly
+cloudy or misty, the heat radiated from the earth will be partly restored
+by counter-radiation from the visible vapour; the cooling of the earth's
+surface will, therefore, take place slowly, and little dew will be
+deposited. On the other hand, if the air contain transparent vapour, and
+the sky appear clear, the counter-radiation will be less, the earth will
+cool rapidly, and the deposit of dew will be copious; provided the night
+be comparatively calm, for, when the wind blows, the circulating air
+supplies heat to the radiating substances, and prevents any considerable
+chilling.
+
+The dew which falls in tropical countries greatly exceeds in abundance
+what we experience in our climate; because the air is there, from the
+great heat, capable of sustaining a large amount of vapour in the
+transparent state, and the conditions most favourable for a maximum
+reduction of temperature by radiation are present. At those places, or
+upon those substances which cool the lowest and most readily, the dew
+falls most copiously.
+
+
+[Illustration: Fig. 83.]
+
+=109. Plan of Exposing Thermometers=, &c.--Figure 83 is an illustration of
+a convenient slab for supporting thermometers in an exposed position
+attached to a stand (such as Glaisher's, described in Chapter XVI.) for
+ordinary scientific observations. It has a projecting ledge, _B_, to carry
+off rain from the instruments, the slab, _A_, being erected vertically.
+The hygrometer is placed at _E_, with the vase of water at _F_. An alcohol
+minimum thermometer is represented at _C_, in the position most favourable
+to its certain action; and at _D_ is shown one of Negretti & Zambra's
+maximum thermometers, the position of which may be more nearly horizontal
+than there exhibited, although a slight depression of the bulb-end of the
+frame is desirable, but not necessary, as this thermometer can be used in
+any position.
+
+
+
+
+CHAPTER XII.
+
+INSTRUMENTS USED FOR MEASURING THE RAINFALL.
+
+
+The instruments in use for measuring the quantity of rain which falls on a
+given spot are of very simple construction. Perhaps the simplest is:--
+
+
+=110. Howard's Rain-Gauge.=--It consists of a copper funnel, a stout glass
+or stone bottle, and a measuring glass. The bottle is to be placed upon
+the ground, with the funnel resting on its neck. A brass band or cylinder
+fixed upon the outer surface of the funnel envelops the neck of the
+bottle, and the pipe of the funnel extends nearly to the bottom of the
+bottle; so that loss by evaporation is avoided as much as possible. The
+receiving space of the funnel is formed by a brass ring, five inches in
+diameter, very accurately turned. The measuring vessel enables the
+observer to note the rainfall in inches, tenths, and hundredths of an
+inch.
+
+
+[Illustration: Fig. 84.]
+
+=111. Glaisher's Rain-Gauge.=--The rain-gauge designed by Mr. Glaisher,
+the well-known meteorologist, and used by most observers of the present
+day, is arranged for the reception of the water which falls upon its
+receiving surface only, and for the prevention of loss by evaporation. The
+rain is first collected in a funnel, _B_, (fig. 84,) the receiving surface
+of which is turned in a lathe. The conical surface of the funnel slopes to
+the pipe, _E_, at an angle of 60� from the horizontal receiving surface.
+The tube, _E_, is of small aperture, and is bent up, in order to retain
+the last few drops of rain, so that the only opening for the escape of
+vapour may be closed as long as possible. The funnel, _B_, fits upon the
+cylinder, _A_, tightly in the groove, _D_. A copper can is placed inside
+the cylinder, _A_, to receive the rain from the funnel. Once or twice a
+day, or after a shower, this can should be taken out, and the water
+measured in the glass measure, _C_, which is graduated to hundredths of an
+inch, according to the calculated quantity of water, determined by the
+area of the receiving space. In use, this gauge should be partly sunk in
+the ground, so that the top may be about five inches above it. Thus
+situated, there will be little or no evaporation from it during any month
+of the year; and the readings need not be taken daily, although desirable.
+
+
+=112. Rain-Gauge with Float.=--In this construction the graduated glass
+measure is dispensed with. The cylinder of the gauge is made less in
+diameter than the funnel, and a hollow, very flattened spheroid of copper
+forming a float, and carrying a vertical graduated boxwood scale which
+moves through the orifice of the funnel, is placed in it. As the rain
+accumulates the float rises, and the amount of rain in the gauge is read
+upon the scale from the top of the gauge, a bar, having a hole at the
+centre for the passage of the scale, being fixed diametrically across the
+receiving space of the funnel. The gauge is provided at the bottom with a
+brass cock, by which the water may be allowed to flow out of it whenever
+necessary.
+
+This form of gauge is not very suitable for the measurement of small
+quantities; but is admirably adapted for localities where the rainfall is
+excessive.
+
+
+[Illustration: Fig. 85.]
+
+=113. Rain-Gauge with Side-Tube.=--This instrument, as represented in fig.
+85, is a cylindrical vessel, mounted on a base shaped as a frustum of a
+cone. This base may be filled with sand or gravel to make the instrument
+stable, so that when placed upon a lawn or in a garden it may have an
+ornamental appearance. The funnel for collecting the rain is larger in
+diameter than the cylinder. Parallel to the cylinder, and communicating
+with the lowest part of the interior and extending to its top, is a
+graduated glass tube, open at both ends. The rain collected will rise as
+high in this tube as in the cylinder, and its amount can therefore be read
+off without any trouble. The gauge is emptied by the brass tap at the
+bottom of the cylinder.
+
+
+=114. Admiral FitzRoy's Rain-Gauge.=--A form of rain-gauge, very well
+adapted for expeditious observation at any time, has been designed by
+Admiral FitzRoy, and extensively employed by his observers. It is
+cylindrical in shape, with the funnel let into the top; and the rainfall
+is collected in an inner and much smaller cylinder, so that a small fall
+is represented by a considerable depth of water in the gauge. The amount
+of rain which has fallen is ascertained by a dipping tube, similar in
+principle to the dipping syphon used by gaugers for taking out specimens
+of wines or spirits from casks by simply removing the bung. A short,
+vertical, tubular opening provided with a cap, which is attached to the
+instrument by a chain that it may not be lost, is formed in the funnel.
+The measuring tube, which has a small hole at each end, should be placed
+upright in the gauge; then the thumb should be pressed over the upper
+aperture, while the tube is lifted gently out, holding in the lower part a
+quantity of water representing the depth of the rain in the gauge, the
+upper edge of which is at the mark to be read off. The glass tube is
+graduated to inches and tenths; hundredths of an inch can be readily
+estimated by the eye. The marks are fixed by actual trial with a standard
+gauge, and are artificial, not true, inches.
+
+
+=115. Self-Registering Rain-Gauge.=--The rain-gauge can be combined with
+clock-work and other mechanism so as to be self-recording of the amount of
+rain, the time, and duration of its fall. For the details of construction
+the reader is referred to the next chapter, where he will find the
+instrument described in connection with Osler's anemometer, as the
+"pluviometer." To observe and duly record the times of commencement and
+termination of rain is very desirable. Scarcely any observer can attempt
+to do this even approximately from personal observation. Hence the want of
+a cheap and simple self-recording rain-gauge is much felt, the present
+construction being too expensive for all but a few individuals.
+
+In 1862, Mr. R. Strachan estimated the duration and amount of rain in
+London (Gray's Inn Road) as follows:--
+
+  +-------------------------------------------------------------------+
+  |  MONTHS.  |INCHES.|DAYS.|HOURS.||  MONTHS.   |INCHES.|DAYS.|HOURS.|
+  |-----------+-------+-----+------++------------+-------+-----+------|
+  |January.   | 1�86  | 19  |  88  ||July.       | 2�27  | 17  |  68  |
+  |February.  | 0�37  |  9  |  25  ||August.     | 2�45  | 12  |  72  |
+  |March.     | 3�40  | 22  | 130  ||September.  | 1�70  | 12  |  55  |
+  |April.     | 2�34  | 14  |  80  ||October.    | 3�23  | 21  |  94  |
+  |May.       | 3�04  | 16  |  90  ||November.   | 1�12  | 10  |  53  |
+  |June.      | 2�45  | 20  |  83  ||December.   | 1�44  | 17  |  66  |
+  +-------------------------------------------------------------------+
+
+"During the year 1862, the rainfall amounted to 25�67 inches. Rain fell on
+179 days, that is, on nearly every other day. The hours of rain were
+estimated at 904; therefore, if the rain had fallen continuously, it would
+have lasted nearly 38 days and nights."[10] The value of similar estimates
+of the rainfall by numerous observers would be very great to meteorology.
+
+
+=116. The principle of measurement= in all these gauges is the relation
+existing between the areas of the collecting and receiving surfaces; that
+is, between the area of the funnel into which the rain falls, and the area
+of the cylinder which receives it. In Howard's and Glaisher's gauges, this
+cylinder is virtually the measuring glass itself; in the others, above
+described, the measuring scales show the same depth of water as in the
+cylinder of the gauge.
+
+The cylinder being of less diameter than the funnel, and receiving all the
+rain collected by the funnel, it follows that its contents will have an
+increased depth. Now equal cylindrical volumes, having different
+diameters, are to each other in length inversely as the squares of the
+diameters. Hence, if the funnel be 9 inches and the cylinder 3 inches in
+diameter, a fall of 1 inch of rain will be represented in the gauge by 9
+inches; for 3� : 9� :: 1 : _x_ = 9. In this case, therefore, a length of
+nine inches of the measuring glass, tube, or scale, would represent an
+inch of rainfall, and be divided into tenths and hundredths of the
+artificial inch.
+
+
+=117. Position for Rain-Gauge, &c.=--Rain gauges should be placed on the
+ground, in any position exposed to a free fall of rain, snow, or hail,
+where neither walls, buildings, nor trees shelter or cause eddies of wind.
+They should be supported by a frame, or other means, to prevent them being
+blown down by the wind, but so that they can be readily emptied.
+
+During snow or frost, the gauge must be watched, and its contents melted
+by placing it in a warm room, either when the amount is to be measured, or
+the funnel is filled up with snow. A tin vessel of equal area to the
+funnel may at such times be useful as a substitute.
+
+Rain gauges are constructed of metal, usually copper, which, besides being
+readily workable, is little affected by atmospheric influences. If made of
+iron or zinc, they should be well japanned; if of copper, this is not so
+essential. The capacity of a gauge should be sufficient to contain at
+least the probable maximum fall of rain in a day at the locality. Those
+required for rainy districts must be of large size.
+
+
+=118. Causes of Rain.=--When the invisible vapour which is diffused in the
+atmosphere becomes sufficiently cooled, it appears visible as mist or
+cloud, and a further reduction of temperature causes its precipitation as
+rain, hail, or snow. The cooling of the higher regions of the atmosphere
+is doubtless the chief cause of this condensation; but the property which
+aqueous vapour possesses of radiating heat may also contribute to the
+result. Moreover, the law which regulates the amount of vapour which air
+at any particular temperature can sustain in a transparent state,
+determines that when two bodies of air at different temperatures,
+saturated with vapour, intermix, some moisture must be rendered visible;
+and hence, it is not only possible, but highly probable, that rain may
+result from the conflict of different winds. Let us imagine two cubic
+yards of air, both saturated with moisture, but having the respective
+temperatures of 50 and 70 degrees, to come into contact. There will be a
+tendency to equalize the temperature to a mean, which is 60�; and during
+this process, some of the vapour will be condensed.
+
+  For in the air at 50� there is                     110�7 grains of vapour[11]
+  and       "       70      "                        216�0       "
+                                                    ------
+  Total amount of vapour                             326�7       "
+  But two cubic yards of air at 60� can only sustain 313�2       "
+                                                    ------
+  Hence there will be deposited                       13�5       " of rain.
+                                                    ======
+
+It may be conceded, therefore, that when a warm and moist current of air
+encounters a body of cold air which may not be extremely dry, the mixture
+is unable to retain the whole of the vapour in an invisible state; so that
+the excess becomes visible as mist or fog, and, when the temperature has
+become sufficiently lowered, rain. The British Isles are more or less
+enveloped in fog, or mist, at the commencement of easterly winds, which,
+with a sudden change of wind, is exhibited even in summer; while the
+south-westerly winds, warm, and arriving from the ocean, deposit large
+quantities of rain by the cooling effect of the land, colder by reason of
+its latitude. When rain occurs with a northerly wind, it is probably due
+to the deposition from an upper south-westerly current, often apparently
+proved by the movements of the upper clouds.
+
+
+=119. Laws of Rain-fall.=--Tropical countries have a dry and a wet season
+during the year: _dry_, when the sun is at the opposite side of the
+equator; _wet_, when the sun is overhead. With reference to the British
+Isles, the statistics collected by Mr. G. J. Symons indicate that: 1st.
+The stations of least rain are inland, or on the east or south-east
+coasts; the stations of greatest rain are on the western coasts. 2nd. The
+rain-fall is very large in the vicinity of mountain chains or groups,
+unless the station happens to be some miles to the north-eastward.
+
+It may be well to illustrate these remarks by quoting[12] the average fall
+at a few places, grouping them as--
+
+      Westerly.
+                           Inches.
+  Bodmin                    43
+  Bolton (Lancashire)       44
+  Coniston (Windermere)     71
+  Seathwaite               127
+  Torosay (I. of Mull)      75
+  Killaloe (Limerick)       38
+
+     Central.
+                           Inches.
+  Enfield                   23
+  Epping                    23
+  Derby                     24
+  York                      22
+  Stirling                  39
+  Perth                     29
+
+     Easterly.
+                           Inches.
+  Witham (Essex)            21
+  Patrington (Hull)         21
+  Sunderland                17
+  Inveresk (Edinburgh)      25
+  Pittenweem (Fife)         24
+  Dublin                    22
+
+Mr. Green, the celebrated aeronaut, has asserted from his experience,
+"that whenever a fall of rain happens, and the sky is entirely overcast,
+there will invariably be found to exist another stratum of cloud at a
+certain elevation above the former;" and the recent scientific balloon
+ascents by Mr. Glaisher have tended to confirm this theory. Mr. Glaisher
+says, "It would seem to be an established fact, that whenever rain is
+falling from an overcast sky, there is a second stratum above." "It would
+also seem that when the sky is overcast without rain, that there is no
+stratum of cloud above, but that the sun is shining on the upper surface.
+In every instance in which I have been up under these circumstances, I
+have found such to be the case, agreeing in this respect also with Mr.
+Green's observations."
+
+The amount of rain collected in a gauge placed near the surface of the
+earth is larger than in any gauge placed above it; and the higher the
+gauge is placed, the less water is collected. Mr. Glaisher contends that
+his balloon experiments corroborate this law.
+
+
+=120. Utility of Statistics of Rain-fall.=--The utility of knowing the
+rain-fall of any locality is sufficiently obvious, and little need be said
+upon the subject. The rain-gauge should be in the hands of every gardener
+and farmer. In the management of out-door plants and crops, as well as in
+the construction of cisterns and tanks for the supply of water, a rain
+gauge is a valuable assistant. By its use, the gardener will be guided in
+judging how far the supply of moisture to the earth is needed; and he will
+also see how beneficial is even a hasty shower to growing plants, when he
+considers that a fall of rain measuring the tenth of an inch in depth,
+corresponds to the deposit of about forty hogsheads per acre. The study of
+the rain-fall of a country is of considerable interest to agriculturists.
+The health and increase of domestic animals, the development of the
+productions of the land, as well as the daily labours of the farmer, are
+dependent upon the excess or deficiency of rain. "It must be a subject of
+great satisfaction and confidence to the husbandman to know at the
+beginning of a summer, by the certain evidence of meteorological results
+on record, that the season, in the ordinary course of things, may be
+expected to be a dry and warm one; or to find, in a certain period of it,
+that the average quantity of rain to be expected for the month has fallen.
+On the other hand, when there is reason, from the same source of
+information, to expect much rain, the man who has courage to begin his
+operations under an unfavourable sky, but with good ground to conclude,
+from the state of his instruments and his collateral knowledge, that a
+fair interval is approaching, may often be profiting by his observations;
+while his cautious neighbour who waited 'for the weather to settle' may
+find that he has let the opportunity go by. This superiority, however, is
+attainable by a very moderate share of application to the subject; and by
+the keeping of a plain diary of the barometer and rain-gauge, with the
+hygrometer and vane, under his daily notice."[13] The statistics of
+rain-fall are not only valuable and interesting in a meteorological point
+of view, and for agricultural purposes, but are also highly important in
+connection with sanitary arrangements for towns, and engineering
+operations. This is especially evident to the hydraulic engineer. As rain
+is an important source of water-supply to rivers, canals, and reservoirs,
+it is evident that a knowledge of the probable fall for any season or
+month, at a given place, as furnished by averages of the observations of
+former years, will be the data upon which the engineer will base his plans
+for providing for floods or droughts; while the measurement of the actual
+quantity which has just fallen, as gathered from the indications of a
+series of gauges, will suggest to him the precautions to adopt either to
+economise or conduct away the in-pouring waters.
+
+"When a canal is conducted across an undulating country, its course is
+necessarily governed by the accidents of the ground, and it alternately
+rises and falls. In this case, rising by a succession of levels, it
+necessarily arrives at a certain highest level, which is called by
+engineers the _summit level_. From this it again descends by a
+corresponding series of levels. Now, it is evident that, supposing the
+locks to be all equal in magnitude, the ascent of a vessel will require
+the descent of as much water from the summit to the lowest level as would
+fill a single lock; for this quantity of water must be discharged from
+each lock of the series when the vessel passes through it.
+
+"The same may be said of the process by which the vessel descends along
+the series of locks on the other side of the summit. It appears,
+therefore, that a supply of water must always be maintained on the summit
+level sufficient to fill a single lock twice for each vessel which crosses
+the summit.
+
+"It happens, fortunately, that by the laws of natural evaporation, rain is
+precipitated in greater quantities on elevated summits than on the
+intermediate valleys, so that the moving power, in this case, accommodates
+itself to the exigencies of intercommunication."--_Dr. Lardner's "Handbook
+of Natural Philosophy."_
+
+
+=121. New Form of Rain-Gauge.=--Since the foregoing pages were in type, a
+modification of Howard's rain-gauge has been arranged by Mr. Symons, which
+is compact in design, convenient in use, and low in price. It combines the
+advantages of most gauges; having solidity, and facility of measurement.
+The bottle is placed in a tin case, to the bottom of which are attached
+stout spikes, which, when forced into the earth, prevent its being upset
+either by wind or accident. The bottle being transparent, and slits made
+in the case, the fall of rain is seen at a glance, or with a race-glass,
+from a window. The funnel being attached to the cover of the case is
+thereby kept strictly horizontal, and the depth of rain can be accurately
+measured by lifting the bottle from its case and emptying it into a
+graduated glass jar.
+
+The funnel of this gauge is a very deep cone, to prevent the rain drops
+outsplashing. When properly placed, the receiving surface will be twelve
+inches above the ground, which experience has shown to be the most
+advantageous height.
+
+
+
+
+CHAPTER XIII.
+
+APPARATUS EMPLOYED FOR REGISTERING THE DIRECTION, PRESSURE, AND VELOCITY
+OF THE WIND.
+
+
+=122. The Vane.=--The instrument by which the wind's direction is most
+generally noted, is the vane, or weather-cock, and all that need be said
+of it here is that the points north, east, south and west, usually
+attached to it, should indicate the _true_ and not the _magnetic_
+directions; and that care should be taken to prevent its setting fast.
+Very complicated instruments are required for ascertaining the pressure
+and velocity of the wind, and these are called _Anemometers_. The simplest
+is _Lind's_.
+
+
+[Illustration: Fig. 86.]
+
+=123. Lind's Anemometer, or Wind-Gauge= (fig. 86), invented so late as
+1775, for showing the pressure of the wind, consists of a glass syphon,
+the limbs parallel to each other, and each limb the same diameter. One end
+of the syphon is bent at right angles to the limb, so as to present a
+horizontal opening to the wind. A graduated scale, divided to inches and
+tenths, is attached to the syphon tube, reading either way from a zero
+point in the centre of the scale. The whole instrument is mounted on a
+spindle, surmounted by a vane, and is moved freely in any direction by the
+wind, always presenting the open end towards the quarter from which the
+wind blows. To use the instrument, it is simply filled up to the zero
+point with water, and then exposed to the wind; the difference in the
+level of the water gives the force of the wind in inches and tenths, by
+adding together the amount of depression in one limb, and elevation in the
+other, the _sum of the two_ being the height of a column of water which
+the wind is capable of sustaining at that time.
+
+TABLE,
+
+Showing the Force of Wind on a square foot, for different heights of the
+column of Water in Lind's Wind-Gauge.
+
+  +-----------------------------------+
+  |Inches.|Force in|Common designation|
+  |       |  lbs.  |   of such Wind.  |
+  |-------+--------+------------------|
+  |  6    |  31�75 | A Hurricane.     |
+  |  5    |  26�04 | A violent Storm. |
+  |  4    |  20�83 | A great Storm.   |
+  |  3    |  15�62 | A Storm.         |
+  |  2    |  10�42 | A strong Wind.   |
+  |  1    |   5�21 | A high Wind.     |
+  |   �5  |   2�60 | A brisk Wind.    |
+  |   �1  |    �52 | A fresh Breeze.  |
+  |   �05 |    �26 | A gentle Breeze. |
+  | 0.    |   0.   | A Calm.          |
+  +-----------------------------------+
+
+
+=124. Modification of Lind's Gauge.=--_Sir W. Snow Harris_ has effected a
+modification of Lind's anemometer, with a view of obtaining a hand
+instrument for use at sea more especially. At present the force of the
+wind is estimated at sea by an arbitrary scale, suggested by Sir F.
+Beaufort, the late hydrographer; 0 being calm, 12 the strongest hurricane,
+and the intermediate numerals giving the varying strength of the wind.
+There has been a long-felt want of instrumental means for obtaining this
+data at sea, if merely for the sake of checking occasionally personal
+estimations, which may vary considerably among different observers.
+Harris's wind gauge is intended to be held by hand, while facing the wind,
+and keeping it in proper position by attending to a spirit-level attached.
+When in position, and held firmly, the tube has to be opened to the wind
+by pressure of the thumb acting upon jointed levers, controlled by
+springs. The pressure of the wind moves the enclosed liquid; and by
+withdrawing the thumb, the tube is closed so as to keep the liquid in its
+position; the reading is then taken from its scale, either in pounds on
+the square foot, miles per hour, or the ordinary designations of wind, as
+light, fresh, strong, &c.
+
+
+=125. Robinson's Anemometer.=--_Dr. Robinson_, of Armagh, is the inventor
+of a very successful anemometer, which determines the horizontal velocity
+of the wind. It was first used in 1850, in the meteorological and tidal
+observations made on the coast of Ireland under the direction of the Rev.
+Dr. Lloyd. No meteorological observatory should be without this valuable
+instrument, which is essential in determining the average velocity of the
+wind of a locality as distinguished from the most frequent wind of the
+same place. It is represented in fig. 87. Four hollow hemispherical cups,
+_A A_, are extended upon conjugate diameters, or arms, with their
+diametrical planes placed vertically, and facing the same way upon a
+vertical axis, _B_, which has at its lower extremity an endless screw,
+_D_. The axis is supported at _C_ so as to turn with as little friction as
+possible. The endless screw is placed in gear with a train of wheels and
+pinions. Each wheel carries an index over a stationary dial in front; or
+the index is fixed, and the graduations are placed upon the wheels
+themselves.
+
+[Illustration: Fig. 87.]
+
+Dr. Robinson has proved, both by theory and experiment, that the centre of
+any one of the cups so mounted and set in motion by the wind, revolves
+with one-third of the wind's velocity. If, therefore, the diametrical
+distance between the centres of the cups be one foot, the circle described
+by the centres in one revolution is 3�1416 feet, and the velocity of the
+wind will be three times this, or 9�42 feet, which must be referred to
+time for the absolute rate. The instrument is sometimes made with the
+centres of the cups 1�12 feet apart, so that the circle described is
+1/1500 of a mile in circumference. Hence, to produce one revolution of the
+cups, the wind must travel three times as fast, or 1/500 of a mile.
+Therefore, 500 revolutions will be produced by one mile of wind; so that
+the dials may be graduated to register the velocity in miles and tenths of
+miles. The simplest arrangement is with five dials, recording respectively
+10, 100, 1,000, 10,000 and 100,000 revolutions.
+
+_Directions for using Robinson's Anemometer._--The dials read off in the
+same manner as the register of a gas meter, commencing with the dial
+farthest from the endless screw.
+
+"The figures on the first dial indicate so many hundreds of thousands of
+revolutions; those on the second dial so many tens of thousands; those on
+the third, thousands; those on the fourth, hundreds; and those on the
+fifth so many tens.
+
+"The instrument should be read every morning at 9 o'clock; and, usually,
+it will only be necessary to read the first three dials. The figures can
+be entered as they are read off. Should the index point _between_ two
+figures, the less of the two is to be taken.
+
+"For example, if the first dial points to 7, or between 7 and 8; while the
+second dial indicates 4; and the third, 5; the entry to be made is 745
+(indicative of 745 _thousand_ revolutions).
+
+"Every time the index of the first dial is found to have passed zero (0),
+a cross or star is to be prefixed to the next (a lower) reading.
+
+"To ascertain how many _thousands_ of revolutions have been made during
+the month, it will simply be necessary to subtract the first reading from
+the last, and prefix to the three figures thus obtained a figure
+corresponding to the number of stars in the column. For every _thousand_
+revolutions there are two miles of wind: we have therefore only to
+multiply by 2 to find how many miles of wind have passed during the month.
+
+"Two entries must be made for the last day of each month (the one being
+written under the other), so as to bring the readings down to 9 A.M. on
+the 1st of the following month. The same entry which ends one month, will
+therefore begin the next. This repetition of one entry is necessary, in
+order to prevent losing a day's wind.
+
+  "The accompanying example of the           687
+  readings of an Anemometer for 13 days      773
+  will illustrate the method of making       822
+  the entries, &c.                           855
+                                             900
+  "In this instance, the first reading       953
+  (687) is less than the last (793).         990
+  When the first reading is greater than    *066
+  the last, it will be necessary to borrow   197
+  1,000 in making the subtractions,          323
+  and then deduct one from the number        414
+  of stars. Thus, if the first reading       597
+  of the series on the margin had            712
+  been 887, the result would have been       793
+  906 instead of 1106.                      ----
+                                            1106 thousands of revolutions.
+                                               2
+                                          +-----
+                                       13 | 2212 miles of wind in period.
+                                          +-----
+                                             170 miles of wind per day, on
+                                                 an average.
+
+"The foregoing directions are all which require to be regularly attended
+to. But it may be interesting at times to find the velocity of the wind
+during a period of a few minutes. This may be ascertained by observing the
+difference of two readings of all the dials, with an interval of some
+minutes between them, when a very brief calculation will suffice; but
+perhaps the simplest method is the following:--
+
+"Take two readings, with an interval of 12 minutes between them. The
+difference of these readings, divided by 10, is the velocity of the wind
+in miles per hour. Thus--if the reading of the five dials (from left to
+right) at noon is 15206, and at 12 minutes past 12 is 15348, the velocity
+of the wind is 14�2 miles per hour."--_Admiral FitzRoy, F.R.S._
+
+A lever and clutch are sometimes fitted to this anemometer, as in fig. 88,
+for throwing the train out of gear when not required to register. It may
+also be connected with clock-work so as to be self-recording, by causing
+the mechanism to impress a mark upon prepared paper moved by the
+apparatus, at certain intervals of time.
+
+[Illustration: Fig. 88.]
+
+This anemometer should be fixed in an exposed situation, as high above
+ground as may be convenient for reading. It may be made very portable, by
+the arms which carry the cups being fitted to unscrew or to fold down.
+When fitted in gimbals, it can be used at sea with much advantage.
+
+The pressure of the wind has been experimentally proved to vary as the
+square of the velocity; the relation being _V�_ = 200 � _P_. From this
+formula, therefore, the pressure can be calculated corresponding to the
+observed velocity.
+
+
+=126. Whewell's Anemometer.=--This apparatus, the invention of the
+celebrated Dr. W. Whewell, registers the horizontal motion of the air with
+the direction. Its mechanism may be described in general terms, as
+follows:--
+
+A horizontal brass plate is attached to a vertical spindle, which passes
+through the axis of a fixed cylinder, being supported by a bearing at the
+lower end, and working in a collar at the upper. A vane is attached, by
+which the plate is moved about according to the direction of the wind. A
+fly, having eight fans, each fixed at an angle of 45� with the axle, is
+placed upon the plate so that the axle is in the line of direction of the
+vane. An endless screw on the axle turns a vertical wheel having one
+hundred teeth, the axle to which has also an endless screw working into a
+horizontal wheel, having a like number of teeth, and which communicates
+motion to a vertical screw fifteen inches long. On this screw is placed a
+moveable nut, which carries a pencil. Round the cylinder is wrapped daily
+a paper divided for the points of the compass. The wind acting upon the
+vane will cause the plate to turn; and the screw which carries the pencil
+will travel with it, so that the pencil will mark upon the paper the
+direction of the wind. The fly will also be set in motion, and thereby the
+nut upon the screw will descend, so that the attached pencil will trace a
+vertical line upon the paper. When the fans on the axle are 2�3 inches
+from axis to end, and 1�9 inches wide, and the thread of the screw such
+that forty-five revolutions will cause the nut to descend two inches,
+75�85 miles of wind will cause the pencil to descend through a vertical
+space of two inches; but the actual trace upon the paper will be longer in
+proportion to the magnitude of change of azimuth, or direction, of the
+wind.
+
+
+=127. Osler's Anemometer, and Pluviometer.=--Mr. Follet Osler is the
+inventor of a self-recording apparatus which registers the direction and
+pressure of the wind, and the amount and duration of rain, upon the same
+sheet of paper. His apparatus has met with very much approbation, and has
+been erected in many observatories. The mechanism may be modified in
+various ways, and the following is a description of the simplest and most
+recent arrangement.
+
+[Illustration: Fig. 89.]
+
+The instrument, of which fig. 89 is a diagram rather than a picture,
+consists, first, of a vane, _V_, of a wedge-shape form, which is found to
+answer better than a flat vane; for the latter is always in a neutral
+line, and therefore is not sufficiently sensitive. A wind-mill governor
+has been substituted for the vane to get the direction of the wind, with
+advantage. At the lower end of the tube, _T T_, is a small pinion, working
+in a rack, _r_, which moves backwards and forwards as the wind presses the
+vane. To this rack a pencil, _x_, is attached, which marks the direction
+of the wind on a properly ruled paper, placed horizontally beneath, and so
+adjusted as to progress at the rate of half an inch per hour, by means of
+a simple contrivance connecting it with a good clock. The paper is shown
+in the illustration upon the table of the instrument.
+
+The pressure plate, _F_, for ascertaining the force of the wind, is one
+foot square, placed immediately beneath, and at right angles with the
+vane; it is supported by light bars, running horizontally on friction
+rollers, and communicating with flattened springs, 1, 2, 3, so that the
+plate, when affected by the pressure of the wind, acts upon them, and they
+transfer such action to a copper chain passing down the interior of the
+direction tube, and over a pulley at the bottom. A light copper wire
+connects this chain with the spring lever, _y y_, carrying a pencil which
+records the pressure upon the paper below. Mr. Osler much prefers a spring
+to any other means for ascertaining the force of the wind, because it is
+of the highest importance to have as little matter in motion as possible,
+otherwise the momentum acquired will cause the pressure plate to give very
+erroneous indications. The pressure plate is as light as is consistent
+with strength. It is kept before the wind by the vane, and is urged out by
+three or more springs, so that with light winds one only is compressed,
+and two, or more, according to the strength of the wind.
+
+The _pluviometer_ is placed on the right in the figure, _P P_ being the
+plane of the roof of the building. The rain funnel, _R_, exposes an area
+of about 200 square inches. The water collected in it is conveyed by a
+tube through the roof of the building into a glass vessel, _G_, so
+adjusted and graduated as to indicate a quarter of an inch of rain for
+every 200 square inches of surface, _i. e._ 50 cubic inches. _G_ is
+supported by spiral springs, _b b_, which are compressed by the
+accumulating rain. A glass tube, open at both ends, is cemented into the
+bottom of _G_, and over it is placed a larger one closed at the top like a
+bell glass. The smaller tube thus forms the long leg of a syphon, and the
+larger tube acts as the short leg. The water, having risen to the level of
+the top of the inner tube, drops over into a little copper tilt, _t_, in
+the globe, _S_, beneath the reservoir. This tilt is divided into two equal
+partitions by a slip of copper, and placed upon an axis not exactly
+balanced, but so that one end or the other preponderates. The water then
+drops into the end of the tilt which happens to be uppermost, and when
+quite full it falls over, throwing the water into the globe, _S_, from
+which it flows away by the waste pipe. In this way an imperfect vacuum is
+produced in the globe, quite sufficient to produce a draught in the small
+tube of the syphon, or the long leg; and the whole contents of the
+reservoir, _G_, immediately run off, and the spiral springs, _b b_,
+elevate the reservoir to its original position. To produce this action, a
+quarter of an inch of rain must have fallen. The registration is easily
+understood. A spring lever, _z_, carrying a pencil, is attached by a
+cord, _c_, to _S_. This spring always keeps the cord tight, so that as the
+apparatus descends during the fall of rain, the spring advances the pencil
+more and more from the zero of the scale upon the paper beneath, until a
+quarter of an inch has fallen, when the pencil is drawn back to zero by
+the ascent of the reservoir.
+
+The clock movement carries the registering paper forward by one of the
+wheels working into a rack attached to the frame.
+
+The adjustment of the instrument should be carefully made at its first
+erection. The scale for pressure should be established experimentally, by
+applying weights of 2, 4, 6, &c., lbs., to move the pressure plate.
+
+The registration trace for twenty-four hours is readily understood. The
+direction is recorded on the centre part; the pressure on one side, and
+the rain on the other. Lines parallel to the length of the paper show no
+rain, steady wind, and constant pressure. On the rain trace, a line
+parallel to the width of the paper shows that the pencil had been drawn
+back to zero, a quarter of an inch of rain having fallen. The hour lines
+are in the direction of the width of the paper.
+
+At the International Exhibition 1862, Messrs. Negretti and Zambra
+exhibited an improved Osler's anemometer, having combined with it
+Robinson's cups, so that the pressure and velocity appear on the same
+sheet, on which a line an inch in length is recorded at every ten miles;
+thus the complete instrument shows continuously the direction, pressure,
+and velocity of the wind.
+
+
+=128. Beckley's Anemometer.=--Mr. R. Beckley, of the Kew Observatory, has
+devised a self-registering anemometer, which consists of three principal
+parts: Robinson's cups for the determination of velocity; a double fan, or
+wind-mill governor, for obtaining the direction; and a clock to move a
+cylinder, around which registration paper is wrapped. The paper records
+the time, velocity, and direction of the wind for twenty-four hours, when
+it must be replaced. It has a cast-iron tubular support, or pedestal to
+carry the external parts--the cups and the fans,--which must be erected
+upon the roof of the building upon which it is desired to mount the
+instrument.
+
+The fans keep their axis at right angles to the wind; and with any change
+of direction they move, carrying with them an outer brass tube, which
+rests upon friction balls on the top of the pedestal, and is attached to a
+tubular shaft passing through the interior of the pedestal, and
+terminating with a mitre wheel. The mitre wheel, working with other cogged
+wheels, communicates the motion of the direction shaft to a cylinder
+carrying a pencil, to record the direction.
+
+The shaft carrying the cups is supported upon friction balls, placed in a
+groove formed on the top of the direction shaft, and passing through the
+interior of that shaft, comes out below the mitre wheel, where it is
+terminated in an endless screw, or worm.
+
+Upon the wind moving the cups, motion is given to the innermost shaft,
+thence to the worm-wheel, whence motion is given to a pencil which
+registers the velocity.
+
+De la Rue's metallic paper is used in registration, it having the property
+of receiving a trace from a brass pencil. The pencils can, therefore, be
+made in the most convenient form. Mr. Beckley forms each pencil of a strip
+of brass wrapped round a cylinder, making a very thin threaded screw, so
+that the contact of the pencil cylinder and the clock cylinder is a mere
+point of the metallic thread. The pencil cylinders are placed side by side
+upon the cylinder turned by the clock, and require no spring or other
+appliance to keep them to their work, but always make contact with the
+registration paper by their own gravity. They therefore require no
+attention, and being as long as the trace which they make, they will last
+a long time.
+
+The velocity pencil has only one turn on the cylinder, and its pitch is
+equal to a scale of fifty miles upon the paper. The direction pencil has
+likewise one turn on its cylinder, its pitch being equal to a scale of the
+cardinal points of the compass upon the paper.
+
+The clock gives a uniform motion of half an inch per hour to the cylinder
+upon which the paper is fastened.
+
+The registering mechanism of the instrument is very compact, requiring
+only a space of about 18 inches by 8 inches.
+
+In the Report of the British Association for 1858, Mr. Beckley has given a
+detailed description of his anemometer, with drawings of all the parts.
+
+
+=129. Self-Registering Lind's Anemometer.=--A Lind's wind-gauge, designed
+to register the maximum pressure, was exhibited at the International
+Exhibition 1862, by Mr. E. G. Wood. The bend of the syphon is contracted
+to obtain steadiness. On the leeward limb a hole is drilled corresponding
+in size with the contracted portion of the tube. The edge of the hole
+corresponds with the zero of the scale. On the pressure of the wind
+increasing, as much of the water as would have risen above the aperture
+flows away, and therefore the quantity left indicates the greatest
+pressure of the wind since the last setting of the instrument, which is
+done by filling it with water up to the zero point.
+
+
+=130. Anemometric Observations.=--To illustrate the value of anemometric
+observations, we quote from a paper by Mr. Hartnup, on the results
+obtained from Osler's Anemometer, at the Liverpool Observatory. The six
+years' observations, ending 1857, gave for the yearly average of the
+winds: North-easterly, on 60 days, at 7�8 miles per hour; North-westerly,
+on 112 days, at 15�4 miles per hour; South-easterly, on 115 days, at 11�0
+miles per hour; South-westerly, on 77 days, at 13�8 miles per hour; and
+one day calm. From the same observations, the average variation in the
+strength of the wind during the 24 hours is:--11 miles per hour, the
+minimum force, occurring at 1-1/2 a.m.; until 6 a.m. it remains much the
+same, being then 11�3 miles per hour; at 10 a.m. it is 13�4 miles per
+hour; at 1-1/2 p.m. the wind is at its maximum strength, being 14�8 miles
+per hour; at 5 p.m. it is again 13�4 miles per hour, and at 9 p.m. 11�3
+miles per hour. Hence it appears that the wind falls to its minimum force
+much more gradually than it rises to its maximum; that the decrease and
+increase are equal and contrary, so that the curve is symmetrical; and
+that generally the force of wind is less at night than during the day.
+
+"There is evidence," says Admiral FitzRoy, "in Mr. Hartnup's very valuable
+anemometrical results, which seems to prove that to his observatory, in a
+valley, with buildings and hills to the north-eastward, the real polar
+current does not blow from N.E., but nearer S.E. By his reliable digest of
+winds experienced there, it appears that those most prevalent were from
+W.N.W. and S.S.E. But in England, generally, the _prevailing_ winds are
+_believed_ to be westerly, inclining to south-westerly, and
+north-easterly; while of all winds, the south-easterly is about the
+rarest.
+
+"At Lord Wrottesley's observatory, in Staffordshire, about 530 feet above
+the sea, there appears to be considerably less strength of wind at any
+given time, when a gale is blowing _generally_, than occurs simultaneously
+at places along the sea-coast: whence the inference is, that undulations
+of the land's surface and hills, diminish the strength of wind materially
+by frictional resistance.
+
+"All the synoptic charts hitherto advanced at the Board of Trade exhibit a
+marked diminution of force inland compared with that on the sea-coast.
+Indeed, the coast itself offers similar evidence, in its stunted, sloping
+trees, and comparative barrenness."[14]
+
+
+
+
+CHAPTER XIV.
+
+INSTRUMENTS FOR INVESTIGATING ATMOSPHERIC ELECTRICITY.
+
+
+=131. Atmospheric Electroscope.=--The simplest instrument for ascertaining
+at any time the electric condition of the atmosphere is an electroscope
+composed of two equal pieces of gold leaf, suspended from a brass support,
+and insulated, as well as protected from the movement of the air, by a
+glass covering. Fig. 90 represents such an instrument. The cap of the
+brass support is fitted for the reception, in the vertical direction, of a
+metallic rod, not less than two or three feet in length. The top of the
+rod carries a clip. The instrument acts according to the law, that bodies
+similarly electrified repel each other; but when dissimilarly electrified,
+they attract each other. To make an observation, the instrument is placed
+in the open air, and a lighted piece of cigar fusee, or touch-paper, is
+fixed in the clip. The electricity of the air is collected by the
+substance undergoing combustion, and conducted by the rod to the gold
+leaf; and the pieces, being similarly electrified, separate more or less
+according to the amount of electricity present. The kind is determined by
+the effect of either an excited stick of sealing-wax or rod of glass upon
+the electrified gold leaf. A rod of glass, when rubbed briskly with a silk
+handkerchief or piece of woollen cloth, becomes positively electrified, or
+excited, as it is termed. A stick of sealing-wax, similarly treated,
+acquires the negative state. If, therefore, an excited glass rod be
+presented to the cap of the instrument, and it cause the pieces of gold
+leaf to diverge still further, the electric state of the air must be
+analogous to that of the glass, that is, _positive_; if they approach, it
+is _negative_. On the contrary, if a stick of sealing-wax be used, the
+pieces will be repelled more apart if they have acquired negative
+electricity from the air; and they will converge if they have a positive
+charge.
+
+[Illustration: Fig. 90.]
+
+By means of this very simple instrument, meteorological observers can
+readily ascertain the electric condition of the lower air at any time.
+
+NOTE.--A book containing strips of gold leaf is sent with the Electrometer
+to replace the gold leaves when torn or broken in use.
+
+To mount fresh gold leaves, unscrew the brass plate to which is attached
+the rod supporting the leaves; then moisten with the breath the flat piece
+of brass, and press it gently down on one strip of gold, whilst the book
+is only partly opened; the second leaf is attached in the same manner.
+
+
+=132. Volta's Electrometer= is similar to the instrument just described,
+except that instead of gold leaf two light pieces of straw, or two pith
+balls, are freely suspended from the conductor; the amount of the electric
+charge being estimated from the degrees of divergence, shown by a
+graduated arc.
+
+
+=133. Peltier's Electrometer= is a much superior instrument in point of
+sensibility. A tall glass tube an inch or more in diameter, is connected
+to a glass receiver, mounted on a base fitted with levelling screws. At
+the top of the tube is formed a globe from four to five inches in
+diameter, which is thickly gilt on the exterior, so as to form a good
+conducting surface. A wire passes from the ball down the tube into the
+receiver, where it is bent up, and ends in a steel point over the centre
+of the base. A bent wire, carrying a small magnetic needle, is balanced on
+the steel point, so that the magnet, with the fine wire, arranges itself
+horizontally in the direction of the magnetic meridian. If any cloud or
+portion of air in the neighbourhood be in an electrical state, it will act
+by induction upon the gilt ball, and the needle will be deflected from its
+north and south direction.
+
+A graduated circle indicates the number of degrees of the deflection,
+which will be greater or less according to the tension of the electricity.
+To ascertain whether the electricity is positive or negative, a stick of
+shellac or glass must be employed, as already described.
+
+
+=134. Bohnenberger's Electroscope= may be fitted with a metallic
+conductor, and used with great advantage for observing atmospheric
+electricity. "The principal parts of the instrument, as improved by
+Becqu�rel, are the following:--_A B_, fig. 91, is a small dry galvanic
+pile of from 500 to 800 pairs, about a quarter of an inch in diameter;
+when the plates are pressed together, such a pile will be from 2 to 2-1/2
+inches in length. The wires, which are bent so as to stand above the pile,
+terminate in two plates, _P_ and _M_, which are the poles of the pile.
+These plates, which are 2 inches by 1/2 an inch, are parallel and opposite
+to each other. It is convenient for their opposite sides to be slightly
+convex, for them to be gilded or coated with platinum, and for them to run
+on the polar wires, by the latter being made to pass through a small hole
+in them. One of these plates will always be in a state of positive, and
+the other of negative, electricity; between them suspend the very fine
+gold leaf, _D G_, which is attached to the conductor, _C D_, of copper
+wire. If the leaf hang exactly between the two plates, it is equally
+attracted by each, and will therefore be in a state of repose. The
+apparatus should be protected by a bell-glass, fitting exactly, and having
+an opening at the top through which the copper wire, _C D_, passes; the
+wire, however, is insulated by its being contained in a glass tube, which
+is made to adhere to the bell-glass by means of a small portion of shellac
+or gum-lac. Screw on a metal ball or plate, to impart to it the
+electricity you wish to test, which will be conveyed by the copper wire to
+the gold leaf, and the latter will immediately move towards the plate
+which has the opposite polarity. This electroscope is, beyond doubt, one
+of the most delicate ever constructed, and is well adapted to show small
+quantities of positive and negative electricity.
+
+[Illustration: Fig. 91.]
+
+"To ensure the susceptibility of electroscopes and electrometers placed
+under bell-glasses, precautions should be taken to render the air they
+contain as dry as possible, which may be effected by enclosing in a
+suitable vessel a little melted chloride of calcium beneath the glass."
+
+The galvanic pile employed in this electroscope is that invented by
+Zamboni. "It differs from the common hydro-electric batteries principally
+in this, that the presence of the electromotive liquid is dispensed with,
+and that in its place is substituted some moist substance of low
+conducting power, generally paper. The electromotors in these piles are
+composed for the most part of Dutch gold (copper) and silver (zinc) paper
+pressed one on the other, with their paper sides together, out of which
+discs are cut with a diameter of from a quarter of an inch to an inch.
+More powerful pairs of plates may be obtained by using only the silver
+paper and smearing its paper side with a thin coat of honey, on which some
+finely pulverized peroxide of manganese has been sprinkled, and all the
+sides similarly coated are presented one way. Powerful pairs of plates may
+also be made by pasting pure gold leaf on the paper side of zinc-paper.
+These plates are then to be arranged, just as in the ordinary voltaic
+pile, one above the other, so that the similar metallic surfaces may all
+lie one way; press them tightly together; tie them with pretty stout silk
+threads, and press them into a glass tube of convenient size. The metal
+rims of the tubes, which must be well connected with the outermost pairs
+of plates, form the poles of the pile, the negative pole being in the
+extreme zinc surface, and the positive in the extreme copper or manganese
+surface.
+
+"The electromotive energy called into action in these dry piles is less
+than that excited in the moist or hydro-electric piles, principally on
+account of the imperfect conduction of the paper. The accumulation of
+electricity at their poles also goes on less rapidly, and consequently the
+electrical tension continues for a long while unaltered; whereas, in all
+moist piles, even in the most constant of them, the tension is maintained,
+comparatively speaking, for but a short time, on account of the chemical
+action and decomposition of the electromotive fluid--causes of disturbance
+which do not exist in the dry pile."[15]
+
+
+=135. Thomson's Electrometer.=--Professor W. Thomson, of Glasgow, has
+devised an atmospheric electrometer, which is likely to become eminently
+successful, in the hands of skilful observers. It is mainly a torsion
+balance combined with a Leyden-jar. The index is an aluminium needle
+strung on a fine platinum wire, passing through its centre of gravity, and
+stretched firmly between two points. The needle and wire are carefully
+insulated from the greater part of the instrument, but are in metallic
+communication with two small plates fixed beside the two ends of the
+needle, and termed the repelling plates. A second pair of larger plates
+face the repelling plates, on the opposite side of the needle, but
+considerably farther from it. These plates are in connection with the
+inner coating of a Leyden-jar, and are termed the attracting plates. The
+whole instrument is enclosed in a metal cage, to protect the glass
+Leyden-jar and the delicate needle.
+
+The Leyden-jar should be charged when the instrument is used. Its effect
+is two-fold: it increases greatly the sensibility of the instrument, and
+enables the observer to distinguish between positive and negative
+electrification.
+
+The air inside the jar is kept dry by pumice-stone, slightly moistened
+with sulphuric acid; by which means very perfect insulation is maintained.
+
+Electrodes, or terminals, are brought outside the instrument, by which the
+Leyden-jar can be charged, and the needle system connected with the body,
+the electric state of which is to be tested.
+
+For the purpose of testing the electric state of the atmosphere, the
+instrument is provided with a conductor and support for a burning match,
+or, preferably, with an arrangement termed a water-dropping collector; by
+either of which means the electricity of the air is conveyed to the needle
+system.
+
+The needle abuts upon the repelling plates when not influenced by
+electricity, in which position it is at zero. It can always be brought
+back to zero by a torsion-head, turning one end of the platinum wire, but
+insulated from it, and provided with a graduated circle, so that the
+magnitude of the arc, that the torsion-head is moved through to bring the
+needle to zero, measures the force tending to deflect it.
+
+The action of the instrument is as follows:--The Leyden-jar is to be
+highly charged, say negatively; and the repelling plates are to be
+connected with the earth. The needle will then be deflected against a
+stop, under the combined influence of attraction from the Leyden-jar, or
+attracting plates, and repulsion from the repelling plates due to the
+positive charge induced on the needle and its plates by the Leyden-jar
+plates. The platinum wire must then be turned round by the torsion-head so
+as to bring back the needle to zero; and the number of degrees of torsion
+required will measure the force with which the needle is attracted. Next,
+let the needle plates be disconnected from the earth, and connected with
+the insulated body, the electric state of which is to be tested. In
+testing the atmosphere, the conductor and lighted match, or water-dropping
+apparatus, must be applied.
+
+If the electricity of the body be positive, it will augment the positive
+charge in the needle plates, induced by the Leyden-jar plates; and
+consequently the needle will be more deflected than by the action of the
+jar alone. If the electricity of the body be negative, it will tend to
+neutralize the positive charge; and the needle will be less deflected.
+Hence the kind of electricity present in the air becomes at once apparent,
+without the necessity of an experimental test. The platinum wire must then
+be turned till the needle is brought to zero, and the number of degrees
+observed; which is a measure of the intensity of the electrification.
+
+Any loss of charge from the Leyden-jar which may from time to time occur,
+reducing the sensibility inconveniently, may be made good by additions
+from a small electrophorus which accompanies the instrument.[16]
+
+The instrument may be made self-recording by the aid of clockwork and
+photography. To effect this, a clock gives motion to a cylinder, upon
+which photographic paper is mounted. The needle of the electrometer is
+made to carry a small reflector; and rays from a properly adjusted source
+of light are thrown by the reflector, through a small opening, upon the
+photographic paper. It is evident, that as the cylinder revolves, a trace
+will be left upon the paper, showing the magnitude of, and variations in,
+the deflection of the needle.
+
+
+=136. Fundamental Facts regarding Atmospheric Electricity.=--The _general_
+electrical condition of the atmosphere is _positive_ in relation to the
+surface of the earth and ocean, becoming more and more positive as the
+altitude increases. When the sky is overcast, and the clouds are moving in
+different directions, it is subject to great and sudden variations,
+changing rapidly from positive to negative, and the reverse. During fog,
+rain, hail, sleet, snow, and thunderstorms, the electrical state of the
+air undergoes many variations. The intensity of the electricity increases
+with hot weather following a series of wet days, or of wet weather coming
+after a continuance of dry days. The atmospheric electricity, in fact,
+seems to depend for its intensity and kind upon the direction and
+character of the prevailing wind, under ordinary circumstances. It has an
+annual and a diurnal variation. There is a greater diurnal change of
+tension in winter than in summer. By comparing observations from month to
+month, a gradual increase of tension is perceived from July to February,
+and a decrease from February to July. The intensity seems to vary with the
+temperature. The diurnal variation exhibits two periods of greatest and
+two of least intensity. In summer, the _maxima_ occur about 10 a.m. and 10
+p.m.; the _minima_ about 2 a.m. and noon. In winter, the _maxima_ take
+place near 10 a.m. and 8 p.m.; the _minima_ near 4 a.m. and 4 p.m.
+
+The researches of Saussure, Beccaria, Crosse, Qu�t�let, Thompson, and
+FitzRoy have tended to show that during the prevalence of polar currents
+of air positive electricity is developed, and becomes more or less active
+according to the greater or less coldness and strength of wind; but with
+winds from the equatorial direction there is little evidence of sensitive
+electricity, and when observable, it is of the negative kind. Storms and
+gales of wind are generally attended, in places, with lightning and
+thunder; and as the former are very often attributed to the conflict of
+polar and equatorial winds, the difference of the electric tension of
+these winds may account for the latter phenomena. It is not our intention
+to enter upon the general consideration of thunderstorms; the facts which
+we have given may be of service to the young observer; and finally, as it
+is interesting to be able to judge of the locality of a thunderstorm, the
+following simple rule will be of service, and sufficiently accurate:--Note
+by a second's watch the number of seconds which elapse from the sight of
+the lightning to the commencement of the thunder; divide them by five, and
+the quotient will be the distance in miles. Thus, if thunder is heard ten
+seconds after the lightning was seen, the distance from the seat of the
+storm will be about two miles. The interval between the flash and the roll
+has seldom been observed greater than seventy-two seconds.
+
+
+=137. Lightning Conductors.=--"The line of danger, whether from the
+burning or lifting power of lightning, is the line of strong and
+obstructed currents of air, of the greatest aerial friction."[17] Trees,
+church spires, wind-mills and other tall structures, obstruct the aerial
+currents, and hence their exposure to danger. The highest objects of the
+landscape, especially those that are nearest the thunder cloud, will
+receive the lightning stroke. The more elevated the object, the more
+likely is it to be struck. Of two or more objects, equally tall and near,
+the lightning is invariably found to select the best conductor of
+electricity, and even to make a circuitous path to get to it. Hence the
+application and evident advantage of metallic rods, called _lightning
+conductors_, attached to buildings and ships. A lightning conductor should
+be pointed at top, and extend some feet above the highest part of the
+edifice, or mast. It should be made of copper, which is a better
+conducting medium than iron, and more durable, being less corrosive. It
+must be unbroken throughout its length, and extend to the bottom of the
+building, and even some distance into the ground, so as to conduct the
+electricity into a well or moist soil. If it be connected with the lead
+and iron work in the structure of the house, it will be all the better, as
+affording a larger surface, and a readier means of exit for the fluid. In
+a ship, the lower end of the conductor should be led into communication
+with the hull, if of iron, and with the copper sheathing, if a wooden
+vessel; so that, spread over a large surface, it may escape more readily
+to the water.
+
+
+=138. Precautions against Lightning.=--Experience seems to warrant the
+assumption that any building or ship, fitted with a substantial lightning
+conductor, is safe from danger during a thunderstorm. Should a house or
+vessel be undefended by a conductor, it may be advisable to adopt a few
+precautions against danger. In a house, the fire-place should be avoided,
+because the lightning may enter by the chimney, its sooty lining being a
+good conductor. "Through chimneys, lightning has a way into most houses;
+and therefore, it is wise, by opening doors or windows, to give it a way
+out. Wherever the aerial current is fiercest, there the danger is
+greatest; and if we kept out of the way of currents or draughts, we keep
+out of the way of the lightning."[18] Lightning evinces as it were a
+preference for metallic substances, and will fly from place to place, even
+out of the direct line of its passage to the earth, to enter such bodies.
+It is therefore well to avoid, as much as possible, gildings, silvered
+mirrors, and articles of metal. The best place is perhaps the middle of
+the room, unless a draught passes, or a metallic lamp or chandelier should
+be hanging from the ceiling. The neighbourhood of bad conductors, such as
+glass windows, not being open, and on a thick bed of mattrasses, are safe
+places. The quality of trees as lightning conductors is considered to
+depend upon their height and moisture, those which are taller and
+relatively more humid being struck in preference to their fellows;
+therefore, it is unwise to seek shelter under tall and wet trees during a
+thunderstorm. In the absence of any other shelter, it would be better to
+lie down on the ground.
+
+
+
+
+CHAPTER XV.
+
+OZONE AND ITS INDICATORS.
+
+
+=139. Nature of Ozone.=--During the action of a powerful electric machine,
+and in the decomposition of water by the voltaic battery, a peculiar odour
+is perceptible, which is considered to arise from the generation of a
+substance to which the term ozone has been given, on account of its having
+been first detected by smell, which, for a long time after its discovery,
+was its only known characteristic. A similar odour is evolved by the
+influence of phosphorus on moist air, and in other cases of slow
+combustion. It is also traceable, by the smell, in air,--where a flash of
+lightning has passed immediately before. Afterwards it was established
+that the same element possessed an oxidising property. It was found to be
+liberated at the oxygen electrode when water was decomposed by an electric
+current; and has been regarded by some chemists as what is termed an
+_allotropic_ form of oxygen, while others speak of it as oxygen in the
+_nascent_ state, and some even regard it as intimately related to
+chlorine. So various are the existing notions of the nature of this
+obscure agent.
+
+Its oxidising property affords a ready means for its detection, even when
+the sense of smell completely fails. The methods of noting the presence
+and measuring the amount of ozone present in the air, are very simple;
+being the free exposure to the air, defended from rain and the direct rays
+of the sun, of prepared test-papers. There are two kinds of test-papers.
+One kind was invented by Dr. Schonbein, the original discoverer of ozone;
+and the other, which is more generally approved, by Dr. Moffat.
+
+
+=140. Schonbein's Ozonometer= consists of strips of paper, previously
+saturated with a solution of starch and iodide of potassium, and dried.
+The papers are suspended in a box, or otherwise properly exposed to the
+air, for a given interval, as twenty-four hours. The presence of ozone is
+shown by the test-paper acquiring a purple tint when momentarily immersed
+in water. The amount is estimated by the depth of the tint, according to a
+scale of ten tints furnished for the purpose, which are distinguished by
+numbers from 1 to 10. The ozone decomposes the compound which iodine forms
+with hydrogen, and, it is presumed, combines as oxygen with hydrogen,
+while the iodine unites with the starch, giving the blue colour when
+moist.
+
+
+=141. Dr. Moffat's Ozonometer= consists of papers prepared in a somewhat
+similar manner to Schonbein's; but they do not require immersion in water.
+The presence of ozone is shown by a brown tint, and the amount by the
+depth of tint according to a scale of ten tints, which is furnished with
+each box of the papers.
+
+Moffat's have the advantage of preserving their tint for years, if kept in
+the dark, or between the leaves of a book; and are simpler to use.
+
+
+[Illustration: Fig. 92.]
+
+=142. Sir James Clark's Ozone Cage= (fig. 92), consists of two cylinders
+of very fine wire gauze, one fitting into the other; the wire gauze being
+of such a fineness as to permit the free ingress of air, at the same time
+that it shuts out all light that would act injuriously on the test-paper,
+which is suspended by a clip or hook attached to the upper part of the
+inner cylinder.
+
+
+=143. Distribution and Effects of Ozone.=--Mr. Glaisher has found that
+"the amount of ozone at stations of low elevation is small; at stations of
+high elevation, it is almost always present; and at other and intermediate
+stations, it is generally so. The presence and amount of ozone would seem
+to be graduated by the elevation, and to increase from the lowest to the
+highest ground. The amount of ozone is less in towns than in the open
+country at the same elevation; and less at inland than at sea-side
+stations." It seems to abound most with winds from the sea, and to be most
+prevalent where the air is considered the purest and most salubrious. This
+may seem, says Admiral FitzRoy, in _The Weather Book_, to point to a
+connection between ozone and chlorine gas, which is in and over sea-water,
+and which _must_ be brought by any wind that blows from the sea. It
+prevails more over the ocean and near it than over land, especially land
+remote from the sea; and, says the Admiral, it affects the gastric juice,
+improves digestion, and has a tanning effect. Dr. Daubeny, in his
+_Lectures on Climate_, writes: "Its presence must have a sensible
+influence upon the purity of the air, by removing from it foetid and
+injurious organic effluvia. It is also quite possible that ozone may play
+an important part in regulating the functions of the vegetable kingdom
+likewise; and although it would be premature at present to speculate upon
+its specific office, yet, for this reason alone, it may be well to note
+the fact of its frequency, in conjunction with the different phases which
+vegetation assumes, persuaded that no principle can be generally diffused
+throughout nature, as appears to be the case, with this, without having
+some important and appropriate use assigned for it to fulfil."
+
+
+=144. Registering Ozonometer.=--Dr. E. Lancaster has contrived an
+ozonometer, the object of which is to secure the constant registration of
+ozone, so that the varying quantities present in the atmosphere may be
+detected and registered. For this purpose, an inch of ozone paper passes
+in each hour, by clock-work, beneath an opening in the cover of the
+instrument.
+
+
+
+
+CHAPTER XVI.
+
+INSTRUMENTS NOT DESCRIBED IN THE PRECEDING CHAPTERS.
+
+
+=145. Chemical Weather Glass.=--This curious instrument appears to have
+been invented more than a hundred years ago, but the original maker is not
+known. It is simply a glass vial about ten inches long and three quarters
+of an inch in diameter, which is nearly filled, and hermetically sealed,
+with the following mixture:--Two drachms of camphor, half a drachm of
+nitrate of potassium, half a drachm of chlorate of ammonium, dissolved in
+about two fluid ounces of absolute alcohol mixed with two ounces of
+distilled water. All the ingredients should be as pure as possible, and
+each vial filled separately. When the instruments are made in numbers and
+filled from a common mixture, some get more than the due proportion of the
+solid ingredients, and consequently such glasses do not exhibit that
+uniformity of appearance and changes, that undoubtedly should accompany
+similar influencing circumstances. It is in consequence of a want of
+precision and fixed principle of manufacture, that these interesting
+instruments are not properly appreciated, and more generally used.
+
+The glass should be kept quite undisturbed, exposed to the north, and
+shaded from the sun. Camphor is soluble in alcohol, but not in water,
+while both water and alcohol have different solvent powers, according to
+the temperature; hence, the solid ingredients being in excess for certain
+conditions of solution, depending upon temperature chiefly, and perhaps
+electricity and the action of light also, appear as crystals and disappear
+with the various changes that occur in the weather.
+
+The various appearances thus presented in the menstruum have been inferred
+to prognosticate atmospheric changes. The following rules have been
+deduced from careful study of the glass and weather:--
+
+1. During cold weather, beautiful fern-like or feathery crystallization is
+developed at the top, and sometimes even throughout the liquid. This is
+the normal state of the glass during winter. The crystallization increases
+with the coldness; and if the structure grows downward, the cold will
+continue.
+
+2. During warm and serene weather, the crystals dissolve, the upper and
+greater part of the liquid becoming perfectly clear. This is the normal
+state of the glass during summer. The less amount of crystallization, that
+is, the greater the clear portion of the liquid (for there is always some
+of the composition visible at the bottom), the greater the probability of
+continued fine dry weather.
+
+3. When the upper portion is clear, and flakes of the composition rise to
+the top and aggregate, it is a sign of increasing wind and stormy
+weather.
+
+4. In cold weather, if the top of the liquid becomes thick and cloudy, it
+denotes approaching rain.
+
+5. In warm weather, if small crystals rise in the liquid, which still
+maintains its clearness, rain may be expected.
+
+6. Sharpness in the points and features of the fern-like structure of the
+crystals, is a sign of fine weather; but when they begin to break up, and
+are badly defined, unsettled weather may be expected.
+
+Admiral FitzRoy, in _The Weather Book_, writes of this instrument as
+follows:--"Since 1825, we have generally had some of these glasses, as
+curiosities rather than otherwise; for nothing certain could be made of
+their variations until lately, when it was fairly demonstrated that if
+fixed undisturbed in free air, not exposed to radiation, fire, or sun, but
+in the ordinary light of a well-ventilated room, or, _preferably_, in the
+outer air, the chemical mixture in a so-called storm-glass varies in
+character with the _direction_ of the wind--not its force, _specially_
+(though it _may_ so vary in _appearance_, only from another cause,
+_electrical tension_).
+
+"As the atmospheric current veers toward, comes from, or is only
+_approaching_ from the polar direction, this chemical mixture--if closely,
+even microscopically watched--is found to grow like _fir_, _yew_, fern
+leaves, or hoar-frost--or like crystallizations.
+
+"As the wind, or great body of air, tends more from the _opposite_
+quarter, the lines or spikes--all regular, hard, or crisp
+features--gradually diminish, till they vanish.
+
+"Before, and in a continued southerly wind, the mixture sinks slowly
+downward in the vial, till it becomes shapeless, like melting white sugar.
+
+"Before, or during the continuance of a northerly wind (polar current),
+the crystallizations are beautiful (if the mixture is correct, the glass a
+_fixture_, and duly _placed_); but the least motion of the liquid disturbs
+them.
+
+"When the main currents meet, and turn _toward the west_, making
+_easterly_ winds, stars are more or less numerous, and the liquid dull, or
+less clear. When, and while they _combine by the west_, making westerly
+winds, the liquid is clear, and the crystallization well-defined, without
+loose stars.
+
+"While _any hard_ or _crisp_ features are visible below, above, or at the
+top of the liquid (where they form for polar winds), there is _plus_
+electricity in the air; a _mixture_ of polar current co-existing _in that
+locality_ with the opposite, or southerly.
+
+"When nothing but soft, melting, sugary substance is seen, the atmospheric
+current (feeble or strong as it may be) is southerly with _minus_
+electricity, unmixed with, and _uninfluenced_ by, the contrary wind.
+
+"Repeated trials with a delicate galvanometer, applied to measure electric
+tension in the air, have proved these facts, which are now found useful
+for aiding, with the barometer and thermometer, in forecasting weather.
+
+"Temperature affects the mixture much, but not solely; as many comparisons
+of winter with summer changes of temperature have fully proved.
+
+"A confused appearance of the mixture, with flaky spots, or stars, in
+motion, and less clearness of the liquid, indicates south-easterly wind,
+probably strong to a gale.
+
+"Clearness of the liquid, with more or less perfect crystallizations,
+accompanies a combination, or a contest, of the main currents, by the
+_west_, and very remarkable these differences are,--the results of these
+air currents acting on each other _from_ eastward, or from an entirely
+opposite direction, the _west_.
+
+"The glass should be wiped clean now and then,--and once or twice a year
+the mixture should be disturbed, by inverting and gently shaking the glass
+vial."
+
+
+[Illustration: Fig. 93.]
+
+=146. Leslie's Differential Thermometer.=--A glass tube having a large
+bulb at each extremity, and bent twice at right angles, as represented in
+figure 93, containing strong sulphuric acid tinged with carmine, and
+supported at the centre by a wooden stand, constitutes the differential
+thermometer as invented by Professor Leslie. The instrument is designed to
+exhibit and measure small differences of temperature. Each leg of the
+instrument is usually from three to six inches long, and the balls are
+about four inches apart. The calibre of the legs is about 1/50 inch, not
+more; the other part of the tube may be wider. The tube is filled with the
+liquid, the bulbs contain air. When both bulbs are heated alike, each
+scale indicates zero. The scale is divided so that the space between the
+freezing and the boiling-points of water is equal to 1,000 parts. When one
+bulb is heated more than the other, the difference of temperature is
+delicately shown by the descent of the coloured fluid from the heated
+ball. It is uninfluenced by changes in the temperature of the atmosphere;
+hence it is admirably adapted for experiments of radiant heat. The theory
+of the instrument is that gases expand equally for uniform increments of
+heat.
+
+
+=147. Rumford's Differential Thermometer= differs from that just described
+in simply containing only a small bubble of liquid, which lies in the
+centre of the tube, when both bulbs are similarly influenced. The bulbs
+and other parts of the tube contain air. When one bulb is more heated than
+the other, the bubble moves towards the one less heated; and the scale
+attached to the horizontal part of the tube affords a measurement of the
+difference of temperature.
+
+
+[Illustration: Fig. 94.]
+
+=148. Glaisher's Thermometer Stand.=--The thermometer stand consists of a
+horizontal board as a base, of a vertical board projecting upwards from
+one edge of the horizontal one, and of two parallel inclined boards,
+separated from each other by blocks of three inches in thickness,
+connected at the top with the vertical, and at the bottom with the
+horizontal board, and the air passes freely about and between them all. To
+the top of the inclined boards is connected a small projecting roof to
+prevent the rain falling on the bulbs of the instrument, which are carried
+on the face of the vertical board, with their bulbs projecting below it,
+so that the air plays freely on the bulbs from all sides. The whole frame
+revolves on an upright post firmly fixed to the ground, as shown in the
+engraving, fig. 94; and in use, the inclined side is always turned towards
+the sun.
+
+
+=149. Thermometer Screen, for use at Sea.=--This screen, or shade, was
+designed by Admiral FitzRoy, and has been in use for several years on
+board H.M. vessels and many merchant-ships. It is about twenty-four inches
+long by twelve wide and eight deep; having lattice-work sides, door, and
+bottom; with perforation also at top, so contrived that the air has free
+access to the interior, while the direct rays of the sun, rain, and sea
+spray are effectually excluded from the thermometers mounted inside. There
+is ample space for two thermometers placed side by side on brackets, at
+least three inches from each other or any part of the exterior of the
+screen. One thermometer should be fitted up as a "wet bulb" (see p. 105).
+A small vessel of water can easily be fixed inside the screen so as to
+retain its position and contents under the usual motions of the ship; and
+by means of a piece of cotton-wick, or muslin rag tied round the bulb of
+the thermometer and trailing into the cup of water, keep the bulb
+constantly moist.
+
+Self-registering thermometers should be protected by a similar screen. It
+has been found that thermometric observations made at sea are not valuable
+for scientific purposes unless the instruments have been duly protected by
+such a screen.
+
+
+=150. Anemoscope=, or Portable Wind Vane for travellers, with compass, bar
+needle, &c., shows the direct course of the wind to half a point of the
+compass.
+
+
+[Illustration: Fig. 95.]
+
+=151. Evaporating Dish, or Gauge= (fig. 95), for showing the amount of
+evaporation from the earth's surface. This gauge consists of a brass
+vessel, the area or evaporating surface of which is accurately determined;
+and also a glass cylindrical measure, graduated into inches, tenths, and
+hundredths of inches. In use, the evaporating gauge is nearly filled with
+water, the quantity having been previously measured by means of the glass
+cylinder; it is then placed out of doors, freely exposed to the action of
+the atmosphere; after exposure, the water is again measured, and the
+difference between the first and second measurement shows the amount of
+evaporation that has taken place. If rain has fallen during the exposure
+of the gauge, the quantity collected by it must be deducted from the
+measured quantity; the amount is shown by the quantity of rain collected
+in the rain gauge. The wire cage round the gauge is to prevent animals,
+birds, &c., from drinking the water.
+
+
+=152. Dr. Babington's Atmidometer=, or instrument for measuring the
+evaporation from water, _ice or snow_, consists of an oblong hollow bulb
+of glass or copper, beneath which and communicating with it by a
+contracted neck is a second globular bulb, duly weighted with mercury or
+shot. The upper bulb is surmounted by a small glass or metal stem, having
+a scale graduated to grains and half-grains; on the top of which is fixed
+horizontally a shallow metal pan. The bulbs are immersed in a vessel of
+water having a circular hole in the cover through which the stem rises.
+Distilled water is then gradually poured into the pan above, until the
+zero of the stem sinks to a level with the cover of the vessel. Thus
+adjusted, as the water in the pan evaporates, the stem ascends, and the
+amount of evaporation is indicated in grains. This instrument affords a
+means of measuring evaporation from _ice or snow_. An adjustment for
+temperature is necessary.
+
+
+=153. Cloud Reflector.=--At the International Exhibition 1862, Mr. J. T.
+Goddard exhibited a cloud mirror, for ascertaining the direction in which
+the clouds are moving.
+
+The mirror is laid on a horizontal support near a window, and fastened so
+that the point marked north may coincide with the south point of the
+horizon,--the several points will consequently be reversed. The edge of a
+conspicuous cloud is brought to the centre of the mirror, and the observer
+keeps perfectly still until it passes off at the margin, where the true
+point of the horizon _from which_ the clouds are coming can be read off.
+
+
+=154. Sunshine Recorder.=--Mr. Goddard also exhibited an instrument which
+he calls by this name. It works by letting the sun's rays pass through a
+narrow slit, and fall on photographic paper wound round a barrel moved by
+clock-work; the paper being changed daily, and the photographic impression
+developed and fixed in the usual manner.[19]
+
+
+155. SET OF PORTABLE INSTRUMENTS.
+
+In a small box, 8 in. by 8 in. by 4 in., a complete set of meteorological
+instruments have been packed. The lid of the box, by an ingenious
+arrangement, is made to take off and hang up; on it are permanently fixed
+for observation, a maximum and minimum, and a pair of dry and wet bulb
+thermometers. The interior of the box contains a maximum thermometer in
+vacuo for solar radiation, and a minimum for terrestrial purposes, one of
+Negretti and Zambra's small pocket aneroid barometers, pedometer for
+measuring distances, pocket compass, clinometer, and lastly a rain gauge.
+This latter instrument consists of an accurately turned brass ring having
+an india rubber body fastened to it to receive the rain, which is measured
+off by a small graduated glass, also contained in the box. Gentlemen
+travelling will find this compact observatory all that can be desired for
+meteorological observations.
+
+
+156. IMPLEMENTS.
+
+The practical meteorologist will find the following articles very useful,
+if not necessary. They scarcely require description; an enumeration will
+therefore suffice:--_Weather Diagrams_, or prepared printed and ruled
+forms, whereon to exhibit graphically the readings of the various
+instruments to render their indications useful in foretelling weather,
+&c.;--_Meteorological Registers_, or Record Books, for recording all
+observations, and the deductions;--_Cloud Pictures_, by which the clouds
+can be readily referred to their particular classification, very necessary
+to the inexperienced and learners;--Cyclone Glasses, or Horns, outline
+Maps with Wind-markers, are also useful, especially in forecasting
+weather.
+
+
+157. HYDROMETER.
+
+A simple kind of hydrometer is very much used at sea, as "a sea-water
+test;" and as the observations are usually recorded in a meteorological
+register or the ship's log-book, it may not be altogether out of place to
+give a description of it here.
+
+[Illustration: Fig. 96.]
+
+[Illustration: Fig. 97.]
+
+It is constructed of glass. If made of brass, the corrosive action of
+salt-water soon renders the instrument erroneous in its indications. The
+shapes usually given to the instruments are shown in figs. 96 and 97. A
+globular bulb is blown, and partly filled with mercury or small shot, to
+make the instrument float steadily in a vertical position. From the neck
+of the bulb the glass is expanded into an oval or a cylindrical shape, to
+give the instrument sufficient volume for flotation; finally, it is
+tapered off to a narrow upright stem which encloses an ivory scale, and is
+closed at the top. The divisions on the scale read downward, so as to
+measure the length of the stem which stands above the surface of any
+liquid in which the hydrometer is floated. The denser the fluid, the
+higher will the instrument rise; the rarer, the lower it will sink.
+
+The indications depend upon the hydrostatic principle, that floating
+bodies displace a quantity of the fluid which sustains them equal to their
+own weight. According, therefore, as the specific gravities of fluids
+differ from each other, so will vary the quantities of the fluids
+displaced by the same body when floated successively in each.
+
+The specific gravity of distilled water, at the temperature of 62� _F_,
+being taken as unity, the depth to which the instrument sinks when gently
+immersed in such water is the zero of the scale. The graduations extend
+from 0 to 40; the latter being the mark which will be level with the
+surface when the instrument is placed in water, the specific gravity of
+which is 1�040. In recording observations, the last two figures
+only--being the figures on the scale--are written down. Sea-water usually
+ranges from 1�020 to 1�036.
+
+A small tin, copper, or glass cylinder is useful for containing the water
+to be tested. It should be wider than the hydrometer, and always filled to
+the brim. If fitted to a stand, which is supported by gimbals, it will be
+very convenient. Water in a bucket, basin, or other wide vessel, acquires
+motion at sea, and the eye cannot be brought low enough (on account of the
+edges) to read off the scale accurately.
+
+Errors of observation may occur with the hydrometer, if it be put into
+water without being clean, or without being carefully wiped. The
+instrument is extremely accurate if correctly used. It should be kept free
+from contact with the sides of the vessel; and all dust, smears, or
+greasiness, should be scrupulously avoided, by carefully wiping it with a
+clean cloth before and after use.
+
+Whenever the temperature of the water tested differs from 62�, a
+correction to the reading is necessary, for the expansion or contraction
+of the glass, as well as the water itself, in order to reduce all
+observations to one generally adopted standard.
+
+Negretti and Zambra's hydrometer, with thermometer in the stem, shows the
+density and temperature in one instrument.
+
+For the following Tables we are indebted to the kindness of Admiral
+FitzRoy:--
+
+TABLE for reducing observations made with a BRASS HYDROMETER, assuming the
+linear expansion of brass to be 0�000009555 for 1� F. The correction is
+additive for all temperatures above 62�, and subtractive for temperatures
+below 62�.
+
+  +----------------------------------------------------------------------+
+  |_t�_|Correction.||_t�_|Correction.||_t�_|Correction.||_t�_|Correction.|
+  |----+-----------++----+-----------++----+-----------++----+-----------|
+  | 32 | -0�0014   || 48 | -0�0010   || 64 | +0�0002   || 80 | +0�0020   |
+  | 33 |   �0014   || 49 |   �0009   || 65 |   �0003   || 81 |   �0021   |
+  | 34 |   �0014   || 50 |   �0009   || 66 |   �0004   || 82 |   �0023   |
+  | 35 |   �0014   || 51 | -0�0008   || 67 |   �0005   || 83 |   �0024   |
+  | 36 |   �0014   || 52 |   �0008   || 68 | +0�0006   || 84 |   �0026   |
+  | 37 |   �0014   || 53 |   �0007   || 69 |   �0007   || 85 | +0�0027   |
+  | 38 | -0�0014   || 54 |   �0006   || 70 |   �0008   || 86 |   �0029   |
+  | 39 |   �0013   || 55 |   �0006   || 71 |   �0009   || 87 |   �0030   |
+  | 40 |   �0013   || 56 | -0�0005   || 72 |   �0010   || 88 |   �0032   |
+  | 41 |   �0013   || 57 |   �0004   || 73 |   �0011   || 89 |   �0033   |
+  | 42 |   �0013   || 58 |   �0003   || 74 | +0�0013   || 90 | +0�0035   |
+  | 43 |   �0012   || 59 |   �0003   || 75 |   �0014   || 91 |   �0036   |
+  | 44 | -0�0012   || 60 |   �0002   || 76 |   �0015   || 92 |   �0038   |
+  | 45 |   �0011   || 61 | -0�0001   || 77 |   �0016   || 93 |   �0040   |
+  | 46 |   �0011   || 62 |  0�0000   || 78 |   �0018   || 94 |   �0041   |
+  | 47 | -0�0010   || 63 | +0�0001   || 79 | +0�0019   || 95 | +0�0043   |
+  +----------------------------------------------------------------------+
+
+TABLE for reducing observations made with a GLASS HYDROMETER, assuming the
+linear expansion of glass to be 0�00000463 for 1� F. The correction is
+additive for temperatures above 62�, and subtractive for temperatures
+below 62�.
+
+  +----------------------------------------------------------------------+
+  |_t�_|Correction.||_t�_|Correction.||_t�_|Correction.||_t�_|Correction.|
+  |----+-----------++----+-----------++----+-----------++----+-----------|
+  | 32 | -0�0019   || 48 | -0�0012   || 64 | +0�0002   || 80 | +0�0023   |
+  | 33 |   �0019   || 49 |   �0011   || 65 |   �0003   || 81 |   �0024   |
+  | 34 |   �0018   || 50 |   �0011   || 66 |   �0004   || 82 |   �0026   |
+  | 35 |   �0018   || 51 | -0�0010   || 67 |   �0005   || 83 |   �0027   |
+  | 36 |   �0018   || 52 |   �0009   || 68 | +0�0007   || 84 |   �0029   |
+  | 37 |   �0017   || 53 |   �0008   || 69 |   �0008   || 85 | +0�0031   |
+  | 38 | -0�0017   || 54 |   �0008   || 70 |   �0009   || 86 |   �0032   |
+  | 39 |   �0017   || 55 |   �0007   || 71 |   �0010   || 87 |   �0034   |
+  | 40 |   �0016   || 56 | -0�0006   || 72 |   �0012   || 88 |   �0036   |
+  | 41 |   �0016   || 57 |   �0005   || 73 |   �0013   || 89 |   �0037   |
+  | 42 |   �0015   || 58 |   �0004   || 74 | +0�0014   || 90 | +0�0039   |
+  | 43 |   �0015   || 59 |   �0003   || 75 |   �0016   || 91 |   �0041   |
+  | 44 | -0�0014   || 60 |   �0002   || 76 |   �0017   || 92 |   �0042   |
+  | 45 |   �0014   || 61 | -0�0001   || 77 |   �0018   || 93 |   �0044   |
+  | 46 |   �0013   || 62 |  0�0000   || 78 |   �0020   || 94 |   �0046   |
+  | 47 | -0�0013   || 63 | +0�0001   || 79 | +0�0021   || 95 | +0�0048   |
+  +----------------------------------------------------------------------+
+
+
+158. NEWMAN'S SELF-REGISTERING TIDE-GAUGE.
+
+At places where the phenomena of the tides are of much maritime
+importance, a continuous series of observations upon the rise and fall,
+and times of change, is essentially necessary as a basis for the
+construction of good tide tables; and as such observations should also be
+accompanied with the registration of atmospheric phenomena, we have no
+hesitation in inserting a description of an accurate self-registering
+tide-gauge.
+
+The tide-gauge, as shown in the illustration, consists of a cylinder, _A_,
+which is made to revolve on its axis once in twenty-four hours by the
+action of the clock, _B_. A chain, to which is attached the float, _D_,
+passes over the wheel, _C_, and on the axis of this wheel, _C_ (in about
+the middle of it) is a small toothed wheel, placed so as to be in contact
+with a larger toothed wheel carrying a cylinder, _E_, over which passes
+another smaller chain. This chain, passing along the upper surface of the
+cylinder, _A_, and round a second cylinder, _F_, at its further end, is
+acted on by a spring so as to be kept in a constant state of tension. In
+the middle of this chain a small tube is fixed for carrying a pencil,
+which, being gently pressed down by means of a small weight on the top of
+it, performs the duty of marking on paper placed round the cylinder the
+progress of the rise or fall of the tide as the cylinder revolves, and as
+it is drawn by the chain forward or backward by the rise or fall of the
+float. The paper is prepared with lines equidistant from each other, to
+correspond with the hours of the clock, crossed by others showing the
+number of feet of rise and fall.
+
+[Illustration]
+
+The cylinder while in action revolves from left to right to a spectator
+facing the clock, and the pencil is carried horizontally along the top of
+the cylinder; and the large wheel being made to revolve by the rise and
+fall of the float, turns the wheel with the small cylinder, _E_,
+attached to it. If the tide is _falling_, the small chain is wound round
+the cylinder, _E_, and the pencil is drawn towards the large wheel; but if
+the tide is _rising_, the small chain is wound on to the cylinder, _F_, by
+means of the spring contained in it, which constantly keeps it in a state
+of tension. Thus, by means of the rise and fall of the tide, a lateral
+progress is given to the pencil, while the cylinder is made to revolve on
+its axis by the clock, so that a line is traced on the paper showing the
+exact state of the tide continuously, without further attention than is
+necessary to change the paper once every day, and to keep the pencil
+carefully pointed; or a metallic pencil may be used, which will require
+little, if any, attention.
+
+A good self-registering tide-gauge is a valuable and important acquisition
+wherever tidal observations are required, and the only perfectly efficient
+instrument of this kind is that invented by the late Mr. John Newman, of
+Regent Street, London. It is now in action in several parts of the world,
+silently and _faithfully_ performing its duty, requiring no other kind of
+attention than that of a few minutes daily, and thus admitting the
+employment of the person on any other service whose duty it would
+otherwise have been to have registered the tide. It has done much by its
+faithful records in contributing to the construction of good tide tables
+for many places; for those unavoidable defects dependent on merely
+watching the surface on a divided scale are set aside by it, all erroneous
+conclusions excluded, and a true delineation of Nature's own making is
+preserved by it for the theorist.
+
+
+
+
+ADDENDA.
+
+
+1. French barometers are graduated to millimetres. An English inch is
+equal to 25�39954 millimetres. Hence, 30 inches on the English barometer
+scales correspond to 762 millimetres on the French barometer scales.
+Conversions from one scale to another can be effected by the following
+formul�:--
+
+  (1) Inches = millimetres divided by         25�39954
+  (2) Millimetres = inches multiplied by      25�39954
+
+Of course, a table of equivalent values should be drawn up and employed,
+when a large number of observations are to be converted from one scale to
+the other.
+
+
+2. In Germany, barometers are sometimes graduated with old French inches
+and lines,--the vernier generally indicating the tenth of a line.
+
+OLD FRENCH LINEAL MEASURE.
+
+                                              English Inches.
+  1 douzi�me, or point                        =  0�0074
+  12 points              = 1 ligne            =  0�0888
+  12 lignes              = 1 pouce            =  1�065765
+  12 pouces              = 1 pied             = 12�7892
+  1 pied                 = 324�7 millimetres.
+
+"The Germans indicate inches by putting two accents after the number;
+lines, by putting three accents; 27" 3'''�85, means 27 inches 3 lines 85
+hundredths of a line; more frequently, they give the height in lines, and
+the preceding number becomes 327'''�85."--_Kaemtz._
+
+
+3. _Rule for finding Diameter of Bore of a Barometer Tube._
+
+"If the maker has not taken care to measure the interior diameter
+directly, it may be deduced from the exterior diameter. The exterior
+diameter is first measured by calipers, and, by deducting from this
+diameter 0�1 of an inch for tubes from �3 to �5 of an inch in external
+diameter, we have an approximation to the interior diameter of the
+tube."--_Kaemtz._
+
+
+4. WIND SCALES.
+
+  Sea Scale.       Wind.      Land Scale.
+  ----------       -----      -----------
+   0 to 3     =   Light     =   0 to 1
+   3 "  5     =   Moderate  =   1 "  2
+   5 "  7     =   Fresh     =   2 "  3
+   7 "  8     =   Strong    =   3 "  4
+   8 " 10     =   Heavy     =   4 "  5
+  10 " 12     =   Violent   =   5 "  6
+
+
+    Pressure in                  Velocity in
+      Pounds      (Land Scale).     Miles
+  (Avoirdupois)                   (Hourly).
+  -------------    ---------     -----------
+      1/2       =      1       =     10
+       5        =      2       =     32
+      10        =      3       =     45
+      21        =      4       =     65
+      26        =      5       =     72
+      32        =      6       =     80
+
+
+5. Letters to Denote the State of the Weather.
+
+  _b_ denotes blue sky, whether with clear or hazy atmosphere.
+  _c_    "    cloudy, that is detached opening clouds.
+  _d_    "    drizzling rain.
+  _f_    "    fog.
+  _h_    "    hail.
+  _l_    "    lightning.
+  _m_    "    misty, or hazy so as to interrupt the view.
+  _o_    "    overcast, gloomy, dull.
+  _p_    "    passing showers.
+  _q_    "    squally.
+  _r_    "    rain.
+  _s_    "    snow.
+  _t_    "    thunder.
+  _u_    "    ugly, threatening appearance of sky.
+  _v_    "    unusual visibility of distant objects.
+  _w_    "    wet, that is dew.
+
+A letter repeated denotes much, as _r r_, heavy rain; _f f_, dense fog;
+and a figure attached denotes duration in hours, as 14 _r_, 14 hours rain.
+
+By the combination of these letters, all the ordinary phenomena of the
+weather may be recorded with certainty and brevity.
+
+EXAMPLES.--_b c_, blue sky with less proportion of cloud. 2 _r r l l t_,
+heavy rain for two hours, with much lightning, and some thunder.
+
+The above methods of recording the force of wind and state of weather
+were originally proposed by Admiral Sir Francis Beaufort. They are now in
+general use at sea, and by many observers on land.
+
+
+6. Table of Expansion by Heat from 32� to 212� F.
+
+  Platinum              0�0008842  of the length.
+  Glass, Flint          0�0008117      "
+    "    with Lead      0�0008622      "
+  Brass                 0�0018708      "
+  Mercury               0�0180180      "
+  Water                 0�0433200, from 39� to 212�
+  Alcohol               0�1100       "  32� to 174�
+  Nitric Acid           0�1100
+  Sulphuric Acid        0�0600
+
+
+7. Table of Specific Gravity of Bodies at 32� F. except water, which is
+taken at 39��4.
+
+  Water                            1�000
+  Alcohol, pure                    0�791
+     "     proof                   0�916
+  Mercury                         13�596
+  Glass                      3 to  2�7
+  Brass                    7�8 to  8�54
+  Platinum                  21 to 22�00
+
+Weight of a cubic foot of water, at the temperature of comparison, 62�425
+lbs. avoirdupois.
+
+The pound avoirdupois contains 7,000 grains.
+
+Air is 813�67 times lighter than water.
+
+The linear expansions are the mean values of the results of various
+experimentalists. The specific gravities are as given in Professor
+Rankine's _Applied Mechanics_.
+
+
+8. Important Temperatures. Under the circumstances of--
+
+                                                        �
+  Water                          boiling at            212
+  Mercury                         boils at             660
+  Sulphuric Acid                     "                 590
+  Oil of Turpentine                  "                 560
+  Nitric Acid                        "                 242
+  Alcohol                            "                 174
+  A Saturated Solution of Salt       "                 218
+  Vital Heat                                            96
+  Olive Oil begins to solidify                          36
+  Fresh Water freezes                                   32
+  Sea Water freezes                                     28
+  Mercury freezes                                      -39
+
+
+9. TABLE OF METEOROLOGICAL ELEMENTS, FORMING EXPONENTS OF THE CLIMATE OF
+LONDON.
+
+  --------------------------------------------------------------------+
+  1841     |Mean Height of Barometer, reduced to 32� F., at the mean  |
+   to      |    sea-level.                                            |
+  1861.    |       +--------------------------------------------------|
+           |       |Mean Monthly Range of Barometer.                  |
+  Months.  |       |    +---------------------------------------------|
+           |       |    |Mean of all the Highest Temperatures.        |
+           |       |    |    +----------------------------------------|
+           |       |    |    |Mean of all the Lowest Temperatures.    |
+           |       |    |    |    +-----------------------------------|
+           |       |    |    |    |Mean Temperature.                  |
+           |       |    |    |    |    +------------------------------|
+           |       |    |    |    |    |Mean Temperature of Dew-point.|
+           |       |    |    |    |    |    +-------------------------|
+           |       |    |    |    |    |    |Mean Degree of Humidity. |
+           |       |    |    |    |    |    |  +----------------------|
+           |       |    |    |    |    |    |  |Mean Number of  Rainy |
+           |       |    |    |    |    |    |  |  Days.               |
+           |       |    |    |    |    |    |  |   +------------------|
+           |       |    |    |    |    |    |  |   |Average Rainfall. |
+           |       |    |    |    |    |    |  |   |    +-------------|
+           |       |    |    |    |    |    |  |   |    |Average      |
+           |       |    |    |    |    |    |  |   |    |Amount of    |
+           |       |    |    |    |    |    |  |   |    |Cloud (10=   |
+           |       |    |    |    |    |    |  |   |    | overcast).  |
+           |       |    |    |    |    |    |  |   |    |   +---------|
+           |       |    |    |    |    |    |  |   |    |   |Prevalent|
+           |       |    |    |    |    |    |  |   |    |   |Winds.   |
+  ---------+-------+----+----+----+----+----+--+---+----+---+---------+
+           |Inches.| In.| �  | �  | �  | �  |  |   | In.|   |         |
+  ---------+-------+----+----+----+----+----+--+---+----+---+---------+
+  January  | 29�932|1�44|43�2|33�7|38�3|35�4|89| 11| 1�8|7�7|W. to N. |
+           |       |    |    |    |    |    |  |   |    |   |         |
+  February | 29�962|1�22|44�7|33�2|38�4|34�4|85| 10| 1�6|7�4|S. to W. |
+           |       |    |    |    |    |    |  |   |    |   |         |
+  March    | 29�967|1�23|50�0|35�3|41�7|36�4|82| 10| 1�5|6�6|N. to E. |
+           |       |    |    |    |    |    |  |   |    |   |         |
+           |       |    |    |    |    |    |  |   |    |   |         |
+  April    | 29�907|1�06|56�8|38�6|46�3|39�9|79| 11| 1�8|6�1|N. to E. |
+           |       |    |    |    |    |    |  |   |    |   |         |
+  May      | 29�931|1�02|64�4|44�2|52�8|45�5|76| 11| 2�1|6�1|S. to W. |
+           |       |    |    |    |    |    |  |   |    |   |         |
+  June     | 29�960|0�89|71�2|50�2|59�2|50�8|74| 11| 1�9|6�1|W. to N. |
+           |       |    |    |    |    |    |  |   |    |   |         |
+  July     | 29�970|0�79|73�8|53�2|61�9|53�9|76| 11| 2�7|6�9|W. to N. |
+           |       |    |    |    |    |    |  |   |    |   |         |
+  August   | 29�954|0�97|72�8|53�4|61�3|54�1|77| 11| 2�4|6�5|W. to N. |
+           |       |    |    |    |    |    |  |   |    |   |         |
+  September| 29�997|0�95|67�4|48�9|56�9|51�1|81| 12| 2�4|5�9|S. to W. |
+           |       |    |    |    |    |    |  |   |    |   |         |
+  October  | 29�860|1�33|58�3|43�7|50�2|46�0|87| 13| 2�8|6�9|S. to W. |
+           |       |    |    |    |    |    |  |   |    |   |         |
+  November | 29�929|1�53|49�3|37�7|43�4|40�1|89| 12| 2�4|7�2|  S.W.   |
+           |       |    |    |    |    |    |  |   |    |   |         |
+  December | 29�979|1�52|45�0|35�5|40�1|36�9|89| 12| 1�9|7�4|   W.    |
+           |       |    |    |    |    |    |  |   |    |   |         |
+  ---------+-------+----+----+----+----+----+--+---+----+---+---------+
+  Year     | 29�946|1�16|58�0|42�3|49�2|43�7|82|133|25�3|6�7|   --    |
+  ---------+-------+----+----+----+----+----+--+---+----+---+---------+
+               1     2     3    4    5    6   7  8    9   10    11
+  ---------------------------------------------------------------------
+
+             +-----------------------------------------------------+
+             |Sun above the Horizon on Middle Day.                 |
+             |       +---------------------------------------------|
+             |       |REMARKS.                                     |
+    ---------+-------+---------------------------------------------|
+             | Hours.|                                             |
+    ---------+-------+---------------------------------------------|
+    January  |  8-1/2|The majority of the nights are frosty.       |
+             |       |                                             |
+    February | 10    |10 frosty nights on the average.             |
+             |       |                                             |
+    March    | 12    |12 ditto   ditto     ditto.                  |
+             |       |   Strong winds.                             |
+             |       |                                             |
+    April    | 14    | 6 ditto   ditto     ditto.                  |
+             |       |                                             |
+    May      | 15-1/2|Very rarely frost.                           |
+             |       |                                             |
+    June     | 16-1/2|Sun attains greatest North Declination, 21st.|
+             |       |                                             |
+    July     | 16    |                                             |
+             |       |                                             |
+    August   | 14-1/2|                                             |
+             |       |                                             |
+    September| 12-1/2|                                             |
+             |       |                                             |
+    October  | 10-1/2|A few frosty nights. Heavy gales.            |
+             |       |                                             |
+    November |  9    | 11 nights frosty.                           |
+             |       |                                             |
+    December |  8    |Sun attains greatest South Declination, 21st.|
+             |       |                                             |
+    ---------+-------+---------------------------------------------|
+    Year     |   --  |                                             |
+    ---------+-------+---------------------------------------------|
+                 12               13                               |
+    ---------------------------------------------------------------+
+
+In the above Table, columns 1 to 10 are results obtained at the Royal
+Observatory, Greenwich, by J. Glaisher, Esq., F.R.S. The data contained in
+columns 2 and 10, are deduced from observations extending over the years
+1841 to 1855 inclusive, and are copied from Edward Hughes' _Third Reading
+Book_; the other columns are results of observations made during the
+twenty years ending 1861. The rest of the information is from Luke
+Howard's _Climate of London_.
+
+These valuable data indicate the characteristics of the weather in each
+month in the suburbs of London, and will be found tolerably accurate as
+indications of weather, and serviceable as standards for comparisons of
+observed results, at most places in England.
+
+
+
+
+STANDARD WORKS ON METEOROLOGY
+
+SUPPLIED BY NEGRETTI & ZAMBRA.
+
+  THE WEATHER BOOK: A MANUAL OF PRACTICAL METEOROLOGY.
+  By Vice-Admiral FITZROY, F.R.S., M.I.F., &c.
+  _Price_, �0 15 6
+
+  THE LAW OF STORMS,
+  By H. W. DOVE, F.R.S.
+  Translated by R. H. SCOTT, M.A.
+  _Price_, �0 10 6
+
+  L. F. K�MTZ'S "COMPLETE COURSE OF METEOROLOGY,"
+  Translated by C. V. WALKER, Esq.
+  _Price_, �0 12 6
+
+  PRACTICAL METEOROLOGY,
+  By JOHN DREW, Ph.D., F.R.A.S.
+  _Price_, �0 5 0
+
+  HYGROMETRICAL TABLES,
+  Adapted to the use of the Wet and Dry Bulb Thermometer,
+  By JAMES GLAISHER, Esq., F.R.S.
+  _Price_, �0 2 6
+
+  TABLES OF THE CORRECTIONS FOR TEMPERATURES,
+  To reduce observations to the 32� Fahrenheit, for Barometers with brass
+  scales extending from the cistern to the top of the mercurial column,
+  By JAMES GLAISHER, Esq., F.R.S.
+  _Price_, �0 1 0
+
+  TABLE OF THE DIURNAL RANGE OF THE BAROMETER,
+  By JAMES GLAISHER, Esq., F.R.S.
+  _Price_, �0 0 6
+
+  TABLES FOR CALCULATION OF HEIGHTS FROM OBSERVATIONS
+  ON THE BOILING-POINT OF WATER,
+  Adapted to the use of Negretti and Zambra's Boiling-point Apparatus.
+  _Price_, �0 1 0
+
+  A THERMOMETRICAL TABLE,
+  ON THE SCALES OF FAHRENHEIT, REAUMUR, AND CENTIGRADE,
+  By ALFRED S. TAYLOR, Esq., M.D., &c.
+  _Price_, in Sheet, with explanatory Pamphlet, �0 1 6
+
+  METEOROLOGICAL TABLES,
+  For the reduction of Barometrical and Hygrometrical Observations,
+  Determination of Heights by the Barometer and Boiling-point
+  Thermometer, &c.
+  By G. HARVEY SIMMONDS, M.B.M.S.
+  _Price_, �0 2 6
+
+  BAROMETER MANUAL,
+  Compiled by Vice-Admiral FITZROY, F.R.S.,
+  For the Board of Trade.
+  _Price_, �0 0 6
+
+  POCKET METEOROLOGICAL REGISTER AND NOTE-BOOK,
+  With Diagrams for exhibiting the Fluctuations of Barometer, &c.
+  Printed on metallic paper. _Price_, with Pencil, �0 3 0
+
+
+  LONDON:
+  PRINTED BY STRAHAN AND WILLIAMS,
+  7 LAWRENCE LAND, CHEAPSIDE, E.C.
+
+
+
+
+NEGRETTI & ZAMBRA'S
+
+PATENT RECORDING AND DEEP-SEA THERMOMETER.[20]
+
+
+This Thermometer differs from all other Registering or Recording
+Thermometers in the following important particulars:--
+
+I. The Thermometer contains only Mercury without any admixture of Alcohol
+or other fluid.
+
+II. It has no indices or springs, and its indications are by the column of
+Mercury only.
+
+III. It can be carried in any position, and cannot possibly be put out of
+order except by actual breakage of the instrument.
+
+And lastly, it will indicate and record the exact temperature at any hour
+of the day or night, or the exact temperature at any depth of the sea,
+irrespective of either warm or cold currents, or stratum through which the
+Thermometer may have to pass in its descent or ascent, this last very
+special quality renders this Thermometer superior for deep-sea
+temperatures to any others; for those now being used in the "Challenger"
+sounding expedition are liable to give erroneous indications owing to
+their indices slipping, and otherwise getting deranged--(This was proved
+by Messrs. Negretti and Zambra at a Meeting of the British Meteorological
+Society,) and _under certain conditions of temperature_ it is not possible
+by the old Thermometers to obtain true temperatures at certain depths
+which might be required. _Annexed is a copy of a report to the Admiralty
+from Captain G. S. Nares, of H.M.S. "Challenger," dated Melbourne, March
+25th, 1874, which we have taken from NATURE, July 30th, 1874, proving
+the assertion._
+
+"In the report to the Admiralty of Capt. G. S. Nares, of H.M.S.
+_Challenger_ dated Melbourne, March 25, 1874, Capt. Nares, speaking of the
+temperature of the ocean, especially near the pack edge of the ice,
+says:--'At a short distance from the pack, the surface water rose to 32�,
+but at a depth of 40 fathoms we always found the temperature to be 29�;
+this continued to 300 fathoms, the depth in which most of the icebergs
+float, after which there is a stratum of slightly warmer water of 33� or
+34�. As the thermometers had to pass through these two belts of water
+before reaching the bottom, the indices registered those temperatures, and
+it was impossible to obtain the exact temperature of the bottom whilst
+near the ice, but the observations made in lower latitudes show that it is
+about 31�. More exact results could not have been obtained even had Mr.
+Siemens's apparatus been on board.' It seems to us that the difficulty
+mentioned is one which would certainly have been surmounted by Messrs.
+Negretti and Zambra's new Recording Thermometers, a description of which
+appeared in NATURE, vol. ix. p. 387; this being exactly one of the cases
+to which this instrument is peculiarly adapted. We believe the inventors
+and makers have greatly improved their Thermometer since our description
+appeared, and no doubt means will be taken by the Admiralty to transmit
+one to the _Challenger_."
+
+
+DESCRIPTION OF THE DEEP-SEA RECORDING THERMOMETER.
+
+In the first place, it must be observed that the bulb of the Thermometer
+is protected so as to resist the pressure of the ocean, which varies
+according to depth that of three thousand fathoms being something like
+three tons pressure on the square inch. The manner of protecting the bulb
+was invented by Messrs. Negretti and Zambra in 1857, and has been latterly
+copied by other persons and brought out as a new invention. The manner of
+protecting the bulb has been described by the late Admiral R. FitzRoy, in
+the first number of Meteorological Papers, page 55, published July 5th,
+1857, as follows:
+
+    "Referring to the erroneous readings of all thermometers, consequent
+    on their delicate bulbs being compressed by the great pressure of the
+    ocean, he says:--'With a view to obviate this failing, Messrs.
+    Negretti and Zambra undertook to make a case for the weak bulbs, which
+    should transmit temperature, but resist pressure. Accordingly a tube
+    of thick glass is sealed outside the delicate bulb, between which and
+    the casing is a space all round, which is nearly filled with mercury.
+    The small space not so filled is a vacuum, into which the mercury can
+    be expanded, or forced by heat or mechanical compression, without
+    doing injury to or even compressing the inner or much more delicate
+    bulb.'"
+
+[Illustration: Fig. 1.]
+
+The construction of this instrument for deep-sea temperatures is as
+follows:--
+
+    In shape it is like a syphon with parallel legs, all in one piece and
+    having a continuous communication, as in the annexed figure. The scale
+    of the Thermometer is pivoted on a centre and being attached in a
+    perpendicular position to a simple apparatus (which will be presently
+    described), is lowered to any depth that may be desired. In its
+    descent the Thermometer acts as an ordinary instrument, the mercury
+    rising or falling according to the temperature of the stratum through
+    which it passes; but so soon as the descent ceases, and a reverse
+    motion is given to the line, so as to pull the Thermometer towards the
+    surface, the instrument turns once on its centre, first bulb
+    uppermost, and afterwards bulb downwards. This causes the mercury,
+    which was in the left-hand column, first to pass into the dilated
+    syphon bend at the top, and thence into the right-hand tube, where it
+    remains, indicating on a graduated scale the exact temperature at the
+    time it was turned over. The woodcut, Fig. 1, shows the position of
+    the mercury _after_ the instrument has been thus turned on its centre.
+    A is the bulb; B the outer coating or protecting cylinder; C is the
+    space of rarefied air, which is reduced if the outer casing be
+    compressed; D is a small glass plug on the principle of Negretti and
+    Zambra's Patent Maximum Thermometer, which cuts off, in the moment of
+    turning, the mercury in the tube from that of the bulb, thereby
+    ensuring that none but the mercury in the tube can be transferred into
+    the indicating column; E is an enlargement made in the bend so as to
+    enable the mercury to pass quickly from one tube to another in
+    revolving; and F is the indicating tube or Thermometer proper. In its
+    action, as soon as the Thermometer is put in motion, and immediately
+    the tube has acquired a slightly oblique position, the mercury breaks
+    off at the point D, runs into the curved and enlarged portion E, and
+    eventually falls into the tube F when this tube resumes its original
+    perpendicular position.
+
+The contrivance for turning the Thermometer over may be described as a
+frame with a vertical propeller; to this frame the instrument is pivoted.
+On its descent through the water the propeller is lifted out of gear and
+revolves freely on its axis; but so soon as the instrument is pulled
+towards the surface the propeller falls into gear and revolves in the
+contrary direction, turning the Thermometer over once, and then becoming
+locked and immovable.
+
+
+_Directions for adjusting the Thermometer previous to its being lowered in
+the Sea._
+
+    I. The mercury must all be in the left-hand column.
+
+    II. The short peg at the back of the thermometer must be in front of
+    the stop plate S +; in order to effect this, pull the knob which stops
+    the Thermometer, and slightly turn the propeller, to make the
+    Thermometer advance sufficiently to escape the stop plate.
+
+Negretti & Zambra's Patent Atmospheric Recording Thermometer, Fig. 3,
+differs from the Deep-sea Thermometer by its not having the double or
+protected bulb, it not being required for resisting pressures. In this
+case the instrument is turned over by a simple clock movement, which can
+be set to any hour it may be desirable; the Thermometer is fixed on the
+clock, and when the hand arrives at the hour determined upon, and to which
+the clock is set as in setting an alarum clock, a spring is released and
+the Thermometer turns over as before described.
+
+[Illustration: Fig. 2.]
+
+[Illustration: Fig. 3.]
+
+Messrs. Negretti and Zambra have arranged a Wet and Dry Bulb Hygrometer
+upon the same plan.
+
+
+
+
+NEGRETTI & ZAMBRA'S PRICE LIST OF STANDARD METEOROLOGICAL AND OTHER
+PHILOSOPHICAL INSTRUMENTS.
+
+_The marginal figures in this List and the numbers of the wood engravings
+refer to paragraphs in "Negretti & Zambra's Treatise on Meteorological
+Instruments."_
+
+
+                                                                  �  s. d.
+
+  4 =Standard Barometers=, Fortin's arrangement, as Fig. 3
+  with mahogany board                                             8   8  0
+
+      Ditto    ditto with Millemetre and English scales           9   9  0
+
+      Ditto    ditto with tube, 0�45 internal diameter and
+                     millimetre scale                            10  10  0
+
+  =Observatory Standard Barometers=, extra large tubes and
+  cisterns                                          �25  0  0    35   0  0
+
+      Ditto    ditto arranged for observations being taken
+                     by the Cathetometer                         18  18  0
+
+  =Cathetometer=, for use with above                             21   0  0
+
+  9 =Self Compensating Standard Barometer=, Fig. 6               20   0  0
+
+  10 =Standard Barometer=, with electrical adjustment            15  15  0
+
+  11 =Pediment Barometers=, Fig 7                    �1  1  0     2   2  0
+
+     Ditto      ditto     Fig. 8                     �3  3  0     3  10  0
+
+     Ditto      ditto     Fig. 9                     �4 10  0     5  10  0
+
+     Ditto      ditto     Fig. 10                                 8  10  0
+
+     Ditto      ditto     ditto handsome carved mountings,
+     in mahogany, oak, or walnut wood    �8  8  0   �10 10  0    12  12  0
+
+  14 =FitzRoy's Storm or Sea Coast Fishery Barometer=, Fig. 12    5   5  0
+
+     Ditto    ditto with two verniers                             6   6  0
+
+     Ditto    ditto mounted in ornamental carved frames, oak,
+     walnut, or mahogany                             �6 10  0     8   8  0
+
+  19 =Marine Barometers=, ordinary forms, Figs. 13 and 14
+                                         �2  2s.     �2 10s.      3   3  0
+
+     Ditto    ditto   Best mounted                   �5  5s.      6   6  0
+
+  20 =The Board of Trade or Kew Marine Barometer=, Fig. 15,
+                                         �4  4s.     �5  5s.      6   6  0
+
+  22 =Negretti and Zambra's FitzRoy Marine or Gun Barometer=,
+       Fig. 16, with N. and Z.'s Patent Porcelain Scales, as
+       used in Her Majesty's Navy                                 5  10  0
+
+     Extra Tube for ditto                                         1  15  0
+
+  25 =Negretti and Zambra's Farmer's Barometer or Domestic
+  Weather Glass=, Fig. 17                                         2  10  0
+
+  28 =Negretti and Zambra's Miner's Barometers=
+                                         �1  1s.     �2  2s.      3   3  0
+
+  31 =Dial or Wheel Barometers=, Figs. 18, 19, 20, 21
+                                         �3  3s.     �4  4s.      5   5  0
+
+     Ditto  ditto  in carved ornamental mountings
+                             �5 10s.     �6  6s.     �8  8s.     10  10  0
+
+     Ditto  ditto  rosewood, inlaid with pearl or metal. Made
+     to order, Figs. 22 and 23. Price varying with size, &c.
+
+  37 =Gay Lussac's Syphon Tube Mountain Barometer=   �6  6  0     8   8  0
+
+  32 =Standard Syphon Barometer=, Gay Lussac's arrangement,
+     Fig. 24                                                      5   5  0
+
+  38 =Negretti and Zambra's Standard Mountain Barometer=, with
+     Fortin's cistern, with tripod stand and travelling case,
+     Fig. 30                                                     10  10  0
+
+  34 =Barograph, or Self-registering Barometer=, with syphon
+     mercurial tube. Negretti and Zambra's improved arrangement,
+     Fig. 26                                         18 18  0    25   0  0
+
+  =Negretti and Zambra's Self-recording Aneroid Barometer=,
+  with =Clock=                                                   22   0  0
+
+
+  48. =ANEROID BAROMETERS.=
+
+  =Aneroid Barometers,= with card dials 4-1/2 inches diameter,
+  best quality.                                                   2  10  0
+
+    Ditto    ditto      with silvered metal dial                  3   0  0
+
+    Ditto    ditto      with ditto and thermometer                3  10  0
+
+    Ditto    ditto         ditto   with corrected scale, as
+    supplied by Negretti and Zambra to the Royal Navy             5   5  0
+
+  =Aneroid Barometers=, with elegantly-chased dials               4   4  0
+
+    Ditto    ditto      with raised ring on dial                  5   5  0
+
+    Ditto    ditto         ditto    with thermometer              6   6  0
+
+  =Aneroid Barometer=, for altitude measurements with revolving
+  ring, carrying index, range of scale 20,000 feet 4-1/2 inches
+  diameter, with magnifier                                        8   8  0
+
+
+  =POCKET ANEROID BAROMETERS.= Fig. 34.
+
+  49 =Pocket Aneroid Barometer=, 2-3/4 inches diameter, with
+     silvered metal scale                                         3   3  0
+
+     Ditto   ditto  for measuring altitudes to 10,000 feet
+     compensated for temperature, in leather case                 5   5  0
+
+     Ditto   ditto  ditto to 20,000 feet, with magnifier          6   6  0
+
+
+  50 =WATCH-SIZE ANEROID BAROMETERS= in gilt metal cases (see figure 35.)
+
+  =Watch-Size Aneroid Barometer=, weather range      �3  3  0     4   4  0
+
+  Ditto      ditto  of best construction, extra thin, for
+  meteorological observations or altitude measurements to
+  10,000 feet                                                     5   5  0
+
+  Ditto      ditto  ditto  to 20,000 feet, compensated for
+  temperature                                                     6   6  0
+
+  Either of the above Watch-size Barometers may be had in Stout
+  Silver Cases at a cost of �2 2s. extra
+
+  _Watch-size Aneroid Barometers in Solid Gold, highly-finished cases.
+  �15 15s. to �21._
+
+  Table Stands for Aneroid Barometers of Carved Oak or other woods,
+  10s. 6d., 25s., 35s., to �5 5s.
+
+  =Ships' Aneroid Barometers=, in suitable mountings �2 10s. �3 3s.
+  �5 5s. �6 6s.
+
+
+  47 =Sympiesometer=, for Ship use                   �3  3  0     4  10  0
+
+     Ditto     ditto  Pocket form, Fig. 32                        4   4  0
+
+  The Sympiesometer is now rarely used, the Aneroid Barometer being
+  found equally sensitive and less liable to derangement.
+
+
+  56 =Independent Standard Thermometers=, Fig. 36                 5   5  0
+
+  57 =Standard Thermometers=, for Boiling Point Apparatus         1  10  0
+
+  =Chemists' or Brewers' Standard Reference Thermometers=
+                                                     �1  1s.      2   2  0
+
+  47 =Chemical Thermometers=, divided on the stem. Fig. 48.
+                                                    10s. 6d.      0  15  0
+
+  =Chemical Thermometers=, Boxwood Scale  7s.  6d.  10s. 6d.      0  12  6
+
+  64 =Thermometers on Boxwood Scales=, Fig. 37.
+                         1s.   1s.  6d.   2s.  6d.   3s. 6d.      0   4  6
+
+     Ditto   ditto   larger sizes                    7s. 6d.      0  12  6
+
+     Ditto   ditto   Engraved glass scales, Fig. 39
+                                    15s.   �1  1s.   �1  5s.      1  10  0
+
+
+  =POCKET THERMOMETERS, IN VARIOUS MOUNTINGS.=
+
+  Fig. 38   10s. 6d.    Fig. 40   10s. 6d. 15s.
+  Fig. 41    5s. 6d.   8s. 6d.   12s. 6d.
+
+  63 =Thermometers of extreme Sensitiveness=, 15s.   �1 10s.      2   2  0
+
+
+  =Drawing Room or Mantel Thermometers=, various mountings,
+
+  Figs. 42 and 43.  12s. 6d.  15s.  �1 1s.  �1 10s.  �2  2s.      2  10  0
+
+  =Bracket Window Thermometers=, Fig. 46
+                                  12s. 6d.    15s.   �1  1s.      1  10  0
+
+  =Bath Thermometers=, Figs. 44                     7s.  6d.      0  15  0
+
+  66 =Sugar Boiling Thermometers=           �1 12s.  �2  2s.      3   3  0
+
+  67 =Earth Thermometers=, Fig. 51                   �1 10s.      2   2  0
+
+     Hot Bed      Ditto                    12s. 6d.  �1  5s.      1  10  0
+
+  68 =Marine Thermometer=, Fig. 52          7s. 6d.  8s. 6d.      0  10  6
+
+  65 =Super Heated or Steam Pressure Thermometers=, Fig. 74,
+     Figs. 49 and 50               �1 5s.   �1 10s.  �2  2s.      2  10  0
+
+
+  =SELF-REGISTERING THERMOMETERS FOR HEAT.=
+
+  72 =Negretti and Zambra's Patent Standard Maximum
+     Self-Registering Thermometer=,[21] Fig. 54                   1   1  0
+
+  72 =Negretti and Zambra's Patent Maximum Thermometer=, on
+  boxwood scale                                                   0  10  6
+
+     Ditto,  ditto,  on Negretti and Zambra's Patent Solid
+     Porcelain or Metal Scales on oak mounting                    0  12  6
+
+  70 =Rutherford's Maximum Thermometer=, on boxwood or metal
+     scale, with steel index                5s. 6d.  7s. 6d.      0  10  6
+
+  71 =Phillip's Maximum Thermometer=, on boxwood or metal
+     scale, with air index                  7s. 6d.  10  6        0  12  6
+
+
+  =SELF-REGISTERING THERMOMETERS FOR COLD.=
+
+  73 =Negretti and Zambra's Standard Minimum Self-Registering
+     Thermometer=, Fig. 55                                        1   1  0
+
+  73 =Rutherford's Minimum Thermometer=, on boxwood or metal scale
+                                   3s. 6d.    5s. 6d.   7s. 6d.   0  10  6
+
+  73 =Rutherford's Minimum Thermometer=, on Negretti and Zambra's
+     Patent solid porcelain scale                      10s. 6d.   0  12  6
+
+     Ditto, on Negretti and Zambra's porcelain or metal scales
+     and oak mounting                                             0  12  6
+
+  74 =Negretti and Zambra's Horticultural Self-Registering
+     Thermometer.= The scale is made of stout zinc, enclosing
+     the tube; the figures and divisions are boldly marked for
+     quickly and easily reading the indications, Fig. 56          0   3  6
+
+
+  83 =Negretti and Zambra's Patent Solar Radiation Thermometer=,
+     Fig. 63                                                      1   5  0
+
+  84 Ditto,   ditto,   ditto, in vacuo, Fig. 64                   1  10  0
+
+     Ditto,   ditto,   ditto, improved form, with test gauge      2   2  0
+
+  85 =Negretti and Zambra's Terrestrial Radiation Thermometer=    1   5  0
+
+  Brass Stands for above, Fig. 65                                 0   5  0
+
+  76 and 77 =Negretti and Zambra's Patent Mercurial Minimum
+     Thermometers=                                   �2 10s.      2   2  0
+
+  81 =Maxima and Minima Thermometers=, on Sixe's arrangement,
+     Fig. 62, various forms of mounting
+                                  12s. 6d.    14s.    21s. 30s.   2   2  0
+
+  =Pocket Maxima and Minima Thermometers=, Negretti and Zambra's
+  Patent, in convenient cases                        �2  2s.      2  10  0
+
+  89 =Deep Sea Registering Thermometer=, with Negretti and
+     Zambra's improved protected bulb, in copper cylinder,
+     Fig. 69.                                                     2  10  0
+
+  89[21] =Negretti and Zambra's Improved Deep Sea Thermometer=,
+     with vulcanite mountings, in copper cylinder, with door,
+     small size                                                   2   5  0
+
+  90 =Negretti and Zambra's Patent Recording Deep Sea
+     Thermometer=                                                10  10  0
+
+  91 Ditto,   ditto,   ditto   =Recording Thermometer=            4   4  0
+
+  92 Ditto,   ditto,   ditto   =Hygrometer=                       6   6  0
+
+  93 =Improved Boiling Point Mountain Thermometer=, or
+     Hypsometric Apparatus, with Tables, Figs. 72 and 73, in
+     leather case with strap                                      5   5  0
+
+     Extra Thermometer for Ditto                                  1  10  0
+
+  106 =Negretti and Zambra's Standard Wet and Dry Bulb
+      Hygrometer=, Fig. 79                                        2   2  0
+
+      Wet and Dry Bulb Hygrometers, various mountings
+                                   30s.   25s.   21s.   14s.      0  10  6
+
+      Pocket Hygrometers, in box                     �2  2s.      2  10  0
+
+  103 =Daniell's Hygrometer=, Fig. 77                             3   3  0
+
+  104 =Regnault's Hygrometer=, Fig. 78               �3 10s.      5   5  0
+
+      Aspirator for Ditto                            �1 15s.      2  15  0
+
+
+  110 =Howard's Rain Gauge=, has a 5-inch copper Funnel, with
+      turned brass rim fitted to a stout stone-ware or glass
+      bottle, with a graduated glass measure, divided to 100ths
+      of an inch                                                  0  10  6
+
+      =Symons' Portable Rain Gauge=, (5-inch) with graduated
+      glass measure, japanned tin                                 0  10  6
+
+      Ditto   ditto   in stout copper                             0  15  0
+
+  111 =Glaisher's Rain Gauge=, the receiving surface is 8-inches
+      diameter, of stout japanned metal, with graduated glass
+      measure, Fig. 84                                            1   1  0
+
+      Ditto   ditto,  of stout copper                             1  10  0
+
+      Receiving Pots for ditto, extra 2s. and 3s. 6d.
+
+  113 =Rain Gauge=, having a receiving surface of 12 inches
+      diameter, and graduated glass gauge tube, divided to
+      hundredths of an inch, in japanned metal, with brass tap    2  10  0
+
+      Ditto   ditto, Fig. 85, in copper                           3  10  0
+
+      Ditto   ditto, with sliding rod instead of graduated tube,
+      japanned tin                                                2   2  0
+
+      =Rain Gauges=, of any form or area made to order, with suitable
+      measuring glasses.
+
+
+  123 =Lind's Anemometer=, Fig. 86                                2   2  0
+
+  125 =Robinson's Anemometer=, Fig. 87                            3   3  0
+
+      Ditto   ditto,   Improved arrangement          �4 10s.      5  15  0
+
+      Ditto   ditto,   with clutch movement, Fig. 88              6  15  0
+
+      =Negretti and Zambra's Improved Air Meter=, of extreme
+      sensitiveness, very portable                                4   4  0
+
+      Large Air Meters made to order.
+
+  127 =Osler's Self-Registering Anemometer and Rain Gauge=,
+      Fig. 89                                            �84 to 150   0  0
+
+  128 =Berkley's Anemometers= fitted up to order, _to suit the
+      Observatory_.
+
+  131 =Gold Leaf Electrometer=, Fig. 90                           1   1  0
+
+  133 =Peltier's Electrometer=                                    4   4  0
+
+  134 =Bohnenberger's Electroscope=, Fig. 91                      8   8  0
+
+  135 =Thompson's Electrometer=, to order
+
+      =Lightning Conductors= fitted up to order.
+
+  142 =Ozone Cage=, Fig. 92                                       0  18  0
+
+      Ditto  ditto, copper                                        1   5  0
+
+  146 =Leslie's Differential Thermometer=, Fig. 93    �1 10s.     2   2  0
+
+  148 =Thermometer Stand (Glaisher's)=                            3   3  0
+
+  149 =Thermometer Screen= for Sea use                            3   3  0
+
+  150 =Anemoscope=, or Portable Vane, Fig. 94                     2   5  0
+
+  151 =Evaporating Dish=, Fig. 95                                 1   2  6
+
+  157 =Sea Water Hydrometers=, Board of Trade Marine, Figs. 96
+      and 97                                                      0   5  6
+
+  158 =Newman's Self-Registering Tide Gauge=, Fig. 158, fitted
+      to the Building to order                              From 50   0  0
+
+
+  _Further Information as to Price, &c., will be found in_
+  NEGRETTI & ZAMBRA'S ENCYCLOP�DIC CATALOGUE OF
+  MATHEMATICAL, PHILOSOPHICAL, OPTICAL, PHOTOGRAPHIC,
+  AND STANDARD METEOROLOGICAL INSTRUMENTS,
+
+  _Containing very numerous Comparative Tables of Reference,
+  and Illustrated by upwards of_
+
+  ELEVEN HUNDRED ENGRAVINGS.
+
+  Royal 8vo. Cloth, Gilt Lettered--Price 5s. 6d.
+
+
+
+
+FOOTNOTES:
+
+[1] Second Number of "Meteorological Papers," issued by the Board of
+Trade.
+
+[2] With reference to these barometers, we have received the subjoined
+testimonial, with permission to use it as we please.
+
+    "_Meteorologic Office, 12th June, 1863._
+
+    "MESSRS. NEGRETTI & ZAMBRA,
+
+    "The barometers which you have lately supplied to Her Majesty's ships
+    through this Office are much approved, being good for general service,
+    afloat or on land.
+
+    "(Signed)       R. FITZROY."
+
+[3] _Vide_ C. Daubeny, F.R.S., "On Climate."
+
+[4] _Vide_ Report of the British Association, 1862.
+
+[5] See page 42 for the Tables.
+
+[6] The quotations in this section are from Tyndall's _Heat considered as
+a Mode of Motion_.
+
+[7] Dr. Daubeny, F.R.S., _On Climate_.
+
+[8] Leslie _On the Relations of Air, Heat, and Moisture_.
+
+[9] Tyndall's _Heat considered as a Mode of Motion_.
+
+[10] Vide _Horological Journal_, Vol. V.
+
+[11] _Hygrometrical Tables_, by J. Glaisher, Esq., F.R.S.
+
+[12] Vide _Report of the British Association_, 1862. It may be added, for
+the information of those who are about to commence observing, that Mr.
+Symons, of Camden Road Villas, London, is desirous of securing returns of
+rain-fall from as many stations as possible, in order to render more
+complete his annual reports to the British Association.
+
+[13] Luke Howard's _Climate of London_.
+
+[14] Vide _Third Number of Meteorological Papers_, issued by the Board of
+Trade.
+
+[15] _Elements of Physics_, by C. F. Peschel.
+
+[16] This description is modified from that in Report of the Jurors for
+Class XIII. International Exhibition, 1862.
+
+[17] _All the Year Round_, No. 224.
+
+[18] _All the Year Round_, No. 224.
+
+[19] Vide _Jurors' Reports_.
+
+[20] See also page 90 of this Treatise.
+
+[21] These Instruments are the only Maximum Thermometers that can be
+recommended, as unless they be broken, they cannot be put out of
+adjustment. Fully described under the head of Standard Maximum
+Thermometers in our large Catalogue, and page 72 of our _Treatise on
+Meteorological Instruments_.
+
+
+
+
+
+
+End of the Project Gutenberg EBook of A Treatise on Meteorological
+Instruments, by Henry Negretti and Joseph Zambra
+
+*** END OF THIS PROJECT GUTENBERG EBOOK TREATISE ON METEOROLOGICAL INSTRUMENTS ***
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+<pre>
+
+The Project Gutenberg EBook of A Treatise on Meteorological Instruments, by 
+Henry Negretti and Joseph Zambra
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: A Treatise on Meteorological Instruments
+       Explanatory of Their Scientific Principles, Method of
+              Construction, and Practical Utility
+
+Author: Henry Negretti
+        Joseph Zambra
+
+Release Date: June 18, 2011 [EBook #36457]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK TREATISE ON METEOROLOGICAL INSTRUMENTS ***
+
+
+
+
+Produced by The Online Distributed Proofreading Team at
+https://www.pgdp.net (This file was produced from images
+generously made available by The Internet Archive.)
+
+
+
+
+
+
+</pre>
+
+
+
+
+<p class="center"><span class="huge">A TREATISE</span></p>
+<p class="center">ON</p>
+<p class="center"><span class="huge">METEOROLOGICAL INSTRUMENTS.</span></p>
+<p>&nbsp;</p><p>&nbsp;</p>
+<p class="center"><small>LONDON:<br />
+PRINTED BY WILLIAMS AND STRAHAN,<br />
+7 LAWRENCE LANE, CHEAPSIDE, E.C.</small></p>
+
+<p>&nbsp;</p><p>&nbsp;</p><p>&nbsp;</p>
+<p class="center"><span class="huge">A TREATISE</span></p>
+<p class="center">ON</p>
+<p class="center"><span class="giant">METEOROLOGICAL<br />INSTRUMENTS:</span></p>
+<p>&nbsp;</p>
+<p class="center"><small>EXPLANATORY OF</small></p>
+<p class="center"><span class="large">THEIR SCIENTIFIC PRINCIPLES,</span></p>
+<p class="center"><span class="large">METHOD OF CONSTRUCTION, AND PRACTICAL UTILITY.</span></p>
+<p>&nbsp;</p>
+<p class="center">BY</p>
+<p class="center"><span class="large">NEGRETTI &amp; ZAMBRA,</span></p>
+<p class="center"><small>METEOROLOGICAL INSTRUMENT MAKERS TO THE QUEEN, THE ROYAL OBSERVATORY, GREENWICH,<br />
+THE BRITISH METEOROLOGICAL SOCIETY, THE BRITISH AND FOREIGN GOVERNMENTS,<br />
+ETC. ETC. ETC.</small></p>
+<p>&nbsp;</p>
+<p class="center">LONDON:<br />
+<small>PUBLISHED AND SOLD AT NEGRETTI &amp; ZAMBRA&#8217;S ESTABLISHMENTS:<br />
+1 HATTON GARDEN, E.C., 59 CORNHILL, E.C., 122 REGENT STREET W.,<br />
+AND 153 FLEET STREET, E.C.</small></p>
+<p class="center">1864.</p>
+<p class="center"><i>Price Five Shillings.</i></p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_v" id="Page_v">[Pg v]</a></span></p>
+<h2>PREFACE.</h2>
+
+<p>The national utilisation of Meteorology in forewarning of storms, and the
+increasing employment of instruments as weather indicators, render a
+knowledge of their construction, principles, and practical uses necessary
+to every well-informed person. Impressed with the idea that we shall be
+supplying an existing want, and aiding materially the cause of
+Meteorological Science, in giving a plain description of the various
+instruments now in use, we have endeavoured, in the present volume, to
+condense such information as is generally required regarding the
+instruments used in Meteorology; the description of many of which could
+only be found in elaborate scientific works, and then only briefly touched
+upon. Every Meteorological Instrument now in use being fully described,
+with adequate directions for using, the uninitiated will be enabled to
+select those which seem to them best adapted to their requirements. With
+accounts of old or obsolete instruments we have avoided troubling the
+reader; on the other hand, we were unwilling to neglect those which,
+though of no great practical importance, are still deserving of notice
+from their being either novel or ingenious, or which, without being
+strictly scientific, are in great demand as simple weather-glasses and
+articles of trade.</p>
+
+<p>We trust, therefore, that the work (however imperfect), bearing in mind
+the importance of the subject, will be acceptable to general readers, as
+well as to those for whose requirements it has been prepared.</p>
+
+<p>The rapid progress made in the introduction of new apparatus of
+acknowledged superiority has rendered the publication of some description
+absolutely necessary. The Report of the Jurors for Class XIII. of the
+International Exhibition, 1862, on Meteorological Instruments, fully bears
+out our assertion, as shown by the following extract:&mdash;</p>
+
+<p><span class="pagenum"><a name="Page_vi" id="Page_vi">[Pg vi]</a></span>&#8220;The progress in the English department has been very great;&mdash;in
+barometers, thermometers, anemometers, and in every class of instruments.
+At the close of the Exhibition of 1851, there seemed to have arisen a
+general anxiety among the majority of makers to pay every attention to all
+the essentials necessary for philosophical instruments, not only in their
+old forms, but also with the view of obtaining other and better forms.
+This desire has never ceased; and no better idea can be given of the
+continued activity in these respects, than the number of patents taken out
+for improvements in meteorological instruments in the interval between the
+recent and preceding exhibitions, which amount to no less than forty-two.&#8221;
+* * * &#8220;In addition to numerous improvements patented by Messrs. Negretti
+and Zambra, there is another of great importance, which they did not
+patent, viz. enamelling the tubes of thermometers, enabling the makers to
+use finer threads of mercury in the construction of all thermometers; for
+the contrast between the opaque mercury and the enamel back of the tubes
+is so great, that the finest bore or thread of mercury, which at one time
+could not be seen without the greatest difficulty, is now seen with
+facility; and throughout the British and Foreign departments, the makers
+have availed themselves of this invention, the tubes of all being made
+with enamelled backs. It is to be hoped that the recent exhibition will
+give a fresh stimulus to the desire of improvement, and that the same rate
+of progress will be continued.&#8221;</p>
+
+<p>To fulfil the desire of the International Jury in the latter portion of
+the above extract will be the constant study of</p>
+
+<p><span style="margin-left: 8em;">NEGRETTI &amp; ZAMBRA.</span></p>
+
+<p><i>1st January, 1864.</i></p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_vii" id="Page_vii">[Pg vii]</a></span></p>
+<h2>TABLE OF CONTENTS.</h2>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td colspan="2" align="center"><a href="#CHAPTER_I">CHAPTER I.</a></td></tr>
+<tr><td colspan="2" align="center"><span class="smcap">Instruments for Ascertaining the Atmospheric Pressure.</span></td></tr>
+<tr><td><small>SECTION</small></td></tr>
+<tr><td align="right">1.</td><td>Principle of the Barometer.</td></tr>
+<tr><td align="right">2.</td><td>Construction of Barometers.</td></tr>
+<tr><td align="right">3.</td><td>Fortin&#8217;s Barometer Cistern.</td></tr>
+<tr><td align="right">4.</td><td><span class="smcap">Standard Barometer.</span></td></tr>
+<tr><td align="right">5.</td><td>Correction due to Capillarity.</td></tr>
+<tr><td align="right">6.</td><td><span class="spacer">&nbsp;</span>"<span class="spacer"> &nbsp; &nbsp; </span>" &nbsp;&nbsp; Temperature.</td></tr>
+<tr><td align="right">7.</td><td><span class="spacer">&nbsp;</span>"<span class="spacer"> &nbsp; &nbsp; </span>" &nbsp;&nbsp; Height.</td></tr>
+<tr><td align="right">8.</td><td>The Barometer Vernier.</td></tr>
+<tr><td align="right">9.</td><td><span class="smcap">Self-compensating Standard Barometer.</span></td></tr>
+<tr><td align="right">10.</td><td><span class="smcap">Barometer with Electrical Adjustment.</span></td></tr>
+<tr><td align="right">11.</td><td><span class="smcap">Pediment Barometers.</span></td></tr>
+<tr><td align="right">12.</td><td>The Words on the Scale.</td></tr>
+<tr><td align="right">13.</td><td>Correction due to Capacity of Cistern.</td></tr>
+<tr><td align="right">14.</td><td><span class="smcap">Public Barometers.</span></td></tr>
+<tr><td align="right">15.</td><td><span class="smcap">Fishery or Sea-Coast Barometers.</span></td></tr>
+<tr><td align="right">16.</td><td>Admiral FitzRoy&#8217;s Words for the Scale.</td></tr>
+<tr><td align="right">17.</td><td>Instructions for Sea-coast Barometer.</td></tr>
+<tr><td align="right">18.</td><td>French Sea-coast Barometer.</td></tr>
+<tr><td align="right">19.</td><td><span class="smcap">Common Marine Barometer.</span></td></tr>
+<tr><td align="right">20.</td><td><span class="smcap">The Kew Marine Barometer.</span></td></tr>
+<tr><td align="right">21.</td><td>Method of verifying Barometers.</td></tr>
+<tr><td align="right">22.</td><td><span class="smcap">FitzRoy&#8217;s Marine Barometer.</span></td></tr>
+<tr><td align="right">23.</td><td>Words for its Scale.</td></tr>
+<tr><td align="right">24.</td><td>Trials of this Barometer under Gun-fire.</td></tr>
+<tr><td align="right">25.</td><td><span class="smcap">Negretti and Zambra&#8217;s Farmer&#8217;s Barometer andDomestic Weather-Glass.</span></td></tr>
+<tr><td align="right">26.</td><td>Rules for Foretelling the Weather.</td></tr>
+<tr><td align="right">27.</td><td>Causes which may bring about a Fall or a Rise in the Barometer.</td></tr>
+<tr><td align="right">28.</td><td>Use of the Barometer in the Management of Mines.</td></tr>
+<tr><td align="right">29.</td><td>Use of the Barometer in estimating the Height of Tides.</td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td colspan="2" align="center"><a href="#CHAPTER_II">CHAPTER II.</a></td></tr>
+<tr><td colspan="2" align="center"><span class="smcap">Syphon Tube Barometers.</span></td></tr>
+<tr><td align="right">30.</td><td>Principle of.</td></tr>
+<tr><td align="right">31.</td><td><span class="smcap">Dial, or Wheel, Barometers.</span></td></tr>
+<tr><td align="right">32.</td><td><span class="smcap">Standard Syphon Barometer.</span></td></tr>
+<tr><td>&nbsp;<span class="pagenum"><a name="Page_viii" id="Page_viii">[Pg viii]</a></span></td></tr>
+<tr><td colspan="2" align="center"><a href="#CHAPTER_III">CHAPTER III.</a></td></tr>
+<tr><td colspan="2" align="center"><span class="smcap">Barographs, or Self-Registering Barometers.</span></td></tr>
+<tr><td align="right">33.</td><td><span class="smcap">Milne&#8217;s Self-Registering Barometer.</span></td></tr>
+<tr><td align="right">34.</td><td><span class="smcap">Modification of Milne&#8217;s Barometer.</span></td></tr>
+<tr><td align="right">35.</td><td><span class="smcap">King&#8217;s Self-Registering Barometer.</span></td></tr>
+<tr><td align="right">36.</td><td><span class="smcap">Syphon, with Photographic Registration.</span></td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td colspan="2" align="center"><a href="#CHAPTER_IV">CHAPTER IV.</a></td></tr>
+<tr><td colspan="2" align="center"><span class="smcap">Mountain Barometers.</span></td></tr>
+<tr><td align="right">37.</td><td><span class="smcap">Gay Lussac&#8217;s Mountain Barometer.</span></td></tr>
+<tr><td align="right">38.</td><td><span class="smcap">Fortin&#8217;s Mountain Barometer.</span></td></tr>
+<tr><td align="right">39.</td><td><span class="smcap">Newman&#8217;s Mountain Barometer.</span></td></tr>
+<tr><td align="right">40.</td><td><span class="smcap">Negretti and Zambra&#8217;s Patent Mountain and other Barometers.</span></td></tr>
+<tr><td align="right">41.</td><td>Short Tube Barometer.</td></tr>
+<tr><td align="right">42.</td><td>Method of Calculating Heights by the Barometer; Tables and Examples.</td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td colspan="2" align="center"><a href="#CHAPTER_V">CHAPTER V.</a></td></tr>
+<tr><td colspan="2" align="center"><span class="smcap">Secondary Barometers.</span></td></tr>
+<tr><td align="right">43.</td><td>Desirability of Magnifying the Barometer Range.</td></tr>
+<tr><td align="right">44.</td><td><span class="smcap">Howson&#8217;s Long-Range Barometer.</span></td></tr>
+<tr><td align="right">45.</td><td><span class="smcap">McNeil&#8217;s Long-Range Barometer.</span></td></tr>
+<tr><td align="right">46.</td><td>The Water-glass Barometer.</td></tr>
+<tr><td align="right">47.</td><td><span class="smcap">Sympiesometers.</span></td></tr>
+<tr><td align="right">48.</td><td><span class="smcap">Aneroids.</span></td></tr>
+<tr><td align="right">49.</td><td><span class="smcap">Small Size Aneroids.</span></td></tr>
+<tr><td align="right">50.</td><td><span class="smcap">Watch Aneroid.</span></td></tr>
+<tr><td align="right">51.</td><td>Measurement of Heights by the Aneroid; Example.</td></tr>
+<tr><td align="right">52.</td><td><span class="smcap">Metallic Barometer.</span></td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td colspan="2" align="center"><a href="#CHAPTER_VI">CHAPTER VI.</a></td></tr>
+<tr><td colspan="2" align="center"><span class="smcap">Instruments for Ascertaining Temperature.</span></td></tr>
+<tr><td align="right">53.</td><td>Temperature.</td></tr>
+<tr><td align="right">54.</td><td>Thermometric Substances.</td></tr>
+<tr><td align="right">55.</td><td>Description of the Thermometer.</td></tr>
+<tr><td align="right">56.</td><td><span class="smcap">Standard Thermometer.</span></td></tr>
+<tr><td align="right">57.</td><td>Method of ascertaining the exact Boiling Temperature; Tables, &amp;c.</td></tr>
+<tr><td align="right">58.</td><td>Displacement of the Freezing Point.</td></tr>
+<tr><td align="right">59.</td><td>The Scale.</td></tr>
+<tr><td align="right">60.</td><td>The method of testing Thermometers.</td></tr>
+<tr><td align="right">61.</td><td>Porcelain Scale-Plates.</td></tr>
+<tr><td align="right">62.</td><td>Enamelled Tubes.</td></tr>
+<tr><td align="right"><span class="pagenum">[Pg ix]</span>63.</td><td><span class="smcap">Thermometers of Extreme Sensitiveness.</span></td></tr>
+<tr><td align="right">64.</td><td><span class="smcap">Varieties of Thermometers.</span></td></tr>
+<tr><td align="right">65.</td><td><span class="smcap">Superheated Steam Thermometer.</span></td></tr>
+<tr><td align="right">66.</td><td><span class="smcap">Thermometer for Sugar Boiling.</span></td></tr>
+<tr><td align="right">67.</td><td><span class="smcap">Earth Thermometer.</span></td></tr>
+<tr><td align="right">68.</td><td><span class="smcap">Marine Thermometer.</span></td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td colspan="2" align="center"><a href="#CHAPTER_VII">CHAPTER VII.</a></td></tr>
+<tr><td colspan="2" align="center"><span class="smcap">Self-registering Thermometers.</span></td></tr>
+<tr><td align="right">69.</td><td>Importance of.</td></tr>
+<tr><td align="right">70.</td><td><span class="smcap">Rutherford&#8217;s Maximum Thermometer.</span></td></tr>
+<tr><td align="right">71.</td><td><span class="smcap">Phillips&#8217;s<span class="spacer">&nbsp; &nbsp; &nbsp;</span>ditto<span class="spacer">&nbsp; &nbsp;</span>ditto.</span></td></tr>
+<tr><td align="right">72.</td><td><span class="smcap">Negretti and Zambra&#8217;s Patent Maximum Thermometer.</span></td></tr>
+<tr><td align="right">73.</td><td><span class="smcap">Rutherford&#8217;s Alcohol Minimum Thermometer.</span></td></tr>
+<tr><td align="right">74.</td><td><span class="smcap">Horticultural Minimum Thermometer.</span></td></tr>
+<tr><td align="right">75.</td><td><span class="smcap">Baudin&#8217;s Alcohol Minimum Thermometer.</span></td></tr>
+<tr><td align="right">76.</td><td>Mercurial Minima Thermometers desirable.</td></tr>
+<tr><td align="right">77.</td><td><span class="smcap">Negretti and Zambra&#8217;s Patent Mercurial Minimum Thermometer.</span></td></tr>
+<tr><td align="right">78.</td><td><span class="smcap">Negretti and Zambra&#8217;s Second Patent Mercurial Minimum Thermometer.</span></td></tr>
+<tr><td align="right">79.</td><td><span class="smcap">Casella&#8217;s Patent Mercurial Minimum Thermometer.</span></td></tr>
+<tr><td align="right">80.</td><td>Day and Night Thermometer.</td></tr>
+<tr><td align="right">81.</td><td><span class="smcap">Sixe&#8217;s Self-registering Thermometer.</span></td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td colspan="2" align="center"><a href="#CHAPTER_VIII">CHAPTER VIII.</a></td></tr>
+<tr><td colspan="2" align="center"><span class="smcap">Radiation Thermometers.</span></td></tr>
+<tr><td align="right">82.</td><td>Solar and Terrestrial Radiation considered.</td></tr>
+<tr><td align="right">83.</td><td><span class="smcap">Solar Radiation Thermometer.</span></td></tr>
+<tr><td align="right">84.</td><td><span class="smcap">Vacuum Solar Radiation Thermometer.</span></td></tr>
+<tr><td align="right">85.</td><td><span class="smcap">Terrestrial Radiation Thermometer.</span></td></tr>
+<tr><td align="right">86.</td><td><span class="smcap">&AElig;thrioscope.</span></td></tr>
+<tr><td align="right">87.</td><td><span class="smcap">Pyrheliometer.</span></td></tr>
+<tr><td align="right">88.</td><td><span class="smcap">Actinometer.</span></td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td colspan="2" align="center"><a href="#CHAPTER_IX">CHAPTER IX.</a></td></tr>
+<tr><td colspan="2" align="center"><span class="smcap">Deep-Sea Thermometers.</span></td></tr>
+<tr><td align="right">89.</td><td><span class="smcap">On Sixe&#8217;s Principle.</span></td></tr>
+<tr><td align="right">90.</td><td><span class="smcap">Johnson&#8217;s Metallic Thermometer.</span></td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td colspan="2" align="center"><a href="#CHAPTER_X">CHAPTER X.</a></td></tr>
+<tr><td colspan="2" align="center"><span class="smcap">Boiling-Point Thermometers.</span></td></tr>
+<tr><td align="right">91.</td><td>Ebullition.</td></tr>
+<tr><td align="right">92.</td><td>Relation between Boiling-Point and Elevation.</td></tr>
+<tr><td align="right"><span class="pagenum"><a name="Page_x" id="Page_x">[Pg x]</a></span>93.</td><td><span class="smcap">Hypsometric Apparatus.</span></td></tr>
+<tr><td align="right">94.</td><td>Precautions to ensure Correct Graduation.</td></tr>
+<tr><td align="right">95.</td><td>Method of Calculating Heights from Observations with the Mountain Thermometer; Example.</td></tr>
+<tr><td align="right">96.</td><td><span class="smcap">Thermometers for Engineers.</span></td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td colspan="2" align="center"><a href="#CHAPTER_XI">CHAPTER XI.</a></td></tr>
+<tr><td colspan="2" align="center"><span class="smcap">Instruments for Ascertaining the Humidity of the Air.</span></td></tr>
+<tr><td align="right">97.</td><td>Hygrometric Substances.</td></tr>
+<tr><td align="right">98.</td><td><span class="smcap">Saussure&#8217;s Hygrometer.</span></td></tr>
+<tr><td align="right">99.</td><td>Dew-Point.</td></tr>
+<tr><td align="right">100.</td><td><span class="smcap">Drosometer.</span></td></tr>
+<tr><td align="right">101.</td><td>Humidity.</td></tr>
+<tr><td align="right">102.</td><td><span class="smcap">Leslie&#8217;s Hygrometer.</span></td></tr>
+<tr><td align="right">103.</td><td><span class="smcap">Daniel&#8217;s Hygrometer.</span></td></tr>
+<tr><td align="right">104.</td><td><span class="smcap">Regnault&#8217;s Condenser Hygrometer.</span></td></tr>
+<tr><td align="right">105.</td><td>Temperature of Evaporation.</td></tr>
+<tr><td align="right">106.</td><td><span class="smcap">Mason&#8217;s Hygrometer.</span></td></tr>
+<tr><td align="right">107.</td><td><span class="smcap">Self-registering Hygrometer.</span></td></tr>
+<tr><td align="right">108.</td><td>Causes of Dew.</td></tr>
+<tr><td align="right">109.</td><td>Plan of Exposing Thermometers.</td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td colspan="2" align="center"><a href="#CHAPTER_XII">CHAPTER XII.</a></td></tr>
+<tr><td colspan="2" align="center"><span class="smcap">Instruments used for Measuring the Rainfall.</span></td></tr>
+<tr><td align="right">110.</td><td><span class="smcap">Howard&#8217;s Rain-Gauge.</span></td></tr>
+<tr><td align="right">111.</td><td><span class="smcap">Glaisher&#8217;s Rain-Gauge.</span></td></tr>
+<tr><td align="right">112.</td><td><span class="smcap">Rain-Gauge with Float.</span></td></tr>
+<tr><td align="right">113.</td><td><span class="smcap">Rain-Gauge with Side Tube.</span></td></tr>
+<tr><td align="right">114.</td><td><span class="smcap">FitzRoy&#8217;s Rain-Gauge.</span></td></tr>
+<tr><td align="right">115.</td><td><span class="smcap">Self-Registering Rain-Gauge.</span></td></tr>
+<tr><td align="right">116.</td><td>The principle of Measurement.</td></tr>
+<tr><td align="right">117.</td><td>Position for Rain-gauge, &amp;c.</td></tr>
+<tr><td align="right">118.</td><td>Cause of Rain.</td></tr>
+<tr><td align="right">119.</td><td>Laws of Rainfall.</td></tr>
+<tr><td align="right">120.</td><td>Utility of Statistics of Rainfall.</td></tr>
+<tr><td align="right">121.</td><td><span class="smcap">New Form of Rain-gauge.</span></td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td colspan="2" align="center"><a href="#CHAPTER_XIII">CHAPTER XIII.</a></td></tr>
+<tr><td colspan="2" align="center"><span class="smcap">Apparatus employed for Registering the Direction, Pressure, and Velocity of the Wind.</span></td></tr>
+<tr><td align="right">122.</td><td><span class="smcap">The Vane.</span></td></tr>
+<tr><td align="right">123.</td><td><span class="smcap">Lind&#8217;s Wind-Gauge.</span></td></tr>
+<tr><td align="right">124.</td><td><span class="smcap">Harris&#8217;s Wind-Gauge.</span></td></tr>
+<tr><td align="right"><span class="pagenum"><a name="Page_xi" id="Page_xi">[Pg xi]</a></span>125.</td><td><span class="smcap">Robinson&#8217;s Anemometer.</span></td></tr>
+<tr><td align="right">126.</td><td><span class="smcap">Whewell&#8217;s Anemometer.</span></td></tr>
+<tr><td align="right">127.</td><td><span class="smcap">Osler&#8217;s Anemometer and Pluviometer.</span></td></tr>
+<tr><td align="right">128.</td><td><span class="smcap">Beckley&#8217;s Anemometer.</span></td></tr>
+<tr><td align="right">129.</td><td><span class="smcap">Self-Registering Wind-Gauge.</span></td></tr>
+<tr><td align="right">130.</td><td>Anemometric Observations.</td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td colspan="2" align="center"><a href="#CHAPTER_XIV">CHAPTER XIV.</a></td></tr>
+<tr><td colspan="2" align="center"><span class="smcap">Instruments for Investigating Atmospheric Electricity.</span></td></tr>
+<tr><td align="right">131.</td><td><span class="smcap">Atmospheric Electroscope.</span></td></tr>
+<tr><td align="right">132.</td><td><span class="smcap">Volta&#8217;s Electrometer.</span></td></tr>
+<tr><td align="right">133.</td><td><span class="smcap">Peltier&#8217;s Electrometer.</span></td></tr>
+<tr><td align="right">134.</td><td><span class="smcap">Bohnenberger&#8217;s Electroscope.</span></td></tr>
+<tr><td align="right">135.</td><td><span class="smcap">Thomson&#8217;s Electrometer.</span></td></tr>
+<tr><td align="right">136.</td><td>Fundamental Facts.</td></tr>
+<tr><td align="right">137.</td><td>Lightning Conductors.</td></tr>
+<tr><td align="right">138.</td><td>Precautions against Lightning.</td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td colspan="2" align="center"><a href="#CHAPTER_XV">CHAPTER XV.</a></td></tr>
+<tr><td colspan="2" align="center"><span class="smcap">Ozone and its Indicators.</span></td></tr>
+<tr><td align="right">139.</td><td>Nature of Ozone.</td></tr>
+<tr><td align="right">140.</td><td><span class="smcap">Schonbein&#8217;s Ozonometer.</span></td></tr>
+<tr><td align="right">141.</td><td><span class="smcap">Moffat&#8217;s Ozonometer.</span></td></tr>
+<tr><td align="right">142.</td><td><span class="smcap">Clark&#8217;s Ozone Cage.</span></td></tr>
+<tr><td align="right">143.</td><td>Distribution and Effects of Ozone.</td></tr>
+<tr><td align="right">144.</td><td><span class="smcap">Lancaster&#8217;s Registering Ozonometer.</span></td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td colspan="2" align="center"><a href="#CHAPTER_XVI">CHAPTER XVI.</a></td></tr>
+<tr><td colspan="2" align="center"><span class="smcap">Miscellaneous Instruments.</span></td></tr>
+<tr><td align="right">145.</td><td><span class="smcap">Chemical Weather Glass.</span></td></tr>
+<tr><td align="right">146.</td><td><span class="smcap">Leslie&#8217;s Differential Thermometer.</span></td></tr>
+<tr><td align="right">147.</td><td><span class="smcap">Romford&#8217;s Differential Thermometer.</span></td></tr>
+<tr><td align="right">148.</td><td><span class="smcap">Glaisher&#8217;s Thermometer Stand.</span></td></tr>
+<tr><td align="right">149.</td><td><span class="smcap">Thermometer Screen, for use at Sea.</span></td></tr>
+<tr><td align="right">150.</td><td><span class="smcap">Anemoscope.</span></td></tr>
+<tr><td align="right">151.</td><td><span class="smcap">Evaporating Dish, or Gauge.</span></td></tr>
+<tr><td align="right">152.</td><td><span class="smcap">Admidometer.</span></td></tr>
+<tr><td align="right">153.</td><td><span class="smcap">Cloud Reflector.</span></td></tr>
+<tr><td align="right">154.</td><td><span class="smcap">Sunshine Recorder.</span></td></tr>
+<tr><td align="right">155.</td><td><span class="smcap">Set of Portable Instruments.</span></td></tr>
+<tr><td align="right">156.</td><td><span class="smcap">Implements.</span></td></tr>
+<tr><td align="right">157.</td><td><span class="smcap">Hydrometer.</span></td></tr>
+<tr><td align="right">158.</td><td><span class="smcap">Newman&#8217;s Self-Registering Tide-Gauge.</span></td></tr></table>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_xii" id="Page_xii">[Pg xii]</a></span></p>
+<h2>TABLES.</h2>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td>&nbsp;</td><td align="right"><small>PAGE</small></td></tr>
+<tr><td>Table of Corrections, for Capillary Depression of the Mercury in Boiled and in Unboiled Barometer-Tubes</td><td align="right"><a href="#Page_6">6</a></td></tr>
+<tr><td>Tables for Deducing Heights by means of the Barometer:&mdash;</td></tr>
+<tr><td><span style="margin-left: 2em;">No. 1. Approximate Height due to Barometric Pressure</span></td><td align="right"><a href="#Page_42">42</a></td></tr>
+<tr><td><span style="margin-left: 2em;">No. 2. Correction for Mean Temperature of Air</span></td><td align="right"><a href="#Page_44">44</a></td></tr>
+<tr><td><span style="margin-left: 2em;">No. 3. Correction due to Latitude</span></td><td align="right"><a href="#Page_44">44</a></td></tr>
+<tr><td><span style="margin-left: 2em;">No. 4. Correction due to Approximate Elevation</span></td><td align="right"><a href="#Page_45">45</a></td></tr>
+<tr><td>Tables for Determining the Temperature of the Vapour of Boiling Water at any Place:&mdash;</td></tr>
+<tr><td><span style="margin-left: 2em;">No. 5. Factor due to Latitude</span></td><td align="right"><a href="#Page_62">62</a></td></tr>
+<tr><td><span style="margin-left: 2em;">No. 6. Temperature and Tension</span></td><td align="right"><a href="#Page_62">62</a></td></tr>
+<tr><td>Table of Temperature of the Soil</td><td align="right"><a href="#Page_69">69</a></td></tr>
+<tr><td>Table of Difference of Elevation corresponding to a fall of 1&deg; in the Boiling-point of Water</td><td align="right"><a href="#Page_98">98</a></td></tr>
+<tr><td>Table showing Proportion of Salt for various Boiling Temperatures of Sea-Water</td><td align="right"><a href="#Page_100">100</a></td></tr>
+<tr><td>Table for finding the Degree of Humidity from Observations with Mason&#8217;s Hygrometer</td><td align="right"><a href="#Page_108">108</a></td></tr>
+<tr><td>Table showing Amount and Duration of Rain at London, in 1862</td><td align="right"><a href="#Page_112">112</a></td></tr>
+<tr><td>Table of Average British Rainfall in Westerly, Central, and Easterly districts</td><td align="right"><a href="#Page_114">114</a></td></tr>
+<tr><td>Table showing Force of Wind, for use with Lind&#8217;s Wind-Gauge</td><td align="right"><a href="#Page_118">118</a></td></tr>
+<tr><td>Tables for Correcting Observations made with&mdash;</td></tr>
+<tr><td><span style="margin-left: 1em;">Brass Hydrometers</span></td><td align="right"><a href="#Page_142">142</a></td></tr>
+<tr><td><span style="margin-left: 1em;">Glass Hydrometers</span></td><td align="right"><a href="#Page_143">143</a></td></tr></table>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<h2>ADDENDA.</h2>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td colspan="2">&nbsp;</td><td align="right"><small>PAGE</small></td></tr>
+<tr><td align="right">1.</td><td>Rule for converting Millimetres into Inches, et vice versa</td><td align="right"><a href="#Page_146">146</a></td></tr>
+<tr><td align="right">2.</td><td>Old French Lineal Measure, with English Equivalents</td><td align="right"><a href="#Page_146">146</a></td></tr>
+<tr><td align="right">3.</td><td>Rule for finding Diameter of Bore of Barometer Tube</td><td align="right"><a href="#Page_146">146</a></td></tr>
+<tr><td align="right">4.</td><td>Wind Scales</td><td align="right"><a href="#Page_147">147</a></td></tr>
+<tr><td align="right">5.</td><td>Letters to denote the State of the Weather</td><td align="right"><a href="#Page_147">147</a></td></tr>
+<tr><td align="right">6.</td><td>Table of Expansion of Bodies</td><td align="right"><a href="#Page_148">148</a></td></tr>
+<tr><td align="right">7.</td><td>Table of Specific Gravity of Bodies</td><td align="right"><a href="#Page_148">148</a></td></tr>
+<tr><td align="right">8.</td><td>Important Temperatures</td><td align="right"><a href="#Page_148">148</a></td></tr>
+<tr><td align="right">9.</td><td>Table of Meteorological Elements, forming Exponents of the Climate of London</td><td align="right"><a href="#Page_149">149</a></td></tr>
+<tr><td align="right">10.</td><td>List of Works on Meteorology</td><td align="right"><a href="#Page_151">151</a></td></tr></table>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_1" id="Page_1">[Pg 1]</a></span></p>
+<h2>METEOROLOGICAL INSTRUMENTS.</h2>
+
+<p>In the pursuits and investigations of the science of Meteorology, which is
+essentially a science of observation and experiment, instruments are
+required for ascertaining, 1. the pressure of the atmosphere at any time
+or place; 2. the temperature of the air; 3. the absorption and radiation
+of the sun&#8217;s heat by the earth&#8217;s surface; 4. the humidity of the air; 5.
+the amount and duration of rainfall; 6. the direction, the horizontal
+pressure, and the velocity of winds; 7. the electric condition of the
+atmosphere, and the prevalence and activity of ozone.</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<h2><a name="CHAPTER_I" id="CHAPTER_I"></a>CHAPTER I.</h2>
+<p class="center"><span class="large">INSTRUMENTS FOR ASCERTAINING THE ATMOSPHERIC PRESSURE.</span></p>
+
+
+<div class="figright">Fig. 1.<br /><img src="images/fig_1.jpg" alt="" /></div>
+
+<p><br /><b>1. Principle of the Barometer.</b>&mdash;The first instrument which gave the exact
+measure of the pressure of the atmosphere was invented by Torricelli, in
+1643. It is constructed as follows:&mdash;A glass tube, CD (fig. 1), about 34
+inches long, and from two to four-tenths of an inch in diameter of bore,
+having one end closed, is filled with mercury. In a cup, B, a quantity of
+mercury is also poured. Then, placing a finger securely over the open end,
+C, invert the tube vertically over the cup, and remove the finger when the
+end of the tube dips into the mercury. The mercury in the tube then partly
+falls out, but a column, AB, about 30 inches in height, remains supported.
+This column is a weight of mercury, the pressure of which upon the surface
+of that in the cup is precisely equivalent to the corresponding pressure
+of the atmosphere which would be exerted in its place if the tube were
+removed. As the atmospheric pressure varies, the length of this mercurial
+column also changes. It is by no means constant in its height; in fact, it
+is very seldom stationary, but is constantly rising or falling through a
+certain extent of the tube, at the level of the sea, near which the above
+experiment is supposed to be performed. It is, therefore, an instrument by
+which the fluctuations taking place in the<span class="pagenum"><a name="Page_2" id="Page_2">[Pg 2]</a></span> pressure of the atmosphere,
+arising from changes in its weight and elasticity, can be shown and
+measured. It has obtained the name <i>Barometer</i>, or measurer of
+heaviness,&mdash;a word certainly not happily expressive of the utility of the
+invention. If the bore of the barometer tube be uniform throughout its
+length, and have its sectional area equal to a square inch, it is evident
+that the length of the column, which is supported by the pressure of the
+air, expresses the number of cubic inches of mercury which compose it. The
+weight of this mercury, therefore, represents the statical pressure of the
+atmosphere upon a square inch of surface. In England the annual mean
+height of the barometric column, reduced to the sea-level and to the
+temperature of 32&deg; Fahrenheit, is about 29&middot;95 inches. A cubic inch of
+mercury at this temperature has been ascertained to weigh 0&middot;48967 lbs.
+avoirdupois. Hence, 29&middot;95 &times; 0&middot;48967= 14&middot;67 lbs., is the mean value of the
+pressure of the atmosphere on each square inch of surface, near the
+sea-level, about the latitude of 50 degrees. Nearer the equator this mean
+pressure is somewhat greater; nearer the poles, somewhat less. For common
+practical calculations it is assumed to be 15 lbs. on the square inch.
+When it became apparent that the movements of the barometric column
+furnished indications of the probable coming changes in the weather, an
+attempt was made to deduce from recorded observations the barometric
+height corresponding to the most notable characteristics of weather. It
+was found that for fine dry weather the mercury in the barometer at the
+sea-level generally stood above 30 inches; changeable weather happened
+when it ranged from 30 to 29 inches, and when rainy or stormy weather
+occurred it was even lower. Hence, it became the practice to place upon
+barometer scales words indicatory of the weather likely to accompany, or
+follow, the movements of the mercury; whence the instruments bearing them
+obtained the name &#8220;Weather Glasses.&#8221;</p>
+
+
+<p><br /><b>2. Construction of Barometers.</b>&mdash;In order that the instrument may be
+portable, it must be made a fixture and mounted on a support; and,
+further, to render it scientifically or even practically useful, many
+precautions are required in its construction. The following remarks apply
+to the construction of all barometers:&mdash;Mercury is universally employed,
+because it is the heaviest of fluids, and therefore measures the
+atmospheric pressure by the shortest column. Water barometers have been
+constructed, and they require to be at least 34 feet long. Oil, or other
+fluids, might be used. Mercury, however, has other advantages: it has
+feeble volatility, and does not adhere to glass, if pure. Oxidised, or
+otherwise impure mercury, may adhere to glass; moreover, such mercury
+would not have the density of the pure metal, and therefore the barometric
+column would be either greater or less than it should be. The mercury of
+commerce generally contains lead; sometimes traces of iron and sulphur. It
+is necessary, therefore, for the manufacturer to purify the mercury; and
+this is done by washing it with diluted acetic, or sulphuric acid, which
+dissolves the impurities. No better test can be found for ascertaining if
+the mercury be pure than that of filling a delicate thermometer tube; if,
+on exhausting<span class="pagenum"><a name="Page_3" id="Page_3">[Pg 3]</a></span> the air from this thermometer, the mercury will freely run
+up and down the bore, which is probably one thousandth of an inch in
+diameter, the mercury from which this thermometer was made will be found
+fit for any purpose, and with it a tube may be filled and boiled, not only
+of one inch, but even of two inches diameter. In all barometers it is
+requisite that the space above the mercurial column should be completely
+void of air and aqueous vapour, because these gases, by virtue of their
+elasticity, would depress the column. To exclude these the mercury is
+introduced, and boiled in the tube, over a charcoal fire, kept up for the
+purpose. In this manner the air and vapour which adhere to the glass are
+expanded, and escape away. One can tell whether a barometer has been
+properly &#8220;boiled,&#8221; as it is termed, by simply holding the tube in a
+slanting direction and allowing the mercury to strike the top. If the
+boiling has been well performed, the mercury will give a clear, metallic
+sound; if not, a dull, flat sound, showing some air to be present.</p>
+
+<p>When the mercury in a barometer tube rises or falls, the level of the
+mercury in the cup, or <i>cistern</i>, as it is generally termed, falls or
+rises by a proportionate quantity, which depends upon the relative areas
+of the interior of the tube and of the cistern. It is necessary that this
+should be taken into consideration in ascertaining the exact height of the
+column. If a fixed scale is applied to the tube, the correct height may be
+obtained by applying a correction for capacity. A certain height of the
+mercury is ascertained to be accurately measured by the scale, and should
+be marked on the instrument as the <i>neutral point</i>. Above this point the
+heights measured are all less, and below, all more, than they should be.
+The ratio between the internal diameters of the tube and cistern (which
+should also be stated on the instrument, as, for instance, capac. <span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">50</span>)
+supplies the data for finding the correction to be applied. This
+correction is obviated by constructing the cistern so as to allow of the
+surface of the mercury in it being adjustable to the commencement of the
+fixed scale, as by Fortin&#8217;s or Negretti&#8217;s plan. It is also unnecessary in
+barometers constructed on what is now called the &#8220;Kew method.&#8221; These will
+all be detailed in their proper place. The tube, being fixed to the
+cistern, may have a moveable scale applied to it. But such an arrangement
+requires the utmost care and skill in observing, and is seldom seen except
+in first-class Observatories.</p>
+
+
+<div class="figright"><img src="images/fig_2tmb.jpg" alt="" /><br /><a href="images/fig_2.jpg"><small>Larger Image</small></a><br />Fig. 2.</div>
+
+<p><br /><b>3. Fortin&#8217;s Barometer.</b>&mdash;Fortin&#8217;s plan of constructing a barometer cistern
+is shown in fig. 2. The cistern is formed of a glass cylinder, which
+allows of the level of the mercury within being seen. The bottom of the
+cylinder is made of sheep-skin or leather, like a bag, so as to allow of
+being pushed up or lowered by means of a screw, D B, worked from beneath.
+This screw moves through the bottom of a brass cylinder, C C, which is
+fixed outside, and protects the glass cylinder containing the mercury. At
+the top of the interior of the cistern is fixed a small piece of ivory, A,
+the point of which<span class="pagenum"><a name="Page_4" id="Page_4">[Pg 4]</a></span> exactly coincides with the zero of the scale. This
+screw and moveable cistern-bottom serve also to render the barometer
+portable, by confining the mercury in the tube, and preventing its coming
+into the cistern, which is thus made too small to receive it.</p>
+
+<div class="figleft"><img src="images/fig_3.jpg" alt="" /><br />Fig. 3.</div>
+
+<p class="center"><br />4. STANDARD BAROMETER.</p>
+
+<p>Fig. 3 represents a Standard Barometer on Fortin&#8217;s principle. The
+barometer tube is enclosed and protected by a tube of brass extending
+throughout its whole length; the upper portion of the brass tube has two
+longitudinal openings opposite each other; on one side of the front
+opening is the barometrical scale of English inches, divided to show, by
+means of a vernier, <span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">500</span>th of an inch; on the opposite side is sometimes
+divided a scale of French millimetres, reading also by a vernier to <span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">10</span>th
+of a millimetre (see directions for reading the vernier, <a href="#Page_7">page 7</a>). A
+thermometer, C, is attached to the frame, and divided to degrees, which
+can be read to tenths; it is necessary for ascertaining the temperature of
+the instrument, in order to correct the observed height of the barometer.</p>
+
+<p>As received by the observer, the barometer will consist of two parts,
+packed separately for safety in carriage,&mdash;1st, the barometer tube and
+cistern, filled with mercury, the brass tube, with its divided scale and
+thermometer; and 2nd, a mahogany board, with bracket at top, and brass
+ring with three adjusting screws at bottom.</p>
+
+<p><i>Directions for fixing the Barometer.</i>&mdash;In selecting a position for a
+barometer, care should be taken to place it so that the sun cannot shine
+upon it, and that it is not affected by direct heat from a fire. The
+cistern should be from two to three feet above the ground, which will give
+a height for observing convenient to most persons. A standard barometer
+should be compared with an observatory standard of acknowledged accuracy,
+to determine its index error; which, as such instruments are graduated by
+micrometrical apparatus of great exactitude, will be constant for all
+parts of the scale. It should be capable of turning on its axis by a
+movement of the hand, so that little difficulty can ever be experienced in
+obtaining a good light for observation. Having determined upon the
+position in which to place the instrument, fix the mahogany board as
+nearly vertical as possible, and ascertain if the barometer is perfect and
+free<span class="pagenum"><a name="Page_5" id="Page_5">[Pg 5]</a></span> from air, in the following manner:&mdash;lower the screw at the bottom of
+the cistern several turns, so that the mercury in the tube, when held
+upright, may fall two or three inches from the top; then slightly incline
+the instrument from the vertical position, and if the mercury in striking
+the top elicit a sharp tap, the instrument is perfect. Supposing the
+barometer to be in perfect condition, as it is almost sure to be, it is
+next suspended on the brass bracket, its cistern passing through the ring
+at bottom, and allowed to find its vertical position, after which it is
+firmly clamped by means of the three thumb-screws.</p>
+
+<p><i>To Remove the Instrument when fixed to another Position.</i>&mdash;If it should
+be necessary to remove the barometer,&mdash;first, by means of the adjusting
+screw, drive the mercury to the top of the tube, turning it gently when it
+is approaching the top, and cease directly any resistance is experienced;
+next, remove from the upper bracket or socket; lift the instrument and
+invert it, carrying it with its lower end upwards.</p>
+
+<p><i>Directions for taking an Observation.</i>&mdash;Before making an observation, the
+mercury in the cistern must be raised or lowered by means of the
+thumb-screw, F, until the ivory point, E, and its reflected image in the
+mercury, D, are just in contact; the vernier is then moved by means of the
+milled head, until its lower termination just excludes the light from the
+top of the mercurial column; the reading is then taken by means of the
+scale on the limb and the vernier. The vernier should be made to read
+upward in all barometers, unless for a special object, as this arrangement
+admits of the most exact setting. In observing, the eye should be placed
+in a right line with the fore and back edges of the lower termination of
+the vernier; and this line should be made to form a tangent to the apex of
+the mercurial column. A small reflector placed behind the vernier and
+moving with it, so as to assist in throwing the light through the back
+slit of the brass frame on to the glass tube, is advantageous; and the
+observer&#8217;s vision may be further assisted by the aid of a reading lens.
+The object is, in these Standard Barometers, to obtain an exact reading,
+which can only be done by having the eye, the fore part of the zero edge
+of the vernier, the top of the mercurial column, and the back of the
+vernier, in the same horizontal plane.</p>
+
+<p><i>Uniformity of Calibre.</i>&mdash;The diameter of that part of the tube through
+which the oscillations of the mercury will take place is very carefully
+examined to insure uniformity of calibre, and only those tubes are used
+which are as nearly as possible of the same diameter throughout. The size
+of the bore should be marked on the frame of the barometer in tenths and
+hundredths of an inch. A correction due to capillary action, and depending
+on the size of the tube, must be applied to the readings.</p>
+
+
+<p><br /><b>5. Correction due to Capillarity.</b>&mdash;When an open tube of small bore is
+plunged into mercury, the fluid will not rise to the same level inside as
+it has outside.<span class="pagenum"><a name="Page_6" id="Page_6">[Pg 6]</a></span> Hence, the effect of capillary action is to depress the
+mercurial column; and the more so the smaller the tube. The following
+table gives the correction for tubes in ordinary use:&mdash;</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td align="center"><i>Diameter of<br />tube.</i></td><td><span class="spacer">&nbsp;</span></td>
+    <td align="center"><i>Depression, in<br />boiled tubes.</i></td><td><span class="spacer">&nbsp;</span></td>
+    <td align="center"><i>Depression, in<br />unboiled tubes.</i></td></tr>
+<tr><td align="center"><span class="smcaplc">INCH.</span></td><td>&nbsp;</td>
+    <td align="center"><span class="smcaplc">INCH.</span></td><td>&nbsp;</td>
+    <td align="center"><span class="smcaplc">INCH.</span></td></tr>
+<tr><td align="center">0&middot;60</td><td>&nbsp;</td>
+    <td align="center">0&middot;002</td><td>&nbsp;</td>
+    <td align="center">0&middot;004</td></tr>
+<tr><td align="center">0&middot;55</td><td>&nbsp;</td>
+    <td align="center">0&middot;003</td><td>&nbsp;</td>
+    <td align="center">0&middot;005</td></tr>
+<tr><td align="center">0&middot;50</td><td>&nbsp;</td>
+    <td align="center">0&middot;003</td><td>&nbsp;</td>
+    <td align="center">0&middot;007</td></tr>
+<tr><td align="center">0&middot;45</td><td>&nbsp;</td>
+    <td align="center">0&middot;005</td><td>&nbsp;</td>
+    <td align="center">0&middot;010</td></tr>
+<tr><td align="center">0&middot;40</td><td>&nbsp;</td>
+    <td align="center">0&middot;007</td><td>&nbsp;</td>
+    <td align="center">0&middot;015</td></tr>
+<tr><td align="center">0&middot;35</td><td>&nbsp;</td>
+    <td align="center">0&middot;010</td><td>&nbsp;</td>
+    <td align="center">0&middot;021</td></tr>
+<tr><td align="center">0&middot;15</td><td>&nbsp;</td>
+    <td align="center">0&middot;044</td><td>&nbsp;</td>
+    <td align="center">0&middot;029</td></tr>
+<tr><td align="center">0&middot;10</td><td>&nbsp;</td>
+    <td align="center">0&middot;070</td><td>&nbsp;</td>
+    <td align="center">0&middot;041</td></tr>
+<tr><td align="center">0&middot;30</td><td>&nbsp;</td>
+    <td align="center">0&middot;014</td><td>&nbsp;</td>
+    <td align="center">0&middot;058</td></tr>
+<tr><td align="center">0&middot;25</td><td>&nbsp;</td>
+    <td align="center">0&middot;020</td><td>&nbsp;</td>
+    <td align="center">0&middot;086</td></tr>
+<tr><td align="center">0&middot;20</td><td>&nbsp;</td>
+    <td align="center">0&middot;029</td><td>&nbsp;</td>
+    <td align="center">0&middot;140</td></tr></table>
+
+<p>This correction is always additive to the observed reading of the
+barometer.</p>
+
+
+<p><br /><b>6. Correction due to Temperature.</b>&mdash;In all kinds of mercurial barometers
+attention must be given to the temperature of the mercury. As this metal
+expands and contracts very much for variations of temperature, its density
+alters correspondingly, and in consequence the height of the barometric
+column also varies. To ascertain the temperature of the mercury, a
+thermometer is placed near the tube, and is sometimes made to dip into the
+mercury in the cistern. The freezing point of water, 32&deg;F., is the
+temperature to which all readings of barometers must be reduced, in order
+to make them fairly comparable. The reduction may be effected by
+calculation, but the practical method is by tables for the purpose; and
+for these tables we refer the reader to the works mentioned at the end of
+this book.</p>
+
+
+<p><br /><b>7. Correction due to Height above the Half-tide Level.</b>&mdash;Further, in order
+that barometrical observations generally may be made under similar
+circumstances, the readings, corrected for capacity, capillarity, and
+temperature, should be reduced to what they would be at the sea-level, by
+adding a correction corresponding to the height above the mean level of
+the sea, or of half-tide. For practical purposes of comparison with
+barometric pressure at other localities, add one-tenth of an inch to the
+reading for each hundred feet of elevation above the sea. For scientific
+accuracy this will not suffice, but a correction must be obtained by means
+of Schuckburg&#8217;s formula, or tables computed therefrom.</p>
+
+<div class="figleft">Fig. 4.<br /><img src="images/fig_4.jpg" alt="" /></div>
+
+<div class="figright">Fig. 5.<br /><img src="images/fig_5.jpg" alt="" /></div>
+
+<p><br /><b>8. The Barometer Vernier.</b>&mdash;The <i>vernier</i>, an invaluable contrivance for
+measuring small spaces, was invented by Peter Vernier, about the year
+1630. The barometer scale is divided into inches and tenths. The vernier
+enables us to accurately subdivide the tenths into hundredths, and, in
+first-class instruments, even<span class="pagenum"><a name="Page_7" id="Page_7">[Pg 7]</a></span> to thousandths of an inch. It consists of a
+short scale made to pass along the graduated fixed scale by a sliding
+motion, or preferably by a rack-and-pinion motion, the vernier being fixed
+on the rack, which is moved by turning the milled head of the pinion. The
+principle of the vernier, to whatever instrumental scale applied, is that
+the divisions of the moveable scale are to those in an equal length of the
+fixed scale in the proportion of two numbers which differ from each other
+by unity.</p>
+
+<p>The scales of standard barometers are usually divided into half-tenths, or
+&middot;05, of an inch, as represented, in fig. 5, by AB. The vernier, CD, is
+made equal in length to twenty-four of these divisions, and divided into
+twenty-five equal parts; consequently one space on the scale is larger
+than one on the vernier by the twenty-fifth part of &middot;05, which is &middot;002
+inch, so that such a vernier shows differences of &middot;002 inch. The vernier
+of the figure reading upwards, the lower edge, D, will denote the top of
+the barometer column; and is the zero of the vernier scale. In fig. 4, the
+zero being in line exactly with 29 inches and five-tenths of the fixed
+scale, the barometer reading would be 29&middot;500 inches. It will be seen that
+the vernier line, <i>a</i>, falls short of a division of the scale by, as we
+have explained, &middot;002 inch; <i>b</i>, by &middot;004; <i>c</i>, by &middot;006; <i>d</i>, by &middot;008; and
+the next line by one hundredth. If, then, the vernier be moved so as to
+make <i>a</i> coincide with <i>z</i>, on the scale, it will have moved through &middot;002
+inch; and if 1 on the vernier be moved into line with <i>y</i> on the scale,
+the space measured will be &middot;010. Hence, the figures 1, 2, 3, 4, 5 on the
+vernier measure hundredths, and the intermediate lines even thousandths of
+an inch. In fig. 5, the zero of the vernier is intermediate 29&middot;65 and
+29&middot;70 on the scale. Passing the eye up the vernier and scale, the second
+line above 3 is perceived to lie evenly with a line of the scale. This
+gives &middot;03 and &middot;004 to add to 29&middot;65, so that the actual reading is 29&middot;684
+inches. It may happen that no line on the vernier <i>accurately</i> lies in the
+same straight line with one on the scale; in such a case a doubt will
+arise as to the selection of one from two equally coincident, and the
+intermediate thousandth of an inch should be taken.</p>
+
+<p>For the ordinary purposes of the barometer as a &#8220;weather-glass,&#8221; such
+minute measurement is not required. Hence, in household and marine
+barometers the scale need only be divided to tenths, and the vernier
+constructed to measure <span class="pagenum"><a name="Page_8" id="Page_8">[Pg 8]</a></span>hundredths of an inch. This is done by making the
+vernier either 9 or 11-10ths of an inch long, and dividing it into ten
+equal parts. The lines above the zero line are then numbered from 1 to 10;
+sometimes the alternate divisions only are numbered, the intermediate
+numbers being very readily inferred. Hence, if the first line of the
+vernier agrees with one on the scale, the next must be out one-tenth of a
+tenth, or &middot;01 of an inch from agreement with the next <i>scale</i> line; the
+following vernier line must be &middot;02 out, and so on. Consequently, when the
+vernier is set to the mercurial column, the difference shown by the
+vernier from the tenth on the scale is the hundredths to be added to the
+inches and tenths of the scale.</p>
+
+<p>A little practice will accustom a person to set and read any barometer
+quickly; an important matter where accuracy is required, as the heat of
+the body, or the hand, is very rapidly communicated to the instrument, and
+may vitiate, to some extent, the observation.</p>
+
+<div class="figleft">Fig. 6.<br /><img src="images/fig_6.jpg" alt="" /></div>
+
+<p class="center"><br />9. SELF-COMPENSATING STANDARD BAROMETER.</p>
+
+<p>This barometer has been suggested to Messrs. Negretti and Zambra by
+Wentworth Erk, Esq. It consists of a regular barometer; but attached to
+the vernier is a double rack worked with one pinion, so that in setting or
+adjusting the vernier in one position, the second rack moves in directly
+the opposite direction, carrying along with it a plug or plunger the exact
+size of the internal diameter of the tube dipping in the cistern, so that
+whatever the displacement that has taken place in the cistern, owing to
+the rise or fall of the mercury, it is exactly compensated by the plug
+being more or less immersed in the mercury, so that no capacity correction
+is required.</p>
+
+<p>A barometer on this principle is, however, no novelty, for at the Royal
+Society&#8217;s room a very old instrument may be seen reading somewhat after
+the same manner.</p>
+
+<p>Fig. 6 is an illustration of the appearance of this instrument. The
+cistern is so constructed that the greatest amount of light is admitted to
+the surface of the mercury.</p>
+
+
+<p class="center"><br />10. BAROMETER WITH ELECTRICAL ADJUSTMENT.</p>
+
+<p>This barometer is useful to persons whose eyesight may be defective; and
+is capable of being read off to greater accuracy than ordinary barometers,
+as will be seen by the following description:&mdash;The barometer consists of
+an upright tube dipping into a cistern, so contrived, that an up-and-down
+movement, by means of a screw, can be imparted to it. In the top of the
+tube a piece of platina wire is hermetically sealed. The cistern also has
+a metallic connection, so that by means of<span class="pagenum"><a name="Page_9" id="Page_9">[Pg 9]</a></span> covered copper wires (in the
+back of the frame) a circuit is established; another connection also
+exists by means of a metallic point dipping into the cistern. The circuit,
+however, can be cut off from this by means of a switch placed about midway
+up the frame; on one side of the tube is placed a scale of inches; a small
+circular vernier, divided into 100 parts, is connected with the dipping
+point, and works at right angles with this scale.</p>
+
+<p>To set the instrument in action for taking an observation, a small battery
+is connected by means of two small binding screws at the bottom of the
+frame. The switch is turned upwards, thereby disconnecting the dipping
+point; the cistern is then screwed up, so that the mercury in the tube is
+brought into contact with the platina wire at the top; the instant this is
+effected the magnetic needle seen on the barometer will be deflected. The
+switch is now turned down; by so doing the connection with the upper wire
+or platina is cut off, and established instead only between the dipping
+point carrying the circular vernier and the bottom of the cistern; the
+point is now screwed by means of the milled head until the needle is again
+deflected. We may now be sure that the line on the circular vernier that
+cuts the division on the scale is the exact height of the barometer.
+Although the description here given may seem somewhat lengthy, the
+operation itself is performed in less time than would be taken in reading
+off an ordinary instrument.</p>
+
+
+<p class="center"><br />11. PEDIMENT BAROMETERS.</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td align="center">Fig. 7.</td><td><span class="spacer2">&nbsp;</span></td>
+    <td align="center">Fig. 8.</td><td><span class="spacer2">&nbsp;</span></td>
+    <td align="center">Fig. 9.</td><td><span class="spacer2">&nbsp;</span></td>
+    <td align="center">Fig. 10.</td><td><span class="spacer2">&nbsp;</span></td>
+    <td align="center">Fig. 11.</td></tr>
+<tr><td align="center"><img src="images/fig_7.jpg" alt="" /></td><td>&nbsp;</td>
+    <td align="center"><img src="images/fig_8.jpg" alt="" /></td><td>&nbsp;</td>
+    <td align="center"><img src="images/fig_9.jpg" alt="" /></td><td>&nbsp;</td>
+    <td align="center"><img src="images/fig_10.jpg" alt="" /></td><td>&nbsp;</td>
+    <td align="center"><img src="images/fig_11.jpg" alt="" /></td></tr></table>
+
+<p>These Barometers, generally for household purposes, are illustrated by
+figs. 7 to 11.<span class="pagenum"><a name="Page_10" id="Page_10">[Pg 10]</a></span> They are intended chiefly for &#8220;weather glasses,&#8221; and are
+manufactured to serve not only a useful, but an ornamental purpose as
+well. They are usually framed in wood, such as mahogany, rosewood, ebony,
+oak or walnut, and can be obtained either plain or handsomely and
+elaborately carved and embellished, in a variety of designs, so as to be
+suitable for private rooms, large halls, or public buildings. The scales
+to the barometer and its attached thermometer may be ivory, porcelain, or
+silvered metal. It is not desirable that the vernier should read nearer
+than one-hundredth of an inch. Two verniers and scales may be fitted one
+on either side of the mercurial column, so that one can denote the last
+reading, and thus show at a glance the extent of rise or fall in the
+interval. The scale and thermometer should be covered with plate glass. A
+cheap instrument has an open face and plain frame, with sliding vernier
+instead of rack-and-pinion motion. The barometer may or may not have a
+moveable bottom to the cistern, with screw for the purpose of securing the
+mercury for portability. The cistern should not, however, require
+adjustment to a zero or fiducial point. It should be large enough to
+contain the mercury, which falls from 31 to 27 inches, without any
+appreciable error on the height read off on the scale.</p>
+
+
+<p><br /><b>12. The Words on the Scale.</b>&mdash;The following words are usually engraved on
+the scales of these barometers, although they are not now considered of so
+much importance as formerly:&mdash;</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td>At</td><td>31</td><td>inches</td><td><span class="spacer">&nbsp;</span></td><td>Very dry.</td></tr>
+<tr><td align="center">"</td><td>30&middot;5</td><td align="center">"</td><td>&nbsp;</td><td>Settled fair.</td></tr>
+<tr><td align="center">"</td><td>30</td><td align="center">"</td><td>&nbsp;</td><td>Fair.</td></tr>
+<tr><td align="center">"</td><td>29&middot;5</td><td align="center">"</td><td>&nbsp;</td><td>Changeable.</td></tr>
+<tr><td align="center">"</td><td>29</td><td align="center">"</td><td>&nbsp;</td><td>Rain.</td></tr>
+<tr><td align="center">"</td><td>28&middot;5</td><td align="center">"</td><td>&nbsp;</td><td>Much rain.</td></tr>
+<tr><td align="center">"</td><td>28</td><td align="center">"</td><td>&nbsp;</td><td>Stormy.</td></tr></table>
+
+<p>The French place upon their barometers a similar formula:&mdash;</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td>At</td><td>785</td><td>millim&egrave;tres</td><td><span class="spacer">&nbsp;</span></td><td>Tr&egrave;s-sec.</td></tr>
+<tr><td align="center">"</td><td>776</td><td align="center">"</td><td>&nbsp;</td><td>Beau-fixe.</td></tr>
+<tr><td align="center">"</td><td>767</td><td align="center">"</td><td>&nbsp;</td><td>Beau temps.</td></tr>
+<tr><td align="center">"</td><td>758</td><td align="center">"</td><td>&nbsp;</td><td>Variable.</td></tr>
+<tr><td align="center">"</td><td>749</td><td align="center">"</td><td>&nbsp;</td><td>Pluie ou vent.</td></tr>
+<tr><td align="center">"</td><td>740</td><td align="center">"</td><td>&nbsp;</td><td>Grande pluie.</td></tr>
+<tr><td align="center">"</td><td>731</td><td align="center">"</td><td>&nbsp;</td><td>Temp&ecirc;te.</td></tr></table>
+
+<p>Manufacturers of barometers have uniformly adopted these indications for
+all countries, without regard to the elevation above the sea, or the
+different geographical conditions; and as it can readily be shown that the
+height and variations of the barometer are dependent on these, it follows
+that barometers have furnished indications which, under many
+circumstances, have been completely false. Even in this country, and near
+the sea-level, storms are frequent with the barometer not below<span class="pagenum"><a name="Page_11" id="Page_11">[Pg 11]</a></span> 29; rain
+is not uncommon with the glass at 30; even fine weather sometimes occurs
+with a low pressure; while it is evident that at an elevation of a few
+thousand feet the mercury would never rise to 30 inches; hence, according
+to the scale, there should never be fair weather there. If tempests
+happened as seldom in our latitude as the barometer gets down to 28
+inches, the maritime portion of the community at least would be happy
+indeed. These words have long been ridiculed by persons acquainted with
+the causes of the barometric fluctuations; nevertheless opticians continue
+to place them on the scales, evidently only because they appear to add to
+the importance of the instrument in the eyes of those who have not learned
+their general inutility. In different regions of the world, the
+indications of the barometer are modified by the conditions peculiar to
+the geographical position and elevation above the sea, and it is necessary
+to take account of these in any attempt to found rules of general utility
+in connection with the barometer as a weather guide. All that can be said
+in favour of these words is, that within a few hundred feet of the
+sea-level, when the column rises or falls gradually during two or three
+days towards &#8220;Fair&#8221; or &#8220;Rain,&#8221; the indications they afford of the coming
+weather are generally extremely probable; but when the variations are
+quick, upward or downward, they presage unsettled or stormy weather.</p>
+
+<p>Admiral FitzRoy writes:&mdash;&#8220;The words on the scales of barometers should not
+be so much regarded, for weather indications, as the rising or falling of
+the mercury; for if it stands at <i>Changeable</i>, and then rises a little
+towards <i>Fair</i>, it presages a change of wind or weather, though not so
+great as if the mercury had risen higher; and, on the contrary, if the
+mercury stands above <i>Fair</i> and falls, it presages a change, though not to
+so great a degree as if it had stood lower; besides which, the direction
+and force of wind are not in any way noticed. It is not from the point at
+which the mercury stands that we are alone to form a judgment of the state
+of the weather, but from its <i>rising</i> or <i>falling</i>; and from the movements
+of immediately preceding days as well as hours, keeping in mind effects of
+change of <i>direction</i> and dryness, or moisture, as well as alteration of
+force or strength of wind.&#8221;<small><a name="f1.1" id="f1.1" href="#f1">[1]</a></small></p>
+
+
+<p><br /><b>13. Correction due to Capacity of Cistern.</b>&mdash;These barometers, having no
+adjustment for the zero of the scale, require a correction for the varying
+level of the mercury in the cistern, when the observations are required
+for strict comparison with other barometric observations, or when they are
+registered for scientific purposes; but for the common purpose of
+predicting the weather, this correction is unnecessary. The neutral point,
+and the ratio of the bore of the tube to the diameter of the cistern, must
+be known (see <a href="#Page_3">p. 3</a>). Then the capacity correction, as it is termed, is
+found as follows:&mdash;Take the fractional part, expressed by the capacity
+ratio, of the difference between the observed reading and the height of
+the neutral point; then, if the mercury stand <i>below</i> the neutral point,
+<i>subtract</i> this result from the reading; if it stand <i>above</i>, <i>add</i> it to
+the reading.</p>
+
+<p><span class="pagenum"><a name="Page_12" id="Page_12">[Pg 12]</a></span>For example, suppose the neutral point to be 29&middot;95 inches, and the
+capacity ratio <span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">50</span>, required the correction when the barometer reads
+30&middot;78.</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td>Here</td>
+    <td><span class="spacer">&nbsp;</span></td>
+    <td colspan="2">30&middot;78 - 29&middot;95</td>
+    <td>=</td><td>0&middot;83</td></tr>
+<tr><td rowspan="2">Correction</td>
+    <td>&nbsp;</td>
+    <td rowspan="2" valign="middle">=</td>
+    <td align="center" class="botbor">0&middot;83</td>
+    <td rowspan="2">=</td><td rowspan="2">+0&middot;02</td><td rowspan="2">nearly.</td></tr>
+<tr><td>&nbsp;</td><td align="center">50</td></tr>
+<tr><td>Scale reading</td><td colspan="4">&nbsp;</td><td class="botbor">30&middot;78</td></tr>
+<tr><td>Correct reading</td><td colspan="4">&nbsp;</td><td class="botbor2">30&middot;80</td></tr></table>
+
+<p>Of course the correction could as easily be found to three decimal places,
+if desirable. It is evident that the correction is more important the
+greater the distance of the top of the mercury from the neutral point.</p>
+
+
+<p class="center"><br />14. PUBLIC BAROMETERS.</p>
+
+<p>Since the increased attention paid to the signs of forthcoming weather of
+late years, and the good which has resulted therefrom to farmers,
+gardeners, civil engineers, miners, fishermen, and mariners generally, by
+forewarning of impending wet or stormy weather, the desirability of having
+good barometers exposed in public localities has become evident.</p>
+
+<p>Barometers may now be seen attached to drinking fountains, properly
+protected, and are frequently consulted by the passers-by. But it is among
+those whose lives are endangered by sudden changes in the weather,
+fishermen especially, that the warning monitor is most urgently required.
+Many poor fishing villages and towns have therefore been provided by the
+Board of Trade, at the public expense, and through the humane effort of
+Admiral FitzRoy, with first-class barometers, each fixed in a conspicuous
+position, so as to be easily accessible to all who desire to consult it.
+Following this example, the Royal National Life Boat Institution has
+supplied each of its stations with a similar storm warner; the Duke of
+Northumberland and the British Meteorological Society have erected several
+on the coast of Northumberland; and many other individuals have presented
+barometers to maritime places with which they are connected.</p>
+
+<p>These barometers have all been manufactured by Messrs. Negretti and
+Zambra. The form given to the instrument seems well adapted for public
+purposes.</p>
+
+
+<div class="figleft">Fig. 12.<br /><img src="images/fig_12.jpg" alt="" /></div>
+
+<p><br /><b>15. Fishery or Sea-coast Barometers.</b>&mdash;Fig. 12 gives a representation of
+these coast and fishery barometers. The frame is of<span class="pagenum"><a name="Page_13" id="Page_13">[Pg 13]</a></span> solid oak, firmly
+screwed together. The scales are very legibly engraved on porcelain by
+Negretti and Zambra&#8217;s patent process. The thermometer is large, and easily
+read; and as this instrument is exposed, it will indicate the actual
+temperature sufficiently for practical purposes. The barometer tube is
+three-tenths of an inch in diameter of bore, exhibiting a good column of
+mercury; and the cistern is of such capacity, in relation to the tube,
+that the change of height in the surface of the mercury in the cistern
+corresponding to a change of height of three inches of mercury in the
+tube, is less than one-hundredth of an inch, and therefore, as the
+readings are only to be made to this degree of accuracy, this small error
+is of no importance. The cistern is made of boxwood, which is sufficiently
+porous to allow the atmosphere to influence the mercurial column; but the
+top is plugged with porous cane, to admit of free and certain play.</p>
+
+
+<p><br /><b>16. Admiral FitzRoy&#8217;s Scale Words.</b>&mdash;The directions given on the scales of
+these barometers were drawn up by Admiral FitzRoy, F.R.S. They appear to
+be founded on the following considerations:&mdash;</p>
+
+<p>Supposing a compass diagram, with the principal points laid down, the N.E.
+is the wind for which the barometer stands highest; for the S.W. wind it
+is lowest. This is found to be so in the great majority of cases; but
+there are exceptions to this, as to all rules. The N.E. and S.W. may
+therefore be regarded as the poles of the winds, being opposite each
+other. When the wind veers from the S.W. through W. and N. to N.E., the
+barometer gradually rises; on the contrary, when the wind veers from N.E.
+and E. to S.E., S. and S.W., the mercury falls. A similar curious law
+exists in relation to the veering of the wind, and the action of the
+thermometer. As the wind veers from the S.W. to W. and N., the thermometer
+falls; as it veers from N.E. to E. and S., it rises, because the wind gets
+from a colder to a warmer quarter. The polar winds are cold, dry, and
+heavy. Those from the equatorial regions are warm, moist, and
+comparatively light.</p>
+
+<p>These laws have been clearly developed and expressed by Professor Dov&eacute; in
+his work on the &#8220;Law of Storms.&#8221; The warm winds of Europe are those which
+bring the greatest quantity of rain, as they blow from the ocean, and come
+heavily laden with moisture. The cold winds, besides containing less
+moisture, blow more from the land. The weight of the vapour of the warm
+winds tends to raise the barometric column; but, at the same time, the
+increased dilatation of the air tends to lower it. This latter influence
+being the stronger, the barometer always falls for these winds; and in
+regions where they traverse a large extent of land, retain their heat, and
+become necessarily very dry, the fall in the barometer will be greater.
+Admiral FitzRoy&#8217;s words for the scales of barometers for use in northern
+latitudes, then, are as follows:&mdash;</p>
+
+<p><span class="pagenum"><a name="Page_14" id="Page_14">[Pg 14]</a></span></p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td align="center"><i>RISE.</i></td><td><span class="spacer">&nbsp;</span></td><td align="center"><i>FALL.</i></td></tr>
+<tr><td align="center">FOR</td><td>&nbsp;</td><td align="center">FOR</td></tr>
+<tr><td align="center"><span class="smcap">N. Ely.</span></td><td>&nbsp;</td><td align="center"><span class="smcap">S. Wly.</span></td></tr>
+<tr><td align="center">NW.&mdash;N.&mdash;E.</td><td>&nbsp;</td><td align="center">SE.&mdash;S.&mdash;W.</td></tr>
+<tr><td align="center">DRY</td><td>&nbsp;</td><td align="center">WET</td></tr>
+<tr><td align="center">OR</td><td>&nbsp;</td><td align="center">OR</td></tr>
+<tr><td align="center">LESS</td><td>&nbsp;</td><td align="center">MORE</td></tr>
+<tr><td align="center">WIND.</td><td>&nbsp;</td><td align="center">WIND.</td></tr>
+<tr><td align="center">&mdash;&mdash;&mdash;</td><td>&nbsp;</td><td align="center">&mdash;&mdash;&mdash;</td></tr>
+<tr><td align="center">EXCEPT</td><td>&nbsp;</td><td align="center">EXCEPT</td></tr>
+<tr><td align="center">WET FROM</td><td>&nbsp;</td><td align="center">WET FROM</td></tr>
+<tr><td align="center"><span class="smcap">N. Ed.</span></td><td>&nbsp;</td><td align="center"><span class="smcap">N. Ed.</span></td></tr>
+<tr><td align="center">&mdash;&mdash;&mdash;</td><td>&nbsp;</td><td align="center">&mdash;&mdash;&mdash;</td></tr>
+<tr><td>Long foretold, long last;<br />Short notice, soon past.</td><td>&nbsp;</td><td>First rise after low,<br />Foretells stronger blow.</td></tr></table>
+
+<p>It will be perceived that the exception in each case applies to N.E.
+winds. The barometer may fall with north-easterly winds, but they will be
+violent and accompanied with rain, hail, or snow; again, it will rise with
+these winds accompanied with rain, when they are light, and bring only
+little rain. It rises, however, highest with the dry and light N.E. winds.</p>
+
+<p>These directions are very practically useful; they provide for
+geographical position&mdash;also for elevation above the sea&mdash;since they are
+not appended to any particular height of the column. They are suited to
+the northern hemisphere generally, as well as around the British Isles.
+The same directions are adapted to the southern hemisphere, by simply
+substituting for the letter N the letter S, reading south for north, and
+<i>vice versa</i>. South of the equator the cold winds come from the south; the
+warm, from the north. The S.E. wind in the southern hemisphere corresponds
+to the N.E. in the northern. The laws there are, while the wind veers from
+S.E. through E. to N. and N.W., the barometer falls and the thermometer
+rises. As the wind veers from N.W. through W. and S. to S.E., the
+barometer rises and the thermometer falls.</p>
+
+
+<p><br /><b>17. Instructions for the Sea-coast Barometer.</b>&mdash;The directions for fixing
+the barometer, and making it portable when it has to be removed, should be
+attended to carefully. The barometer should be suspended against a frame
+or piece of wood, so that light may be seen <i>through</i> the tube. Otherwise
+a piece of paper, or a <i>white place</i>, should be behind the upper or <i>scale
+part</i> of the <i>tube</i>.</p>
+
+<p>When suspended on a hook, or stout nail, apply the milled-head key (which
+will be found just below the scales) to the square brass pin at the lower
+end of the instrument, and turn <i>gently</i> toward the left hand till the
+screw stops; then take off the key and replace it for use, near the scale,
+as it was before. The cistern bottom being thus <i>let down</i>, the mercury
+will sink to its proper level quickly.</p>
+
+<p>In removing this barometer it is necessary to <i>slope it gradually</i>, till
+the mercury<span class="pagenum"><a name="Page_15" id="Page_15">[Pg 15]</a></span> is at the top of the tube, and then, with the instrument
+reversed, to screw up the cistern bottom, or bag, by the key, used
+<i>gently</i>, till it stops. It will then be portable, and may be carried with
+the <i>cistern</i> end <i>upwards</i>, or lying flat; but it must not be jarred, or
+receive a concussion.</p>
+
+
+<p><br /><b>18. French Sea-coast Barometer.</b>&mdash;The French have imitated this form of
+barometer for coast service, and have translated Admiral FitzRoy&#8217;s
+indications for the scale as follows:&mdash;</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td colspan="3" align="center"><span class="smcaplc">LA</span></td><td><span class="spacer">&nbsp;</span></td>
+    <td colspan="3" align="center"><span class="smcaplc">LA</span></td></tr>
+<tr><td colspan="3" align="center">HAUSSE</td><td>&nbsp;</td>
+    <td colspan="3" align="center">BAISSE</td></tr>
+<tr><td colspan="3" align="center"><span class="smcaplc">INDIQUE.</span></td><td>&nbsp;</td>
+    <td colspan="3" align="center"><span class="smcaplc">INDIQUE.</span></td></tr>
+<tr><td colspan="3" align="center">&mdash;&mdash;&mdash;</td><td>&nbsp;</td>
+    <td colspan="3" align="center">&mdash;&mdash;&mdash;</td></tr>
+<tr><td colspan="3" align="center"><span class="smcap">des Vents de la</span></td><td>&nbsp;</td>
+    <td colspan="3" align="center"><span class="smcap">des Vents de la</span></td></tr>
+<tr><td colspan="3" align="center"><span class="smcaplc">PARTIE DU</span></td><td>&nbsp;</td>
+    <td colspan="3" align="center"><span class="smcaplc">PARTIE DU</span></td></tr>
+<tr><td colspan="3" align="center">N.E.</td><td>&nbsp;</td>
+    <td colspan="3" align="center">S.O.</td></tr>
+<tr><td rowspan="2" align="center"><span class="giant">(</span></td><td align="center"><span class="smcap">du N.O.</span> &aacute; l&#8217;E</td><td rowspan="2" align="center"><span class="giant">)</span></td>
+    <td>&nbsp;</td>
+    <td rowspan="2" align="center"><span class="giant">(</span></td><td align="center"><span class="smcap">du S.E.</span> &aacute; l&#8217;O.</td><td rowspan="2" align="center"><span class="giant">)</span></td></tr>
+<tr><td align="center"><span class="smcap">par le NORD</span>.</td><td>&nbsp;</td><td align="center"><span class="smcap">par le SUD</span>.</td></tr>
+
+<tr><td colspan="3" align="center"><span class="smcaplc">DE LA</span></td><td>&nbsp;</td>
+    <td colspan="3" align="center"><span class="smcaplc">DE</span></td></tr>
+<tr><td colspan="3" align="center">S&Eacute;CHERESSE.</td><td>&nbsp;</td>
+    <td colspan="3" align="center">L&#8217;HUMIDIT&Eacute;.</td></tr>
+<tr><td colspan="3" align="center">&mdash;&mdash;&mdash;</td><td>&nbsp;</td>
+    <td colspan="3" align="center">&mdash;&mdash;&mdash;</td></tr>
+<tr><td colspan="3" align="center"><span class="smcap">un VENT</span></td><td>&nbsp;</td>
+    <td colspan="3" align="center"><span class="smcap">un VENT</span></td></tr>
+<tr><td colspan="3" align="center"><span class="smcaplc">PLUS FAIBLE</span></td><td>&nbsp;</td>
+    <td colspan="3" align="center"><span class="smcaplc">PLUS FORT</span></td></tr>
+<tr><td colspan="3" align="center"><span class="smcaplc">EXCEPT&Eacute; S&#8217;IL PLEUT</span></td><td>&nbsp;</td>
+    <td colspan="3" align="center"><span class="smcaplc">EXCEPT&Eacute; S&#8217;IL PLEUT</span></td></tr>
+<tr><td colspan="3" align="center"><span class="smcaplc">AVEC DE FORTES BRISES</span></td><td>&nbsp;</td>
+    <td colspan="3" align="center"><span class="smcaplc">AVEC DE PETITES BRISES</span></td></tr>
+<tr><td colspan="3" align="center"><span class="smcap">du N.E.</span></td><td>&nbsp;</td>
+    <td colspan="3" align="center"><span class="smcap">du N.E.</span></td></tr>
+<tr><td colspan="3" align="center">&mdash;&mdash;&mdash;</td><td>&nbsp;</td>
+    <td colspan="3" align="center">&mdash;&mdash;&mdash;</td></tr>
+<tr><td colspan="3">Mouvements lents,<br />Temps durable.<br /><span style="margin-left: 2em;">&mdash;&mdash;&mdash;</span><br />Mouvements rapides,<br />Temps variable.</td><td>&nbsp;</td>
+    <td colspan="3" align="center">Le commencement<br />de la hausse,<br />apr&egrave;s une grande<br />baisse pr&eacute;sage<br />un Vent violent.</td></tr></table>
+
+
+<div class="figleft">Fig. 13.<br /><img src="images/fig_13.jpg" alt="" /></div>
+
+<div class="figright">Fig. 14.<br /><img src="images/fig_14.jpg" alt="" /></div>
+
+<p class="center"><br />MARINE BAROMETERS.</p>
+
+<p><b>19. The Common Form.</b>&mdash;The barometer is of great use to the mariner, who,
+by using it as a &#8220;weather glass,&#8221; is enabled to foresee and prepare for
+sudden changes in the weather. For marine purposes, the lower portion of
+the glass tube of the barometer must be contracted to a fine bore, to
+prevent oscillation in the mercurial column, which would otherwise be
+occasioned by the movements of the ship. This tube is cemented to the
+cistern, which is made of boxwood, and has a moveable leathern bottom, for
+the purpose of rendering the instrument portable,<span class="pagenum"><a name="Page_16" id="Page_16">[Pg 16]</a></span> by screwing up the
+mercury compactly in the tube. The tube is enclosed in a mahogany frame,
+which admits of a variety of style in shape, finish, and display, to meet
+the different fancies and means of purchasers. The frame is generally
+enlarged at the upper part to receive the scales and the attached
+thermometer, which are covered by plate glass. The cistern is encased in
+brass for protection, the bottom portion unscrewing to give access to the
+portable screw beneath the cistern. Figs. 13 and 14 illustrate this form
+of barometer. Marine barometers require to be suspended, so that they may
+remain in a vertical position under the changeable positions of a vessel
+at sea. To effect this they are suspended in gimbals by a brass arm. The
+gimbals consist of a loose ring fastened by thumb-screws to the middle
+part of the frame of the barometer, in front and back. The forked end of
+the arm supports this ring at the sides, also by the aid of thumb-screws.
+Hence the superior weight of the cistern end is always sufficient to cause
+the instrument to move on its bearing screws, so as always to maintain a
+perpendicular position; in fact, it is so delicately held that it yields
+to the slightest disturbance in any direction. The other end of the arm is
+attached to a stout plate, having holes for screws, or fitted to slip into
+a staple or bracket, by which it may be fixed to any part of the cabin of
+a ship; the arm is hinged to the plate, for the purpose of turning the arm
+and barometer up whenever it is desirable.</p>
+
+<p>Other forms of barometer (to be immediately described) have superseded
+this in the British Marine, but the French still give the preference to
+the wooden frames. They think the barometer can be more securely mounted
+in wood, is more portable, and less liable to be broken by a sudden
+concussion than if fitted in a metal frame. The English deem the ordinary
+wooden barometers not sufficiently accurate, owing to the irregular
+expansion of wood, arising from its hygrometric properties. Some of the
+English opticians have shown that very portable, and really accurate
+barometers can be made in brass frames, and therefore the preference is
+now given to this latter material.</p>
+
+
+<p><br /><b>20. The Kew Marine Barometer.</b>&mdash;The form of barometer so-called, is that
+recommended by the Congress of Brussels, held in 1853, for the purpose of
+devising a systematic plan of promoting meteorological observations at
+sea.</p>
+
+<p>The materials employed in its construction are mercury, glass, iron, and
+brass. The upper part of the tube is carefully calibrated to ensure
+uniformity of bore, as this is a point upon which the accuracy of the
+instrument to some extent depends. At sea, the barometer has never been
+known to stand above 31 inches, nor below 27. These extremes have been
+attained with instruments of undoubted accuracy, but they are quite
+exceptional. It is not necessary, therefore, to carry the scales of marine
+barometers beyond these limits, but they should not be made shorter. If
+the vernier is adjusted to read upward, the scale should extend to 32
+inches, to allow room for the vernier to be set to 31 inches at least.
+Cases have occurred in which this could not be done, and rare, but
+valuable observations have been lost in consequence. If the scale part of
+the tube be not uniform in bore, the index error<span class="pagenum"><a name="Page_17" id="Page_17">[Pg 17]</a></span> will be irregular
+throughout the scale. Whether the bore of the rest of the tube varies in
+diameter, is of no moment. From two to three inches below the measured
+part, the bore is contracted very much, to prevent the pulsations in the
+mercurial column&mdash;called &#8220;pumping&#8221;&mdash;which, otherwise, would occur at sea
+from the motion of the ship. In ordinary marine barometers, this
+contraction extends to the end of the tube. Below the contracted part is
+inserted a pipette&mdash;or Gay Lussac air-trap&mdash;which is a little elongated
+funnel with the point downwards. Its object is to arrest any air that may
+work in between the glass and the mercury. The bubble of air lodges at the
+shoulder, and can go up no farther. It is one of those simple contrivances
+which turn out remarkably useful. If any air gets into the tube, it does
+not get to the top, and therefore does not vitiate the performance of the
+barometer; for the mercury itself works up and down through the funnel.
+Below this, the tube should not be unnecessarily contracted.</p>
+
+<div class="figright">Fig. 15.<br /><img src="images/fig_15.jpg" alt="" /></div>
+
+<p>The open end of the tube is fixed into an iron cylinder, which forms the
+cistern of the barometer. Iron has no action upon mercury, and is
+therefore used instead of any other metal. One or two holes are made in
+the top of the cistern, which are covered on the inside with strong
+sheep-skin leather, so as to be impervious to mercury, but sufficiently
+porous for the outer air to act upon the column. The cistern is of
+capacity sufficient to receive the mercury which falls out of the tube
+until the column stands lower than the scale reads; and when the tube is
+completely full, there is enough mercury to cover the extremity so as to
+prevent access of air. There is no screw required for screwing up the
+mercury.</p>
+
+<p>The glass tube thus secured to the cistern is protected by a brass tubular
+frame, into which the iron cistern fits and screws compactly. Cork is used
+to form bearings for the tube. A few inches above the cistern is placed
+the attached thermometer. Its bulb is enclosed in the frame, so as to be
+equally affected by heat with the barometric column. The upper end of the
+frame is fitted with a cap which screws on, and embraces a glass shield
+which rests in a gallery formed on the frame below the scale, and serves
+to protect the silvered scale, as well as the inner tube, from dust and
+damp. A ring, moveable in a collar fixed on the frame above the centre of
+gravity of the instrument, is attached to gimbals, and the whole is
+supported by a brass arm in the usual manner; so that the instrument can
+be moved round its axis to bring any source of light upon it, and will
+remain vertical in all positions of the ship. The vernier reads to
+five-hundredths of an inch. No words are placed upon the scale, as the old
+formulary was deemed misleading. The vernier can be set with great
+exactness, as light is admitted to the top of the<span class="pagenum"><a name="Page_18" id="Page_18">[Pg 18]</a></span> mercury by a front and
+a back slit in the frame. The lower edge of the vernier should be brought
+to the top of the mercury, so as just to shut out the light.</p>
+
+<p>It is evident that this form of barometer must be more reliable in its
+indications than those in wooden frames. The graduations can be accurately
+made, and they will be affected only by well-known alterations due to
+temperature. Some think the tube is too firmly held, and therefore liable
+to be broken by concussion more readily than that of an inferior
+instrument. This, however, appears a necessary consequence of greater
+exactness. It is an exceedingly good portable instrument, and can be put
+up and taken down very readily. These barometers are preferred to marine
+barometers in wood, wherever they have been used. In merchant ships, and
+under careful treatment, they have been found very durable. They may be
+sent with safety by railway, packed carefully in a wooden box.</p>
+
+<p><i>Directions for Packing.</i>&mdash;In removing this barometer it is necessary to
+slope it gradually till the mercury reaches the top of the tube. It is
+then portable, if carried cistern end upwards or lying flat. If carried
+otherwise, it will very probably be broken by the jerking motion of the
+heavy mercury in the glass tube. Of course it must not be jarred, or
+receive concussion.</p>
+
+<p><i>Position for Marine Barometer.</i>&mdash;Admiral FitzRoy, to whose valuable
+papers we are much indebted, writes in his &#8220;Barometer Manual&#8221;:&mdash;&#8220;It is
+desirable to place the barometer in such a position as not to be in danger
+of a side blow, and also sufficiently far from the deck above to allow for
+the spring of the metal arm in cases of sudden movements of the ship.</p>
+
+<p>&#8220;If there is risk of the instrument striking anywhere when the vessel is
+much heeled, it will be desirable to put some soft padding on that place,
+or to check movement in that direction by a light elastic cord; in fixing
+which, attention must be paid to have it acting only where risk of a blow
+begins, not interfering otherwise with the free swing of the instrument: a
+very light cord attached above, when possible, will be least likely to
+interfere injuriously.&#8221;</p>
+
+
+<p><br /><b>21. Method of verifying Marine and other Barometers.</b>&mdash;&#8220;In nearly all the
+barometers which had been employed at sea till recently the index
+correction varied through the range of scale readings, in proportion to
+the difference of capacity between the cistern and the tube. To find the
+index correction for a land barometer, comparison with a standard, at any
+part of the scale at which the mercury may happen to be, is generally
+considered sufficient. To test the marine barometer is a work of much more
+time, since it is necessary to find the correction for scale readings at
+about each half inch throughout the range of atmospheric pressure to which
+it may be exposed; and it becomes necessary to have recourse to artificial
+means of changing the pressure of the atmosphere on the surface of the
+mercury in the cistern.</p>
+
+<p>&#8220;The barometers to be thus tested are placed, together with a standard, in
+an air-tight chamber, to which an air-pump is applied, so that, by
+partially exhausting the air, the standard can be made to read much lower
+than the lowest pressure to<span class="pagenum"><a name="Page_19" id="Page_19">[Pg 19]</a></span> which marine barometers are likely to be
+exposed; and by compressing the air it can be made to read higher than the
+mercury ever stands at the level of the sea. The tube of the standard is
+contracted similarly to that of the marine barometer, but a provision is
+made for adjusting the mercury in its cistern to the zero point. Glass
+windows are inserted in the upper part of the iron air-chamber, through
+which the scales of the barometers may be seen; but as the verniers cannot
+be moved in the usual way from outside the chamber, a provision is made
+for reading the height of the mercury independent of the verniers attached
+to the scales of the respective barometers. At a distance of some five or
+six feet from the air-tight chamber a vertical scale is fixed. The
+divisions on this scale correspond exactly with those on the tube of the
+standard barometer. A vernier and telescope are made to slide on the scale
+by means of a rack and pinion. The telescope has two horizontal wires, one
+fixed and the other moveable by a micrometer screw, so that the difference
+between the height of the column of mercury and the nearest division on
+the scale of the standard, and also of all the other barometers placed by
+the side of it for comparison, can be measured either with the vertical
+scale and vernier or the micrometer wire. The means are thus possessed of
+testing barometers for index error in any part of the scale, through the
+whole range of atmospheric pressure to which they are likely to be
+exposed; and the usual practice is to test them at every half inch from
+27&middot;5 to 31 inches.</p>
+
+<p>&#8220;In this way barometers of various other descriptions have been tested,
+and some errors found to be so large that a few barometers read half an
+inch and upwards too high, while others read as much too low. In some
+cases those which were correct in one part of the scale were found to be
+from half an inch to an inch wrong in other parts. These barometers were
+of an old and ordinary, not to say inferior, construction. In some the
+mercury would not descend lower than about 29 inches, owing to a fault
+very general in the construction of many common barometers till lately in
+frequent use:&mdash;the <i>cistern was not large enough</i> to hold the mercury
+which descended from the tube in a <i>low atmospheric pressure</i>.</p>
+
+<p>&#8220;When used on shore, this contraction of the tube causes the marine
+barometer to be <i>sometimes</i> a little behind an ordinary land barometer,
+the tube of which is not contracted. The amount varies according to the
+rate at which the mercury is rising or falling, and ranges from 0&middot;00 to
+0&middot;02 of an inch. As the motion of the ship at sea causes the mercury to
+pass more rapidly through the contracted tube, the readings are almost the
+same there as they would be if the tube were not contracted, and in no
+case do they differ enough to be of importance in maritime use.&#8221;</p>
+
+<p>The cistern of this marine barometer is generally made an inch and a
+quarter in diameter, and the scale part of the tube a quarter of an inch
+in bore. The inches on the scale, instead of being true, are shortened by
+&middot;04 of an inch, in order to avoid the necessity of applying a correction
+due to the difference of capacity of the tube and cistern. This is done
+with much perfection, and the errors of the instruments, when compared
+with a standard by the apparatus used at Kew and<span class="pagenum"><a name="Page_20" id="Page_20">[Pg 20]</a></span> Liverpool Observatories,
+are determined to the thousandth of an inch, and are invariably very
+uniform and small. The error so determined includes the correction due to
+capillarity, capacity, and error of graduation, and forms a constant
+correction, so that only one variable correction, that due to temperature,
+need be applied, when the barometer is suspended near the water line of
+the ship, to make the observations comparable with others. With all the
+advantages of this barometer, however, it has recently been superseded, to
+some extent, because it was found to require more care than could
+ordinarily be expected to be given to it by the commander of a ship.
+Seamen do not exactly understand the value of such nice accuracy as the
+thousandth part of an inch, but prefer an instrument that reads only to a
+hundredth part.</p>
+
+
+<p class="center"><br />22. THE FITZROY MARINE BAROMETER.</p>
+
+<p>Admiral FitzRoy deemed it desirable to construct a form of barometer as
+practically useful as possible for marine purposes. One that should be
+less delicate in structure than the Kew barometer, and not so finely
+graduated. One that could be set at a glance and read easily; that would
+be more likely to bear the common shocks unavoidable in a ship of war.
+Accordingly, the Admiral has devised a barometer, which he has thus
+described:&mdash;</p>
+
+<p>&#8220;This marine barometer, for Her Majesty&#8217;s service, is adapted to <i>general</i>
+purposes.</p>
+
+<p>&#8220;It differs from barometers hitherto made in points of detail, rather than
+principle:&mdash;1. The glass tube is packed with vulcanised india-rubber,
+which checks vibration from concussion; but does not hold it rigidly, or
+prevent expansion. 2. It does not oscillate (or pump), though extremely
+sensitive. 3. The scale is porcelain, <i>very legible</i>, and not liable to
+change. 4. There is no iron anywhere (<i>to rust</i>). 5. Every part can be
+unscrewed, examined, or cleaned, by any careful person. 6. There is a
+<i>spare</i> tube, fixed in a cistern, filled with boiled mercury, and <i>marked</i>
+for adjustment in this, or <i>any similar</i> instrument.</p>
+
+<p>&#8220;These barometers are graduated to hundredths, and they will be found
+accurate to <i>that</i> degree, namely the second decimal of an inch.</p>
+
+<p>&#8220;They are packed with vulcanised caoutchouc, in order that (by this, and
+by a peculiar strength of glass tube) guns may be fired near these
+instruments without causing injury to them by ordinary concussion.</p>
+
+<p>&#8220;It is hoped that all such instruments, for the public service at sea,
+will be quite similar, so that any spare tube will fit <i>any</i> barometer.</p>
+
+<div class="figright">Fig. 16.<br /><img src="images/fig_16.jpg" alt="" /></div>
+
+<p>&#8220;<i>To Shift a Tube.</i>&mdash;Incline the barometer slowly, and then take it down,
+after allowing the mercury to fill the upper part. Lay the instrument on a
+table, unscrew the outer cap at the joining just below the cistern swell,
+then unscrew the tube <i>and</i> cistern, by turning the cistern gently,
+against the sun, or to <i>the left</i>, and draw out the tube very carefully
+<i>without bending it in the least</i>, <i>turning</i> it a little, if required, as
+moved. Then insert the new tube very cautiously, screw in, and adjust to
+the<span class="pagenum"><a name="Page_21" id="Page_21">[Pg 21]</a></span> diamond-cut mark for 27 inches. Attach the cap, and suspend the
+barometer for use.</p>
+
+<p>&#8220;If the mercury does not immediately quit the top of the tube, tap the
+cistern end rather sharply. In a well-boiled tube, with a good vacuum, the
+mercury hangs, at times, so adhesively as to deceive, by causing a
+supposition of some defect.</p>
+
+<p>&#8220;In about ten minutes the mercurial column should be nearly right; but as
+local temperature affects the brass, as well as the mercury, slowly and
+unequally, it may be well to defer any <i>exact comparisons with other
+instruments</i> for some few hours.&#8221;</p>
+
+<p>Messrs. Negretti and Zambra are the makers of these barometers for the
+Royal Navy. Fig. 16 is an illustration.</p>
+
+<p>The tube is fixed to a boxwood cistern, which is plugged with very porous
+cane at the top, to allow of the ready influence of a variation in
+atmospheric pressure upon the mercury. Round the neck of the cistern is
+formed a brass ring, with a screw thread on its circumference. This screws
+into the frame, and a mark on the tube is to be adjusted to 27 inches on
+the scale, the cistern covering screwed on, and the instrument is ready to
+suspend. The frame and all the fittings are brass, without any iron
+whatever; because the contact of the two metals produces a galvanic
+action, which is objectionable. The spare tube is fitted with
+india-rubber, and ready at any time to replace the one in the frame. The
+ease with which a tube can be replaced when broken is an excellent feature
+of the instrument. The spare tube is carefully stowed in a box, which can
+also receive the complete instrument when not in use. All the parts are
+made to a definite gauge; the frames are, therefore, all as nearly as
+possible similar to each other, and the tubes&mdash;like rifle bullets&mdash;are
+adjustible to any frame. If, then, the tube in use gets broken, the
+captain can replace it by the other; but, as it is securely packed with
+india-rubber, there is very little liability of its being broken by fair
+usage. Every person who knows the importance of the barometer on board
+ship, will acknowledge that the supplementary tube is a decided
+improvement. Many instruments of this description are afloat in the Royal
+Navy, and in a short time it may be expected that all the frames and tubes
+of barometers in the public service at sea will be similar in size and
+character; so that should a captain have the misfortune to get both his
+tubes broken, he would be able to borrow another from any ship he fell in
+with that had one to spare, which would be perfectly accurate, because it
+would have been verified before it was sent out.</p>
+
+
+<p><br /><b>23. Admiral FitzRoy&#8217;s Words for the Scale.</b>&mdash;The graduation of inches and
+decimals are placed in this barometer on the right-hand side of the tube;
+and on a<span class="pagenum"><a name="Page_22" id="Page_22">[Pg 22]</a></span> similar piece of porcelain, on the left-hand, are engraved, as
+legibly as they are expressed succinctly, the following words, of
+universal application in the interpretation of the barometer movements:&mdash;</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td align="center"><i>RISE</i></td><td><span class="spacer">&nbsp;</span></td>
+    <td align="center"><i>FALL</i></td></tr>
+<tr><td align="center"><span class="smcaplc">FOR</span></td><td>&nbsp;</td>
+    <td align="center"><span class="smcaplc">FOR</span></td></tr>
+<tr><td align="center">COLD</td><td>&nbsp;</td>
+    <td align="center">WARM</td></tr>
+<tr><td align="center">DRY</td><td>&nbsp;</td>
+    <td align="center">WET</td></tr>
+<tr><td align="center"><span class="smcaplc">OR</span></td><td>&nbsp;</td>
+    <td align="center"><span class="smcaplc">OR</span></td></tr>
+<tr><td align="center">LESS</td><td>&nbsp;</td>
+    <td align="center">MORE</td></tr>
+<tr><td align="center">WIND.</td><td>&nbsp;</td>
+    <td align="center">WIND.</td></tr>
+<tr><td align="center">&mdash;&mdash;&mdash;</td><td>&nbsp;</td>
+    <td align="center">&mdash;&mdash;&mdash;</td></tr>
+<tr><td align="center"><span class="smcaplc">EXCEPT</span></td><td>&nbsp;</td>
+    <td align="center"><span class="smcaplc">EXCEPT</span></td></tr>
+<tr><td align="center"><span class="smcaplc">WET FROM</span></td><td>&nbsp;</td>
+    <td align="center"><span class="smcaplc">WET FROM</span></td></tr>
+<tr><td align="center"><span class="smcaplc">COOLER SIDE.</span></td><td>&nbsp;</td>
+    <td align="center"><span class="smcaplc">COOLER SIDE.</span></td></tr></table>
+
+<p>Reverting to the explanation of the words on the &#8220;Coast&#8221; barometers (at
+<a href="#Page_14">page 14</a>), and comparing and considering them as given for northern
+latitudes, and as they must be altered for southern latitudes, it will be
+perceived, that for all <i>cold</i> winds the barometer rises; and falls for
+<i>warm</i> winds. The mercury also falls for <i>increased</i> strength of wind; and
+rises as the wind <i>lulls</i>. Likewise before or with rain the column of
+mercury falls; but it rises with fine dry weather. Putting these facts
+together, and substituting for the points of the compass the terms &#8220;cold&#8221;
+and &#8220;warm,&#8221; the appropriateness of the words on the scale of this
+barometer is readily perceived. These concise and practical indications of
+the movements in the barometer are applicable for instruments intended for
+use in any region of the world, and are in perfect accordance with the
+laws of winds and weather deduced by Dov&eacute; and other meteorologists. There
+is nothing objectionable in them, and being founded upon experience and
+the deductions made from numerous recorded observations of the weather in
+all parts of the world, as well as confirmed by the theories of science,
+they may consequently be considered as generally reliable. They involve no
+conjecture, but express succinctly scientific principles.</p>
+
+
+<p><br /><b>24. Trials of the FitzRoy Marine Barometer under Fire of Guns.</b>&mdash;Some of
+the first barometers made by Messrs. Negretti and Zambra on Admiral
+FitzRoy&#8217;s principle were severely tried under the heaviest naval gun
+firing, on board H.M.S. <i>Excellent</i>; and under all the circumstances, they
+withstood the concussion. The purpose of the trials was &#8220;to ascertain
+whether the <i>vulcanized india-rubber packing</i> round the glass tube of a
+<i>new marine barometer</i> did check the vibration caused by firing, and
+whether guns might be fired close to these instruments without causing
+injury to them.&#8221; In the first and second series of experiments, a marine
+barometer on Admiral FitzRoy&#8217;s plan was tried against a marine barometer
+on the Kew principle, both instruments being new, and treated<span class="pagenum"><a name="Page_23" id="Page_23">[Pg 23]</a></span> in all
+respects similarly. They were &#8220;hung over the gun, under the gun, and by
+the side of the gun, the latter both inside and outside a bulkhead,&mdash;in
+fact, in all ways that they would be tried in action with the bulkheads
+cleared away.&#8221; The result was that the Kew barometer was broken and
+rendered useless, while the new pattern barometer was not injured in the
+least. In a third series of experiments, Mr. Negretti being present, five
+of the new pattern barometers were subjected to the concussion produced by
+firing a 68-pounder gun with shot, and 16 lbs. charge of powder. They were
+suspended from a beam immediately under the gun, then from a beam
+immediately over the gun, and finally they were suspended by the arm to a
+bulkhead, at a distance of only 3 ft. 6 in. from the axis of the gun; and
+the result was, according to the official report, &#8220;that all these
+barometers, however suspended, would stand, without the slightest injury,
+the most severe concussion that they would ever be likely to experience in
+any sea-going man-of-war.&#8221; These trials were conducted under the
+superintendence of Captain Hewlett, C.B., and the guns were fired in the
+course of his <i>usual</i> instructions. His reports to Admiral FitzRoy, giving
+all the particulars of the trials, are published in the &#8220;Ninth Number of
+Meteorological Papers,&#8221; issued by the Board of Trade.<small><a name="f2.1" id="f2.1" href="#f2">[2]</a></small></p>
+
+
+<p class="center"><br />25. NEGRETTI AND ZAMBRA&#8217;S FARMER&#8217;S BAROMETER AND DOMESTIC WEATHER-GLASS.</p>
+
+<p>It is a well-known fact that the barometer is as much, or even more
+affected by a change of wind as it is by rain; and the objection raised
+against a simple barometer reading, as leaving the observer in doubt
+whether to expect wind or rain, is removed by the addition of the
+Hygrometer, an instrument indicating the comparative degree of dryness or
+dampness of the air;&mdash;a most important item in the determination of the
+coming weather.</p>
+
+<p>The farmer should not be content to let his crops lie at the mercy, so to
+speak, of the weather, when he has within his command instruments which
+may be the means of preventing damage to, and in cases total loss of, his
+crops.</p>
+
+<p>The farmer hitherto has had to depend for his prognostication of the
+weather on his own unassisted &#8220;Weather Wisdom;&#8221; and it is perfectly
+marvellous how expert he has become in its use. Science now steps in, not
+to ignore this experience, but on the contrary, to give it most valuable
+assistance by extending it, and enabling it to predict, with an accuracy
+hitherto unknown, the various changes that take place in this most
+variable of climates.</p>
+
+<p>To the invalid, the importance of predicting with tolerable accuracy the
+changes<span class="pagenum"><a name="Page_24" id="Page_24">[Pg 24]</a></span> that are likely to occur in the weather, cannot be over-rated.
+Many colds would be prevented, if we could know that the morning so balmy
+and bright, would subside into a cold and cheerless afternoon. Even to the
+robust, much inconvenience may be prevented by a due respect to the
+indications of the hygrometer and the barometer, and the delicate in
+health will do well to regard its warnings.</p>
+
+<div class="figleft">Fig. 17.<br /><img src="images/fig_17.jpg" alt="" /></div>
+
+<p><i>Description of the Instrument.</i>&mdash;The farmer&#8217;s barometer, as figured in
+the margin, consists of an upright tube of mercury inverted in a cistern
+of the same fluid; this is secured against a strong frame of wood, at the
+upper end of which is fixed the scale, divided into inches and tenths of
+an inch. On either side of the barometer, or centre tube, are two
+thermometers&mdash;that on the left hand has its bulb uncovered and freely
+exposed, and indicates the temperature of the air at the place of
+observation; that on the right hand has its bulb covered with a piece of
+muslin, from which depend a few threads of soft lamp cotton; this cotton
+is immersed in the small cup situated just under the thermometer, this
+vessel being full of water; the water rises by capillary attraction to the
+muslin-covered bulb, and keeps it in a constantly moist state.</p>
+
+<p>These two thermometers, which we distinguish by the names &#8220;Wet Bulb&#8221; and
+&#8220;Dry Bulb,&#8221; form the Hygrometer; and it is by the simultaneous reading of
+these two thermometers, and noting the difference that exists between
+their indications, that the humidity in the atmosphere is determined.</p>
+
+<p>Admiral FitzRoy&#8217;s words (see <a href="#Page_22">p. 22</a>) are placed upon the scale of the
+barometer, as the value of a reading depends, not so much on the actual
+height of the mercury in the tube, as it does on whether the column is
+rising, steady, or falling.</p>
+
+<p>The moveable screw at the bottom of the cistern is for the purpose of
+forcing the mercury to the top of the tube when the instrument is being
+carried from place to place, and it must always be unscrewed to its utmost
+limit when the barometer is hung in its proper place. After this it should
+never be touched.</p>
+
+<p>The manner in which the Hygrometer acts is as follows: It is a pretty
+well-known fact that water or wine is often cooled by a wet cloth being
+tied round the bottle, and then being placed in a current of air. The
+evaporation that takes place in the progressive drying of the cloth causes
+the temperature to fall considerably below that of the surrounding
+atmosphere, and the contents of the bottle are thus cooled. In the same
+manner, then, the covered wet bulb thermometer will be found <i>invariably</i>
+to read lower than the uncovered one; and the greater the dryness of the
+air, the greater will be the difference<span class="pagenum"><a name="Page_25" id="Page_25">[Pg 25]</a></span> between the indications of the
+two thermometers; and the more moisture that exists in the air, the more
+nearly they will read alike.</p>
+
+<p>The cup must be kept filled with pure water, and occasionally cleaned out,
+to remove any dirt. The muslin, or cotton-wick, should also be renewed
+every few weeks. The hygrometer may be had separate from the barometer, if
+the combined instruments cannot be sufficiently exposed to the external
+air, this being essential for the successful use of the hygrometer.</p>
+
+<p>This farmer&#8217;s weather-glass, then, consists of three distinct instruments:
+the barometer, the thermometer, and the hygrometer. He has thus at command
+the three instrumental data necessary for the prediction of the weather.
+And now to describe&mdash;</p>
+
+<p><i>How to Use the Instrument.</i>&mdash;The observations should be taken twice a
+day, say at 9 <span class="smcaplc">A.M.</span> and 3 <span class="smcaplc">P.M.</span>; and should be entered on a slip of paper,
+or a slate hung up by the barometer. The observer will then be able to see
+the different values of the readings from time to time, and to draw his
+conclusions therefrom.</p>
+
+<p>The thermometer on the left hand should first be read, and a note made of
+its indication, which is the temperature of the air. The wet bulb
+thermometer should now be read, and also noted; and the difference should
+be taken of these two readings. Next read the barometer by moving the
+small index at the side of the tube until it is on a level with the top of
+the mercury. Having noted the number of inches at which the column stands,
+compare with the last observation, and see immediately whether the
+barometer is rising, steady, or falling.</p>
+
+<p>Now, having taken the observations as above, we naturally ask the
+question, <i>What are we to predict from them?</i></p>
+
+<p>And, probably, the best way of answering this query will be by giving an
+example. We will suppose that our readings yesterday were as
+follows:&mdash;Temperature, 70&deg;; Wet Bulb, 69&deg;; Difference, 1&deg;; =very moist
+air. Barometer, 29&middot;5, and that rain has fallen.</p>
+
+<p>To-day, we read:&mdash;Temperature, 60&deg;; Wet Bulb, 55&deg;; Difference, 5&deg;; =dryer
+air. Barometer, 30. We may safely predict that the rain will cease, and
+probably we may have wind from the northward.</p>
+
+<p>In spring or autumn, if the barometric height be steady any where between
+29&middot;5 and 30 inches, with the temperature about 60&deg;, fresh to moderate
+south-westerly winds, with cloudy sky, will probably characterize the
+weather; the indications of the hygrometer being then specially
+serviceable in enabling us to foretell rain; but if the mercury become
+steady at about 30&middot;5 inches, with temperature about 40&deg;, north-easterly
+winds, dry air, and clear sky, may be confidently expected.</p>
+
+<p>Many cases will doubtless suggest themselves to the observer where these
+figures do not occur, and where he might find a difficulty in interpreting
+the indications of his instruments. We have, therefore, drawn up some
+concise rules for his guidance; and although they will not prove
+absolutely infallible guides to this acknowledged most difficult problem,
+still, they will be found of much service in foretelling the weather, when
+added to an intelligent observation of ordinary atmospheric <span class="pagenum"><a name="Page_26" id="Page_26">[Pg 26]</a></span>phenomena, as
+force and direction of wind, nature of any particular season, and the time
+of year.</p>
+
+
+<p class="center"><br />26. RULES FOR FORETELLING THE WEATHER.</p>
+
+<p class="center"><br />A RISING BAROMETER.</p>
+
+<div class="blockquot"><p>A &#8220;Rapid&#8221; rise indicates unsettled weather.</p>
+
+<p>A &#8220;Gradual&#8221; rise indicates settled weather.</p>
+
+<p>A &#8220;Rise,&#8221; with dry air, and cold increasing in summer, indicates wind
+from northward; and if rain has fallen, better weather is to be
+expected.</p>
+
+<p>A &#8220;Rise,&#8221; with moist air and a low temperature, indicates wind and
+rain from northward.</p>
+
+<p>A &#8220;Rise,&#8221; with southerly wind, indicates fine weather.</p></div>
+
+<p class="center"><br />A STEADY BAROMETER,</p>
+
+<div class="blockquot"><p>With dry air and a seasonable temperature, indicates a continuance of
+very fine weather.</p></div>
+
+<p class="center"><br />A FALLING BAROMETER.</p>
+
+<div class="blockquot"><p>A &#8220;Rapid&#8221; fall indicates stormy weather.</p>
+
+<p>A &#8220;Rapid&#8221; fall, with westerly wind, indicates stormy weather from
+northward.</p>
+
+<p>A &#8220;Fall,&#8221; with a northerly wind, indicates storm, with rain and hail
+in summer, and snow in winter.</p>
+
+<p>A &#8220;Fall,&#8221; with increased moisture in the air, and the heat increasing,
+indicates wind and rain from southward.</p>
+
+<p>A &#8220;Fall,&#8221; with dry air, and cold increasing (in winter), indicates
+snow.</p>
+
+<p>A &#8220;Fall,&#8221; after very calm and warm weather, indicates rain with
+squally weather.</p></div>
+
+
+<p><br /><b>27. Causes which may bring about a Fall or a Rise in the Barometer.</b><small><a name="f3.1" id="f3.1" href="#f3">[3]</a></small>&mdash;As
+heat produces rarefaction, a sudden rise of temperature in a distant
+quarter may affect the weight of the atmosphere over our heads, by
+producing an aerial current outwards, to supply the place of the lighter
+air which has moved from its former position; in which case the barometer
+will fall. Now such a movement in the atmosphere is likely to bring about
+an intermixture of currents of air of different temperatures, and from
+this intermixture rain is likely to result.</p>
+
+<p>On the other hand, as cold produces condensation, any sudden fall of
+temperature causes the column of air over the locality to contract and
+sink to a lower level, whilst other air rushes in from above to supply the
+void; and, accordingly, the barometer rises. Should this air, as often
+happens, proceed from the north, it will contain in general but little
+moisture; and hence, on reaching a warmer latitude, will take up the
+vapour of the air, so that dry weather will result.</p>
+
+<p>It is generally observed, that wind causes a fall in the instrument; and,
+indeed, in those greater movements of the atmosphere which we denominate
+storms or<span class="pagenum"><a name="Page_27" id="Page_27">[Pg 27]</a></span> hurricanes, the depression is so considerable as to forewarn
+the navigator of his impending danger. It is evident, that a draught of
+air in any direction must diminish the weight of the column overhead, and
+consequently cause the mercury in the barometer to sink.</p>
+
+<p>The connection, therefore, of a sinking of the barometric column with rain
+is frequently owing to the wind causing an intermixture of the aerial
+currents which, by their motion, diminish the weight of the atmosphere
+over our heads; whilst a steady rise in the column indicates the absence
+of any great atmospheric changes in the neighbourhood, and a general
+exemption from those causes which are apt to bring about a precipitation
+of vapour.</p>
+
+
+<p><br /><b>28. Use of the Barometer in the management of Mines.</b>&mdash;The inflammable and
+suffocating gases, known to coal-miners as fire-damp and choke-damp, are
+specifically heavier than air; and as they issue from the fissures of the
+mine, or are released from the coal, the atmospheric pressure tends to
+drive them into the lowest and least ventilated galleries. Consequently a
+greatly reduced atmospheric pressure will favour a sudden outflow or
+advance of gas; whence may result cases of explosion or suffocation. It
+has been found that these accidents occur for the most part about the time
+of a low barometric column. A reliable barometer should, therefore, be
+systematically consulted by those entrusted with the management or control
+of coal-mines, so that greater vigilance and caution may be enjoined on
+the miners whenever the mercury falls low, especially after it has been
+unusually high for some days.</p>
+
+
+<p><br /><b>29. Use of the Barometer in estimating the Height of Tides.</b>&mdash;The pressure
+of the atmosphere affects the height of the tide, the water being in
+general higher as the barometer is lower. The expressions of seamen, that
+&#8220;frost nips the tide,&#8221; and &#8220;fog nips the tide,&#8221; are explained by the high
+barometer which usually accompanies frost and fog. M. Daussy, Sir J. C.
+Ross, and others, have established that a rise of one inch in the
+barometer will have a corresponding fall in the tide of about one foot.
+Therefore navigators and pilots will appreciate the following suggestion
+of Admiral FitzRoy:&mdash;</p>
+
+<p>&#8220;Vessels sometimes enter docks, or even harbours, where they have scarcely
+a foot of water more than their draught; and as docking, as well as
+launching large ships, requires a close calculation of height of water,
+the state of the barometer becomes of additional importance on such
+occasions.&#8221;</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_28" id="Page_28">[Pg 28]</a></span></p>
+<h2><a name="CHAPTER_II" id="CHAPTER_II"></a>CHAPTER II.</h2>
+<p class="center"><span class="large">SYPHON TUBE BAROMETERS.</span></p>
+
+<p><br /><b>30. Principle of.</b>&mdash;If some mercury, or any other fluid, be poured into a
+tube of glass, bent in the form of <big><strong>&#8746;</strong></big>, and open at both ends, it will rise
+to the same height in both limbs, the tube being held vertically. If
+mercury be poured in first, and then water upon it at one end, these
+liquids will not come to the same level; the water will stand much higher
+than the mercury. If the height of the mercury, above the line of meeting
+of the fluids, be one inch, that of the water will be about
+thirteen-and-a-half inches. The explanation of this is, that the two
+columns balance each other. The pressure of the atmosphere in each limb is
+precisely similar; but the one column stands so much higher than the
+other, because the fluid of which it is composed is so much lighter, bulk
+for bulk, than the other. If one end of the tube be hermetically closed,
+the other limb be cut off within a few inches of the bend, and the tube
+carefully filled with mercury; by placing it in a vertical position, the
+mercury will fall, if the closed limb be long enough, until it is about
+thirty inches higher than that in the exposed limb, where it will remain.
+Here the atmosphere presses upon the short column; but not upon the long
+one. It is this pressure, therefore, which maintains the difference of
+level. In fact, it forms a barometer without a cistern, the short limb
+answering the purpose of a cistern. The first barometers on this principle
+were devised by the celebrated philosopher, Dr. Hook, as described in the
+next section.</p>
+
+
+<p class="center"><br />31. DIAL, OR WHEEL BAROMETERS.</p>
+
+<p>The familiar household &#8220;Weather Glasses&#8221; are barometers on the syphon
+principle. The portions of the two limbs through which the mercury will
+rise and fall with the varying pressure of the atmosphere are made of
+precisely the same diameter; while the part between them is contracted. On
+the mercury, in the exposed limb, rests a round float of ivory or glass;
+to this a string is attached and passed over and around a brass pulley,
+the other end carrying another lighter weight. The weight resting on the
+mercury rises and falls with it. On the spindle of the pulley, which
+passes through the frame and centre of the dial-plate, is fixed a light
+steel hand, which revolves as the pulley turns round. When the mercury
+falls for a decrease of atmospheric pressure, it rises by the same
+quantity in the short tube, and pushes up the float, the counterpoise
+falls, and thus moves the hand or pointer<span class="pagenum"><a name="Page_29" id="Page_29">[Pg 29]</a></span> to the left. When the pressure
+increases, the pointer is drawn in a similar manner to the right.</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td align="center">Fig. 18.</td><td><span class="spacer2">&nbsp;</span></td>
+    <td align="center">Fig. 19.</td><td><span class="spacer2">&nbsp;</span></td>
+    <td align="center">Fig. 20.</td></tr>
+<tr><td align="center"><img src="images/fig_18.jpg" alt="" /></td><td>&nbsp;</td>
+    <td align="center"><img src="images/fig_19.jpg" alt="" /></td><td>&nbsp;</td>
+    <td align="center"><img src="images/fig_20.jpg" alt="" /></td></tr></table>
+
+<p>The dials are generally made of metal silvered over or enamelled, but
+porcelain may be used. If the circumference of the pulley, or &#8220;wheel,&#8221; be
+two inches, it will revolve once for an alteration of level amounting to
+two inches in each tube, or four inches in the height of the barometric
+column; and as the dial may be from twenty to thirty-six inches in
+circumference, five to nine inches on the graduated scale corresponds to
+one inch of the column; and hence the sub-divisions are distinctly
+perceptible, and a vernier is not necessary.</p>
+
+<p>The motion of the pointer alone is visible; and a mahogany, or rosewood,
+frame, supports, covers, and renders the instrument ornamental and
+portable. In the back of the frame is a hinged door, which covers the
+cavity containing the tube and fixtures. The dial is covered by a glass in
+a brass rim, similar to a clock face. A brass index, working over the
+dial, moveable by a key or button, may be applied, and will serve to
+register the position of the hand when last observed. These instruments
+are usually fitted with a thermometer, and a spirit level; the latter for
+the purpose of getting the instrument perfectly vertical. They sometimes
+have, in addition, a hygrometer, a sympiesometer, an aneroid, a mirror, or
+a clock, &amp;c., singly or combined. The frame admits of much variety of
+style and decoration. It may be carved or inlaid. The usual adjustment of
+scale is suited for localities<span class="pagenum"><a name="Page_30" id="Page_30">[Pg 30]</a></span> at no considerable elevation above the
+sea. Accordingly, being commercial articles, they have been found
+frequently quite out of place. When intended for use at high elevations,
+they should have a special adjustment of scale. As household instruments
+they are serviceable, and ornamental. But the supply-and-demand principle
+upon which they are sold, has entailed upon those issued by inferior
+makers a generally bad adjustment of scale. The illustrations are those of
+ordinary designs.</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td align="center">Fig. 21.</td><td><span class="spacer2">&nbsp;</span></td>
+    <td align="center">Fig. 22.</td><td><span class="spacer2">&nbsp;</span></td>
+    <td align="center">Fig. 23.</td></tr>
+<tr><td align="center"><img src="images/fig_21.jpg" alt="" /></td><td>&nbsp;</td>
+    <td align="center"><img src="images/fig_22.jpg" alt="" /></td><td>&nbsp;</td>
+    <td align="center"><img src="images/fig_23.jpg" alt="" /></td></tr></table>
+
+<p>Dial barometers required for transmission to distant parts, as India and
+the Colonies, are furnished with a steel stop-cock, to render them
+portable more effectually than can be done by the method of <i>plugging</i> the
+tube.</p>
+
+<div class="figleft">Fig. 24.<br /><img src="images/fig_24.jpg" alt="" /></div>
+
+
+
+<p class="center"><br />32. STANDARD SYPHON BAROMETER.</p>
+
+<p>Fig. 24 represents the most accurate form of the Gay Lussac barometer. The
+short limb is closed at the top, after the mercury is introduced, and a
+small lateral<span class="pagenum"><a name="Page_31" id="Page_31">[Pg 31]</a></span> puncture is made at <i>a</i>, which is covered over with a
+substance which permits the access of air, but prevents the escape of any
+mercury when the instrument is packed for travelling. The bent part of the
+tube is contracted to a capillary bore; and just above this, in the long
+limb, is placed the air-trap, already described (see <a href="#Page_17">p. 17</a>), and here
+illustrated (fig. 25). <span class="figright">Fig. 25.<br /><img src="images/fig_25.jpg" alt="" /></span> When reversed, as it must be for portability, the
+capillary attraction keeps the mercury in the long branch. Should the
+mercury of the short column get detached, some small quantity of air <i>may</i>
+pass; but it will be arrested at the pipette, and will not vitiate the
+length of the barometric column. It can be easily expelled by gently
+shaking or tapping the instrument before suspending it for observation. In
+the illustration, the zero of the scale is placed at Z, near the middle of
+the tube; and the graduations extend above and below. In making an
+observation, it is necessary to take the reading ZA on the long branch,
+and ZB on the short one. The sum of the two gives the height of the
+barometer. The zero of the scale in some instruments is placed low down,
+so as to require the difference of the two readings to be taken. A
+thermometer is attached to the frame as usual.</p>
+
+<p>These instruments can be very accurately graduated, and are very exact in
+their indications, provided great care has been exercised in selecting the
+tubes, which must be of the same calibre throughout the parts destined to
+measure the variations of atmospheric pressure. They should be suspended
+so as to insure their hanging vertically.</p>
+
+<p>The syphon barometer does not require correction for capillarity nor for
+capacity, as each surface of the mercury is equally depressed by capillary
+attraction, and the quantity of mercury which falls from the long limb of
+the tube occupies the same length in the short one. The barometric height
+must, however, be corrected for temperature, as in the cistern barometer.
+Tables containing the temperature corrections to be applied to barometer
+readings for scales engraved on the glass tube, or on brass or wood
+frames, are published.</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_32" id="Page_32">[Pg 32]</a></span></p>
+<h2><a name="CHAPTER_III" id="CHAPTER_III"></a>CHAPTER III.</h2>
+<p class="center"><span class="large">BAROGRAPHS, OR SELF-REGISTERING BAROMETERS.</span></p>
+
+<p><br /><b>33. Milne&#8217;s Self-Registering Barometer.</b>&mdash;For a long time a good and
+accurate self-recording barometer was much desired. This want is now
+satisfactorily supplied, not by one, but by several descriptions of
+apparatus. The one first to be described was the design of Admiral Sir A.
+Milne, who himself constructed, in 1857, we believe, the original
+instrument, which he used with much success. Since that time several of
+these instruments have been made, and have performed satisfactorily. The
+barometer tube is a syphon of large calibre, provided with a Gay Lussac
+pipette, or air-trap; and fitted with a float, a wheel, and a pointer, as
+in the &#8220;Dial&#8221; barometer. The float is attached to a delicate watch-chain,
+which passes over the wheel and is adequately counterpoised. Behind the
+indicating extremity of the pointer or hand is a projecting point, which
+faces the frame of the instrument, and is just within contact with the
+registering paper. A clock is applied, and fitted with auxiliary
+mechanism, so as to be able to move the mounted paper with regularity
+behind the pointer, and at designed equal intervals of time to release a
+system of levers and springs, so as to cause the marker to impress a dot
+on the paper, either by puncture or pencil-mark. The paper is ruled with
+horizontal lines for the range of the mercurial column, and parallel arcs
+of circles for the hours. Thus the barometer is rendered self-recording,
+by night or day, for a week or more; hence the great value of the
+instrument. The clock, index, and registering mechanism are protected from
+dust and interference by a glass front, hinged on and locked. As the
+temperature of the mercury is not registered, there is fixed to the frame
+a Sixe&#8217;s thermometer to record the maximum and minimum temperatures, which
+should be noted at least every twenty-four hours.</p>
+
+<p>Admiral FitzRoy has suggested the name &#8220;Atmoscope&#8221; for Admiral Milne&#8217;s
+barometer; and he has also termed it a &#8220;Barograph.&#8221; This latter word
+appears to be applicable to all kinds of self-registering barometers
+hitherto designed. Of the arrangement under consideration Admiral FitzRoy
+writes:&mdash;&#8220;It shows the alterations in tension, or the pulsations, so to
+speak, of atmosphere, on a large scale, by hourly marks; and the diagram
+expresses, to a practised observer, what the &#8216;indicator card&#8217; of a
+steam-cylinder shows to a skilful engineer, or a stethescope to a
+physician.&#8221;</p>
+
+
+<div class="figright">Fig. 26.<br /><img src="images/fig_26tmb.jpg" alt="" /><br /><a href="images/fig_26.jpg"><small>Larger Image</small></a></div>
+
+<p><br /><b>34. Modification of Milne&#8217;s Barometer.</b>&mdash;The great difficulty to be
+overcome in Milne&#8217;s barometer, is to adjust the mechanism for obtaining
+registration so that the action of the striker upon the pointer should not
+in the slightest degree<span class="pagenum"><a name="Page_33" id="Page_33">[Pg 33]</a></span> move it from its true position. A different mode
+of registration, capable of recording accurately the least appreciable
+movement of the mercurial column, has been effected. In this instrument
+the registering paper is carried upon a cylinder or drum. By reference to
+the illustration, Fig. 26, the details of construction will be readily
+understood. It should, however, be mentioned, that it is not a picture of
+the outward appearance of the instrument. The position of the barometer
+should be behind the clock; it is represented on one side merely for the
+purpose of clearly illustrating the arrangement and principles. The
+instrument has a large syphon barometer tube, in which the mercurial
+column is represented. On the mercury at <i>A</i>, in its open end, rests a
+glass float, attached to a watch-chain, or suitable silken cord, the other
+end of which is connected to the top of the arched head on the short arm
+of a lever-beam. The long arm of the beam is twice the length of the short
+arm, for the following reason. As the mercury falls in the long limb, it
+rises through an equal space in the short limb of the tube, and <i>vice
+versa</i>. But the barometric column is the difference of height of the
+mercury in the two limbs; hence the rise or fall of the float through
+half-an-inch will correspond to a decrease or an increase of the
+barometric column of one inch. In order, then, to record the movements of
+the barometric column, and not those of the float, the arm of the beam
+connected with the float is only half the radius of the other arm. Both
+arms of the beam carry circular-arched heads, which are similar portions
+of the complete circles, the centre of curvature being the fulcrum, or
+axis. This contrivance maintains the leverage on each extremity of the
+beam always at the same distance from the fulcrum. From the top of the
+large arched head a piece of watch-chain descends, and is attached to the
+marker, <i>B</i>, which properly counterpoises the float, <i>A</i>, and is capable
+of easy movement along a groove in a brass bar, so as to indicate the
+barometric height on an ivory scale, <i>C</i>, fixed on the same vertical
+framing. On the opposite side of the marker, <i>B</i>, is formed a metallic
+point, which faces the registration sheet and is nearly in contact with
+it. The framing, which carries the scale and marker, is an arrangement of
+brass bars, delicately adjusted and controlled by springs, so as to permit
+of a quick horizontal motion, in a small arc, being communicated to it by
+the action of the hammer, <i>E</i>, of the clock, whereby the point of the
+marker is caused to impress a<span class="pagenum"><a name="Page_34" id="Page_34">[Pg 34]</a></span> dot upon the paper. The same clock gives
+rotation to the hollow wooden cylinder, <i>D</i>, upon which is mounted the
+registering paper. The clock must be rewound when a fresh paper is
+attached to the cylinder, which may be daily, weekly, or monthly,
+according to construction; and the series of dots impressed upon the paper
+shows the height of the barometric column every hour by day and night. The
+space traversed by the marker is precisely equal to the range of the
+barometric column.</p>
+
+<div class="figleft">Fig. 27.<br /><img src="images/fig_27.jpg" alt="" /></div>
+
+<p><br /><b>35. King&#8217;s Self-Registering Barometer.</b>&mdash;Mr. Alfred King, Engineer of the
+Liverpool Gas-light Company, designed, so long ago as 1854, a barometer to
+register, by a continuous pencil-tracing, the variations in the weight of
+the atmosphere; and a highly-satisfactory self-recording barometer, on his
+principle and constructed under his immediate superintendence, has quite
+recently been erected at the Liverpool Observatory.</p>
+
+<p>Fig. 27 is the front elevation of this instrument. <i>A</i>, the barometer
+tube, is three inches in internal diameter, and it floats freely (not
+being fixed as usual) in the fixed cistern, <i>B</i>, guided by
+friction-wheels, <i>W</i>. The top end of the tube is fastened to a peculiar
+chain, which passes over a grooved wheel turning on finely-adjusted
+friction rollers. The other end of the chain supports the frame, <i>D</i>,
+which carries the tracing pencil. The frame is suitably weighted and
+guided, and faces the cylinder, <i>C</i>, around which the tracing paper is
+wrapped, and which rotates once in twenty-four hours by the movement of a
+clock. Mr. Hartnup, Director of the Liverpool Observatory, in his Annual
+Report, 1868, says:&mdash;&#8220;For one inch change in the mercurial column the
+pencil is moved through five inches, so that the horizontal lines on the
+tracing, which are half an inch apart, represent one-tenth of an inch
+change in the barometer. The vertical lines are hour lines, and being
+nearly three-quarters of an inch apart, it will be seen that the smallest
+appreciable change in the barometer, and the time of its occurrence, are
+recorded.&#8221;</p>
+
+<p>&#8220;It has been remarked by persons in the habit of reading barometers with
+large<span class="pagenum"><a name="Page_35" id="Page_35">[Pg 35]</a></span> tubes, that, in squally weather, sudden and frequent oscillations
+of the mercurial column are sometimes seen. Now, to register these small
+oscillations must be a very delicate test of the sensitiveness of a
+self-registering barometer, as the time occupied by the rise and fall of
+the mercury in the tube in some cases does not exceed one minute.&#8221; Mr.
+Hartnup affirms that the tracing of this instrument exhibits such
+oscillations whenever the wind blows strong and in squalls.</p>
+
+<p>As the barometer in this instrument is precisely similar to the &#8220;Long
+Range Barometer&#8221; invented by Mr. McNeild (and which will be found
+described at page 48), it may be desirable to quote the following, from
+Mr. Hartnup&#8217;s Report:&mdash;&#8220;Mr. King constructed a small model instrument to
+illustrate the principle. This instrument was entrusted to my care for
+examination, and it was exhibited to the scientific gentlemen who visited
+the Observatory in 1854, during the meeting of the British Association for
+the Advancement of Science.&#8221;</p>
+
+
+<p><br /><b>36. Syphon, with Photographic Registration.</b>&mdash;A continuous self-registering
+barometer has been constructed, in which photography is employed. Those
+who may wish to adopt a similar apparatus, or thoroughly to understand the
+arrangements and mode of observation, should consult the detailed
+description given in the <i>Greenwich Magnetical and Meteorological
+Observations</i>, 1847. As the principles are applicable to photographic
+registration of magnetic and electric as well as meteorologic variations
+in instrumental indications, it would be beside our purpose to describe
+fully the apparatus.</p>
+
+<p>The barometer is a large syphon tube; the bore of the upper and lower
+extremities, through which the surfaces of the mercury rise and fall, is
+1<span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">10</span> inch in diameter. The glass float in the open limb is attached to a
+wire, which moves a delicately-supported light lever as it alters its
+elevation. The fulcrum of the lever is on one side of the wire; the
+extremity on the other side, at four times this distance from the fulcrum,
+carries a vertical plate of opaque mica, having a small aperture. Through
+this hole the light of a gas-jet shines upon photographic paper wrapped
+round a cylinder placed vertically, and moved round its axis by a clock
+fixed with its face horizontal. The cylinder is delicately supported, and
+revolves in friction rollers. A bent wire on the axis is embraced by a
+prong on the hour hand of the time-piece; therefore the cylinder is
+carried round once in twelve hours. It might be arranged for a different
+period of rotation.</p>
+
+<p>As the cylinder rotates, the paper receives the action of the light, and a
+photographic trace is left of the movements of the barometer four times
+the extent of the oscillations of the float, or twice the length of the
+variations in the barometric column. Certain chemical processes are
+required in the preparation of the paper, and in developing the trace. The
+diagram which we give on the next page, with the explanation, taken from
+Drew&#8217;s <i>Practical Meteorology</i>, will enable the above description to be
+better understood:</p>
+
+<p>&nbsp;<span class="pagenum"><a name="Page_36" id="Page_36">[Pg 36]</a></span></p>
+<p class="center">Fig. 28.</p>
+<div class="figcenter"><img src="images/fig_28.jpg" alt="" /></div>
+<p>&nbsp;</p>
+
+<p>&#8220;<i>Q e</i> is a lever whose fulcrum is <i>e</i>, the counterpoise <i>f</i> nearly
+supporting it; <i>s</i> is an opaque plate of mica, with a small aperture at
+<i>p</i>, through which the light passes, having before been refracted by a
+cylindrical lens into a long ray, the portion only of which opposite the
+aperture <i>p</i> impinges on the paper; <i>d</i> is a wire supported by a float on
+the surface of the mercury; <i>G H</i> is the barometer; <i>p</i>, the vertical
+cylinder charged with photographic paper; <i>r</i>, the photographic trace;
+<i>I</i>, the timepiece, carrying round the cylinder by the projecting arm <i>t</i>.
+It is evident that the respective distances of the float and the aperture
+<i>p</i> from the fulcrum may be regulated so that the rise and fall of the
+float may be multiplied to any extent required.&#8221; When <i>only</i> the lower
+surface of the mercury in a syphon barometer is read, as in the instrument
+just described, a correction for temperature is strictly due to the height
+of the quicksilver in the <i>short</i> tube; but this in so short a column will
+rarely be sensible.</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_37" id="Page_37">[Pg 37]</a></span></p>
+<h2><a name="CHAPTER_IV" id="CHAPTER_IV"></a>CHAPTER IV.</h2>
+<p class="center"><span class="large">MOUNTAIN BAROMETERS.</span></p>
+
+<p><br /><b>37. The Syphon Tube Mountain Barometer, on Gay Lussac&#8217;s principle</b>,
+constructed as described at <a href="#Page_31">page 31</a>, and fixed in a metallic tubular
+frame, forms a simple and light travelling instrument. The graduations are
+made upon the frame, and it is suspended for reading by a ring at the top,
+from beneath an iron tripod stand, which is usually supplied with it.
+Considerable care is requisite in adjusting the verniers, so as to keep
+the instrument steady and vertical. A drawback to the convenience of this
+barometer is the movement of the mercury in the short limb, which is
+generally not confined, and hence has every facility for becoming quickly
+oxidised in travelling. To remedy this, Messrs. Negretti and Zambra so
+construct the Mountain Syphon Barometer that by a simple half turn of a
+screw the mercury can be confined for portability, while the lower limb
+can be taken out for cleaning whenever found requisite.</p>
+
+
+<p><br /><b>38. Mountain Barometer on Fortin&#8217;s principle.</b>&mdash;This barometer, with
+Fortin&#8217;s cistern, as arranged by Messrs. Negretti and Zambra, is an
+elegant, manageable, and very accurate instrument for travelling purposes,
+and well adapted for careful measurement of heights. The cistern is made
+large enough to receive all the mercury that will fall from the tube at
+the highest attainable elevation. The screw at the bottom confines the
+mercury securely for carriage, and serves to adjust the surface of the
+mercury to the zero of the scale when making an observation. The vernier
+reads to &middot;002 of an inch, and slides easily on the brass frame, which is
+made as small in diameter as is compatible with the size of the tube. The
+tube in this barometer should be altogether without contractions, so that
+the mercury will readily fall when it is set up for observation. It must
+be carefully calibrated, and its internal diameter ascertained, in order
+that correction may be made for capillarity. This correction, however,
+should be combined with the error of graduation, and form a permanent
+index error, ascertainable at any time by comparison with an acknowledged
+standard barometer.</p>
+
+<p>The barometer is supported in the tripod stand (furnished as part of the
+instrument) when used for observation. It is suspended by placing two
+studs, in the ring on the frame, in slots formed on the top of the stand,
+so that it hangs freely and<span class="pagenum"><a name="Page_38" id="Page_38">[Pg 38]</a></span> vertically in gimbals. To the metal top of
+the stand, mahogany legs are hinged. To make the barometer portable, it
+must be lifted out of the stand, sloped gently until the mercury reaches
+the top, turning the screw at the bottom meanwhile; then invert and screw
+until the mercury is made tight. The inverted instrument packs in the
+stand, the legs being formed to fit round the frame; and receptacles are
+scooped out for the cistern, thermometer, gimbals, and vernier; so that
+the instrument is firmly surrounded by the wooden legs, which are held
+fast together by brass rings passed over them.</p>
+
+<div class="figleft">Fig. 29.<br /><img src="images/fig_29.jpg" alt="" /></div>
+
+<p><br /><b>39. Newman&#8217;s Mountain Barometer.</b>&mdash;Fig. 29 is an illustration of the
+mountain barometer known as Newman&#8217;s. The cistern consists of two separate
+compartments;&mdash;the top of the lower and the bottom of the upper, being
+perfectly flat, are pivoted closely together at the centres, so that the
+lower can move through a small arc, when turned by the hand. This movement
+is limited by two stops. The top of the lower compartment and the bottom
+of the upper have each a circular hole, through which the mercury
+communicates. When the instrument is required for observation, the cistern
+is turned close up to the stop marked &#8220;<i>open</i>&#8221; or &#8220;<i>not portable</i>.&#8221; When
+it is necessary to pack it for travelling, the mercurial column must be
+allowed to fill the tube by sloping the barometer gently; then invert it,
+and move the cistern to the stop marked &#8220;<i>shut</i>&#8221; or &#8220;<i>portable</i>.&#8221; In this
+condition, the upper compartment is completely filled with mercury, and
+consequently that in the tube cannot move about, so as to admit air or
+endanger the tube. Nor can the mercury pass back to the lower compartment,
+as the holes are not now coincident, and the contact is made too perfect
+to allow the mercury to creep between the surfaces. The tube does not
+enter the lower compartment, which is completely full of mercury when the
+instrument is arranged for observation. The spare capacity of the upper
+cistern is sufficient to receive the mercury which descends from the tube
+to the limit of the engraved scale, which in these barometers generally
+extends only to about 20 inches. A lower limit could of course be given by
+increasing the size of the cisterns, which it is not advisable to do
+unless for a special purpose. This barometer may be had mounted in wood,
+or in brass frame. If in wood, it has a brass shield, which slides round
+the scale part of the frame, so as to be easily brought in front of the
+tube and scale as a protection in travelling; the vernier screw, in this
+case, being placed at the top of the instrument. When the scale is
+graduated with true inches, the neutral point, the capacity and
+capillarity corrections should be marked on the frame. The graduated
+scales, however, placed on these barometers in brass frames, are usually
+artificial inches, like the Kew plan of graduation; the advantage being
+that one simple correction only is required, viz. one for index error and
+capillarity combined, which can always be readily determined by comparison
+with a standard barometer; moreover, as no adjustment of cistern is<span class="pagenum"><a name="Page_39" id="Page_39">[Pg 39]</a></span>
+required in reading, the instrument can be verified by artificial pressure
+throughout the scale, by the plan practised at Kew, Liverpool, &amp;c., and
+already described (see <a href="#Page_18">p. 18</a>).</p>
+
+
+<p class="center"><br />40. NEGRETTI &amp; ZAMBRA&#8217;S PATENT MOUNTAIN AND OTHER BAROMETERS.</p>
+
+<div class="figright">Fig. 30.<br /><img src="images/fig_30.jpg" alt="" /></div>
+
+<p>This invention is intended to make mountain and other barometers of
+standard accuracy stronger, more portable, and less liable to derangement,
+when being carried about, than heretofore, by dispensing with the ordinary
+flexible cistern containing the mercury at the bottom of the instrument,
+and adapting in lieu thereof a rigid cistern constructed of glass and
+iron. The cistern is composed of a glass cylinder, which is secured in a
+metallic tube or frame. In order to render the cistern mercury-tight at
+top and bottom, metal caps are screwed into the tube or frame, and bear
+against leather washers placed between them and the edges of the glass
+cylinder. The upper cap of the cistern is tapped with a fine threaded
+screw to receive the iron plug or socket, into which the barometer tube is
+securely fixed. The whole length of this plug has a fine screw cut upon it
+by which the cistern can be screwed up or down. At the side of this plug
+or socket, extending from the lower end to within half an inch of the top,
+is cut a groove for admitting the air to the surface of the mercury within
+the cistern when the barometer is in use. An ivory point is screwed into
+the under surface of the plug, carrying the barometer tube. This ivory
+point is very carefully adjusted by measurement to be the zero point of
+the instrument, from which the barometer scale of inches is divided. The
+surface of the mercury in the cistern is adjusted to the zero point by
+screwing the cistern up or down until the ivory point and its reflected
+image are in contact.</p>
+
+<p><span class="pagenum"><a name="Page_40" id="Page_40">[Pg 40]</a></span>The instrument (fig. 30) is shown in a state of adjustment, ready to take
+an observation; but <i>when it is desired to render it portable, it must be
+inclined, until mercury from the cistern fills the tube; the cistern must
+then be screwed up on the socket</i>, so as to bring the face of the upper
+cap against the under side of the shoulder of the cover immediately above
+it; the instrument may then be carried without being liable to
+derangement.</p>
+
+<p><i>Precautions necessary in using the Mountain Barometer.</i>&mdash;On removing the
+barometer from its case after a journey, allow it to remain with its scale
+end downward, whilst the cistern is unscrewed to the extent of <i>one turn
+of the screw</i>, after which slightly shake the cistern; the mercury in it
+will then completely fill the end of the barometer tube, should any
+portion of it have escaped therefrom.</p>
+
+<p>The barometer is then inverted, and if it be desired to make an
+observation, suspend it vertically from its stand by the ring at top. The
+cistern must then be unscrewed, until the surface of the mercury is
+brought just level with the extreme end of the ivory or zero point fixed
+to the iron plug on which the glass cistern moves up and down.</p>
+
+<p>Should the elevation of the place where the barometer is to be used be
+considerably above the sea level, it will be well&mdash;after suspending it
+from the stand&mdash;to unscrew the cistern several turns, <i>holding the
+barometer in an oblique position</i>, as at great heights the mercury will
+fall considerably quicker than the cistern can be unscrewed, thereby
+filling it to overflowing; but by partly unscrewing the cistern first,
+room is given for the reception of a fall of mercury to the extent of
+several inches.</p>
+
+<p>The cistern must not be unscrewed when the <i>Instrument is</i> <span class="smcaplc">INVERTED</span> <i>more
+than</i> two turns of the screw, otherwise the mercury will flow out through
+the groove.</p>
+
+<p>It is found safer when travelling to carry the barometer in a horizontal
+position, or with its cistern end uppermost.</p>
+
+<p><i>To clean the Barometer.</i>&mdash;Should at any time the mercury in the cistern
+become oxidised, and reading from its surface be difficult, it can be
+readily cleaned by removing the cistern and its contained mercury from the
+barometer frame by unscrewing it <i>when in a horizontal position</i>; this
+precaution is necessary that the mercury in the tube may not escape, and
+thereby allow air to enter. The cistern must then be emptied, and with a
+dry clean leather, or silk handkerchief, well cleaned.</p>
+
+<p>The operation of cleaning being performed, return the cistern to the
+frame, and screw it until the face is brought up against the under side of
+the shoulder, still keeping the instrument <i>horizontal</i>. The cistern is
+now ready for re-filling, to do which stand the barometer on end <i>head
+downwards</i>, and remove the small screw at bottom; through the aperture
+thus opened, pour in mercury, passing it through a paper funnel with a
+very small aperture. It is well to pass the mercury through a very small
+funnel two or three times before returning it to the barometer cistern, as
+by this process all particles of dust or oxide adhere to the paper, and
+are effectually removed.</p>
+
+<p><span class="pagenum"><a name="Page_41" id="Page_41">[Pg 41]</a></span>Should any small quantity of the mercury be lost during the operation of
+cleaning, it is of no importance so long as sufficient remains to allow of
+adjustment to the zero point. This latter constitutes one of the great
+advantages of this new instrument over the ordinary barometer; for, in the
+majority of cases, after an instrument has been compared carefully with a
+standard, should mercury be lost, there is no means of correcting the
+error unless a standard barometer be at hand; the new barometer is, in
+this respect, independent, a little mercury more or less being
+unimportant.</p>
+
+
+<p><br /><b>41. Short Tube Barometer.</b>&mdash;This is simply a tube shorter, as may be
+required, than that necessary to show the atmospheric pressure at the sea
+level. It is convenient for balloon purposes, and for use at mountain
+stations, being of course a special construction.</p>
+
+
+<p><br /><b>42. Method of Calculating Heights by the Barometer.</b>&mdash;The pressure of the
+atmosphere being measured by the barometer, it is evident that as the
+instrument is carried up a high mountain or elevated in a balloon, the
+length of the column must decrease as the atmospheric pressure decreases,
+in consequence of a stratum of air being left below. The pressure of air
+arises from its weight, or the attraction of gravitation upon it, and
+therefore the quantity of air below the barometer cistern cannot influence
+the height of the column. Hence it follows that a certain relation must
+exist between the difference of the barometric pressure at the foot and at
+the top of a hill or other elevation, and the difference of the absolute
+heights above the sea. Theoretical investigation, abundantly confirmed by
+practical results, has determined that the strata of air decrease in
+density in a geometrical proportion, while the elevations increase in an
+arithmetical one. Hence we have a method of determining differences of
+level, by observations made on the density of the air by means of the
+barometer. It is beyond our purpose to explain in detail the principles
+upon which this method is founded, or to give its mathematical
+investigation. We append Tables, which will be useful to practical
+persons,&mdash;surveyors, engineers, travellers, tourists, &amp;c.,&mdash;who may carry
+a barometer as a travelling companion.</p>
+
+<p>Table I. is calculated from the formula, height in feet = 60,200 (log.
+29&middot;922 - log. B) + 925; where 29&middot;922 is the mean atmospheric pressure at
+32&deg; F., and the mean sea-level in latitude 45&deg;; and B is any other
+barometric pressure; the 925 being added to avoid minus signs in the
+Table.</p>
+
+<p>Table II. contains the correction necessary for the mean temperature of
+the stratum of air between the stations of observation; and is computed
+from Regnault&#8217;s co-efficient for the expansion of air, which is &middot;002036 of
+its volume at 32&deg; for each degree above that temperature.</p>
+
+<p>Table III. is the correction due to the difference of gravitation in any
+other latitude, and is found from the formula, <i>x</i> = 1 + &middot;00265 cos. 2
+lat.</p>
+
+<p><span class="pagenum"><a name="Page_42" id="Page_42">[Pg 42]</a></span>Table IV. is to correct for the diminution of gravity in ascending from
+the sea-level.</p>
+
+<p>To use these Tables: The barometer readings at the upper and lower
+stations having been corrected and reduced to temperature 32&deg; F., take out
+from Table I. the numbers opposite the corrected readings, and subtract
+the lower from the upper. Multiply this difference successively by the
+factors found in Tables II. and III. The factor from Table III. may be
+neglected unless precision is desired. Finally, add the correction taken
+from Table IV.</p>
+
+<p class="center"><span class="smcap">Table I.</span></p>
+<p class="center"><i>Approximate Height due to Barometric Pressure.</i></p>
+
+<table border="0" cellpadding="0" cellspacing="0" summary="table">
+<tr><td class="btlr" align="center">Inches.</td>
+    <td class="btrdoub" align="center">Feet.</td>
+    <td class="btr" align="center">Inches.</td>
+    <td class="btrdoub" align="center">Feet.</td>
+    <td class="btr" align="center">Inches.</td>
+    <td class="btr" align="center">Feet.</td></tr>
+<tr><td class="btlr" align="center">31&middot;0</td>
+    <td class="btrdoub" align="center"><span style="margin-left: 1.5em;">0</span></td>
+    <td class="btr" align="center">28&middot;2</td>
+    <td class="btrdoub" align="center">2475</td>
+    <td class="btr" align="center">25&middot;4</td>
+    <td class="btr" align="center">5209</td></tr>
+<tr><td class="blr" align="center">30&middot;9</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">84</span></td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;1</span></td>
+    <td class="brdoub" align="center">2568</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;3</span></td>
+    <td class="br" align="center">5312</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;8</span></td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">169</span></td>
+    <td class="br" align="center">28&middot;0</td>
+    <td class="brdoub" align="center">2661</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;2</span></td>
+    <td class="br" align="center">5415</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;7</span></td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">254</span></td>
+    <td class="br" align="center">27&middot;9</td>
+    <td class="brdoub" align="center">2754</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;1</span></td>
+    <td class="br" align="center">5519</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;6</span></td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">339</span></td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;8</span></td>
+    <td class="brdoub" align="center">2848</td>
+    <td class="br" align="center">25&middot;0</td>
+    <td class="br" align="center">5623</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;5</span></td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">425</span></td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;7</span></td>
+    <td class="brdoub" align="center">2942</td>
+    <td class="br" align="center">24&middot;9</td>
+    <td class="br" align="center">5728</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;4</span></td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">511</span></td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;6</span></td>
+    <td class="brdoub" align="center">3037</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;8</span></td>
+    <td class="br" align="center">5833</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;3</span></td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">597</span></td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;5</span></td>
+    <td class="brdoub" align="center">3132</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;7</span></td>
+    <td class="br" align="center">5939</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;2</span></td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">683</span></td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;4</span></td>
+    <td class="brdoub" align="center">3227</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;6</span></td>
+    <td class="br" align="center">6045</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;1</span></td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">770</span></td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;3</span></td>
+    <td class="brdoub" align="center">3323</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;5</span></td>
+    <td class="br" align="center">6152</td></tr>
+<tr><td class="blr" align="center">30&middot;0</td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">857</span></td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;2</span></td>
+    <td class="brdoub" align="center">3419</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;4</span></td>
+    <td class="br" align="center">6259</td></tr>
+<tr><td class="blr" align="center">29&middot;9</td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">944</span></td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;1</span></td>
+    <td class="brdoub" align="center">3515</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;3</span></td>
+    <td class="br" align="center">6366</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;8</span></td>
+    <td class="brdoub" align="center">1032</td>
+    <td class="br" align="center">27&middot;0</td>
+    <td class="brdoub" align="center">3612</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;2</span></td>
+    <td class="br" align="center">6474</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;7</span></td>
+    <td class="brdoub" align="center">1120</td>
+    <td class="br" align="center">26&middot;9</td>
+    <td class="brdoub" align="center">3709</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;1</span></td>
+    <td class="br" align="center">6582</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;6</span></td>
+    <td class="brdoub" align="center">1208</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;8</span></td>
+    <td class="brdoub" align="center">3806</td>
+    <td class="br" align="center">24&middot;0</td>
+    <td class="br" align="center">6691</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;5</span></td>
+    <td class="brdoub" align="center">1296</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;7</span></td>
+    <td class="brdoub" align="center">3904</td>
+    <td class="br" align="center">23&middot;9</td>
+    <td class="br" align="center">6800</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;4</span></td>
+    <td class="brdoub" align="center">1385</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;6</span></td>
+    <td class="brdoub" align="center">4002</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;8</span></td>
+    <td class="br" align="center">6910</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;3</span></td>
+    <td class="brdoub" align="center">1474</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;5</span></td>
+    <td class="brdoub" align="center">4100</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;7</span></td>
+    <td class="br" align="center">7020</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;2</span></td>
+    <td class="brdoub" align="center">1563</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;4</span></td>
+    <td class="brdoub" align="center">4199</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;6</span></td>
+    <td class="br" align="center">7131</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;1</span></td>
+    <td class="brdoub" align="center">1653</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;3</span></td>
+    <td class="brdoub" align="center">4298</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;5</span></td>
+    <td class="br" align="center">7242</td></tr>
+<tr><td class="blr" align="center">29&middot;0</td>
+    <td class="brdoub" align="center">1743</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;2</span></td>
+    <td class="brdoub" align="center">4398</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;4</span></td>
+    <td class="br" align="center">7353</td></tr>
+<tr><td class="blr" align="center">28&middot;9</td>
+    <td class="brdoub" align="center">1833</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;1</span></td>
+    <td class="brdoub" align="center">4498</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;3</span></td>
+    <td class="br" align="center">7465</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;8</span></td>
+    <td class="brdoub" align="center">1924</td>
+    <td class="br" align="center">26&middot;0</td>
+    <td class="brdoub" align="center">4598</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;2</span></td>
+    <td class="br" align="center">7577</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;7</span></td>
+    <td class="brdoub" align="center">2015</td>
+    <td class="br" align="center">25&middot;9</td>
+    <td class="brdoub" align="center">4699</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;1</span></td>
+    <td class="br" align="center">7690</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;6</span></td>
+    <td class="brdoub" align="center">2106</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;8</span></td>
+    <td class="brdoub" align="center">4800</td>
+    <td class="br" align="center">23&middot;0</td>
+    <td class="br" align="center">7803</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;5</span></td>
+    <td class="brdoub" align="center">2198</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;7</span></td>
+    <td class="brdoub" align="center">4902</td>
+    <td class="br" align="center">22&middot;9</td>
+    <td class="br" align="center">7917</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;4</span></td>
+    <td class="brdoub" align="center">2290</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;6</span></td>
+    <td class="brdoub" align="center">5004</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;8</span></td>
+    <td class="br" align="center">8032</td></tr>
+<tr><td class="bblr" align="center"><span style="margin-left: 1em;">&middot;3</span></td>
+    <td class="bbrdoub" align="center">2382</td>
+    <td class="bbr" align="center"><span style="margin-left: 1em;">&middot;5</span></td>
+    <td class="bbrdoub" align="center">5106</td>
+    <td class="bbr" align="center"><span style="margin-left: 1em;">&middot;7</span></td>
+    <td class="bbr" align="center">8147</td></tr></table>
+
+<p>&nbsp;<span class="pagenum"><a name="Page_43" id="Page_43">[Pg 43]</a></span></p>
+<p class="center"><span class="smcap">Table I.</span>&mdash;<i>continued</i>.</p>
+<p class="center"><i>Approximate Height due to Barometric Pressure.</i></p>
+
+<table border="0" cellpadding="0" cellspacing="0" summary="table">
+<tr><td class="btlr" align="center">Inches.</td>
+    <td class="btrdoub" align="center">Feet.</td>
+    <td class="btr" align="center">Inches.</td>
+    <td class="btrdoub" align="center">Feet.</td>
+    <td class="btr" align="center">Inches.</td>
+    <td class="btr" align="center">Feet.</td></tr>
+<tr><td class="btlr" align="center">22&middot;6</td>
+    <td class="btrdoub" align="center"><span style="margin-left: .5em;">8262</span></td>
+    <td class="btr" align="center">18&middot;9</td>
+    <td class="btrdoub" align="center">12937</td>
+    <td class="btr" align="center">15&middot;2</td>
+    <td class="btr" align="center">18632</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;5</span></td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">8378</span></td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;8</span></td>
+    <td class="brdoub" align="center">13076</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;1</span></td>
+    <td class="br" align="center">18805</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;4</span></td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">8495</span></td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;7</span></td>
+    <td class="brdoub" align="center">13215</td>
+    <td class="br" align="center">15&middot;0</td>
+    <td class="br" align="center">18979</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;3</span></td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">8612</span></td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;6</span></td>
+    <td class="brdoub" align="center">13355</td>
+    <td class="br" align="center">14&middot;9</td>
+    <td class="br" align="center">19154</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;2</span></td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">8729</span></td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;5</span></td>
+    <td class="brdoub" align="center">13496</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;8</span></td>
+    <td class="br" align="center">19330</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;1</span></td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">8847</span></td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;4</span></td>
+    <td class="brdoub" align="center">13638</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;7</span></td>
+    <td class="br" align="center">19507</td></tr>
+<tr><td class="blr" align="center">22&middot;0</td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">8966</span></td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;3</span></td>
+    <td class="brdoub" align="center">13780</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;6</span></td>
+    <td class="br" align="center">19685</td></tr>
+<tr><td class="blr" align="center">21&middot;9</td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">9085</span></td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;2</span></td>
+    <td class="brdoub" align="center">13923</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;5</span></td>
+    <td class="br" align="center">19865</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;8</span></td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">9205</span></td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;1</span></td>
+    <td class="brdoub" align="center">14067</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;4</span></td>
+    <td class="br" align="center">20046</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;7</span></td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">9325</span></td>
+    <td class="br" align="center">18&middot;0</td>
+    <td class="brdoub" align="center">14212</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;3</span></td>
+    <td class="br" align="center">20228</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;6</span></td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">9446</span></td>
+    <td class="br" align="center">17&middot;9</td>
+    <td class="brdoub" align="center">14358</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;2</span></td>
+    <td class="br" align="center">20412</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;5</span></td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">9567</span></td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;8</span></td>
+    <td class="brdoub" align="center">14505</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;1</span></td>
+    <td class="br" align="center">20597</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;4</span></td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">9689</span></td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;7</span></td>
+    <td class="brdoub" align="center">14652</td>
+    <td class="br" align="center">14&middot;0</td>
+    <td class="br" align="center">20783</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;3</span></td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">9811</span></td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;6</span></td>
+    <td class="brdoub" align="center">14800</td>
+    <td class="br" align="center">13&middot;9</td>
+    <td class="br" align="center">20970</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;2</span></td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">9934</span></td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;5</span></td>
+    <td class="brdoub" align="center">14949</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;8</span></td>
+    <td class="br" align="center">21159</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;1</span></td>
+    <td class="brdoub" align="center">10058</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;4</span></td>
+    <td class="brdoub" align="center">15099</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;7</span></td>
+    <td class="br" align="center">21349</td></tr>
+<tr><td class="blr" align="center">21&middot;0</td>
+    <td class="brdoub" align="center">10182</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;3</span></td>
+    <td class="brdoub" align="center">15250</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;6</span></td>
+    <td class="br" align="center">21541</td></tr>
+<tr><td class="blr" align="center">20&middot;9</td>
+    <td class="brdoub" align="center">10307</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;2</span></td>
+    <td class="brdoub" align="center">15402</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;5</span></td>
+    <td class="br" align="center">21734</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;8</span></td>
+    <td class="brdoub" align="center">10432</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;1</span></td>
+    <td class="brdoub" align="center">15554</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;4</span></td>
+    <td class="br" align="center">21928</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;7</span></td>
+    <td class="brdoub" align="center">10558</td>
+    <td class="br" align="center">17&middot;0</td>
+    <td class="brdoub" align="center">15707</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;3</span></td>
+    <td class="br" align="center">22124</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;6</span></td>
+    <td class="brdoub" align="center">10684</td>
+    <td class="br" align="center">16&middot;9</td>
+    <td class="brdoub" align="center">15861</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;2</span></td>
+    <td class="br" align="center">22321</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;5</span></td>
+    <td class="brdoub" align="center">10812</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;8</span></td>
+    <td class="brdoub" align="center">16016</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;1</span></td>
+    <td class="br" align="center">22520</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;4</span></td>
+    <td class="brdoub" align="center">10940</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;7</span></td>
+    <td class="brdoub" align="center">16172</td>
+    <td class="br" align="center">13&middot;0</td>
+    <td class="br" align="center">22720</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;3</span></td>
+    <td class="brdoub" align="center">11069</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;6</span></td>
+    <td class="brdoub" align="center">16329</td>
+    <td class="br" align="center">12&middot;9</td>
+    <td class="br" align="center">22922</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;2</span></td>
+    <td class="brdoub" align="center">11198</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;5</span></td>
+    <td class="brdoub" align="center">16487</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;8</span></td>
+    <td class="br" align="center">23126</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;1</span></td>
+    <td class="brdoub" align="center">11328</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;4</span></td>
+    <td class="brdoub" align="center">16646</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;7</span></td>
+    <td class="br" align="center">23331</td></tr>
+<tr><td class="blr" align="center">20&middot;0</td>
+    <td class="brdoub" align="center">11458</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;3</span></td>
+    <td class="brdoub" align="center">16806</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;6</span></td>
+    <td class="br" align="center">23538</td></tr>
+<tr><td class="blr" align="center">19&middot;9</td>
+    <td class="brdoub" align="center">11589</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;2</span></td>
+    <td class="brdoub" align="center">16967</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;5</span></td>
+    <td class="br" align="center">23746</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;8</span></td>
+    <td class="brdoub" align="center">11721</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;1</span></td>
+    <td class="brdoub" align="center">17129</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;4</span></td>
+    <td class="br" align="center">23956</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;7</span></td>
+    <td class="brdoub" align="center">11853</td>
+    <td class="br" align="center">16&middot;0</td>
+    <td class="brdoub" align="center">17292</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;3</span></td>
+    <td class="br" align="center">24168</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;6</span></td>
+    <td class="brdoub" align="center">11986</td>
+    <td class="br" align="center">15&middot;9</td>
+    <td class="brdoub" align="center">17456</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;2</span></td>
+    <td class="br" align="center">24381</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;5</span></td>
+    <td class="brdoub" align="center">12120</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;8</span></td>
+    <td class="brdoub" align="center">17621</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;1</span></td>
+    <td class="br" align="center">24596</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;4</span></td>
+    <td class="brdoub" align="center">12254</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;7</span></td>
+    <td class="brdoub" align="center">17787</td>
+    <td class="br" align="center">12&middot;0</td>
+    <td class="br" align="center">24813</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;3</span></td>
+    <td class="brdoub" align="center">12389</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;6</span></td>
+    <td class="brdoub" align="center">17954</td>
+    <td class="br" align="center">11&middot;9</td>
+    <td class="br" align="center">25032</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;2</span></td>
+    <td class="brdoub" align="center">12525</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;5</span></td>
+    <td class="brdoub" align="center">18122</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;8</span></td>
+    <td class="br" align="center">25253</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;1</span></td>
+    <td class="brdoub" align="center">12662</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;4</span></td>
+    <td class="brdoub" align="center">18291</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;7</span></td>
+    <td class="br" align="center">25476</td></tr>
+<tr><td class="bblr" align="center">19&middot;0</td>
+    <td class="bbrdoub" align="center">12799</td>
+    <td class="bbr" align="center"><span style="margin-left: 1em;">&middot;3</span></td>
+    <td class="bbrdoub" align="center">18461</td>
+    <td class="bbr" align="center"><span style="margin-left: 1em;">&middot;6</span></td>
+    <td class="bbr" align="center">25700</td></tr></table>
+
+
+<p>&nbsp;<span class="pagenum"><a name="Page_44" id="Page_44">[Pg 44]</a></span></p>
+<p class="center"><span class="smcap">Table</span> II.</p>
+<p class="center"><i>Correction due to Mean Temperature of the Air.</i></p>
+
+<table border="0" cellpadding="0" cellspacing="0" summary="table">
+<tr><td class="btlr" align="center">Mean<br />Temp.</td>
+    <td class="btrdoub" align="center">Factor.</td>
+    <td class="btr" align="center">Mean<br />Temp.</td>
+    <td class="btrdoub" align="center">Factor.</td>
+    <td class="btr" align="center">Mean<br />Temp.</td>
+    <td class="btr" align="center">Factor.</td></tr>
+<tr><td class="btlr" align="center"><span style="margin-left: .35em;">10&deg;</span></td>
+    <td class="btrdoub" align="center">0&middot;955</td>
+    <td class="btr" align="center"><span style="margin-left: .35em;">35&deg;</span></td>
+    <td class="btrdoub" align="center">1&middot;006</td>
+    <td class="btr" align="center"><span style="margin-left: .35em;">60&deg;</span></td>
+    <td class="btr" align="center">1&middot;057</td></tr>
+<tr><td class="blr" align="center">11</td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">&middot;957</span></td>
+    <td class="br" align="center">36</td>
+    <td class="brdoub" align="center">1&middot;008</td>
+    <td class="br" align="center">61</td>
+    <td class="br" align="center">1&middot;059</td></tr>
+<tr><td class="blr" align="center">12</td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">&middot;959</span></td>
+    <td class="br" align="center">37</td>
+    <td class="brdoub" align="center">1&middot;010</td>
+    <td class="br" align="center">62</td>
+    <td class="br" align="center">1&middot;061</td></tr>
+<tr><td class="blr" align="center">13</td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">&middot;961</span></td>
+    <td class="br" align="center">38</td>
+    <td class="brdoub" align="center">1&middot;012</td>
+    <td class="br" align="center">63</td>
+    <td class="br" align="center">1&middot;063</td></tr>
+<tr><td class="blr" align="center">14</td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">&middot;963</span></td>
+    <td class="br" align="center">39</td>
+    <td class="brdoub" align="center">1&middot;014</td>
+    <td class="br" align="center">64</td>
+    <td class="br" align="center">1&middot;065</td></tr>
+<tr><td class="blr" align="center">15</td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">&middot;965</span></td>
+    <td class="br" align="center">40</td>
+    <td class="brdoub" align="center">1&middot;016</td>
+    <td class="br" align="center">65</td>
+    <td class="br" align="center">1&middot;067</td></tr>
+<tr><td class="blr" align="center">16</td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">&middot;967</span></td>
+    <td class="br" align="center">41</td>
+    <td class="brdoub" align="center">1&middot;018</td>
+    <td class="br" align="center">66</td>
+    <td class="br" align="center">1&middot;069</td></tr>
+<tr><td class="blr" align="center">17</td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">&middot;969</span></td>
+    <td class="br" align="center">42</td>
+    <td class="brdoub" align="center">1&middot;020</td>
+    <td class="br" align="center">67</td>
+    <td class="br" align="center">1&middot;071</td></tr>
+<tr><td class="blr" align="center">18</td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">&middot;971</span></td>
+    <td class="br" align="center">43</td>
+    <td class="brdoub" align="center">1&middot;022</td>
+    <td class="br" align="center">68</td>
+    <td class="br" align="center">1&middot;073</td></tr>
+<tr><td class="blr" align="center">19</td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">&middot;974</span></td>
+    <td class="br" align="center">44</td>
+    <td class="brdoub" align="center">1&middot;024</td>
+    <td class="br" align="center">69</td>
+    <td class="br" align="center">1&middot;075</td></tr>
+<tr><td class="blr" align="center">20</td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">&middot;976</span></td>
+    <td class="br" align="center">45</td>
+    <td class="brdoub" align="center">1&middot;026</td>
+    <td class="br" align="center">70</td>
+    <td class="br" align="center">1&middot;077</td></tr>
+<tr><td class="blr" align="center">21</td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">&middot;978</span></td>
+    <td class="br" align="center">46</td>
+    <td class="brdoub" align="center">1&middot;029</td>
+    <td class="br" align="center">71</td>
+    <td class="br" align="center">1&middot;079</td></tr>
+<tr><td class="blr" align="center">22</td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">&middot;980</span></td>
+    <td class="br" align="center">47</td>
+    <td class="brdoub" align="center">1&middot;031</td>
+    <td class="br" align="center">72</td>
+    <td class="br" align="center">1&middot;081</td></tr>
+<tr><td class="blr" align="center">23</td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">&middot;982</span></td>
+    <td class="br" align="center">48</td>
+    <td class="brdoub" align="center">1&middot;033</td>
+    <td class="br" align="center">73</td>
+    <td class="br" align="center">1&middot;083</td></tr>
+<tr><td class="blr" align="center">24</td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">&middot;984</span></td>
+    <td class="br" align="center">49</td>
+    <td class="brdoub" align="center">1&middot;035</td>
+    <td class="br" align="center">74</td>
+    <td class="br" align="center">1&middot;086</td></tr>
+<tr><td class="blr" align="center">25</td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">&middot;986</span></td>
+    <td class="br" align="center">50</td>
+    <td class="brdoub" align="center">1&middot;037</td>
+    <td class="br" align="center">75</td>
+    <td class="br" align="center">1&middot;088</td></tr>
+<tr><td class="blr" align="center">26</td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">&middot;988</span></td>
+    <td class="br" align="center">51</td>
+    <td class="brdoub" align="center">1&middot;039</td>
+    <td class="br" align="center">76</td>
+    <td class="br" align="center">1&middot;090</td></tr>
+<tr><td class="blr" align="center">27</td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">&middot;990</span></td>
+    <td class="br" align="center">52</td>
+    <td class="brdoub" align="center">1&middot;041</td>
+    <td class="br" align="center">77</td>
+    <td class="br" align="center">1&middot;092</td></tr>
+<tr><td class="blr" align="center">28</td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">&middot;992</span></td>
+    <td class="br" align="center">53</td>
+    <td class="brdoub" align="center">1&middot;043</td>
+    <td class="br" align="center">78</td>
+    <td class="br" align="center">1&middot;094</td></tr>
+<tr><td class="blr" align="center">29</td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">&middot;994</span></td>
+    <td class="br" align="center">54</td>
+    <td class="brdoub" align="center">1&middot;045</td>
+    <td class="br" align="center">79</td>
+    <td class="br" align="center">1&middot;096</td></tr>
+<tr><td class="blr" align="center">30</td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">&middot;996</span></td>
+    <td class="br" align="center">55</td>
+    <td class="brdoub" align="center">1&middot;047</td>
+    <td class="br" align="center">80</td>
+    <td class="br" align="center">1&middot;098</td></tr>
+<tr><td class="blr" align="center">31</td>
+    <td class="brdoub" align="center">0&middot;998</td>
+    <td class="br" align="center">56</td>
+    <td class="brdoub" align="center">1&middot;049</td>
+    <td class="br" align="center">81</td>
+    <td class="br" align="center">1&middot;100</td></tr>
+<tr><td class="blr" align="center">32</td>
+    <td class="brdoub" align="center">1&middot;000</td>
+    <td class="br" align="center">57</td>
+    <td class="brdoub" align="center">1&middot;051</td>
+    <td class="br" align="center">82</td>
+    <td class="br" align="center">1&middot;102</td></tr>
+<tr><td class="blr" align="center">33</td>
+    <td class="brdoub" align="center">1&middot;002</td>
+    <td class="br" align="center">58</td>
+    <td class="brdoub" align="center">1&middot;053</td>
+    <td class="br" align="center">83</td>
+    <td class="br" align="center">1&middot;104</td></tr>
+<tr><td class="bblr" align="center">34</td>
+    <td class="bbrdoub" align="center">1&middot;004</td>
+    <td class="bbr" align="center">59</td>
+    <td class="bbrdoub" align="center">1&middot;055</td>
+    <td class="bbr" align="center">84</td>
+    <td class="bbr" align="center">1&middot;106</td></tr></table>
+
+
+<p class="center"><br /><span class="smcap">Table</span> III.</p>
+
+<table border="0" cellpadding="0" cellspacing="0" summary="table">
+<tr><td class="btlr" align="center">Latitude.</td>
+    <td class="btrdoub" align="center">Factor.</td>
+    <td class="btr" align="center">Latitude.</td>
+    <td class="btrdoub" align="center">Factor.</td>
+    <td class="btr" align="center">Latitude.</td>
+    <td class="btr" align="center">Factor.</td></tr>
+<tr><td class="btlr" align="center">80&deg;</td>
+    <td class="btrdoub" align="center">0&middot;99751</td>
+    <td class="btr" align="center">50</td>
+    <td class="btrdoub" align="center">0&middot;99954</td>
+    <td class="btr" align="center">20</td>
+    <td class="btr" align="center">1&middot;00203</td></tr>
+<tr><td class="blr" align="center">75</td>
+    <td class="brdoub" align="center">0&middot;99770</td>
+    <td class="br" align="center">45</td>
+    <td class="brdoub" align="center">1&middot;00000</td>
+    <td class="br" align="center">15</td>
+    <td class="br" align="center">1&middot;00230</td></tr>
+<tr><td class="blr" align="center">70</td>
+    <td class="brdoub" align="center">0&middot;99797</td>
+    <td class="br" align="center">40</td>
+    <td class="brdoub" align="center">1&middot;00046</td>
+    <td class="br" align="center">10</td>
+    <td class="br" align="center">1&middot;00249</td></tr>
+<tr><td class="blr" align="center">65</td>
+    <td class="brdoub" align="center">0&middot;99830</td>
+    <td class="br" align="center">35</td>
+    <td class="brdoub" align="center">1&middot;00090</td>
+    <td class="br" align="center"><span style="margin-left: .5em;">5</span></td>
+    <td class="br" align="center">1&middot;00261</td></tr>
+<tr><td class="blr" align="center">60</td>
+    <td class="brdoub" align="center">0&middot;99868</td>
+    <td class="br" align="center">30</td>
+    <td class="brdoub" align="center">1&middot;00132</td>
+    <td class="br" align="center"><span style="margin-left: .5em;">0</span></td>
+    <td class="br" align="center">1&middot;00265</td></tr>
+<tr><td class="bblr" align="center">55</td>
+    <td class="bbrdoub" align="center">0&middot;99910</td>
+    <td class="bbr" align="center">25</td>
+    <td class="bbrdoub" align="center">1&middot;00170</td>
+    <td class="bbr" align="center">&nbsp;</td>
+    <td class="bbr" align="center">&nbsp;</td></tr></table>
+
+<p>&nbsp;<span class="pagenum"><a name="Page_45" id="Page_45">[Pg 45]</a></span></p>
+<p class="center"><span class="smcap">Table</span> IV.</p>
+
+<table border="0" cellpadding="0" cellspacing="0" summary="table">
+<tr><td class="btlr" align="center">Height in<br />Thousand Feet.</td>
+    <td class="btrdoub" align="center">Correction<br />Additive.</td>
+    <td class="btr" align="center">Height in<br />Thousand Feet.</td>
+    <td class="btr" align="center">Correction<br />Additive.</td></tr>
+<tr><td class="btlr" align="center"><span style="margin-left: .5em;">1</span></td>
+    <td class="btrdoub" align="center"><span style="margin-left: .5em;">3</span></td>
+    <td class="btr" align="center">14</td>
+    <td class="btr" align="center">44</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: .5em;">2</span></td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">5</span></td>
+    <td class="br" align="center">15</td>
+    <td class="br" align="center">48</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: .5em;">3</span></td>
+    <td class="brdoub" align="center"><span style="margin-left: .5em;">8</span></td>
+    <td class="br" align="center">16</td>
+    <td class="br" align="center">52</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: .5em;">4</span></td>
+    <td class="brdoub" align="center">11</td>
+    <td class="br" align="center">17</td>
+    <td class="br" align="center">56</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: .5em;">5</span></td>
+    <td class="brdoub" align="center">14</td>
+    <td class="br" align="center">18</td>
+    <td class="br" align="center">60</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: .5em;">6</span></td>
+    <td class="brdoub" align="center">17</td>
+    <td class="br" align="center">19</td>
+    <td class="br" align="center">65</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: .5em;">7</span></td>
+    <td class="brdoub" align="center">20</td>
+    <td class="br" align="center">20</td>
+    <td class="br" align="center">69</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: .5em;">8</span></td>
+    <td class="brdoub" align="center">23</td>
+    <td class="br" align="center">21</td>
+    <td class="br" align="center">74</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: .5em;">9</span></td>
+    <td class="brdoub" align="center">26</td>
+    <td class="br" align="center">22</td>
+    <td class="br" align="center">78</td></tr>
+<tr><td class="blr" align="center">10</td>
+    <td class="brdoub" align="center">30</td>
+    <td class="br" align="center">23</td>
+    <td class="br" align="center">83</td></tr>
+<tr><td class="blr" align="center">11</td>
+    <td class="brdoub" align="center">33</td>
+    <td class="br" align="center">24</td>
+    <td class="br" align="center">88</td></tr>
+<tr><td class="blr" align="center">12</td>
+    <td class="brdoub" align="center">37</td>
+    <td class="br" align="center">25</td>
+    <td class="br" align="center">93</td></tr>
+<tr><td class="bblr" align="center">13</td>
+    <td class="bbrdoub" align="center">41</td>
+    <td class="bbr" align="center">26</td>
+    <td class="bbr" align="center">98</td></tr></table>
+
+<p><span class="smcap">Example 1.</span> On October 21st, 1852, when Mr. Welsh ascended in a balloon, at
+3h. 30m. p.m., the barometer, corrected and reduced, was 18&middot;85, the air
+temperature 27&deg;, while at Greenwich, 159 feet above the sea, the barometer
+at the same time was 29&middot;97 inches, air temperature 49&deg;, the balloon not
+being more than 5 miles S.W. from over Greenwich; required its elevation.</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td colspan="6">&nbsp;</td><td align="center">Feet.</td></tr>
+
+<tr><td>Barometer</td>
+    <td>in Balloon</td>
+    <td>&nbsp; &nbsp;</td>
+    <td>18&middot;85,</td>
+    <td>Table I.</td>
+    <td align="center">=</td>
+    <td align="right">13007</td></tr>
+<tr><td align="center">"</td>
+    <td>at Greenwich</td>
+    <td>&nbsp; &nbsp;</td>
+    <td>29&middot;97</td>
+    <td align="center">"</td>
+    <td>&nbsp;</td>
+    <td class="botbor" align="right">883</td></tr>
+<tr><td colspan="6">&nbsp;</td>
+    <td align="right">12124</td></tr>
+<tr><td colspan="6">Mean Temperature, 38&deg;, Table II. Factor</td>
+    <td align="right" class="botbor">1&middot;012</td></tr>
+<tr><td colspan="6">&nbsp;</td>
+    <td align="right" class="botbor">12269&middot;</td></tr>
+<tr><td colspan="6">Latitude 51&#189;&deg;, Factor from Table III.</td>
+    <td align="right" class="botbor">&middot;99941</td></tr>
+<tr><td colspan="6">&nbsp;</td>
+    <td align="right">12262</td></tr>
+<tr><td colspan="6">Correction from Table IV.</td>
+    <td align="right" class="botbor">38</td></tr>
+<tr><td colspan="6">&nbsp;</td>
+    <td align="right">12300</td></tr>
+<tr><td colspan="6">Elevation of Greenwich</td>
+    <td align="right" class="botbor">159</td></tr>
+<tr><td colspan="6"><span style="margin-left: 1.75em;">"</span><span style="margin-left: 2.75em;">Balloon</span></td>
+    <td align="right" class="botbor2">12459</td><td>feet.</td></tr></table>
+
+<p>The following examples, from the balloon ascents of J. Glashier, Esq.,
+F.R.S., will serve for practice.<small><a name="f4.1" id="f4.1" href="#f4">[4]</a></small></p>
+
+<p><span class="pagenum"><a name="Page_46" id="Page_46">[Pg 46]</a></span>2. Ascended from Wolverhampton, 18th August, 1862, at 2h. 38m. p.m.;
+barometer (in all cases corrected and reduced to 32&deg; F) was 14&middot;868, the
+temperature of the air 26&deg;; at the same time, at Wrottesley Hall, 531 feet
+above the sea, in latitude 52&#189;&deg; N, the barometer was 29&middot;46, and the
+temperature of the air 65&deg;&middot;4; find the elevation of the balloon above the
+sea.</p>
+
+<p class="right">Height, 18,959 feet.</p>
+
+<p>3. From the same place an ascent was made 5th September, 1862, when at 1h.
+48m. p.m. barometer was 11&middot;954, air O&deg;; at Wrottesley Hall 29&middot;38, air 56&deg;.</p>
+
+<p class="right">Height, 23,923 feet.</p>
+
+<p>4. From the Crystal Palace a balloon ascent was made 20th August, 1862. At
+6h. 47m. p.m. barometer was 25&middot;55, air 50&deg;&middot;5; and at the same time at
+Greenwich Observatory, at 159 feet above the sea, the barometer was 29&middot;81,
+air 63&deg;.</p>
+
+<p class="right">Height, 4,406 feet.</p>
+
+<p>5. From the same place an ascent was made 8th September, 1862. At 5 p.m.,
+the balloon being over Blackheath, barometer was 25&middot;60, and the air 49&deg;&middot;5,
+while at Greenwich, barometer was 29&middot;92, air 66&deg;&middot;4.</p>
+
+<p class="right">Height, 4,461 feet.</p>
+
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_47" id="Page_47">[Pg 47]</a></span></p>
+<h2><a name="CHAPTER_V" id="CHAPTER_V"></a>CHAPTER V.</h2>
+<p class="center"><span class="large">SECONDARY BAROMETERS.</span></p>
+
+<p><br /><b>43. Desirability of Magnifying the Barometer Range.</b>&mdash;The limits within
+which the ordinary barometric column oscillates, do not exceed four inches
+for extreme range, while the ordinary range is confined to about two
+inches; hence it has often been felt that the public utility of the
+instrument would be greatly enhanced if by any means the scale indications
+could be increased in length. This object was sought to be obtained by
+bending the upper part of the tube from the vertical, so that the inches
+on the scale could be increased in length in proportion to the secant of
+the angle it made with the vertical. This was called &#8220;the diagonal
+barometer.&#8221; The upper part of the tube has also been formed into a spiral,
+and the scale, placed along it, is thus greatly enlarged.</p>
+
+<p>But these methods of enlarging the indications cannot be so successfully
+accomplished, nor so cheaply nor so elegantly, as is done by the principle
+employed in the dial barometer. Hence they are not in use.</p>
+
+
+<div class="figleft">Fig. 31.<br /><img src="images/fig_31.jpg" alt="" /></div>
+
+<p><br /><b>44. Howson&#8217;s Long Range Barometer.</b>&mdash;Very recently quite a novel design has
+been patented by Mr. Howson, for a long range barometer. The construction
+requires neither distortion of the tube, nor mechanism for converting a
+short scale into a long one; but the mercury itself rises and falls,
+through an extended range, naturally, and in simple obedience to the
+varying pressure of the atmosphere. The tube is fixed, but its cistern is
+sustained by the mere pressure of the atmosphere. Looking at the
+instrument, it seems a perfect marvel. It appears as though the cistern
+with the mercury in it must fall to the ground. The bore of the tube is
+wide, about an inch across. A long glass rod is fixed to the bottom of the
+glass cistern, where a piece of cork or some elastic substance is also
+placed. The tube is filled with mercury; the glass rod is plunged into the
+tube as it is held top downwards, until the cork gets close up to the tube
+and fits tightly against it. The pressure against the cork simply prevents
+the mercury from coming out while the instrument is being inverted. When
+it is inverted, the mercury partly falls, and forms an ordinary barometric
+column. When the top is held, the cistern and glass rod, instead of
+falling away, remain perfectly suspended. There is no material support to
+the cistern; the tube only is fixed, the cistern hangs to it. Glass is
+many times lighter than mercury. When the glass rod is introduced, it
+displaces an equal volume of mercury. The glass rod, being so much lighter
+than mercury, floats and sustains the additional weight of the cistern by
+its buoyancy. In the mean time, the atmosphere is acting upon the mercury,
+keeping up the ordinary barometric column. Supposing there is a rise in
+the ordinary barometer, the<span class="pagenum"><a name="Page_48" id="Page_48">[Pg 48]</a></span> atmosphere presses some more mercury up the
+tube. This mercury is taken out of the cistern, which of course becomes
+lighter, and therefore the rod and cistern float up a little higher, which
+thus causes the column of mercury to rise still more. The increased
+pressure and buoyancy thus acting together, increase the ascent in the
+barometric column, as shown by the fixed scale. One inch in the barometer
+might be represented by two or more inches in this instrument, according
+to construction. Supposing there was a decrease of pressure, the mercury
+would fall, come into the cistern, make it heavier, and increase the fall
+somewhat. Friction guides, at the top of the rod, prevent it coming into
+contact with the side of the tube when vertically suspended. The
+illustration, Fig. 31, shows the appearance of the instrument as framed in
+wood by the makers, Messrs. Negretti and Zambra.</p>
+
+
+<p><br /><b>45. McNeild&#8217;s Long Range Barometer.</b>&mdash;A barometer designed by a gentleman
+named McNeild is on a directly opposite principle to the one just
+described. The tube is made to float on the mercury in the cistern. It is
+filled with mercury, inverted in the usual manner, then allowed to float,
+being held vertically by glass friction points or guides. By this
+contrivance, the ordinary range of the barometer is greatly increased. One
+inch rise or fall in the standard barometer may be represented by four or
+five inches in this instrument, so that it shows small variations in
+atmospheric pressure very distinctly. As the mercury falls in the tube
+with a decrease of pressure, the surface of the mercury in the cistern
+rises, and the floating tube rises also, which causes an additional
+descent in the column, as shown by fixed graduations on the tube. With an
+increase of pressure, some mercury will leave the cistern and rise in the
+tube, while the tube itself will fall, and so cause an additional ascent
+of mercury. This barometer is identical in principle with King&#8217;s Barograph
+(see <a href="#Page_34">p. 34</a>).</p>
+
+<p>The construction of Howson&#8217;s and McNeild&#8217;s Barometers has been assigned to
+Messrs. Negretti and Zambra. These instruments are usually made for
+domestic purposes with a scale of from three to five, and for public use
+from five to eight times the scale of the ordinary standard. Their
+sensitiveness is consequently increased in an equal proportion, and they
+have the additional advantage of not being affected by differences of
+level in the cistern. However, these novelties have not been sufficiently
+tried to determine their practical value for strictly scientific<span class="pagenum"><a name="Page_49" id="Page_49">[Pg 49]</a></span>
+purposes; but as weather-glasses, for showing minute changes, they are
+superior to the common barometer.</p>
+
+
+<p><br /><b>46. The Water-glass Barometer.</b>&mdash;If a Florence flask, having a long neck,
+have a small quantity of water poured into it, and then be inverted and so
+supported that the open end dips into a vessel containing water, a small
+column of water will be confined in the neck of the bottle, the pressure
+of which, upon the surface of the exposed water, will be equal to the
+difference between the atmospheric pressure and the elasticity of the
+confined air in the body of the bottle. As the pressure of the atmosphere
+varies, this column will alter in height. But the elasticity of the
+confined air is also subject to variations, owing to changes of
+temperature. It follows, then, that the oscillations of the column are
+dependent on alterations of temperature and atmospheric pressure. Such an
+arrangement has been called &#8220;the Water-glass Barometer,&#8221; and bears about
+the same relative value to the mercurial barometer, as an exponent of
+weather changes, that a cat-gut hygrometer bears to a thermometric
+hygrometer, as an indicator of relative moisture.</p>
+
+
+<p class="center"><br />47. SYMPIESOMETER.</p>
+
+<div class="figleft">Fig. 32.<br /><img src="images/fig_32.jpg" alt="" /></div>
+
+<p>Nevertheless the instrument now about to be described, depending upon
+similar principles, but scientifically constructed and graduated, is a
+very useful and valuable substitute for the mercurial barometer. It
+consists of a glass tube, varying, according to the purposes for which the
+instrument is required, from six to twenty-four inches in length. The
+upper end is closed, and formed into a bulb; the lower is turned up,
+formed into a cistern, and open at top, through a pipette, or cone. A
+plug, moveable by a catch from below, can be made to close this opening,
+so as to render the instrument portable.</p>
+
+<p>The upper portion of the tube is filled with air; the lower portion, and
+part of the cistern, with sulphuric acid, coloured so as to render it
+plainly visible. Formerly, hydrogen and oil were used. It was found,
+however, that, by the process known to chemists as <i>osmosis</i>, this light
+gas in time partially escaped, and the remainder became mixed with air,
+the consequence being that the graduations were no longer correct. They
+are more durable as at present constructed. The liquid rises and falls in
+the tube with the variations of atmospheric pressure and temperature
+acting together. If the pressure were constant, the confined air would
+expand and contract for temperature only, and the instrument would act as
+a thermometer. In fact, the instrument is regarded as such in the
+manufacture; and the thermometric<span class="pagenum"><a name="Page_50" id="Page_50">[Pg 50]</a></span> scales are ascertained and engraved on
+the scale. A good mercurial thermometer is also mounted on the same frame.
+If, therefore, at any time the mercurial and the air thermometers do not
+read alike, it must evidently be due to the atmospheric pressure acting
+upon the air in the tube; and it is further evident that, under these
+circumstances, the position of the top of the liquid may be marked to
+represent the barometric pressure at the time. In this manner a scale of
+pressure is ascertained by comparison with a standard barometer, extending
+generally from 27 to 31 inches.</p>
+
+<p>When made correctly, these instruments agree well with the mercurial
+barometer for a number of years, and their subsequent adjustment is not a
+matter of much expense.</p>
+
+<p>For use at sea, the liquid column is contracted at the bend. The
+sympiesometer is very sensitive, and feels the alterations in the
+atmospheric pressure sooner than the ordinary marine barometer.</p>
+
+<p>The scale is usually on silvered brass, mounted on a mahogany or rosewood
+frame, protected in front by plate glass. It is generally furnished with a
+revolving register, to record the observation, in order that it may be
+known whether the pressure has increased or decreased in the interval of
+observation.</p>
+
+<p>Small pocket sympiesometers are sometimes fitted with ivory scales, and
+protected by a neat velvet-lined pasteboard or morocco case.</p>
+
+<p><i>How to take an Observation.</i>&mdash;In practice, the indications of the
+atmospheric pressure are obtained from the sympiesometer by noting, first,
+the temperature of the mercurial thermometer; secondly, adjusting the
+pointer of the pressure scale to the same degree of temperature on the
+scale of the air column; thirdly, reading the height of the liquid on the
+sliding scale.</p>
+
+<p><i>Directions for Use.</i>&mdash;The sympiesometer should be carried and handled so
+as to keep the top always upwards, to prevent the air mechanically mixing
+with the liquid. Care should also be taken to screen it from casual rays
+of the sun or cabin fire.</p>
+
+
+<p class="center"><br />48. ANEROIDS.</p>
+
+<p>The beautiful and highly ingenious instrument called by the name
+<i>Aneroid</i>, is no less remarkable for the scientific principles of its
+construction and action, than for the nicety of its mechanism. It is a
+substitute, and perhaps the best of all substitutes, for the mercurial
+barometer. As its name implies, it is constructed &#8220;without fluid.&#8221; It was
+invented by M. Vidi of Paris. In the general form in which it is made it
+consists of a brass cylindrical case about four inches in diameter and one
+and a half inch deep, faced with a dial graduated and marked similarly to
+the dial-plate of a &#8220;wheel-barometer,&#8221; upon which the index or pointer
+shows the atmospheric pressure in inches and decimals of an inch in
+accordance with the mercurial barometer. Within the case, for ordinary
+sizes, is placed a flat metal box, generally not more than half an inch
+thick and about two inches or a little more in diameter, from which nearly
+all the air is exhausted. The top and bottom of this box is corrugated in
+concentric circles, so as to yield inwardly to external pressure, and
+return when the pressure is removed. The<span class="pagenum"><a name="Page_51" id="Page_51">[Pg 51]</a></span> pressure of the atmosphere,
+acting externally, continually changes, while the elastic pressure of the
+small quantity of air within can only vary by its volume being increased
+or decreased, or by change of temperature. Leaving out of consideration,
+for the moment, the effect of temperature, we can readily perceive that as
+the pressure is lessened upon the outside of the box, the elastic force of
+the air within will force out the top and bottom of the box; and when the
+outer pressure is increased they will be forced in. Thus with the varying
+pressure of the atmosphere, the top and bottom of the box approach to and
+recede from each other by a small quantity; but the bottom being fixed,
+nearly all this motion takes place on the top. Thus the top of the box is
+like an elastic cushion, which rises and falls according as the
+compressing force lessens or increases. To the eye these expansions and
+contractions would not be perceptible, so small is the motion. But they
+are rendered very evident by a nice mechanical arrangement. To the box is
+attached a strong piece of iron, kept pressed upon it by a spring at one
+extremity; so that as the top of the box rises, the motion is made
+sensible at the point held by the spring, and when the top descends the
+spring draws the piece of iron into close contact with it. This piece of
+iron acts as a lever, having its fulcrum at one extremity, the power at
+the centre of the box-top, and the other extremity controlled by the
+spring. Thus it is evident that the small motion of the centre of the
+box-top is much increased at the spring extremity. The motion thus
+obtained is communicated to a system of levers; and, by the intervention
+of a piece of watch-chain and a fine spring passing round the arbour,
+turns the index to the right or left, according as the external pressure
+increases or decreases. Thus, when by increase of pressure the vacuum box
+is compressed, the mechanism transfers the movement to the index, and it
+moves to the right; when the vacuum box bulges out under diminished
+pressure, the mechanical motion is reversed, and the index moves to the
+left. As the index traverses the dial, it shows upon the scale the
+pressure corresponding with that which a good mercurial barometer would at
+the same time and place indicate; that is, supposing it correctly
+adjusted.</p>
+
+<p>A different and more elegant arrangement has since been adopted. A broad
+curved spring is connected to the top of the vacuum box, so as to be
+compressed by the top of the box yielding inward to increased pressure,
+and to relax itself and the box as the pressure is lessened. The system of
+levers is connected to this spring, which augments and transfers the
+motion to the index, in the manner already described. Increase of pressure
+causes the levers to slacken the piece of watch-chain connected with them
+and the arbour of the index. The spring now uncoils, winds the chain upon
+the arbour, and turns the index to the right. Decrease of pressure winds
+the chain off the barrel, tightens the spiral spring, which thus turns the
+index to the left. The graduations of the aneroid scale are obtained by
+comparisons with the correct standard reading of a mercurial barometer,
+under the normal and reduced atmospheric pressure. Reduced pressure is
+obtained by placing both instruments under the receiver of an air pump.</p>
+
+<div class="figleft">Fig. 33.<br /><img src="images/fig_33.jpg" alt="" /></div>
+
+<p><span class="pagenum"><a name="Page_52" id="Page_52">[Pg 52]</a></span>Fig. 33 represents the latest improved mechanism of an aneroid. The outer
+case and the face of the instrument are removed, but the hand is attached
+by its collet to the arbour. <i>A</i> is the corrugated box, which has been
+exhausted of air through the tube, <i>J</i>, and hermetically sealed by
+soldering. <i>B</i> is a powerful curved spring, resting in gudgeons fixed on
+the frame-plate, and attached to a socket behind, <i>F</i>, in the top of the
+box. A lever, <i>C</i>, joined to the stout edge of the spring, is connected,
+by the bent lever at <i>D</i>, with the chain, <i>E</i>, the other end of which is
+coiled round, and fastened to the arbour, <i>F</i>. As the box, <i>A</i>, is
+compressed by the weight of the atmosphere increasing, the spring, <i>B</i>, is
+tightened, the lever, <i>C</i>, depressed, and the chain, <i>E</i>, uncoiled from
+<i>F</i>, which is thereby turned so that the hand, <i>H</i>, moves to the right. In
+the mean while the spiral spring, <i>G</i>, coiled round <i>F</i>, and fixed at one
+extremity to the frame-work and by the other to <i>F</i>, is compressed. When,
+therefore, the pressure decreases, <i>A</i> and <i>B</i> relax, by virtue of their
+elasticity; <i>E</i> slackens, <i>G</i> unwinds, turning <i>F</i>, which carries <i>H</i> to
+the left. Near <i>J</i> is shown an iron pillar, cast as part of the stock of
+the spring, <i>B</i>. A screw works in this pillar through the bottom of the
+plate, by means of which the spring, <i>B</i>, may be so adjusted to the box,
+<i>A</i>, as to set the hand, <i>H</i>, to read on the scale according to the
+indications of a mercurial barometer. The lever, <i>C</i>, is composed of brass
+and steel, soldered together, and adjusted by repeated trials to correct
+for the effects of temperature.</p>
+
+<p>A thermometer is sometimes attached to the aneroid, as it is convenient
+for indicating the temperature of the air. As regards the instrument
+itself, no correction for temperature can be applied with certainty. It
+should be set to read with the mercurial barometer at 32&deg; F. Then the
+readings from it are supposed to require no correction.</p>
+
+<p>In considering the effects of temperature upon the aneroid, they are found
+to be somewhat complex. There is the effect of expansion and contraction
+of the various metals of which the mechanism is composed; and there is the
+effect on the elasticity of the small portion of air in the box. An
+increase of temperature produces greater, a diminution less elasticity in
+this air. The compensation for effects of temperature is adjusted by the
+process of &#8220;trial and error,&#8221; and only a few makers do it well. It is very
+often a mere sham. Admiral FitzRoy writes, in his <i>Barometer Manual</i>, &#8220;The
+known expansion and contraction of metals under varying temperatures,
+caused doubts as to the accuracy of the aneroid under such changes; but
+they were partly removed by introducing into the vacuum box a small
+portion of gas, as a compensation for the effects of heat or cold. The gas
+in the box, changing its bulk on a change of temperature, was intended to
+compensate for the effect on the metals<span class="pagenum"><a name="Page_53" id="Page_53">[Pg 53]</a></span> of which the aneroid is made.
+Besides which, a further and more reliable compensation has lately been
+effected by a combination of brass and steel bars.&#8221;</p>
+
+<p>&#8220;Aneroid barometers, if often compared with good mercurial columns, are
+similar in their indications, and valuable; but it must be remembered that
+they are not independent instruments, that they are set originally by a
+barometer, require adjustment occasionally, and may deteriorate in time,
+though slowly.&#8221;</p>
+
+<p>&#8220;The aneroid is quick in showing the variation of atmospheric pressure;
+and to the navigator who knows the difficulty, at times, of using
+barometers, this instrument is a great boon, for it can be placed
+anywhere, quite out of harm&#8217;s way, and is not affected by the ship&#8217;s
+motion, although faithfully giving indication of increased or diminished
+pressure of air. In ascending or descending elevations, the hand of the
+aneroid may be seen to move (like the hand of a watch), showing the height
+above the level of the sea, or the difference of level between places of
+comparison.&#8221;</p>
+
+<p>In the admiral&#8217;s <i>Notes on Meteorology</i>, he says, &#8220;The aneroid is an
+excellent <i>weather glass</i>, if well made. Compensation for heat or cold has
+lately been introduced by efficient mechanism. In its <i>improved</i>
+condition, when the cost may be about &pound;5, it is fit for measuring heights
+as far as 5,000 feet with approximate accuracy; but even at the price of
+&pound;3, as a <i>weather-glass</i> only, it is exceedingly valuable, because it can
+be carried anywhere; and if now and then compared with a good barometer,
+it may be relied on sufficiently. I have had one in constant use for ten
+years, and it appears to be as good now as at first. For a ship of war
+(considering concussion by the fire of guns), for boats, or to put in a
+drawer, or on a table, I believe there is nothing better than it for use
+as a common weather-glass.&#8221;</p>
+
+<p>Colonel Sir H. James, R.E., in his <i>Instructions for taking Meteorological
+Observations</i>, says of the aneroid, &#8220;This is a most valuable instrument;
+it is extremely portable. I have had one in use for upwards of ten years,
+and find it to be the best form of barometer, as a &#8220;weather-glass,&#8221; that
+has been made.&#8221;</p>
+
+<p>One of the objects of Mr. Glaisher&#8217;s experiments in balloons was &#8220;to
+compare the readings of an aneroid barometer with those of a mercurial
+barometer up to five miles.&#8221; In the comparisons the readings of the
+mercurial barometer were corrected for index-error and temperature. The
+aneroid readings, says Mr. Glaisher, &#8220;prove all the observations made in
+the several ascents may be safely depended upon, and also that an aneroid
+barometer can be made to read correctly to pressures below twelve inches.&#8221;
+As one of the general conclusions derived from his experiments he states,
+&#8220;that an aneroid barometer read correctly to the first place, and probably
+to the second place of decimals, to a pressure as low as seven inches.&#8221;
+The two aneroids used by Mr. Glaisher were by Messrs. Negretti and Zambra.</p>
+
+<p>Aneroids are now manufactured almost perfectly compensated for
+temperature. Such an instrument therefore ought to show the same pressure
+in the external air at a temperature say of 40&deg;, as it would in a room
+where the temperature at the same time may be 60&deg;; provided there is no
+difference of elevation. To test it thoroughly would require an
+examination and a comparison with barometer readings<span class="pagenum"><a name="Page_54" id="Page_54">[Pg 54]</a></span> reduced to 32&deg; F.,
+conducted through a long range of temperature and under artificially
+reduced pressure. A practical method appears to be to compare the aneroid
+daily, or more often, for a few weeks with the readings of a mercurial
+barometer reduced to 32&deg;; and if the error so found be constant, the
+object of the compensation may be assumed to be attained, particularly if
+the temperature during the period has varied greatly.</p>
+
+<p><i>Directions for using the Aneroid.</i>&mdash;Aneroids are generally suspended with
+the dial vertical; but if they be placed with the dial horizontal, the
+indications differ a few hundredths of an inch in the two positions.
+Hence, if their indications are registered, they should be kept in the
+same position.</p>
+
+<p>The aneroid will not answer for exact scientific purposes, as it cannot be
+relied upon for a length of time. Its error of indication changes slowly,
+and hence the necessity of its being set from time to time with the
+reading of a good barometer. To allow of this being done, at the back of
+the outer case is the head of a screw in connection with the spring
+attached to the vacuum box. By applying a small turnscrew to this screw,
+the spring of the vacuum box may be tightened or relaxed, and the index
+made to move correspondingly to the right or left on the dial. By this
+means, besides being enabled to correct the aneroid at any time, &#8220;if the
+measure of a height rather greater than the aneroid will commonly show be
+required, it may be <i>re-set</i> thus: When at the upper station (<i>within its
+range</i>), and having noted the reading carefully, touch the screw behind so
+as to bring back the hand a few inches (if the instrument will admit),
+then read off and start again. <i>Reverse the operation when descending.</i>
+This may add some inches of measure <i>approximately</i>.&#8221;&mdash;<i>FitzRoy.</i></p>
+
+
+<div class="figleft">Fig. 34.<br /><img src="images/fig_34.jpg" alt="" /></div>
+
+<p><br /><b>49. Small Size Aneroids.</b>&mdash;The patent for the Aneroid having expired,
+Admiral FitzRoy urged upon Messrs. Negretti &amp; Zambra the desirability of
+reducing the size at which it had hitherto been made, as well as of
+improving its mechanical arrangement, and compensation for temperature.
+They accordingly engaged skilful workmen, who, under their directions, and
+at their expense, by a great<span class="pagenum"><a name="Page_55" id="Page_55">[Pg 55]</a></span> amount of labour and experiment, succeeded
+in reducing its dimensions to two inches in diameter, and an inch and a
+quarter thick. The exact size and appearance of this aneroid are shown in
+fig. 34. The compensation is carefully adjusted, and the graduations of
+the dial ascertained under reduced pressure, so that they are not quite
+equal, but more accurate.</p>
+
+<div class="figright">Fig. 35.<br /><img src="images/fig_35.jpg" alt="" /></div>
+
+<p><br /><b>50. Watch Aneroid.</b>&mdash;Subsequently the aneroid has been further reduced in
+size and it can now be had from an inch and a quarter to six inches in
+diameter. The smallest size can be enclosed in watch cases, fig. 35, or
+otherwise, so as to be adapted to the pocket. By a beautifully simple
+contrivance, a milled rim is adjusted to move round with hand pressure,
+and carry a fine index or pointer, outside and around the scale engraved
+on the dial, or face, for the purpose of marking the reading, so that the
+subsequent increase or decrease of pressure may be readily seen. These
+very small instruments are found to act quite as correctly as the largest,
+and are much more serviceable. Besides serving the purpose of a
+weather-glass in the house or away from home, if carried in the pocket,
+they are admirably suited to the exigencies of tourists and travellers.
+They may be had with scale sufficient to measure heights not exceeding
+8,000 feet; with a scale of elevation in feet, as well as of pressure in
+inches, engraved on the dial. The scale of elevation, which is for the
+temperature of 50&deg;, was computed by Professor Airy, the Astronomer Royal,
+who kindly presented it to Messrs. Negretti and Zambra, at the same time
+suggesting its application. Moderate-sized aneroids, fitted in leathern
+sling cases, are also good travelling instruments, and will be found
+serviceable to pilots, fishermen, and for use in coasting and small
+vessels, where a mercurial barometer cannot be employed, because requiring
+too much space.</p>
+
+<p>Admiral FitzRoy, in a communication to the <i>Mercantile Marine Magazine</i>,
+December, 1860, says:&mdash;&#8220;Aneroids are now made more portable, so that a
+pilot or chief boatman may carry one in his pocket, as a railway guard
+carries his timekeeper; and, thus provided, pilots cruising for expected
+ships would be able to caution strangers arriving, if bad weather were
+impending, or give warning to coasters or fishing boats. Harbours of
+Refuge, however excellent and important, are not always accessible, even
+when most wanted, as in snow, rain, or darkness, when neither land, nor
+buoy, nor even a lighthouse-light can be seen.&#8221;</p>
+
+
+<p><br /><span class="pagenum"><a name="Page_56" id="Page_56">[Pg 56]</a></span><b>51. Measurement of Heights by the Aneroid.</b>&mdash;For measuring heights not
+exceeding many hundred feet above the sea-level by means of the aneroid,
+the following simple method will suffice:&mdash;</p>
+
+<p>Divide the difference between the aneroid readings at the lower and upper
+stations by &middot;0011; the quotient will give the approximate height in feet.</p>
+
+<p>Thus, supposing the aneroid to read at the</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td>Lower Station</td><td>&nbsp;</td><td>30&middot;385</td><td>inches.</td></tr>
+<tr><td>Upper Station</td><td>&nbsp;</td><td class="botbor">30&middot;025</td></tr>
+<tr><td>&nbsp;</td><td>Difference</td><td class="botbor2">&middot;360</td></tr></table>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td rowspan="2">Divided gives</td><td class="botbor">&middot;360</td><td rowspan="2">= 327 feet.</td></tr>
+<tr><td>&middot;0011</td></tr></table>
+
+<p>As an illustration of the mode in which the aneroid should be used in
+measuring heights, the following example is given:&mdash;</p>
+
+<p>A gentleman who ascended Helvellyn, August 12th, 1862, recorded the
+following observations with a pocket aneroid by Negretti and Zambra:&mdash;</p>
+
+<p>Near 10 a.m., at the first milestone from Ambleside, found by survey to be
+188 feet above the sea, the aneroid read 29&middot;89 inches; about 1 p.m., at
+the summit of Helvellyn, 26&middot;81; and at 5 p.m., at the milestone again,
+29&middot;76. The temperature of the lower air was 57&deg;, of the upper, 54&deg;. Hence
+the height of the mountain is deduced as follows:&mdash;</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td colspan="3">&nbsp;</td>
+    <td align="center">Inches.</td></tr>
+<tr><td>Reading at</td>
+    <td>10 a.m.</td>
+    <td><span class="spacer">&nbsp;</span></td>
+    <td>29&middot;89</td></tr>
+<tr><td align="center">"</td>
+    <td>5 p.m.</td>
+    <td>&nbsp;</td>
+    <td class="botbor">29&middot;76</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Mean</td>
+    <td>&nbsp;</td>
+    <td>29&middot;825</td>
+    <td><span class="spacer2">&nbsp;</span></td>
+    <td>Table I.<small><a name="f5.1" id="f5.1" href="#f5">[5]</a></small></td>
+    <td align="right">1010</td></tr>
+<tr><td colspan="2">Upper Reading</td>
+    <td>&nbsp;</td>
+    <td class="botbor">26&middot;81</td>
+    <td>&nbsp;</td>
+    <td align="center">"</td>
+    <td align="right" class="botbor">3796</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Difference</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">2786</td></tr>
+<tr><td colspan="6">Mean Temperature 55&deg;&middot;5, gives in Table II.</td>
+    <td align="right" class="botbor">1&middot;048</td></tr>
+<tr><td colspan="6">&nbsp;</td>
+    <td align="right">2920</td></tr>
+<tr><td colspan="6">Lat. 55&deg; N., gives in Table III.</td>
+    <td align="right" class="botbor">&middot;9991</td></tr>
+<tr><td colspan="6">&nbsp;</td>
+    <td align="right">2917</td></tr>
+<tr><td colspan="6"><span style="margin-left: 8em;">Table IV.</span></td>
+    <td align="right" class="botbor">5</td></tr>
+<tr><td colspan="6"><span style="margin-left: 4em;">Difference of height</span></td>
+    <td align="right">2922</td></tr>
+<tr><td colspan="6">Height of lower station</td>
+    <td align="right" class="botbor">188</td></tr>
+<tr><td colspan="6"><span style="margin-left: 2em;">"</span><span style="margin-left: 1.5em;">Helvellyn</span></td>
+    <td align="right" class="botbor2">3110</td></tr>
+<tr><td colspan="6">In Sir J. Herschell&#8217;s <i>Physical Geography</i> it is given as</td>
+    <td align="right">3115</td><td>ft.</td></tr></table>
+
+<p><span class="pagenum"><a name="Page_57" id="Page_57">[Pg 57]</a></span>So near an agreement is attributable to the excellence of the aneroid, and
+the careful accuracy of the observer.</p>
+
+
+<p class="center"><br />52. METALLIC BAROMETER.</p>
+
+<p>This instrument, the invention of M. Bourdon, has a great resemblance to
+the aneroid, but is much simpler in arrangement. The inventor has applied
+the same principle to the construction of metallic steam-pressure gauges.
+We are here, however, only concerned with it as constructed to indicate
+atmospheric pressure. It consists of a long slender flattened metallic
+tube, partially exhausted of air, and hermetically closed at each end,
+then fixed upon its centre, and bent round so as to make the ends face
+each other. The transverse section of this tube is an elongated ellipse.
+The principle of action is this: interior pressure tends to straighten the
+tube, external pressure causes it to coil more. Hence as the atmospheric
+pressure decreases, the ends of the tube become more apart.</p>
+
+<p>This movement is augmented and transferred by a mechanical arrangement of
+small metallic levers to a radius bar, which carries a rack formed on the
+arc of its circle. This moves a pinion, upon the arbour of which a light
+pointer, or &#8220;hand,&#8221; is poised, which indicates the pressure upon a dial.
+When the pressure increases, the ends of the tube approach each other, and
+the pointer moves from left to right over the dial. The whole mechanism is
+fixed in a brass case, having a hole at the back for adjusting the
+instrument to the mercurial barometer by means of a key, which sets the
+pointer without affecting the levers. The dial is generally open to show
+the mechanism, and is protected by a glass, to which is fitted a moveable
+index.</p>
+
+<p>This barometer is very sensitive, and has the advantage of occupying
+little space, although it has not yet been made so small as the aneroid.
+Both these instruments admit of a great variety of mounts to render them
+ornamental. The metallic barometer can be constructed with a small clock
+in its centre, so as to form a novel and beautiful drawing-room ornament.</p>
+
+<p>Admiral FitzRoy writes, &#8220;Metallic barometers, by Bourdon, have not yet
+been tested in very moist, hot, or cold air for a sufficient time. They
+are dependent, or secondary instruments, and liable to deterioration. For
+limited employment, when sufficiently compared, they may be very useful,
+especially in a few cases of electrical changes, <i>not foretold or shown by
+mercury</i>, which these seem to indicate remarkably.&#8221;</p>
+
+<p>They are not so well adapted for travellers, nor for measurements of
+considerable elevations, as aneroids.</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_58" id="Page_58">[Pg 58]</a></span></p>
+<h2><a name="CHAPTER_VI" id="CHAPTER_VI"></a>CHAPTER VI.</h2>
+<p class="center"><span class="large">INSTRUMENTS FOR ASCERTAINING TEMPERATURE.</span></p>
+
+<p><br /><b>53. Temperature</b> is the energy with which heat affects our sensation of
+feeling.</p>
+
+<p>Bodies are said to possess the same temperature, when the amounts of heat
+which they respectively contain act outwardly with the same intensity of
+transfer or absorption, producing in the one case the sensation of warmth,
+in the other that of coldness. Instruments used for the determination and
+estimation of temperatures are called <i>Thermometers</i>.</p>
+
+<p>Experience proves that the same body always occupies the same space at the
+same temperature; and that for every increase or decrease of its
+temperature, it undergoes a definite dilatation or contraction of its
+volume. Provided, then, a body suffers no loss of substance or peculiar
+change of its constituent elements or atoms, while manifesting changes of
+temperature it will likewise exhibit alterations in volume; the latter
+may, therefore, be taken as exponents of the former. The expansion and
+contraction of bodies are adopted as arbitrary measures of changes of
+temperature; and any substance will serve for a thermometer in which these
+changes of volume are sensible, and can be rendered measureable.</p>
+
+
+<p><br /><b>54. Thermometric Substances.</b>&mdash;Thermometers for meteorological and domestic
+purposes are constructed with liquids, and generally either mercury or
+alcohol, because their alterations of volume for the same change of
+temperature are greater than those of solids; while being more manageable,
+they are preferred to gases. Mercury is of all substances the best adapted
+for thermometric purposes, as it maintains the liquid state through a
+great alteration of heat, has a more equable co-efficient of expansion
+than any other fluid, and is peculiarly sensitive to changes of
+temperature. The temperature of solidification of mercury, according to
+Fahrenheit&#8217;s scale of temperature, is -40&deg;; and its temperature of
+ebullition is about 600&deg;. Sulphuric ether, nitric acid, oil of sassafras,
+and other limpid fluids, have been employed for thermometers.</p>
+
+
+<p><br /><b>55. Description of the Thermometer.</b>&mdash;The ordinary thermometer consists of
+a glass tube of very fine bore, having a bulb of thin glass at one
+extremity, and closed at the other. The bulb and part of the tube contains
+mercury; the rest of the tube is a vacuum, and affords space for the
+expansion of the liquid. This arrangement renders very perceptible the
+alterations in volume of the mercury due to changes of temperature. It is
+true, the glass expands and contracts also; but only by about
+one-twentieth of the extent of the mercury. Regarding the bulb, then, as
+unalterable in size, all the changes in the bulk of the fluid must take
+place<span class="pagenum"><a name="Page_59" id="Page_59">[Pg 59]</a></span> in the tube, and be exhibited by the expansion and contraction of
+the column, which variations are made to measure changes of temperature.</p>
+
+
+<p class="center"><br />56. STANDARD THERMOMETER.</p>
+
+<div class="figright">Fig. 36<br /><img src="images/fig_36.jpg" alt="" /><br /></div>
+
+<p>The peculiarities in the construction of thermometers will be best
+understood by describing the manufacture of a <i>Standard Thermometer</i>,
+which is one of the most accurate make, and the scale of which is divided
+independently of any comparison with another thermometer. Fig. 36 is an
+illustration of such an instrument, on a silvered brass scale.</p>
+
+<p><i>Selection of Tube.</i>&mdash;In selecting the glass tube, much care is requisite
+to ascertain that its bore is perfectly uniform throughout. As received
+from the glass-house, the tubes are generally, in their interior, portions
+of very elongated cones, so that the bore is wider at one end than at the
+other. With due care, however, a proper length of tube can be selected, in
+which there is no appreciable difference of bore. This is ascertained by
+introducing into the tube a length of mercury of about a half or a third
+of an inch, and accurately measuring it in various positions in the tube.
+To accomplish this, the workman blows a bulb at one end of the tube, and
+heats the bulb a little to drive out some of the air. Then, placing the
+open end in mercury, upon cooling the elasticity of the enclosed air
+diminishes, and the superior pressure of the atmosphere drives in some
+mercury. The workman stops the process so soon as he judges sufficient
+mercury has entered. By cooling or heating the bulb, as necessary, the
+mercury is made to pass from one end of the tube to the other. Should the
+length of this portion of mercury alter in various parts of the bore, the
+tube must be rejected. If it is, as nearly as possible, one uniform
+length, the tube is set aside for filling.</p>
+
+<p>The <i>bulb</i> is never blown by the breath, but by an elastic caoutchouc ball
+containing air, so that the introduction of moisture is avoided. The
+spherical form is to be preferred; for it is best adapted to resist the
+varying pressure of the atmosphere. The bulbs should not be too large, or
+the mercury will take some time to indicate sudden changes of temperature.
+Cylindrical bulbs are sometimes desirable, as they offer larger surfaces
+to the mercury, and enable thermometers to be made more sensitive.</p>
+
+<p>The <i>mercury</i>, with which the bulb is to be filled, should be quite pure,
+and freed from moisture and air by recent boiling.</p>
+
+<p><i>Filling the Tube.</i>&mdash;The filling is effected by heating the bulb with the
+flame of a spirit-lamp, while the open end is embedded in mercury. Upon
+allowing the bulb to cool, the atmospheric pressure drives some mercury
+into it; and the process of heating and cooling is thus continued until
+sufficient mercury is introduced. The mercury is next boiled in the tube,
+to expel any air or moisture that may be present. In order to close the
+tube and exclude all air, the artist ascertains that the tube<span class="pagenum"><a name="Page_60" id="Page_60">[Pg 60]</a></span> contains
+the requisite quantity of mercury; then, by holding the bulb over the
+spirit flame, he causes the mercury to fill the whole of the tube, and
+dexterously removing it from the source of heat, he, at the same instant,
+closes it with the flame of a blow-pipe. If any air remain in the tube, it
+is easily detected; for if the instrument be inverted, the mercury will
+fall to the extremity of the tube, if there is a perfect vacuum, unless
+the tube be so finely capillary that its attraction for the mercury is
+sufficient to overcome the force of gravity, in which case the mercury
+will retain its position in every situation of the instrument. If,
+however, the mercury fall and does not reach quite to the extremity of the
+bore, some air is present, which must be removed.</p>
+
+<p><i>The Graduation.</i>&mdash;The thermometer is now prepared for graduation, the
+first part of which process is the determination of two fixed points.
+These are given by the temperatures of melting ice and of the vapour of
+boiling water. Melting ice has always the same temperature in every place
+and under all circumstances; provided only that the water from which the
+ice is congealed is free from salts. The temperature of the vapour of
+boiling water depends upon the pressure of the atmosphere, but is always
+constant for the same pressure.</p>
+
+<p>The fixed point corresponding to the temperature of melting ice is called
+the <i>freezing point</i>. It is obtained by keeping the bulb and the part of
+the tube occupied by mercury immersed in melting ice, until the mercury
+contracts to a certain point, where it remains stationary. This position
+of the end of the mercury is then marked upon the tube.</p>
+
+<p>The <i>boiling point</i> is not so easily determined, for the barometer must be
+consulted about the same time. The boiling apparatus is generally
+constructed of copper. It consists of a cylindrical boiler, heated from
+the base by a spirit lamp or charcoal fire. An open tube two or three
+inches in diameter and of suitable length enters the top of the boiler.
+This tube is enveloped by another fixed to the top of the boiler but not
+opening into it, and so that the two tubes are about an inch apart. The
+object of the outer tube is to protect the inner tube from the cold
+temperature of the air. The outer tube has an opening at the top for the
+admission of the thermometer, and a hole near the bottom for the escape of
+steam through a spout. When the water is made to boil, the steam rises in
+the inner tube, fills the space between the tubes, and escapes at the
+spout. The thermometer is then passed down into the inner cylinder, and
+held securely from the top by means of a piece of caoutchouc. The tubes or
+cylinders should be of sufficient length to prevent the thermometer
+entering the water. This is necessary because the temperature of boiling
+water is influenced by any substance which it holds in chemical solution;
+and, moreover, its temperature increases with the depth, owing to the
+pressure of the upper stratum. The thermometer being thus surrounded with
+steam, the mercury rises in the tube. As it does so, the tube should be
+depressed so as always to keep the top of the mercury just perceptible.
+When the temperature of the vapour is attained, the mercury ceases to
+rise, and remains stationary. The position of<span class="pagenum"><a name="Page_61" id="Page_61">[Pg 61]</a></span> the end of the mercury is
+now marked upon the tube, and the &#8220;<i>boiling-point</i>&#8221; is obtained.</p>
+
+
+<p><br /><b>57. Methods of ascertaining the exact Boiling Temperature.</b>&mdash;The normal
+boiling temperature of water all nations have tacitly agreed to fix under
+a normal barometric pressure of 29&middot;922 inches of mercury, having the
+temperature of melting ice, in the latitude of 45&deg;, and at the sea-level.
+If the atmospheric pressure at the time or place of graduating a
+thermometer does not equal this, the boiling temperature will be higher or
+lower according as the pressure is greater or less. Hence a reading must
+be taken from a reliable barometer, which must also be corrected for
+errors and temperature, and reduced for latitude, in order to compare the
+actual atmospheric pressure at the time with the assumed normal pressure.
+Tables of vapour tension, as they are termed, have been computed from
+accurate experimental investigations and theory,&mdash;giving the temperatures
+of the vapour of water for all probable pressures; Regnault&#8217;s, the most
+recent, is considered the most accurate; and his investigations are based
+upon the standard pressure given above, and are for the same latitude. His
+Table, therefore, will give the temperature on the thermometric scale
+corresponding to the pressure.</p>
+
+<p>The Commissioners appointed by the British Government to construct
+standard weights and measures, decided that the normal boiling-point,
+212&deg;, on the thermometer should represent the temperature of steam
+generated under an atmospheric pressure equal in inches of mercury, at the
+temperature of freezing water, to 29&middot;922 + (cos. 2 latitude &times; &middot;0766) +
+(&middot;00000179 &times; height in feet above the sea-level). Hence, at London, lat.
+51&deg;30&acute; N., we deduce 29&middot;905 as the barometric pressure representing the
+normal boiling point of water,&mdash;the trifling correction due to height
+being neglected. If then, in the latitude of London, the barometric
+pressure, at the time of fixing the boiling point, be not 29&middot;905 inches,
+that point will be higher or lower, according to the difference of the
+pressure from the normal. Near the sea-level about 0&middot;59 inch of such
+difference is equivalent to 1&deg; Fahrenheit in the boiling point.</p>
+
+<p>Suppose, then, the atmospheric pressure at London to be 30&middot;785 inches, the
+following calculation gives the corresponding boiling temperature for
+Fahrenheit&#8217;s scale:&mdash;</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td>Observed</td><td>pressure</td><td><span class="spacer">&nbsp;</span></td><td align="right">30&middot;785</td></tr>
+<tr><td>Normal</td><td align="center">"</td><td>&nbsp;</td><td align="right" class="botbor">29&middot;905</td></tr>
+<tr><td colspan="2">Difference</td><td>&nbsp;</td><td align="right" class="botbor2">&middot;880</td></tr></table>
+
+<p>As 0&middot;59 is to 0&middot;88, so is 1&deg; to 1&deg;&middot;5.</p>
+
+<p>That is, the water boils at 1&deg;&middot;5 above its normal temperature; so that, in
+this case, the normal temperature to be placed on the scale, viz. 212&deg;,
+must be 1&deg;&middot;5 lower than the mark made on the tube at the height at which
+the mercury stood under the influence of the boiling water.</p>
+
+<p><span class="pagenum"><a name="Page_62" id="Page_62">[Pg 62]</a></span>The temperature of the vapour of boiling water may be found, at any time
+and place, as follows:&mdash;Multiply the atmospheric pressure by the factor
+due to the latitude, given in the annexed Table V., and with the result
+seek the temperature in Table VI.</p>
+
+<table border="0" cellpadding="0" cellspacing="0" summary="table">
+<tr><td colspan="2" align="center"><span class="smcap">Table V.</span></td>
+    <td colspan="4" align="center"><span class="smcap">Table VI.</span></td></tr>
+<tr><td class="btlr" align="center">Latitude.</td>
+    <td class="btrdoub2" align="center">Factor.</td>
+    <td class="btr" align="center">Temperature<br />of Vapour.</td>
+    <td class="btrdoub" align="center">Tension.</td>
+    <td class="btr" align="center">Temperature<br />of Vapour.</td>
+    <td class="btr" align="center">Tension.</td></tr>
+<tr><td class="btlr" align="center">Degrees.</td>
+    <td class="btrdoub2" align="center">&nbsp;</td>
+    <td class="btr" align="center">Degrees.</td>
+    <td class="btrdoub" align="center">Inches.</td>
+    <td class="btr" align="center">Degrees.</td>
+    <td class="btr" align="center">Inches.</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: .5em;">0</span></td>
+    <td class="brdoub2" align="center">0&middot;99735</td>
+    <td class="br" align="center">179</td>
+    <td class="brdoub" align="center">14&middot;934</td>
+    <td class="br" align="center">197</td>
+    <td class="br" align="center">22&middot;036</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: .5em;">5</span></td>
+    <td class="brdoub2" align="center">0&middot;99739</td>
+    <td class="br" align="center">180</td>
+    <td class="brdoub" align="center">15&middot;271</td>
+    <td class="br" align="center">198</td>
+    <td class="br" align="center">22&middot;501</td></tr>
+<tr><td class="blr" align="center">10</td>
+    <td class="brdoub2" align="center">0&middot;99751</td>
+    <td class="br" align="center">181</td>
+    <td class="brdoub" align="center">15&middot;614</td>
+    <td class="br" align="center">199</td>
+    <td class="br" align="center">22&middot;974</td></tr>
+<tr><td class="blr" align="center">15</td>
+    <td class="brdoub2" align="center">0&middot;99770</td>
+    <td class="br" align="center">182</td>
+    <td class="brdoub" align="center">15&middot;963</td>
+    <td class="br" align="center">200</td>
+    <td class="br" align="center">23&middot;456</td></tr>
+<tr><td class="blr" align="center">20</td>
+    <td class="brdoub2" align="center">0&middot;99797</td>
+    <td class="br" align="center">183</td>
+    <td class="brdoub" align="center">16&middot;318</td>
+    <td class="br" align="center">201</td>
+    <td class="br" align="center">23&middot;946</td></tr>
+<tr><td class="blr" align="center">25</td>
+    <td class="brdoub2" align="center">0&middot;99830</td>
+    <td class="br" align="center">184</td>
+    <td class="brdoub" align="center">16&middot;680</td>
+    <td class="br" align="center">202</td>
+    <td class="br" align="center">24&middot;445</td></tr>
+<tr><td class="blr" align="center">30</td>
+    <td class="brdoub2" align="center">0&middot;99868</td>
+    <td class="br" align="center">185</td>
+    <td class="brdoub" align="center">17&middot;049</td>
+    <td class="br" align="center">203</td>
+    <td class="br" align="center">24&middot;952</td></tr>
+<tr><td class="blr" align="center">35</td>
+    <td class="brdoub2" align="center">0&middot;99910</td>
+    <td class="br" align="center">186</td>
+    <td class="brdoub" align="center">17&middot;425</td>
+    <td class="br" align="center">204</td>
+    <td class="br" align="center">25&middot;468</td></tr>
+<tr><td class="blr" align="center">40</td>
+    <td class="brdoub2" align="center">0&middot;99954</td>
+    <td class="br" align="center">187</td>
+    <td class="brdoub" align="center">17&middot;808</td>
+    <td class="br" align="center">205</td>
+    <td class="br" align="center">25&middot;993</td></tr>
+<tr><td class="blr" align="center">45</td>
+    <td class="brdoub2" align="center">1&middot;00000</td>
+    <td class="br" align="center">188</td>
+    <td class="brdoub" align="center">18&middot;197</td>
+    <td class="br" align="center">206</td>
+    <td class="br" align="center">26&middot;527</td></tr>
+<tr><td class="blr" align="center">50</td>
+    <td class="brdoub2" align="center">1&middot;00046</td>
+    <td class="br" align="center">189</td>
+    <td class="brdoub" align="center">18&middot;594</td>
+    <td class="br" align="center">207</td>
+    <td class="br" align="center">27&middot;070</td></tr>
+<tr><td class="blr" align="center">55</td>
+    <td class="brdoub2" align="center">1&middot;00090</td>
+    <td class="br" align="center">190</td>
+    <td class="brdoub" align="center">18&middot;998</td>
+    <td class="br" align="center">208</td>
+    <td class="br" align="center">27&middot;623</td></tr>
+<tr><td class="blr" align="center">60</td>
+    <td class="brdoub2" align="center">1&middot;00132</td>
+    <td class="br" align="center">191</td>
+    <td class="brdoub" align="center">19&middot;409</td>
+    <td class="br" align="center">209</td>
+    <td class="br" align="center">28&middot;185</td></tr>
+<tr><td class="blr" align="center">65</td>
+    <td class="brdoub2" align="center">1&middot;00170</td>
+    <td class="br" align="center">192</td>
+    <td class="brdoub" align="center">19&middot;828</td>
+    <td class="br" align="center">210</td>
+    <td class="br" align="center">28&middot;756</td></tr>
+<tr><td class="blr" align="center">70</td>
+    <td class="brdoub2" align="center">1&middot;00203</td>
+    <td class="br" align="center">193</td>
+    <td class="brdoub" align="center">20&middot;254</td>
+    <td class="br" align="center">211</td>
+    <td class="br" align="center">29&middot;335</td></tr>
+<tr><td class="blr" align="center">75</td>
+    <td class="brdoub2" align="center">1&middot;00230</td>
+    <td class="br" align="center">194</td>
+    <td class="brdoub" align="center">20&middot;688</td>
+    <td class="br" align="center">212</td>
+    <td class="br" align="center">29&middot;922</td></tr>
+<tr><td class="blr" align="center">80</td>
+    <td class="brdoub2" align="center">1&middot;00249</td>
+    <td class="br" align="center">195</td>
+    <td class="brdoub" align="center">21&middot;129</td>
+    <td class="br" align="center">213</td>
+    <td class="br" align="center">30&middot;515</td></tr>
+<tr><td class="bblr" align="center">&nbsp;</td>
+    <td class="bbrdoub2" align="center">&nbsp;</td>
+    <td class="bbr" align="center">196</td>
+    <td class="bbrdoub" align="center">21&middot;578</td>
+    <td class="bbr" align="center">214</td>
+    <td class="bbr" align="center">31&middot;115</td></tr></table>
+
+<p><i>How to use the Tables.</i>&mdash;When the <i>temperature</i> is known to decimals of a
+degree, take out the tension for the degree, and multiply the difference
+between it and the next tension by the decimals of the temperature, and
+add the product to the tension, for the degree.</p>
+
+<p>Required the tension corresponding to 197&deg;&middot;84.</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td align="center">&deg;</td></tr>
+
+<tr><td>197</td><td>&nbsp;</td><td>=</td><td align="right" class="botbor">22&middot;036</td><td><span class="spacer">&nbsp;</span></td>
+    <td>&middot;465 &times; &middot;84</td><td>=</td><td align="right">&middot;391</td></tr>
+
+<tr><td>198</td><td>&nbsp;</td><td>=</td><td align="right" class="botbor">22&middot;501</td><td>&nbsp;</td>
+    <td>197&deg;</td><td>=</td><td align="right" class="botbor">22&middot;036</td></tr>
+
+<tr><td>&nbsp;</td><td>Difference</td><td>&nbsp;</td><td align="right" class="botbor2">&middot;465</td><td>&nbsp;</td>
+    <td>197&middot;84</td><td>=</td><td align="right" class="botbor2">22&middot;427</td></tr></table>
+
+<p>When the <i>tension</i> is given, take the difference between it and the next
+less tension in the Table, and divide this difference by the difference
+between the next less and<span class="pagenum"><a name="Page_63" id="Page_63">[Pg 63]</a></span> next greater tensions. The quotient will be the
+decimals to add to the degree opposite the next less tension.</p>
+
+<p>Thus, for 23&middot;214 inches, required the temperature.</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td>Given</td>
+    <td align="right">23&middot;214</td>
+    <td><span class="spacer2">&nbsp;</span></td>
+    <td>&nbsp;</td><td>&nbsp;</td>
+    <td>Next</td><td>greater</td>
+    <td><span class="spacer2">&nbsp;</span></td>
+    <td align="right">23&middot;456</td></tr>
+<tr><td>&nbsp;</td>
+    <td align="right" class="botbor">22&middot;974</td>
+    <td>&nbsp;</td><td>&nbsp;</td><td>&nbsp;</td>
+    <td>Next</td><td>less</td>
+    <td>&nbsp;</td>
+    <td class="botbor" align="right">22&middot;974</td></tr>
+<tr><td>&nbsp;</td>
+    <td align="right">&middot;240</td>
+    <td colspan="4">&nbsp;</td>
+    <td>Difference</td>
+    <td>&nbsp;</td>
+    <td align="right">&middot;482</td></tr>
+<tr><td rowspan="2" colspan="3">&nbsp;</td>
+    <td rowspan="2">And &nbsp;</td>
+    <td class="botbor">&middot;240</td>
+    <td rowspan="2" colspan="2">&nbsp;</td>
+    <td rowspan="2">=</td>
+    <td rowspan="2" align="right">&middot;5</td></tr>
+<tr><td>&middot;482</td></tr>
+<tr><td colspan="7"><span style="margin-left: 1em;">Temperature opposite next less</span></td>
+    <td>&nbsp;</td>
+    <td align="right" class="botbor">199&middot;0</td></tr>
+<tr><td colspan="7"><span style="margin-left: 1em;">Temperature required</span></td>
+    <td>&nbsp;</td>
+    <td align="right" class="botbor2">199&middot;5</td></tr></table>
+
+<p>A similar method of interpolation in taking out numerical quantities is
+applicable to almost all tables; and should be practised with all those
+given in this work.</p>
+
+<p><i>Example.</i>&mdash;Thus, in Liverpool, lat. 53&deg; 30&acute; N., the barometer reading
+29&middot;876 inches, its attached thermometer 55&deg;, and the correction of the
+instrument being + &middot;015 (including index error, capillarity and capacity),
+what temperature should be assigned for the boiling point marked on the
+thermometer?</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td>Observed barometer</td>
+    <td><span class="spacer2">&nbsp;</span></td>
+    <td align="right">29&middot;876</td></tr>
+<tr><td>Correction</td>
+    <td>&nbsp;</td>
+    <td align="right" class="botbor">+<span class="spacer2">&nbsp;</span>&middot;015</td></tr>
+<tr><td>&nbsp;</td>
+    <td>&nbsp;</td>
+    <td align="right">29&middot;891</td></tr>
+<tr><td>Correction for temperature</td>
+    <td>&nbsp;</td>
+    <td align="right" class="botbor">-<span class="spacer2">&nbsp;</span>&middot;074</td></tr>
+<tr><td>Reduced reading</td>
+    <td>&nbsp;</td>
+    <td align="right">29&middot;817</td></tr>
+<tr><td>Factor from Table V.</td>
+    <td>&nbsp;</td>
+    <td align="right" class="botbor">1&middot;00077</td></tr>
+<tr><td>&nbsp;</td>
+    <td>&nbsp;</td>
+    <td align="right">208719</td></tr>
+<tr><td>&nbsp;</td>
+    <td>&nbsp;</td>
+    <td><span style="margin-left: 1.75em;">208719</span></td></tr>
+<tr><td>&nbsp;</td>
+    <td>&nbsp;</td>
+    <td class="botbor"><span style="margin-left: .25em;">29817</span></td></tr>
+<tr><td>Equivalent for lat. 45&deg;</td>
+    <td>&nbsp;</td>
+    <td align="right" class="botbor2">29&middot;83995909</td></tr></table>
+
+<p>In Table VI., 29&middot;84 gives temperature 211&deg;&middot;86.</p>
+
+
+<p><br /><b>58. Displacement of the Freezing Point.</b>&mdash;Either the prolonged effect of
+the atmospheric pressure upon the thin glass of the bulbs of thermometers,
+or the gradual restoration of the equilibrium of the particles of the
+glass after having been greatly disturbed by the operation of boiling the
+mercury, seems to be the cause of the freezing points of standard
+thermometers reading from a few tenths to a degree higher in the course of
+some years, as has been repeatedly observed. To obviate this small error,
+it is our practice to place the tubes aside for about six months before
+fixing the freezing point, in order to give time for the glass to regain
+its former state<span class="pagenum"><a name="Page_64" id="Page_64">[Pg 64]</a></span> of aggregation. The making of accurate thermometers is a
+task attended with many difficulties, the principal one being the
+liability of the zero or freezing point varying constantly, so much so,
+that a thermometer that is perfectly correct to-day, if immersed in
+boiling water, will be no longer accurate; at least, it will take some
+time before it again settles into its normal state. Then, again, if a
+thermometer is recently blown, filled, and graduated immediately, or, at
+least, before some months have elapsed, though every care may have been
+taken with the production of the instrument, it will require some
+correction; so that the instrument, however carefully made, should from
+time to time be plunged into finely-pounded ice, in order to verify the
+freezing point.</p>
+
+
+<p><br /><b>59. The Scale.</b>&mdash;The two fixed points having been determined, it is
+necessary to apply the scale. The thermometers in general use in the
+United Kingdom, the British Colonies, and North America are constructed
+with Fahrenheit&#8217;s scale. Fahrenheit was a philosophical instrument maker
+of Amsterdam, who, about the year 1724, invented the scale which has given
+his name to the thermometer. The freezing point is marked 32&deg;, the boiling
+point 212&deg;, so that the intermediate space is divided into 180 equal
+parts, called degrees. &#8220;The principle which dictated this <i>peculiar
+division</i> of the scale is as follows:&mdash;When the instrument stood at the
+greatest cold of Iceland, or 0 degree, it was computed to contain 11124
+equal parts of quicksilver, which, when plunged in melting snow, expanded
+to 11156 parts; hence the intermediate space was divided into 32 equal
+portions, and 32 was taken as the freezing point of water: when the
+thermometer was plunged in boiling water, the quicksilver was expanded to
+11336; and therefore 212&deg; was marked as the boiling point of that fluid.
+In <i>practice</i>, Fahrenheit determined the divisions of his scale from two
+fixed points, the freezing and boiling of water. <i>The theory</i> of the
+division, if we may so speak, was derived from the lowest cold observed in
+Iceland, and the expansions of a given portion of mercury&#8221; (<i>Professor
+Trail</i>).</p>
+
+<p>The divisions of the scale can be carried beyond the fixed points, if
+requisite, by equal graduations. Fahrenheit&#8217;s scale is very convenient in
+some respects. The meteorological observer is seldom troubled with
+negative signs, as the zero of the scale is much below freezing. Again,
+the divisions are more numerous, and consequently smaller, than on other
+scales in use; and the further subdivision into tenths of degrees, seems
+to give all the minuteness usually required.</p>
+
+<p><i>Celcius</i>, a Swede, in 1742, proposed zero for the freezing point, and 100
+for the boiling point, all temperatures below zero being distinguishable
+by the sign (&mdash;) minus. This scale is known as the <i>centigrade</i>, and is in
+use in France, Sweden, and the southern part of Europe. It has the
+advantage of the decimal notation, with the embarrassment of the negative
+sign.</p>
+
+<p><i>Reaumur</i>, a Frenchman, proposed zero for the freezing point, and 80&deg; for
+the boiling point, an arrangement inferior to the centigrade. It is,
+however, in use in Spain, Switzerland, and Germany.</p>
+
+<p><span class="pagenum"><a name="Page_65" id="Page_65">[Pg 65]</a></span>It is merely a simple arithmetical operation to change the indications of
+any one of these scales into the equivalents on the others. To facilitate
+such conversions, tables are convenient, when a large number of
+observations are under discussion; and they can be easily formed or
+obtained.</p>
+
+<p>In the absence of such tables, the following formul&aelig; will insure accuracy
+of method, and save thinking, when occasional conversions are wanted to be
+made:&mdash;F. stands for Fahrenheit, C. for Centigrade, and R. for Reaumur.</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td align="center">Given.</td><td><span class="spacer">&nbsp;</span></td>
+    <td align="center">Required.</td><td><span class="spacer">&nbsp;</span></td>
+    <td>&nbsp;</td>
+    <td colspan="2" align="center">Solution.</td></tr>
+<tr><td align="center">F.</td><td>&nbsp;</td>
+    <td align="center">C.</td><td>&nbsp;</td>
+    <td>=</td>
+    <td>&nbsp;</td>
+    <td>(F.-32) <span style="font-size: 0.8em;"><sup>5</sup></span>&frasl;<span style="font-size: 0.6em;">9</span></td></tr>
+<tr><td align="center">F.</td><td>&nbsp;</td>
+    <td align="center">R.</td><td>&nbsp;</td>
+    <td>=</td>
+    <td>&nbsp;</td>
+    <td>(F.-32) <span style="font-size: 0.8em;"><sup>4</sup></span>&frasl;<span style="font-size: 0.6em;">9</span></td></tr>
+<tr><td align="center">C.</td><td>&nbsp;</td>
+    <td align="center">F.</td><td>&nbsp;</td>
+    <td>=</td>
+    <td><span style="font-size: 0.8em;"><sup>9</sup></span>&frasl;<span style="font-size: 0.6em;">5</span></td>
+    <td>C. + 32</td></tr>
+<tr><td align="center">C.</td><td>&nbsp;</td>
+    <td align="center">R.</td><td>&nbsp;</td>
+    <td>=</td>
+    <td><span style="font-size: 0.8em;"><sup>4</sup></span>&frasl;<span style="font-size: 0.6em;">5</span></td>
+    <td>C.</td></tr>
+<tr><td align="center">R.</td><td>&nbsp;</td>
+    <td align="center">F.</td><td>&nbsp;</td>
+    <td>=</td>
+    <td><span style="font-size: 0.8em;"><sup>9</sup></span>&frasl;<span style="font-size: 0.6em;">5</span></td>
+    <td>R. + 32</td></tr>
+<tr><td align="center">R.</td><td>&nbsp;</td>
+    <td align="center">C.</td><td>&nbsp;</td>
+    <td>=</td>
+    <td><span style="font-size: 0.8em;"><sup>5</sup></span>&frasl;<span style="font-size: 0.6em;">4</span></td>
+    <td>R.</td></tr></table>
+
+<p><i>Example.</i>&mdash;Convert 25&deg; of Fahrenheit&#8217;s scale into the corresponding
+temperature on the Centigrade scale.</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td>Here</td><td>C. =</td><td colspan="3"> (25 - 32) <span style="font-size: 0.8em;"><sup>5</sup></span>&frasl;<span style="font-size: 0.6em;">9</span></td></tr>
+<tr><td rowspan="2">&nbsp;</td><td rowspan="2">C. = -</td><td class="botbor" align="center">35</td><td rowspan="2">= -3&middot;9</td></tr>
+<tr><td align="center">9</td></tr></table>
+
+<p>or nearly 4&deg; <i>below</i> zero of the Centigrade scale. The algebraical sign
+must be carefully attended-to in the calculations.</p>
+
+
+<p><br /><b>60. The method of testing Thermometers</b> for meteorological purposes is very
+simple. Such thermometers are seldom required to read above 120&deg;. In these
+the freezing point having been determined, the divisions of the scale are
+ascertained by careful comparisons, with a standard thermometer, in water
+of the requisite temperature. &#8220;For the freezing point, the bulbs, and a
+considerable portion of the tubes of the thermometers, are immersed in
+pounded ice. For the higher temperatures, the thermometers are placed in a
+cylindrical glass vessel containing water of the required heat: the scales
+of the thermometers intended to be tested, together with the Standard with
+which they are to be compared, are read through the glass. In this way the
+scale readings may be tested at any required degree of temperature, and
+the usual practice is to test them at every ten degrees from 32&deg; to 92&deg; of
+Fahrenheit.&#8221;&mdash;<i>FitzRoy.</i></p>
+
+
+<p><br /><b>61. Porcelain Scale Plates.</b>&mdash;Thermometer scales of brass, wood, or ivory,
+either by atmospheric influence or dipping in sea-water, are very liable
+to become soiled and discoloured, so much so that after a very little time
+the divisions are rendered nearly invisible. To obviate this
+inconvenience, Messrs. Negretti and Zambra were the first to introduce
+into extensive use thermometer and barometer scale-plates made of
+porcelain, having the divisions and figures engraved thereon by means of
+fluoric acid, and permanently burnt-in and blackened, so as always to
+present a clear legible scale. That these scales have been found superior
+to all others, may be inferred from the fact that all the thermometers now
+supplied to the various government departments are provided with such
+scales.</p>
+
+<p><span class="pagenum"><a name="Page_66" id="Page_66">[Pg 66]</a></span>They can be adapted to replace any of the old forms of brass or zinc
+scales, the divisions and figures of which have become obliterated or
+indistinct.</p>
+
+
+<p><br /><b>62. Enamelled Tubes.</b>&mdash;Nearly all thermometer tubes are now made with
+enamelled backs. This contrivance of enamelling the backs of the tubes
+enables the makers to use finer threads of mercury than had before been
+found practicable; for were it not for the great contrast between the dark
+thread of mercury and the white enamel on the glass, many of the
+thermometers now in use would be positively illegible. The enamelling of
+thermometers is an invention of Messrs. Negretti and Zambra. It is
+necessary to state this, as many persons, from interested motives, are
+anxious to ignore to whom the credit of the invention is due.</p>
+
+
+<p><br /><b>63. Thermometers of extreme Sensitiveness.</b>&mdash;Thermometers for delicate
+experiments are no novelty. Thermometers have been made with very delicate
+bulbs to contain a very small quantity of mercury. Such instruments have
+also been made with spiral or coiled tubular bulbs, but the thickness of
+glass required to keep these coils or spirals in shape, and in fact to
+prevent their falling to pieces, served to nullify the effect sought to be
+produced, viz. instantaneous action; and where a small thin bulb was
+employed, the indicating column was generally so fine that it was
+positively invisible except by the aid of a powerful lens. Messrs.
+Negretti and Zambra have now introduced a new form of thermometer, which
+combines sensitiveness and quickness of action, together with a good
+visible column. The bulb of this thermometer is of the gridiron form. Care
+has been taken in constructing the bulb, so that the objections attending
+spirals and other forms have been overcome; for whilst the reservoir or
+bulb is made of glass so thin that it is only by a spirit lamp and not a
+glass blower&#8217;s blowpipe that it can be formed, yet it is still so rigid
+(owing to its peculiar configuration) that no variations in its
+indications can be detected, whether it be held in a horizontal, vertical,
+or oblique position, nor will any error be detected if it be stood on its
+own bulb. They have made thermometers with bulbs or reservoirs formed of
+about nine inches of excessively thin cylindrical glass, whose outer
+diameter is not more than a twentieth of an inch; so that, owing to the
+large surface presented, the indications are positively instantaneous.
+This form of thermometer was constructed expressly to meet the
+requirements of scientific balloon ascents, to enable thermometrical
+readings to be taken at the precise elevation. It was contemplated to
+procure a metallic thermometer, but on the production of this perfect
+instrument the idea was abandoned.</p>
+
+
+<p class="center"><br />64. VARIETIES OF THERMOMETERS.</p>
+
+<p>Fig. 37 is an illustration of boxwood scale thermometers for general use
+and common purposes.</p>
+
+<p>Fig. 38, Negretti and Zambra&#8217;s Travelling Thermometer; it is fixed in a
+plated metal (silver or otherwise) case, similar to a pencil-case, and has
+the scale divided upon its stem.</p>
+
+<p><span class="pagenum"><a name="Page_67" id="Page_67">[Pg 67]</a></span>Fig. 39, Thermometer mounted on a slab of glass, upon which the scale is
+etched, the back being either oak, mahogany, or ebony.</p>
+
+<p>Fig. 40, Portable Thermometer, in a bronzed brass or German silver
+revolving case.</p>
+
+<p>Fig. 41, Pocket Thermometer, on ivory or metallic scale, in morocco or
+papier-m&acirc;ch&eacute; case.</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td align="center">Fig. 37.</td><td><span class="spacer2">&nbsp;</span></td>
+    <td align="center">Fig. 38.</td><td><span class="spacer2">&nbsp;</span></td>
+    <td align="center">Fig. 39.</td><td><span class="spacer2">&nbsp;</span></td>
+    <td align="center">Fig. 40.</td><td><span class="spacer2">&nbsp;</span></td>
+    <td align="center">Fig. 41.</td><td><span class="spacer2">&nbsp;</span></td></tr>
+<tr><td align="center"><img src="images/fig_37.jpg" alt="" /></td><td>&nbsp;</td>
+    <td align="center"><img src="images/fig_38.jpg" alt="" /></td><td>&nbsp;</td>
+    <td align="center"><img src="images/fig_39.jpg" alt="" /></td><td>&nbsp;</td>
+    <td align="center"><img src="images/fig_40.jpg" alt="" /></td><td>&nbsp;</td>
+    <td align="center"><img src="images/fig_41.jpg" alt="" /></td><td>&nbsp;</td></tr></table>
+
+
+<p>Fig. 42, an Ornamental Drawing-room Thermometer, on ebony or ivory stand,
+with glass shade.</p>
+
+<p>Fig. 43, representation of highly carved or engine-turned design for
+thermometer mounts, in ivory or wood, for the drawing-room. Some have the
+addition of a sundial or compass at the top; they may also be formed for a
+watch-stand.</p>
+
+<p>Fig. 44, <b>Bath Thermometer</b>, having a float to admit of its being kept in
+the water.</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td align="center">Fig. 42.</td><td><span class="spacer2">&nbsp;</span></td>
+    <td align="center">Fig. 43.</td><td><span class="spacer2">&nbsp;</span></td>
+    <td align="center">Fig. 44.</td><td><span class="spacer2">&nbsp;</span></td></tr>
+<tr><td align="center"><img src="images/fig_42.jpg" alt="" /></td><td>&nbsp;</td>
+    <td align="center"><img src="images/fig_43.jpg" alt="" /></td><td>&nbsp;</td>
+    <td align="center"><img src="images/fig_44.jpg" alt="" /></td><td>&nbsp;</td></tr></table>
+
+<p><span class="pagenum"><a name="Page_68" id="Page_68">[Pg 68]</a></span>Fig. 45, Thermometer with ivory scale in glass cylinder, mounted on oak
+bracket with metal top, for out-door use; as at a window.</p>
+
+<p>Fig. 46, Thermometer for the window, on patent porcelain or glass scale,
+with oak bracket and convenient brass supports, for placing the instrument
+at any angle.</p>
+
+<p>Fig. 47, <b>Chemical Thermometer</b>, on boxwood scale, jointed near the bulb on
+a brass hinge, ranging from 300&deg; to 600&deg;.</p>
+
+<p>Fig. 48, <b>Chemical Thermometer</b>, for acids, graduated on its own stem,
+suitable for insertion in the tubulure of retorts; they are also made
+insulated in glass cylinder to protect the graduated stem; ranging from 0&deg;
+to 600&deg;.</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td align="center">Fig. 45.</td><td><span class="spacer2">&nbsp;</span></td>
+    <td align="center">Fig. 46.</td><td><span class="spacer2">&nbsp;</span></td>
+    <td align="center">Fig. 47.</td><td><span class="spacer2">&nbsp;</span></td>
+    <td align="center">Fig. 48.</td><td><span class="spacer2">&nbsp;</span></td>
+    <td align="center">Fig. 49.</td><td><span class="spacer2">&nbsp;</span></td>
+    <td align="center">Fig. 50.</td><td><span class="spacer2">&nbsp;</span></td></tr>
+<tr><td align="center"><img src="images/fig_45.jpg" alt="" /></td><td>&nbsp;</td>
+    <td align="center"><img src="images/fig_46.jpg" alt="" /></td><td>&nbsp;</td>
+    <td align="center"><img src="images/fig_47.jpg" alt="" /></td><td>&nbsp;</td>
+    <td align="center"><img src="images/fig_48.jpg" alt="" /></td><td>&nbsp;</td>
+    <td align="center"><img src="images/fig_49.jpg" alt="" /></td><td>&nbsp;</td>
+    <td align="center"><img src="images/fig_50.jpg" alt="" /></td><td>&nbsp;</td></tr></table>
+
+
+<p><br /><b>65. Superheated Steam Thermometer.</b>&mdash;The great advantage gained by the use
+of superheated steam in marine and other steam-engines being now generally
+admitted by engineers, reliable thermometers, reading to 600&deg; at least,
+are of the utmost importance. To meet this want, Messrs. Negretti and
+Zambra have constructed for the purpose a substantial form of thermometer,
+on their patent porcelain scales, in strong and convenient metal
+mountings, with perforated protection to the bulb. The scales cannot be
+deteriorated by steam, heat, oil, or dirt; and an occasional wiping will
+be all that is necessary to keep the divisions and figures clean and
+visible for any length of time; while careful calibration of the
+thermometer tubes ensures the most accurate indications attainable. These
+thermometers are illustrated by figs. 49 &amp; 50. A similar,<span class="pagenum"><a name="Page_69" id="Page_69">[Pg 69]</a></span> but cheaper,
+construction is given to thermometers to be used with hot air, or hot
+water, apparatus.</p>
+
+
+<p><br /><b>66. Thermometer for Sugar Boiling</b> is protected by a metallic frame; and is
+usually from three to four feet long, the graduations being confined to a
+space of about twelve inches at the upper part of the instrument, allowing
+the bulb and greater part of the tube to be immersed in the boiling sugar.
+The graduations extend to 270&deg; or further. An index is sometimes attached
+to the scale, which may be set to any degree of heat required to be
+maintained.</p>
+
+
+<p class="center"><br />67. EARTH THERMOMETER.</p>
+
+<p>The Earth Thermometer is for ascertaining the temperature of the soil at
+various depths. It is protected by a brass frame, pointed and strengthened
+at the end to facilitate insertion into the ground, as in fig. 51.</p>
+
+<div class="figright">Fig. 51.<br /><img src="images/fig_51.jpg" alt="" /></div>
+
+<p><i>Utility of a Knowledge of the Temperature of the Soil.</i>&mdash;The temperature
+of the soil is an important element in the consideration of climate, as it
+concerns the vegetable kingdom.</p>
+
+<p>Dr. Daubeny, in his <i>Lectures on Climate</i>, gives the following statement
+with respect to some temperatures which have been observed just beneath
+the earth&#8217;s surface, in different parts of the globe:&mdash;</p>
+
+<table border="0" cellpadding="0" cellspacing="0" summary="table">
+<tr><td class="btr" align="center">Country.</td>
+    <td class="btr" align="center">Temperature.</td>
+    <td class="bt" align="center">Authority.</td></tr>
+<tr><td class="btr">Tropics, often</td>
+    <td class="btr">162-184&deg;</td>
+    <td class="bt">Humboldt.</td></tr>
+<tr><td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td></tr>
+<tr><td class="br">Egypt</td>
+    <td class="br">133-144</td>
+    <td class="dent">Edwards &amp; Colin.</td></tr>
+<tr><td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td></tr>
+<tr><td class="br">Orinoco</td>
+    <td class="br">In white sand, 140</td>
+    <td class="dent">Humboldt.</td></tr>
+<tr><td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td></tr>
+<tr><td class="br">Chili</td>
+    <td class="br">113-118, among dry grass</td>
+    <td class="dent">Boussingault.</td></tr>
+<tr><td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td></tr>
+<tr><td class="br">Cape of Good Hope</td>
+    <td class="br">150, under the soil of a bulb garden</td>
+    <td class="dent">Herschell.</td></tr>
+<tr><td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td></tr>
+<tr><td class="br">Bermuda</td>
+    <td class="br">142, thermometer barely covered in earth</td>
+    <td class="dent">Emmet.</td></tr>
+<tr><td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td></tr>
+<tr><td class="br">China</td>
+    <td class="br">Water of the fields, 113; adjacent sand,<br />much higher; blackened sides of the boat<br />at midday, 142-150</td>
+    <td class="dent">Meyer.</td></tr>
+<tr><td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td></tr>
+<tr><td class="bbr">France</td>
+    <td class="bbr">118-122, and in one instance 127</td>
+    <td class="bb">Arago.</td></tr></table>
+
+<p>&#8220;The importance of this to vegetation may be estimated by the following
+considerations:&mdash;</p>
+
+<p>&#8220;It is known that every plant requires a certain amount of heat, varying
+in<span class="pagenum"><a name="Page_70" id="Page_70">[Pg 70]</a></span> the case of each species, for the renewal of its growth, at the
+commencement of the season.</p>
+
+<p>&#8220;Now when this degree of heat has spurred into activity those parts that
+are above ground, and caused them to elaborate the sap, it is necessary
+that the subterranean portions should at the same time be excited by the
+heat of the ground to absorb the materials which are to supply the plant
+with nourishment. Unless the latter function is provided for, the aerial
+portions of the plant will languish from want of food to assimilate.
+Indeed, it is even advisable that the roots should take the start of the
+leaves, in order to have in readiness a store of food for the latter to
+draw upon.&#8221; In another place the professor remarks:&mdash;&#8220;It has been
+calculated by Mr. Raikes, from experiments made at Chat Moss, that the
+temperature of the soil when drained averages 10&deg; more than it does when
+undrained; and this is not surprising, when we find that 1 lb. of water
+evaporated from 1,000 lbs. of soil will depress the whole by 10&deg;, owing to
+the latent heat which it absorbs in its conversion into vapour.&#8221;</p>
+
+
+<p class="center"><br />68. MARINE THERMOMETER.</p>
+
+<div class="figleft">Fig. 52.<br /><img src="images/fig_52.jpg" alt="" /></div>
+
+<p>This instrument is a special construction to meet the requirements of
+navigation. It consists of a carefully constructed thermometer divided on
+its stem to degrees, which are sufficiently large to admit of subdivision
+into tenths of degrees by estimation, and ranging from 0&deg; to 130&deg;. The
+scale is porcelain, having the degrees etched upon it, and burnt-in a
+permanent black. The instrument is made to slide into a japanned metallic
+case, for handy use and protection. It is therefore adapted for almost any
+ordinary purpose; and cannot be injuriously affected by any chemical
+action arising from air or sea-water. A set of these thermometers consists
+of six, carefully packed in a neat box; two having japanned metallic cases
+(fig. 52), the others being designed for use without the case, or to
+replace a breakage.</p>
+
+<p>This thermometer is employed in the Royal Navy, and for the observations
+made at sea for the Board of Trade.</p>
+
+<p>The thermometer is now considered a necessary instrument on board ship.
+Not only is it of invaluable utility in connection with the barometer as a
+guide to the weather, but its indications are of service in showing the
+presence of a warm or cold current in the sea; many of the great oceanic
+currents being characterised by the warmth or coldness of their waters. In
+seas visited by icebergs, the habitual use of the thermometer would
+indicate their proximity, as the water is rendered colder for some
+distance around by the thawing of huge masses of ice. The water over a
+shoal in the sea is generally colder than the surface-water of the
+surrounding ocean; which may result from the cold water being brought to
+the surface by the current of water encountering the shoal. With this fact
+navigators are well acquainted; and therefore a fall in the sea-water
+thermometer may forebode that shallow water is at hand. It has been
+ascertained that fish<span class="pagenum"><a name="Page_71" id="Page_71">[Pg 71]</a></span> inhabit regions of the oceans and seas having the
+peculiar temperature suitable to their habits. The better and firmer sort
+of fish are found where cold waters exist. Those taken in warmer belts or
+streams of water, even in the same latitude, are far inferior in
+condition, and less approved by the palate. The fish of the Mediterranean,
+a warm sea, are generally poor and scarce. Fish taken in the cold waters
+between the American shore and the Gulf Stream are much esteemed; while in
+and on the other side of the stream they are said to be tasteless, and of
+no flavour. Between the coasts of China and the warm waters of the
+Japanese current, the seas abound with excellent fish; but in the warm
+waters of the current and beyond, they are never seen in such shoals.</p>
+
+<p>In fact, it is clearly ascertained that fishes are adapted to climates,
+like birds and beasts. It has been even affirmed, after careful
+investigation, that herrings, which abound in the British Seas, and form a
+most important branch of our fisheries, can only be found in a temperature
+varying from 54&deg; to 58&deg;. Hence the thermometer, if brought into use by the
+fishermen, would guide them to the spots where they may with the best
+chance cast their nets on dark nights, when other indications are not
+perceptible.</p>
+
+<p>This thermometer in its metallic case is perfectly suited for dipping
+overboard, or placing in a bucket of water just taken from the sea, to
+ascertain its temperature.</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_72" id="Page_72">[Pg 72]</a></span></p>
+<h2><a name="CHAPTER_VII" id="CHAPTER_VII"></a>CHAPTER VII.</h2>
+<p class="center"><span class="large">SELF-REGISTERING THERMOMETERS.</span></p>
+
+<p><br /><b>69. Importance of Self-Registering Thermometers.</b>&mdash;Heat being apparently
+the most effective agent in producing meteorological phenomena, the
+determination of the highest temperature of the day, and the lowest during
+the night, is a prime essential to enable an estimate of the climate of
+any place to be formed. To observe these extremes by means of the ordinary
+thermometer would be impracticable, from the constant watchfulness which
+would be necessary. Hence, the utility and importance of self-recording
+thermometers are evident. A thermometer constructed to <i>register</i> the
+highest temperature is usually called a <i>maximum thermometer</i>; one to show
+the lowest temperature is termed a <i>minimum thermometer</i>; and if made to
+record both extremes of temperature, it is designated a
+<i>maximum-and-minimum thermometer</i>. We will, for the sake of method,
+describe the instruments in use in this order.</p>
+
+<p>It would carry us beyond our scope to explain in detail the methods of
+dealing with temperature observations; but we may remark that half the sum
+of the maximum and minimum temperature of each day of twenty-four hours,
+is not what meteorologists designate the <i>mean daily temperature</i>,
+although it very frequently approximates to it. The mean temperature of
+the day is understood to be the average of twenty-four consecutive hourly
+readings of a thermometer; and meteorology now supplies formul&aelig; whereby
+this result can be deduced from two or three observations only in a day.
+But we would observe that the actual mean temperature of any place has not
+such an important influence upon life, either animal or vegetable, as the
+abruptness and magnitude of the variations of temperature. Climate,
+therefore, should be estimated more by the range of the thermometer than
+by the average of its indications. The Registrar General&#8217;s returns prove
+that with a wide range of the thermometer, the mortality greatly
+increases; and it is now becoming apparent to meteorologists that the
+daily range of the thermometer marks the effects of temperature on the
+health of men, and the success of crops, better than any other
+meteorological fact of which we take cognizance. Now that self-registering
+thermometers are constructed with mercury, the most appropriate of all
+thermometric substances, not only for maxima, but likewise for minima
+temperatures, the determination of the diurnal range of temperature is
+rendered more certain, and observations at different places are more
+strictly comparable.</p>
+
+
+<p class="center"><br /><span class="pagenum"><a name="Page_73" id="Page_73">[Pg 73]</a></span>MAXIMA THERMOMETERS.</p>
+
+<p><br /><b>70. Rutherford&#8217;s Maximum Thermometer.</b>&mdash;The maximum thermometer, invented
+by Dr. John Rutherford, differs from an ordinary thermometer in having a
+small cylinder of steel, porcelain, or aluminium, moving freely in the
+tube beyond the mercury, so as to form an index. The stem of the
+thermometer is fixed horizontally on the frame, which must be suspended in
+the same position, as represented in fig. 53. The instrument is set by
+holding it bulb downward, so as to allow the index to fall by its own
+gravity into contact with the mercury. Increase of heat produces expansion
+of the mercury, which consequently pushes forward the index. When the
+temperature decreases, the mercury recedes from the index, leaving it so
+that the extremity which was in contact with the mercury indicates upon
+the scale the highest temperature since the instrument was last set.</p>
+
+<p>&nbsp;</p>
+<p class="center">Fig. 53.</p>
+<div class="figcenter"><img src="images/fig_53.jpg" alt="" /></div>
+<p>&nbsp;</p>
+
+<p>As it is easily constructed and is comparatively cheap, it is still
+employed for ordinary purposes. Its disadvantages are, firstly, its
+liability of soon getting out of order by the index becoming embedded in
+the mercury, or fixed by oxidation, thus rendering it altogether useless;
+secondly, the ease with which the index can be displaced by the wind
+moving the instrument, or other accidental disturbance, so as to cause it
+to give erroneous indications occasionally; and thirdly, its consequent
+total unfitness for use at sea.</p>
+
+<p>In the part of the tube beyond the mercury, a small quantity of air is
+enclosed for the purpose of preventing the metal flowing freely in the
+tube. This necessitates the construction of a larger bulb, which renders
+the thermometer less sensitive. Moreover, as it frequently happens that
+some mercury passes the index, particles of air insinuate themselves in
+the metal, and cause separations in the column, which very often can be
+removed only by a maker. To facilitate this re-adjustment, a small chamber
+is left at the end of the tube, and the mercury being expanded into it by
+heat until the index and air bubbles are forced into it, if possible, upon
+the cooling down again, by a little management, the mercury will contract,
+leaving the air and index behind. Yet sometimes the index cannot be moved
+in the least from its place of fixture, so that the instrument must be
+virtually reconstructed.</p>
+
+
+<p><br /><b>71. Phillip&#8217;s Maximum Thermometer.</b>&mdash;A maximum thermometer, better perhaps
+in its action than Rutherford&#8217;s, has been suggested by Professor John
+Phillips, of Oxford. A small portion of air is introduced into an ordinary
+thermometer, so as to cut off about half an inch of the mercurial thread
+near its end in the tube. This forms a maximum thermometer, when the stem
+is arranged horizontally. The isolated portion is pushed forward by
+expansion, and is left in this position when the<span class="pagenum"><a name="Page_74" id="Page_74">[Pg 74]</a></span> mercury contracts. The
+end remote from the bulb shows on the scale the maximum temperature.</p>
+
+<p>When made with a capillary tube so fine that the attraction arising from
+capillarity overcomes the force of gravity, and prevents the mercury
+falling to the end of the tube when the instrument is inverted, it forms a
+very serviceable thermometer, quite portable and suitable for use on board
+ship. In such a tube a smart shake from a swing of the hand is required to
+bring the detached portion back to the column, so as to set the instrument
+for future observation; no ordinary motion will move it. When the
+thermometer has not this peculiarity, the mercury will flow to the end, if
+held bulb downward; and in this state it is not at all a satisfactory
+instrument, as the air is likely to be displaced, and a great deal of tact
+is requisite to again get it to divide the column suitably. It has been
+found in practice that the air bubble at different temperatures assumes
+different lengths, and if very small it disappears in a few years by
+oxidation and by diffusion with the mercury, so that the instrument
+becomes defective and uncertain in action,&mdash;results which led to the
+construction of the self-registering mercurial maximum thermometer,
+invented and patented by Messrs. Negretti and Zambra. It has been before
+the public about twelve years; we may therefore, now, safely speak of its
+merits.</p>
+
+
+<p><br /><b>72. Negretti and Zambra&#8217;s Patent Maximum Thermometer</b> consists of a glass
+tube containing mercury fitted on an engraved scale, as shown in fig. 54.
+The part of the thermometer tube above the mercury is entirely free from
+air; and at the point <span class="smcaplc">A</span> in the bend above the bulb, is inserted and fixed
+with the blow-pipe a small piece of solid glass, or enamel, which acts as
+a valve, allowing mercury to pass on one side of it when heat is applied,
+but not allowing it to return when the thermometer cools. When mercury has
+been once made to pass the contraction, which nothing but the expansive
+force of heat can effect, and has risen in the tube, the upper end of the
+column registers the maximum temperature. To return the mercury to the
+bulb, we must apply a force equal to that which raised it in the tube; the
+force employed is gravity, assisted when necessary by a little agitation
+of the instrument.</p>
+
+<p>&nbsp;</p>
+<p class="center">Fig. 54.</p>
+<div class="figcenter"><img src="images/fig_54.jpg" alt="" /></div>
+<p>&nbsp;</p>
+
+<p>The degrees are generally divided on the stems of these thermometers, but
+their frames of course bear a scale as well. The makers have various
+styles of framing in wood, metal, porcelain, and even glass. Each material
+is eligible according to requirements. Porcelain scales, having the marks
+<i>etched</i> upon them by acid and permanently blackened and baked in,&mdash;by a
+process for which the<span class="pagenum"><a name="Page_75" id="Page_75">[Pg 75]</a></span> inventors have a separate patent,&mdash;will be found
+very serviceable, as they do not corrode or tarnish by exposure to any
+kind of weather; while any amount of dust and dirt can readily be cleaned
+off.</p>
+
+<p>The chief recommendation of this thermometer is its simplicity of
+construction, enabling it to be used with confidence and safety. Of no
+other maximum thermometer can it be said that it is impossible to derange
+or put it out of order; hence, as regards durability, it surpasses all
+others. Nothing short of actual breakage can cause it to fail. Hence it is
+the most easily portable of all self-registering thermometers, an
+advantage which renders it suitable for travellers, and for transmission
+abroad. In the year 1852, the British Meteorological Society reported this
+thermometer to be &#8220;the best which has yet been constructed for maximum
+temperature, and particularly for sun observations.&#8221; Since then eleven
+years have elapsed, and it is still without a rival.</p>
+
+<p><i>Directions for use.</i> In using this thermometer for meteorological
+observations, it should be suspended by means of two brass plates <span class="smcaplc">B</span>, <span class="smcaplc">C</span>,
+attached for that purpose, in such manner that it hangs raised up a little
+at <span class="smcaplc">C</span>, and so placed that it is in the shade, with the air passing freely
+to it from all sides; then, on an increase of heat, the mercury will pass
+up the tube as in an ordinary thermometer, and continue doing so as long
+as the heat increases. On a decrease of heat, the contraction of mercury
+will take place <i>below</i> the <i>bend</i> in the tube, leaving the whole column
+of mercury in the tube, thus registering the highest temperature, and
+showing such till the instrument is disturbed.</p>
+
+<p>To prepare the instrument for future observations, remove and hold it
+perpendicularly, with the bulb downward, and then shake it. The mercury
+will then descend in the tube, and indicate the temperature of the air at
+that time; and, when again suspended, is prepared for future observation.</p>
+
+<p>After the temperature has attained a maximum, there will be, with a
+decrease of heat, a slight contraction of mercury in the tube&mdash;as well as
+of that in the bulb&mdash;and hence doubts have arisen as to the accuracy of
+the registration; but calculation shows, and critical trial has proved,
+that the greatest daily range of temperature will not produce an error
+large enough to be appreciable on the scale.</p>
+
+<p>A very great advantage of this thermometer is that the mercury may be
+allowed to flow to the end of the tube without the maximum temperature
+attained during an experiment being lost. It can be employed with the bulb
+uppermost. All that is necessary for reading the maximum temperature is to
+slope the instrument so that the mercury flows gently towards the bulb. It
+will then stop at the contraction so as to show the maximum temperature on
+the scale. Afterwards the mercury is driven into the bulb by agitating the
+instrument while held in the hand. Hence the instrument is invaluable as a
+registering thermometer on board ship, as its indications are in no way
+affected by the motions and tremors of the vessel.</p>
+
+<p>For physiological experiments, such as taking the temperature of the
+mouth<span class="pagenum"><a name="Page_76" id="Page_76">[Pg 76]</a></span> in fever, this thermometer is the only one that can be used with
+certainty, as it can be held in any position, without losing the maximum
+temperature attained.</p>
+
+
+<p class="center"><br />MINIMA THERMOMETERS.</p>
+
+<p><br /><b>73. Rutherford&#8217;s Alcohol Minimum Thermometer</b>, fig. 55, consists of a glass
+tube, the bulb and part of the bore of which is filled with perfectly pure
+spirits of wine, in which moves freely a black glass index. A slight
+elevation of the thermometer, bulb uppermost, will cause the glass index
+to flow to the surface of the liquid, where it will remain, unless
+violently shaken. On a <i>decrease</i> of temperature the alcohol recedes,
+taking with it the glass index; on an <i>increase</i> of temperature the
+alcohol alone ascends in the tube, leaving the end of the index <i>farthest</i>
+from the bulb indicating the minimum temperature.</p>
+
+<p>&nbsp;</p>
+<p class="center">Fig. 55.</p>
+<div class="figcenter"><img src="images/fig_55.jpg" alt="" /></div>
+<p>&nbsp;</p>
+
+<p><i>Directions for using, &amp;c.</i>&mdash;Having caused the glass index to flow to the
+end of the column of spirit, by slightly tilting the thermometer, bulb
+uppermost, suspend the instrument (in the shade with the air passing
+freely to it on all sides) by the two brass plates attached for that
+purpose,&mdash;in such manner that the bulb is about half an inch lower than
+the upper, or the end of the thermometer farthest from the bulb; then, on
+a decrease of temperature, the spirits of wine will descend, carrying with
+it the glass index; on an increase of temperature, however, the spirits of
+wine will ascend in the tube, leaving that end of the small glass index
+farthest from the bulb indicating the minimum temperature. To reset the
+instrument, simply raise the bulb end of the thermometer a little, as
+before observed, and the index will again descend to the end of the
+column, ready for future observation.</p>
+
+<p><i>Precautions.</i>&mdash;1. By no means jerk or shake an alcohol minimum
+thermometer <i>when resetting</i> it, for by so doing it is liable to
+disarrange the instrument, either by causing the index to leave the
+spirit, or by separating a portion of the spirit from the main column.</p>
+
+<p>2. As alcohol thermometers have a tendency to read lower by age, owing to
+the volatile nature of the fluid allowing particles in the form of vapour
+to rise and lodge in the tube, it becomes necessary to compare them
+occasionally with a mercurial thermometer whose index error is known; and
+if the difference be more than a few tenths of a degree, examine well the
+upper part of the tube to see if any alcohol is hanging in the bore
+thereof; if so, the detached portion of it can be joined to the main
+column by swinging the thermometer with a pendulous motion, <i>bulb
+downwards</i>.</p>
+
+<p>3. The spirit column is sometimes much separated by jolting in travelling.
+If the instrument is in such a condition when received, it should be held
+by the right<span class="pagenum"><a name="Page_77" id="Page_77">[Pg 77]</a></span> hand, bulb downward, and the frame tapped smartly, but
+cautiously, against the palm of the left hand. The broken thread of spirit
+will soon begin to join, and by continuing the operation a sufficient time
+all the bubbles will disappear, and the thermometer become as good as
+ever.</p>
+
+
+<p><br /><b>74. Horticultural Minimum Thermometer.</b>&mdash;This instrument, represented in
+fig. 56, is a special construction of Rutherford&#8217;s minimum thermometer to
+meet the requirements of horticulturists. It is desirable, if not
+essential, that gardeners should have the means of ascertaining to what
+temperature stoves and greenhouses descend on cold nights, especially in
+winter. This thermometer is mounted on a strong cast zinc frame, with the
+divisions and figures of the scale raised.</p>
+
+<p>&nbsp;</p>
+<p class="center">Fig. 56.</p>
+<div class="figcenter"><img src="images/fig_56.jpg" alt="" /></div>
+<p>&nbsp;</p>
+
+<p>The sunk surface of the frame is painted dark; the figures and division a
+bright colour, so that observations can be made without a close inspection
+of the instrument.</p>
+
+<p>The directions for using are the same as those given in the preceding
+section. It may be used as an ordinary thermometer, by simply hanging it
+from the top loop, in which position, the coloured liquid will always
+indicate the present temperature.</p>
+
+<p>It was a source of annoyance with the ordinary boxwood and flat metal
+scales, that after a time, exposure to a damp warm atmosphere favoured the
+growth of conferv&aelig; upon them, and obliterated the divisions; the plan of
+raising the figures and divisions of the scale has been found to prevent
+the destruction of the instrument in this way.</p>
+
+
+<p><br /><b>75. Baudin&#8217;s Alcohol Minimum Thermometer.</b>&mdash;This instrument resembles
+Rutherford&#8217;s thermometer in appearance; its indications are given by the
+expansion and contraction of alcohol, and its minimum temperature is
+likewise registered by a glass index being pulled back and left behind by
+the alcohol, as in Rutherford&#8217;s instrument. There is, however, a great
+improvement in Baudin&#8217;s instrument; for whilst Rutherford&#8217;s thermometer
+can only register in a horizontal position, Baudin&#8217;s can be used either
+horizontally or vertically, as necessity may require. This important
+change is effected in the following manner:&mdash;Instead of the index in the
+thermometer being loose and free to run up and down according to the
+position in which the instrument is held, as in Rutherford&#8217;s, the index in
+the new instrument is made to fit the bore of the tube as nearly tight as
+possible, so much so that in holding the thermometer even upside down, or
+shaking it, the index will not shift from its position; but, inasmuch as a
+minimum thermometer with an immoveable index could not be set when
+required for observation, and would consequently be<span class="pagenum"><a name="Page_78" id="Page_78">[Pg 78]</a></span> useless, the inventor
+has introduced behind the index a piece of solid glass, about
+one-and-a-half inch in length, which moves freely in the alcohol. The
+addition of the weight of this piece of glass on the top of the index,
+when turned upside down, forces the index down to the edge of the alcohol;
+and it is there left, as in the case of the ordinary Rutherford&#8217;s
+thermometer. It is, therefore, by turning the thermometer upside down, and
+letting the moveable piece of glass fall on the index, that the index is
+driven to the end of the alcohol; after this operation the thermometer is
+hung up either horizontally or vertically, and will then be ready for use.</p>
+
+<p>The index, although immoveable <i>per se</i>, is by the alcohol drawn back, as
+in the ordinary minimum, and its indications are read off on the scale
+from the top of the index.</p>
+
+
+<p><br /><b>76. Mercurial Minima Thermometers desirable.</b>&mdash;Alcohol does not expand
+equally for equal increments of heat, consequently errors are likely to
+exist in the scale indications unless the graduations are very
+accurately&mdash;not necessarily equally&mdash;made. On this account, as well as
+from the volatility of alcohol, and the intervention of gaseous partitions
+in the tube, a good and thoroughly reliable minimum thermometer was for a
+long time a desideratum. It was desirable to obtain a thermometer which
+should register the lowest temperature by mercury, the fluid in general
+use for meteorological thermometers. Several instruments have recently
+been invented to meet this requirement, which are suitable and
+satisfactory for land purposes, but one well adapted for use on board ship
+is still very much wanted.</p>
+
+<p>For very low temperatures, alcohol thermometers will always be required;
+as mercury freezes at -40&deg; F, and contracts very irregularly much before
+this point, while alcohol has never yet been frozen.</p>
+
+<div class="figleft">Fig. 57.<br /><img src="images/fig_57.jpg" alt="" /></div>
+
+<p><br /><b>77. Negretti and Zambra&#8217;s Patent Mercurial Minimum Thermometer</b>,
+represented by fig. 57, has a cylindrical bulb of large size, which, at
+first sight, might induce the idea that the instrument would not be
+sufficiently sensitive; but as length is given to the cylinder instead of
+increasing its diameter, it will be found as sensitive as a globular bulb
+of the same diameter, and much more so than an ordinary alcohol
+thermometer.</p>
+
+<p>The reason for having the bulb large is to allow the internal diameter of
+the thermometer tube to be larger than that generally used for
+thermometrical purposes, so that a steel index, pointed at both ends, may
+move freely within when required.</p>
+
+<p>The tube is blown, filled and regulated in the usual way, 60&deg; of
+temperature being about half-way up the tube. A small cylindrical bulb is
+then formed at the upper end of the tube, and then is introduced a steel
+needle pointed at both ends, that in contact with the mercury being
+abrupt, the other more prolonged. The open<span class="pagenum"><a name="Page_79" id="Page_79">[Pg 79]</a></span> extremity of the tube is now
+drawn out into a fine capillary tube, and the bulb of the instrument
+warmed so as to cause the mercury to fill the tube completely. When the
+mercury reaches the capillary tube, the flame of a blow-pipe is applied;
+the glass is dexterously melted, the superfluous part taken away, and the
+tube left hermetically closed. During this operation, the steel index has
+been embedded in the heated mercury. As the instrument cools, if held
+upright, the mercury will recede and expose the needle, which will then
+follow the descending column simply by its own gravity. In this condition
+the thermometer resembles Rutherford&#8217;s maximum, being a tube of mercury
+with a steel index floating on its surface; but it possesses these
+important advantages: it is quite free from air, so that the mercury can
+move with perfect freedom; and the index is pointed at both ends, to allow
+the mercury to pass, instead of being ground flat to prevent it.</p>
+
+<div class="figright">Fig. 58.<br /><img src="images/fig_58.jpg" alt="" /></div>
+
+<p><i>To use the Thermometer</i>, it is suspended perpendicularly (figure 57) with
+the steel index resting on the surface of the mercurial column. As the
+mercury in the cylinder contracts, that in the tube descends, and the
+index, of its own gravity, follows it; on the contrary, as the mercury
+expands and rises in the tube, it passes the index on one side, and in
+rising, exerts a lateral pressure on the needle, and jams it to one side
+of the tube, where it remains firmly fixed, leaving the upper point of the
+needle indicating the minimum temperature. In this thermometer, the
+reading is always from the upper point of the needle, and not from the
+mercury itself.</p>
+
+<p><i>To extricate the Needle</i> from the mercury, a magnet is used, when, if the
+needle is embedded only a few degrees, it can readily be withdrawn without
+altering the position of the instrument. Should the magnet not be
+sufficient for the purpose, we simply turn the thermometer on its support
+from the upright position, slightly elevating the bulb (fig. 58 (<b>2</b>)). The
+mercury and index will then flow into the small reservoir. Should the
+index not freely leave the tube with the mercury, assist it with a magnet,
+and when the mercury and index are in the upper bulb (figure <b>2</b>), apply a
+magnet outside, which will attract and hold fast the index; and whilst
+thus holding it, again bring the thermometer to the upright position, when
+the mercury will immediately fall back into the tube, leaving the index
+attached to the magnet (figure <b>4</b>), with which it is guided down to the
+surface of the mercury, ready for another observation.</p>
+
+<p>Care must be taken not to withdraw the magnet until the index is in
+contact with mercury; for, if released before touching, it might plunge
+too deeply, and give a false indication. The rule for re-setting it will
+be to bring the needle-point in contact with the mercury, and then
+withdraw the magnet, having previously ascertained that no particles of
+mercury are attached to the index.</p>
+
+<p>It may sometimes, though rarely, happen, that from the time a<span class="pagenum"><a name="Page_80" id="Page_80">[Pg 80]</a></span> minimum
+temperature is registered by the index, and by the time an observation is
+made, the mercury may have risen so high in the tube as to completely pass
+the index, as shown (figure <b>3</b>). Should it so happen, the space which the
+index occupies will readily be observed, as it will be pressed to one side
+of the tube, causing a different appearance in that part, although the
+point of the needle may not be seen. If such be the case, apply a magnet
+to the spot where you see the index is fixed: this will hold the needle
+firmly. Then, by slightly tilting the thermometer bulb uppermost, the
+mercury will flow into the top bulb, leaving the index attached to the
+magnet, and quite uncovered. Having taken the reading, draw the needle
+into the top bulb, and hold it there whilst you adjust the thermometer by
+again bringing it to the upright position.</p>
+
+<p>By contracting the bore of this thermometer, at the bend of the tube,
+sufficiently to keep the mercury from flowing out of its bulb with too
+much freedom by motion, the instrument becomes perfectly safe for
+transmission abroad.</p>
+
+
+<p><br /><b>78. Negretti &amp; Zambra&#8217;s Second Patent Mercurial Minimum Thermometer.</b>&mdash;In
+this thermometer a principle is used that has been long known to
+scientific men, viz. the affinity of mercury for platinum. If mercury be
+placed in contact with platinum under ordinary circumstances, no effect
+will take place; but if the mercury is once made to attack the platinum,
+the amalgamation is permanent and the contact perfect, so much so, that
+the principle was made use of in constructing standard barometers. A ring
+of platinum was fused round the end of the tube, dipping into the mercury;
+and the contact between the platinum and mercury became so perfect that
+air could not creep down the tube and up the bore, as in ordinary
+barometer tubes. This principle of adhesion or affinity of mercury for
+platinum has been brought into play for the purpose of arresting the
+mercury after it has reached the minimum temperature in a thermometer.
+This thermometer is made as follows:&mdash;behind the bulb is placed a
+supplementary chamber; in the space or neck between the bulb of the
+thermometer and the chamber, is placed a small piece of platinum; this may
+be of any shape or size, but the smaller the better. This is not to fit in
+the neck; it must, on the contrary, be rather loose; it may be fastened in
+position or not. The instrument is represented by fig. 59.</p>
+
+<p>&nbsp;</p>
+<p class="center">Fig. 59.</p>
+<div class="figcenter"><img src="images/fig_59.jpg" alt="" /></div>
+<p>&nbsp;</p>
+
+<p><i>Directions for using.</i>&mdash;Having suspended the thermometer in a horizontal
+position, the mercury is made to stand in exact contact with the platinum
+plug by slightly elevating the bulb end of the instrument. The thermometer
+is now ready for observation. On a decrease of temperature, the mercury
+will endeavour to contract first from the easier passage, viz. behind the
+bulb; but in consequence of the adhesion of the<span class="pagenum"><a name="Page_81" id="Page_81">[Pg 81]</a></span> mercury to the platinum,
+it cannot recede from here, it is therefore forced to contract from the
+indicating tube, and will continue to do so as long as the temperature
+decreases; and as no indices are employed in this thermometer, the extreme
+end of the mercurial column will show &#8220;how cold it has been.&#8221; On an
+increase of temperature the mercury will glide over the platinum plug and
+expand by the easier passage into the supplementary chamber, and there
+remain until a decrease of temperature again takes place, when the mercury
+that had gone into the supplementary chamber will be the first to recede,
+until it reaches the platinum plug, its further progress being arrested;
+it will then fall in the indicating tube, and there remain until re-set.</p>
+
+
+<p><br /><b>79. Casella&#8217;s Mercurial Minimum Thermometer.</b>&mdash;The general form and
+arrangement of this instrument is shown in fig. 60. A tube with large
+bore, <i>a</i>, has at the end a <i>flat glass diaphragm</i> formed by the abrupt
+junction of a small chamber, <i>b c</i>, the inlet to which at <i>b</i> is larger
+than the bore of the indicating tube. The result of this is that on
+setting the thermometer, as described below, the contracting force of the
+mercury in cooling withdraws the fluid in the indicating stem only; whilst
+on its expanding with heat, the long column does not move, the increased
+bulk of mercury finding an easier passage into the small pear-shaped
+chamber attached.</p>
+
+<p>&nbsp;</p>
+<p class="center">Fig. 60.</p>
+<div class="figcenter"><img src="images/fig_60.jpg" alt="" /></div>
+<p>&nbsp;</p>
+
+<p>We believe that a small speck of air must be confined in the chamber, <i>b
+c</i>, to act as a spring to start the mercury from the chamber in the act of
+setting the thermometer. Were this air not present, the mercury would so
+adhere to the glass that no amount of shaking could induce it to flow from
+the chamber.</p>
+
+<p><i>To set the Instrument</i>, place it in a horizontal position, with the back
+plate, <i>d</i>, suspended on a nail, and the lower part supported on a hook,
+<i>e</i>. The bulb end may now be gently raised or lowered, causing the mercury
+to flow slowly until the bent part, <i>a</i>, <i>is full</i> and the chamber, <i>b c</i>,
+<i>quite empty</i>. At this point the flow of mercury in the long stem of the
+tube is arrested, <i>and indicates the exact temperature</i> of the bulb or air
+at the time. On an increase of temperature the mercury will expand into
+the small chamber, <i>b c</i>; and a return of cold will cause its recession
+from this chamber only, until it reaches the diaphragm, <i>b</i>. Any further
+diminution of heat withdraws the mercury down the bore to whatever degree
+the cold may attain, where it remains until farther withdrawn by increased
+cold, or till re-set for future observation.</p>
+
+
+<p class="center"><br /><span class="pagenum"><a name="Page_82" id="Page_82">[Pg 82]</a></span>MAXIMA AND MINIMA THERMOMETERS.</p>
+
+<p><br /><b>80. Rutherford&#8217;s</b> arrangement for obtaining a complete instrument for the
+registration of heat and cold was simply mounting a maximum thermometer
+and a minimum thermometer upon the same frame or slab. Thus constructed,
+they are often called &#8220;day and night&#8221; thermometers, though somewhat
+inappropriately; for in temperate climates the temperature of the night
+sometimes exceeds that of the day, notwithstanding the reverse is the
+general law of temperature. Fig. 61 will explain the arrangement of
+Rutherford&#8217;s day and night thermometer.</p>
+
+<p>&nbsp;</p>
+<p class="center">Fig. 61.</p>
+<div class="figcenter"><img src="images/fig_61.jpg" alt="" /></div>
+<p>&nbsp;</p>
+
+<div class="figleft">Fig. 62.<br /><img src="images/fig_62.jpg" alt="" /></div>
+
+<p><br /><b>81. Sixe&#8217;s Self-Registering Thermometer.</b>&mdash;The very ingenious and certainly
+elegant instrument about to be described was invented by James Sixe, of
+Colchester. It consists of a long cylindrical bulb, united to a tube of
+more than twice its length, bent round each side of it in the form of a
+syphon, and terminated in a smaller, oval-shaped bulb. Figure 62 gives a
+representation of this instrument. The lower portion of the syphon is
+filled with mercury; the long bulb, the other parts of the tube, and part
+of the small bulb, with highly rectified alcohol. A steel index moves in
+the spirit in each limb of the syphon. The two indices are terminated at
+top and bottom with a bead of glass, to enable them to move with the least
+possible friction, and without causing separation of the spirit, or
+allowing mercury to pass easily. They would, from their weight, always
+rest upon the mercury; but each has a fine hair tied to its upper
+extremity and bent against the interior of the tube, which acts as a
+spring with sufficient elasticity to keep the index supported in the
+spirit in opposition to gravity.</p>
+
+<p>The instrument acts as follows:&mdash;A rise of temperature causes the spirit
+in the long bulb to expand and press some of the mercury into the other
+limb of the syphon, into which it rises also from its own expansion, and
+carries the index with it, until the greatest temperature is attained. The
+lower end of this index then indicates upon the engraved scale the maximum
+temperature. As the temperature falls the spirit and the mercury contract,
+and in returning towards the bulb the second index is met and carried up
+by the mercury until the lowest temperature occurs, when it is left to
+indicate upon the scale the minimum temperature. The limb of the syphon
+adjoining the bulb requires, therefore, a descending scale of<span class="pagenum"><a name="Page_83" id="Page_83">[Pg 83]</a></span>
+thermometric degrees; the other limb, an ascending scale. The graduations
+must be obtained by comparisons with a standard thermometer under
+artificial temperatures, which should be done in this way for every 5&deg;, in
+order to correct for the inequality in the bore of the tube, and the
+irregular expansion of the spirit. The instrument is set for observation
+by bringing the indices into contact with the mercury, by means of a small
+magnet, which attracts the steel through the glass, so that it is readily
+drawn up or down. They should be drawn nearly to the top of the limbs when
+it is desired to remove the instrument, which should be carefully carried
+in the vertical position; for should it be inverted, or laid flat, the
+spirit may get among the mercury, and so break up the column as to require
+the skill of a maker to put it in order again. For transmission by
+ordinary conveyances, it requires that attention be given to keep it
+vertical. The entanglement of a small portion of mercury with the indices
+is sometimes a source of annoyance in this instrument, for the readings
+are thereby rendered somewhat incorrect. Small breakages in the mercury,
+either from intervening bubbles of spirit or adhesion to the indices, may
+generally be rectified by cautiously tapping the frame of the instrument,
+so as to cause the mercury to unite by the assistance thus given to its
+superior gravity.</p>
+
+<p>These thermometers, when carefully made and adjusted to a standard
+thermometer, are strongly recommended for ordinary purposes, where strict
+scientific accuracy is not required. This is also the only fluid
+thermometer applicable for determining the temperature of the sea at
+depths.</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_84" id="Page_84">[Pg 84]</a></span></p>
+<h2><a name="CHAPTER_VIII" id="CHAPTER_VIII"></a>CHAPTER VIII.</h2>
+<p class="center"><span class="large">RADIATION THERMOMETERS.</span></p>
+
+<p><br /><b>82. Solar and Terrestrial Radiation considered.</b>&mdash;The surface of the earth
+absorbs the heat of the sun during the day, and radiates heat into space
+during the night. The envelope of gases and vapour, which we call the
+atmosphere, exerts highly important functions upon these processes. Thanks
+to the researches of Professor Tyndall, we are now enabled to understand
+these functions much more clearly than heretofore. His elaborate, patient,
+and remarkably sagacious series of experiments upon radiant heat, have
+satisfactorily demonstrated that <i>dry</i> air is as transparent to radiant
+heat as the vacuum itself; while air <i>perfectly saturated</i> with aqueous
+vapour absorbs more than five per cent. of radiant heat, estimated by the
+thermal unit adopted for the galvanometer indications of the effect upon a
+thermo-electric pile.</p>
+
+<p>Aqueous vapour, in the form of fog or mist, as is well known, gives to our
+sensation a feeling of cold, and interferes with the healthy action of the
+skin and the lungs; the cause being its property of absorbing heat from
+our person.</p>
+
+<p>Air containing moisture in an invisible state likewise exerts a remarkable
+influence in radiating and absorbing heat. By reason of these properties,
+aqueous vapour acts as a kind of blanket upon the ground, and maintains
+upon it a higher temperature than it would otherwise have. &#8220;Regarding the
+earth as a source of heat, no doubt at least ten per cent. of its heat is
+intercepted within ten feet of the surface.&#8221; Thus vapour&mdash;whether
+transparent and invisible, or visible, as cloud, fog, or mist&mdash;is
+intimately connected with the important operations of solar and
+terrestrial radiation. Cloudy, or humid days, diminish the effect upon the
+soil of solar radiation; similar nights retard the radiation from the
+earth. A dry atmosphere is the most favourable for the direct transmission
+of the sun&#8217;s rays; and the withdrawal of the sun from any region over
+which the air is dry, must be followed by very rapid cooling of the soil.
+&#8220;The removal, for a single summer night, of the aqueous vapour from the
+atmosphere which covers England, would be attended by the destruction of
+every plant which a freezing temperature could kill. In Sahara, where &#8216;the
+soil is fire and the wind is flame,&#8217; the refrigeration at night is often
+painful to bear. Ice has been formed in this region at night. In
+Australia, also, the <i>diurnal range</i> of temperature is very great,
+amounting, commonly, to between 40 and 50 degrees. In short, it may be
+safely predicted, that wherever the air is <i>dry</i>, the daily thermometric
+range will be great. This, however, is quite different from saying that
+when the air is <i>clear</i>, the thermometric range will be great. Great
+clearness to light is perfectly compatible with great opacity to heat; the
+atmosphere may be charged with aqueous vapour while a deep blue sky is
+overhead; and on such occasions the terrestrial<span class="pagenum"><a name="Page_85" id="Page_85">[Pg 85]</a></span> radiation would,
+notwithstanding the &#8216;clearness,&#8217; be intercepted.&#8221; The great range of the
+thermometer is attributable to the absence of that protection against gain
+or loss of heat which is afforded when aqueous vapour is present in the
+air; and during such weather the rapid abstraction of moisture from the
+surface of plants and animals is very deleterious to their healthy
+condition. &#8220;The nipping of tender plants by frost, even when the air of
+the garden is some degrees above the freezing temperature, is also to be
+referred to chilling by radiation.&#8221; Hence the practice of gardeners of
+spreading thin mats, of bad radiating material, over tender plants, is
+often attended with great benefit.</p>
+
+<p>By means of the process of terrestrial radiation ice is artificially
+formed in Bengal, &#8220;where the substance is never formed naturally. Shallow
+pits are dug, which are partially filled with straw, and on the straw flat
+pans containing water which had been boiled is exposed to the clear
+firmament. The water is a very powerful radiant, and sends off its heat
+into space. The heat thus lost cannot be supplied from the earth&mdash;this
+source being cut off by the non-conducting straw. Before sunrise a cake of
+ice is formed in each vessel.... To produce the ice in abundance, the
+atmosphere must not only be clear, but it must be comparatively free from
+aqueous vapour.&#8221;</p>
+
+<p>Considering, therefore, the important consequences attending both
+terrestrial and solar radiation, it appears to us that observations from
+radiation thermometers are of much more utility in judging of climate than
+is usually supposed. These observations are very scanty; and what few are
+upon record are not very reliable, principally from bad exposure of the
+instruments, while the want of uniformity in construction may be another
+cause. Herschell&#8217;s actinometer and Pouillet&#8217;s pyrheliometer, instruments
+for ascertaining the absolute heating effect of the sun&#8217;s rays, should,
+however, be more generally employed by meteorologists. In comparing
+observations on radiation it should be kept in mind, that &#8220;the difference
+between a thermometer which, properly confined [or shaded], gives the true
+temperature of the night air, and one which is permitted to radiate freely
+towards space, must be greater at high elevations than at low ones;&#8221;<small><a name="f6.1" id="f6.1" href="#f6">[6]</a></small>
+because the higher the place, the less the thickness of the vapour-screen
+to intercept the radiation.</p>
+
+
+<p><br /><b>83. Solar Radiation Thermometer.</b>&mdash;&#8220;As the interchange of heat between two
+bodies by radiation depends upon the relative temperature which they
+respectively possess, the earth, by the rays transmitted from the sun
+during the day, must be continually gaining an accession of heat, which
+would be far from being counterbalanced by the opposite effect of its own
+radiation into space. Hence, from sunrise till two or three hours after
+mid-day, the earth goes on gradually increasing in temperature, the
+augmentation being greatest where the surface consists of materials
+calculated, from their colour and texture, to absorb heat, and where it is
+deficient in<span class="pagenum"><a name="Page_86" id="Page_86">[Pg 86]</a></span> moisture, which, by its evaporation, would have a tendency
+to diminish it.&#8221;<small><a name="f7.1" id="f7.1" href="#f7">[7]</a></small> It is, therefore, important to have instruments for
+measuring the efficacy of solar radiation, apart from those for exhibiting
+the temperature of the place in the shade.</p>
+
+<p>&nbsp;</p>
+<p class="center">Fig. 63.</p>
+<div class="figcenter"><img src="images/fig_63.jpg" alt="" /></div>
+<p>&nbsp;</p>
+
+<p>Fig. 63 shows the arrangement of Negretti &amp; Zambra&#8217;s maximum thermometer,
+for registering the greatest heat of the sun&#8217;s direct rays, hence called a
+<i>solar radiation thermometer</i>. It has a blackened bulb, the scale divided
+on its own stem, and the divisions protected by a glass shield. In use it
+should be placed nearly horizontally, resting on Y supports of wood or
+metal, with its bulb in the full rays of the sun, resting on grass, and,
+if possible, so that lateral winds should not strike the bulb; and at a
+sufficient distance from any wall, so that it does not receive any
+<i>reflected</i> heat from the sun. Some observers place the thermometer as
+much as two feet from the ground. It would be very desirable if one
+uniform plan could be recognized: that of placing the instrument as
+indicated in the figure appears to be most generally adopted, and the
+least objectionable.</p>
+
+
+<p><br /><b>84. Vacuum Solar Radiation Thermometer.</b>&mdash;In order that the heat absorbed
+by the blackened bulb of the solar radiation thermometer may not in part
+be carried off by the currents of air which would come into contact with
+it, the instrument has been improved by Messrs. Negretti and Zambra into
+the <i>vacuum solar radiation thermometer</i>, as illustrated by fig. 64.</p>
+
+<p>&nbsp;</p>
+<p class="center">Fig. 64.</p>
+<div class="figcenter"><img src="images/fig_64.jpg" alt="" /></div>
+<p>&nbsp;</p>
+
+<p>This consists of a blackened-bulb radiation thermometer, enclosed in a
+glass tube and globe, from which all air is exhausted. Thus protected from
+the loss of heat which would ensue if the bulb were exposed, its
+indications are from 20&deg; to 30&deg; higher than when placed side by side with
+a similar instrument with the bulb exposed to the passing air. At times
+when the air has been in rapid motion, the difference between the reading
+of a thermometer giving the true temperature of the<span class="pagenum"><a name="Page_87" id="Page_87">[Pg 87]</a></span> air in the shade, and
+an ordinary solar radiation thermometer, has been 20&deg; only, whilst the
+difference between the air temperature and the reading of a radiation
+thermometer in vacuo has been as large as 50&deg;. It is also found that the
+readings are almost identical at distances from the earth varying from six
+inches to eighteen inches. By the use of this improvement, it is hoped
+that the amounts of solar radiation at different places may be rendered
+comparable; hitherto they have not been so; the results found at different
+places cannot be compared, as the bulbs of the thermometers are under very
+different circumstances as to exposure and currents of air. Important
+results are anticipated from this arrangement. The observations at
+different places are expected to present more agreement. Observers would
+do well to note carefully the effect of any remarkable degree of intensity
+in the solar heat upon particular plants, crops, fruit or other trees.</p>
+
+
+<p><br /><b>85. Terrestrial Radiation Thermometer</b> is an alcohol minimum thermometer,
+with the graduations etched upon the stem, and protected by a glass
+shield, as shown in figure 65, instead of being mounted on a frame. The
+bulb is transparent; that is to say, the spirit is not coloured.</p>
+
+<p>&nbsp;</p>
+<p class="center">Fig. 65.</p>
+<div class="figcenter"><img src="images/fig_65.jpg" alt="" /></div>
+<p>&nbsp;</p>
+
+<p>In use, it should be placed with its bulb fully exposed to the sky,
+resting on grass, the stem being supported by little forks of wood. The
+precautions required with this thermometer are similar to those for
+ordinary spirit thermometers, explained at <a href="#Page_76">page 76</a>.</p>
+
+<div class="figright">Fig. 66.<br /><img src="images/fig_66.jpg" alt="" /></div>
+
+<p><br /><b>86. &AElig;thrioscope.</b>&mdash;The celebrated experimental philosopher, Sir John
+Leslie, was the inventor of this instrument, the purpose of which is to
+give a comparative idea of the radiation proceeding from the surface of
+the earth towards the sky. It consists, as represented in fig. 66, of two
+glass bulbs united by a vertical glass tube, of so fine a bore that a
+little coloured liquid is supported in it by its own adhesion, there being
+air confined in each of the bulbs. The bulb, <i>A</i>, is enclosed in a highly
+polished brass sphere, <i>D</i>, made in halves and screwed together. The bulb,
+<i>B</i>, is blackened and placed in the centre of a metallic cup, <i>C</i>, which
+is well gilt on the inside, and which may be covered by a top, <i>F</i>. The
+brass coverings defend both bulbs from solar radiation, or any
+adventitious source of heat.<span class="pagenum"><a name="Page_88" id="Page_88">[Pg 88]</a></span> When the top is on, the liquid remains at
+zero of the scale. On removing the top and presenting the instrument to a
+clear sky, either by night or by day, the bulb, <i>B</i>, is cooled by
+terrestrial radiation, while the bulb, <i>A</i>, retains the temperature of the
+air. The air confined in <i>B</i>, therefore, contracts; and the elasticity of
+that within <i>A</i> forces the liquid up the tube, to a height proportionate
+to the intensity of the radiation. Such is the sensitiveness of the
+instrument, that the smallest cloud passing over it checks the rise of the
+liquid. Sir John Leslie says:&mdash;&#8220;Under a clear blue sky, the <i>&aelig;thrioscope</i>
+will sometimes indicate a cold of fifty millesimal degrees; yet, on other
+days, <i>when the air seems equally bright</i>, the effect is hardly 30&deg;.&#8221; This
+anomaly, according to Dr. Tyndall, is simply due to the difference in the
+quantity of aqueous vapour present in the atmosphere. The presence of
+invisible vapour intercepts the radiation from the &aelig;thrioscope, while its
+absence opens a door for the escape of this radiation into space.</p>
+
+<div class="figleft">Fig. 67.<br /><img src="images/fig_67.jpg" alt="" /></div>
+
+<p><br /><b>87. Pouillet&#8217;s Pyrheliometer.</b>&mdash;&#8220;This instrument is composed of a shallow
+cylinder of steel, <i>A</i>, fig. 67, which is filled with mercury. Into the
+cylinder a thermometer, <i>D</i>, is introduced, the stem of which is protected
+by a piece of brass tubing. We thus obtain the temperature of the mercury.
+The flat end of the cylinder is to be turned towards the sun, and the
+surface, <i>B</i>, thus presented is coated with lamp black. There is a collar
+and screw, <i>C</i>, by means of which the instrument may be attached to a
+stake driven into the ground, or into the snow, if the observations are
+made at considerable heights. It is necessary that the surface which
+receives the sun&#8217;s rays should be perpendicular to the rays; and this is
+secured by appending to the brass tube which shields the stem of the
+thermometer, a disk, <i>E</i>, of precisely the same diameter as the steel
+cylinder. When the shadow of the cylinder accurately covers the disk, we
+are sure that the rays fall, as perpendiculars, on the upturned surface of
+the cylinder.</p>
+
+<div class="figright">Fig. 68.<br /><img src="images/fig_68.jpg" alt="" /></div>
+
+<p>&#8220;The observations are made in the following manner:&mdash;First, the instrument
+is permitted, not to receive the sun&#8217;s rays, but to radiate its own heat
+for five minutes against an unclouded part of the firmament; the decrease
+of the temperature of the mercury consequent on this radiation is then
+noted. Next, the instrument is turned towards the sun, so that the solar
+rays fall perpendicularly upon it for five minutes; the augmentation of
+heat is now noted. Finally, the instrument is turned again towards the
+firmament, away from the sun, and allowed to radiate for another five
+minutes, the sinking of the thermometer being noted as before. In order to
+obtain the whole heating power<span class="pagenum"><a name="Page_89" id="Page_89">[Pg 89]</a></span> of the sun, we must add to his observed
+heating power the quantity lost during the time of exposure, and this
+quantity is the mean of the first and last observations. Supposing the
+letter <i>R</i> to represent the augmentation of temperature by five minutes&#8217;
+exposure to the sun, and that <i>t</i> and <i>t&sup1;</i> represent the reductions of
+temperature observed before and after, then the whole force of the sun,
+which we may call <i>T</i>, would be thus expressed:&mdash;<i>T = R + &#189;(t + t&sup1;)</i>.</p>
+
+
+<p>&#8220;The surface on which the sun&#8217;s rays here fall is known; the quantity of
+mercury within the cylinder is also known; hence we can express the effect
+of the sun&#8217;s heat upon a given area, by stating that it is competent, in
+five minutes, to raise so much mercury so many degrees in
+temperature.&#8221;&mdash;<i>Dr. Tyndall&#8217;s &#8220;Heat considered as a Mode of Motion.&#8221;</i></p>
+
+
+<p><br /><b>88. Sir John Herschell&#8217;s Actinometer</b>, for ascertaining the absolute
+heating effect of the solar rays, in which <i>time</i> is considered one of the
+elements of observation, is illustrated by fig. 68. The actinometer
+consists of a large cylindrical thermometer bulb, with a scale
+considerably lengthened, so that minute changes may be easily seen. The
+bulb is of transparent glass filled with a deep blue liquid, which is
+expanded when the rays of the sun fall direct on the bulb. To take an
+observation, the actinometer is placed in the shade for one minute and
+read off; it is then exposed for one minute to sunshine, and its
+indication recorded; it is finally restored to the shade, and its reading
+noted. The mean of the two readings in the shade, subtracted from that in
+the sun, gives the actual amount of expansion of the liquid produced by
+the sun&#8217;s rays in one minute of time. For further information, see <i>Report
+of the Royal Society on Physics and Meteorology</i>; or <i>K&aelig;mtz&#8217;s
+Meteorology</i>, translated by C. V. Walker; or the <i>Admiralty Manual of
+Scientific Instructions</i>.</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_90" id="Page_90">[Pg 90]</a></span></p>
+<h2><a name="CHAPTER_IX" id="CHAPTER_IX"></a>CHAPTER IX.</h2>
+<p class="center"><span class="large">DEEP-SEA THERMOMETERS.</span></p>
+
+<p><br /><b>89. On Sixe&#8217;s Principle.</b>&mdash;Thermometers for ascertaining the temperature of
+the sea at various depths are constructed to register either the maximum
+or minimum temperature, or both. The principle of each instrument is that
+of Sixe. There are very few parts of the ocean in which the temperature
+below is greater than at the surface, except in the Polar Seas, where it
+is generally found to be a few degrees warmer at considerable depths than
+at the surface. When the instrument is required to register only one
+temperature, it can be made narrower and more compact&mdash;a great advantage
+in sounding; and with less length of bulb and glass tube, so that the
+liability of error is diminished. Hence, the minimum is the most generally
+useful for deep-sea soundings. These thermometers must be sufficiently
+strong to withstand the pressure of the ocean at two or three miles of
+depth, where there may be a force exerted to compress them exceeding three
+or four hundred atmospheres (of 15 lbs. to the square inch).</p>
+
+<p>Many have been the contrivances for obtaining correct deep-sea
+indications. Thermometers and machines of various sorts have been
+suggested, adopted, and eventually abandoned as only approximate
+instruments. The principal reason for such instruments failing to give
+correct or reliable indications, has been that the weight or pressure on
+the bulbs at great depths has interfered with the correct reading of the
+instruments. Thermometers have been enclosed in strong water-tight cases
+to resist the pressure; but this contrivance has only had the tendency to
+retard the action, so much so as to throw a doubt on the indications
+obtained by the instrument so constructed.</p>
+
+<p>The thermometers constructed by Messrs. Negretti and Zambra for this
+purpose do not differ materially from those usually made under the
+denomination of Sixe&#8217;s thermometers, except in the following most
+important particular:&mdash;The usual Sixe&#8217;s thermometers have a central
+reservoir or cylinder containing alcohol; this reservoir, which is the
+only portion of the instrument likely to be affected by pressure, has
+been, in Negretti and Zambra&#8217;s new instrument, superseded by a strong
+outer cylinder of glass, containing mercury and rarefied air; by this
+means the portion of the instrument susceptible of compression, has been
+so strengthened that no amount of pressure can possibly make the
+instrument vary. This instrument has been tested in every possible manner,
+and the results have been highly satisfactory, so much so as to place
+their reliability beyond any possible doubt.</p>
+
+<div class="figright">Fig. 69.<br /><img src="images/fig_69.jpg" alt="" /></div>
+
+<p><span class="pagenum"><a name="Page_91" id="Page_91">[Pg 91]</a></span>The scales are made of porcelain, and are firmly secured to a back of oak,
+which holds in a recess the bulb with its protecting shield, and is
+rounded off so as to fit easily and firmly in a stout cylindrical copper
+case, in which the thermometer is sent down when sounding (see fig. 69).
+The lid of the case is made to fit down closely, and water-tight. At the
+bottom of the case is a valve opening upward; and the lid has a similar
+valve. These allow the water to pass through the case as the instrument
+sinks, so that the least amount of obstruction is offered to the descent.
+At the lower end of the case is a stout brass spring, to protect the
+instrument from a sudden jar if it should touch the bottom while
+descending rapidly. As the instrument is drawn up, the valves close with
+the weight of water upon them, and it arrives at the surface filled with
+water brought up from its lowest position. The deep-sea thermometers used
+in the Royal Navy are of this pattern.</p>
+
+
+<p><br /><b>90. Johnson&#8217;s Metallic Deep-Sea Thermometer.</b>&mdash;The objection to the
+employment of mercurial thermometers for ascertaining the temperature of
+the ocean at depths, arising from the compression of the bulbs, which was
+of such serious consequence previous to the modification made in the
+construction of the instrument by Messrs. Negretti and Zambra, led to the
+construction of a metallic thermometer altogether free from liability of
+disturbance from compression by the surrounding water; which, however, is
+certainly not so sensitive to changes of temperature as mercury. This
+instrument is the invention of Henry Johnson, Esq., F.R.A.S., and is thus
+described by him:&mdash;</p>
+
+<p>&#8220;During the year 1844 some experiments were made by James Glaisher, Esq.,
+F.R.S., on the temperature of the water of the Thames near Greenwich at
+the different seasons of the year; when that gentleman found that the
+indications of temperature were greatly affected by the pressure on the
+bulbs of the thermometers. At a depth of 25 feet this pressure would be
+nearly equal to the presence of three-fourths of an atmosphere. These
+observations demonstrate the importance of using in deep-sea soundings an
+instrument free from liability of disturbance from compression by the
+surrounding water, and have ultimately led to the construction of the
+thermometer now to be described.</p>
+
+<p>&#8220;The instrument is composed of solid metals of considerable specific
+gravity, viz. of brass and steel, the specific gravity of these metals
+being 8&middot;39 and 7&middot;81 respectively. They are therefore not liable to
+compression by the water, which under a pressure of 1,120 atmospheres, or
+at a depth of 5,000 fathoms in round numbers, acquires a density or
+specific gravity of 1&middot;06. In the construction of this instrument,
+advantage has been taken of the well-known difference in the ratios of
+expansion and contraction by heat and cold of brass and steel, to form
+compound bars of thin bars of these metals riveted together; and which
+will be found to assume<span class="pagenum"><a name="Page_92" id="Page_92">[Pg 92]</a></span> a slight curve in one direction when heat has
+expanded the brass more than the steel, and a slight one in the contrary
+direction when cold has contracted the brass more than the steel.</p>
+
+<div class="figleft">Fig. 70.<br /><img src="images/fig_70.jpg" alt="" /></div>
+
+<p>&#8220;The indications of the instrument record the motions under changes of
+temperature of such compound bars; in which the proportion of brass, the
+more dilatable metal, is two-thirds, and of steel one-third.</p>
+
+<p>&#8220;Upon one end of a narrow plate of metal about a foot long, <i>a</i>, are fixed
+three scales of temperature, <i>h</i>, which ascend from 25&deg; to 100&deg; F., and
+which are shown more clearly in the drawing detached from the instrument.
+Upon one of these scales the present temperature is shown by the pointer,
+<i>e</i>, which turns upon a pivot in its centre. The register index, <i>g</i>, to
+the maximum temperature, and the index, <i>f</i>, to the minimum temperature,
+are moved along the other scales by the pin upon the moving pointer, at
+<i>e</i>, where they are retained by stiff friction. At equal distances from
+the centre of the pointer are two connecting pieces, <i>d d</i>, by which it is
+attached to the free ends of two compound bars, <i>b b</i>, and its movements
+correspond with the movements of the compound bars under variations of
+temperature. The other ends of the bars are fastened by the plate, <i>c</i>, to
+the plate, <i>a</i>, on which the scales of temperature are fixed. The
+connection of the bars with both sides of the centre of the pointer
+prevents disturbance of indication by lateral concussion. The case of the
+instrument has been improved at the suggestion of Admiral FitzRoy, and now
+presents to the water a smooth cylindrical surface, with rounded ends, and
+without projection of fastenings.</p>
+
+<p>&#8220;In surveying expeditions, this instrument would be found useful in giving
+notice of variation of depth of water, and of the necessity for taking
+soundings. A diminution of the temperature of water has been observed by
+scientific voyagers to accompany diminution of depth, as on nearing land,
+or approaching hidden rocks or shoals. Attention would also thus be
+attracted to the vicinity of icebergs.&#8221;</p>
+
+<div class="figright">Fig. 71.<br /><img src="images/fig_71.jpg" alt="" /></div>
+
+<p>This thermometer might easily be modified to serve for several other
+important purposes, such as the determination of the temperature of
+intermittent hot springs, and mud volcanoes.</p>
+
+<p>The principle of this thermometer is not altogether new; but the duplicate
+arrangement of the bars, which effectually prevents the movement of the
+indices by any shaking, and the application are certainly novel. Professor
+Trail, in the <i>Library of Useful Knowledge</i>, writes:&mdash;&#8220;In 1803, Mr. James
+Crighton, of Glasgow, published a new<span class="pagenum"><a name="Page_93" id="Page_93">[Pg 93]</a></span> &#8216;metallic thermometer,&#8217; in which
+the unequal expansion of zinc and iron is the moving power. A bar is
+formed by uniting a plate of zinc (fig. 71), <i>c d</i>, 8 inches long, 1 inch
+broad, and &#188; inch thick, to a plate of iron, <i>a b</i>, of the same length.
+The lower extremity of the compound bar is firmly attached to a mahogany
+board at <i>e e</i>; a pin, <i>f</i>, fixed to its upper end, plays in the forked
+opening in the short arm of the index, <i>g</i>. When the temperature is
+raised, the superior expansion of the zinc, <i>c d</i>, will bend the whole
+bar, as in the figure; and the index, <i>g</i>, will move along the graduated
+arc, from right to left, in proportion to the temperature. In order to
+convert it into a <i>register thermometer</i>, Crighton applied two slender
+hands, <i>h h</i>, on the axis of the index; these lie below the index, and are
+pushed in opposite directions by the stud, <i>i</i>,&mdash;a contrivance seemingly
+borrowed from the instrument of Fitzgerald,&#8221; a complicated metallic
+thermometer, described by the Professor previously.</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_94" id="Page_94">[Pg 94]</a></span></p>
+<h2><a name="CHAPTER_X" id="CHAPTER_X"></a>CHAPTER X.</h2>
+<p class="center"><span class="large">BOILING-POINT THERMOMETERS.</span></p>
+
+<p><br /><b>91. Ebullition.</b>&mdash;The temperature at which a fluid <i>boils</i> is called the
+<i>boiling-point</i> of that particular fluid. It is different for different
+liquids; and, moreover, in the same liquid it varies with certain changes
+of circumstance. Thus the same liquid in various states of purity would
+have its boiling temperature altered in a slight degree. There is also an
+intimate connection with the pressure under which a fluid is boiled, and
+its temperature of ebullition. Liquids boiled in the open air are
+subjected to the atmospheric pressure, which is well known to vary at
+different times and places; and the boiling-point of the liquid exhibits
+corresponding changes. When the pressure is increased on the surface of
+any fluid, the temperature of ebullition rises; and with a decrease of
+pressure, the boiling goes on at a lower degree of heat.</p>
+
+<p>In the case of water, we commonly state the boiling-point to be 212&deg; F.;
+but it is only so at the level of the sea, under the mean pressure of the
+atmosphere, represented, in the latitude of London, by a column of 29&middot;905
+inches of mercury, at a temperature of 32&deg; F., and when the water is fresh
+and does not contain any matter chemically dissolved in it. When steam is
+generated and confined in a boiler, the pressure upon the boiling water
+may be several times greater than that of the atmosphere. Experimentally
+it has been found, that if the pressure in the boiler be 25 lbs. on the
+square inch, the temperature of the boiling water, and of the steam
+likewise, is raised to 241&deg;; while under the exhausted receiver of an
+air-pump, water will boil at 185&deg;, when the pressure is reduced to 17
+inches of mercury.</p>
+
+
+<p><br /><b>92. Relation between the Boiling-Point and Elevation.</b>&mdash;Now, as the
+atmospheric pressure is diminished by ascent, as shown by the fall of
+mercury in the barometer, it follows that in elevated localities water, or
+any other fluid, heated in the open air, will boil at a temperature lower
+than at the sea-level. Therefore, there must be some relation between the
+height of a hill, or mountain, and the temperature at which a fluid will
+boil at that height. Hence, the thermometer, as used to determine the
+boiling-point of fluids, is also an indicator of the atmospheric pressure;
+and may be used as a substitute for the barometer in measuring elevations.</p>
+
+<p>If the atmospheric pressure were constant at the sea-level, and always the
+same for definite heights, we might expect the boiling-points of fluids
+also to be in exact accordance with height; and the relation once
+ascertained, we could readily, by means of the thermometer and boiling
+water, determine an unknown height, or for a known elevation assert the
+boiling temperature of a liquid. However, as the atmospheric pressure is
+perpetually varying at the same place, within certain limits, so there
+are, as it were, sympathetic changes in the boiling temperatures of
+fluids.<span class="pagenum"><a name="Page_95" id="Page_95">[Pg 95]</a></span> It follows from this, that heights can never be accurately
+measured, either by the barometer or the boiling-point thermometer, by
+simply observing at the places whose elevations are required. To determine
+a height with any approach to accuracy, it is necessary that a similar
+observation should be made at the same time at a lower station, not very
+remote laterally from the upper, and that they should be many times
+repeated. When such observations have been very carefully conducted, the
+height of the upper station above the lower may be ascertained with great
+precision, as has been repeatedly verified by subsequent trigonometrical
+measurement of elevations so determined. If the lower station be at the
+sea-level, of course the absolute height of the upper is at once obtained.</p>
+
+
+<p><br /><b>93. Mountain Thermometer; sometimes called Hypsometric Apparatus.</b>&mdash;We have
+now to examine the construction of the boiling-point thermometer, and its
+necessary appendages, as adapted for the determination of heights.</p>
+
+<div class="figleft">Fig. 72.<br /><img src="images/fig_72.jpg" alt="" /></div>
+
+<div class="figright">Fig. 73.<br /><img src="images/fig_73.jpg" alt="" /></div>
+
+<p>Messrs. Negretti and Zambra&#8217;s arrangement of the instrument is shown in
+figures 72 and 73.</p>
+
+<p>The thermometer is made with an elongated bulb, so as to be as sensitive
+as possible. The scale, about a foot long, is graduated on the stem, and
+ranges from 180&deg; to 214&deg;, each degree being sufficiently large to show the
+divisions of tenths of a degree. A sliding metallic vernier might perhaps
+with advantage be attached to the stem, which would enable the observer to
+mark hundredths of a degree; which, however, he can pretty well do by
+estimation. The boiler is so contrived as to allow, not only the bulb, but
+the stem also of the thermometer, to be surrounded by the steam. The
+arrangement is readily understood by reference to the accompanying
+diagram, fig. 73.</p>
+
+<p><i>C</i>, is a copper boiler, supported by a tripod stand so as to allow a
+spirit-lamp, <i>A</i>, made of metal to be placed underneath. The flame from
+the lamp may be surrounded by a fine wire gauze, <i>B</i>, which will prevent
+it being extinguished when experimenting in the external air. <i>E E E</i>, is
+a three-drawn telescope tube, proceeding from the boiler, and open also at
+top. Another tube, similarly constructed, envelops this, as shown by <i>D D
+D</i>. This tube is screwed to the top of the boiler, and has two openings,
+one at the top to admit the thermometer, the other low<span class="pagenum"><a name="Page_96" id="Page_96">[Pg 96]</a></span> down, <i>G</i>, to give
+vent to the steam. As the steam is generated, it rises in the inner tube,
+passes down between the tubes, and flows away at <i>G</i>. The thermometer is
+passed down, supported by an india-rubber washer, fitting steam tight, so
+as to leave the top of the mercury, when the boiling-point is attained,
+sufficiently visible to make the observation. The telescopic movement, and
+the mode of supporting the thermometer, enable the observer always to keep
+the bulb near the water, and the double tube gives all the protection
+required to obtain a steady boiling-point. Some boiling-point thermometers
+are constructed with their scales altogether exposed to the air, which may
+be very cold, and consequently may contract to some extent the thread of
+mercury outside the boiler. The steam, having the same temperature as the
+boiling water, keeps the tube, throughout nearly its whole length, at the
+same degree of heat, in the apparatus described. The whole can be packed
+in a tin case very compactly and securely for travelling, as in fig. 72.</p>
+
+<p><i>Directions for Using.</i>&mdash;When the apparatus is required for practical use,
+sufficient water must be poured into the boiler to fill it about one
+third, through an opening, <i>F</i>, which must be afterwards closed by the
+screw plug. Then apply the lighted lamp. In a short time steam will issue
+from <i>G</i>; and the mercury in the thermometer, kept carefully immersed,
+will rise rapidly until it attains a stationary point, which is the
+boiling temperature. The observation should now be taken and recorded with
+as much accuracy as possible, and the temperature of the external air must
+be noted at the same time by an ordinary thermometer.</p>
+
+<p>The water employed should be pure. Distilled water would therefore be the
+best. If a substance is held mechanically suspended in water, it will not
+affect the boiling-point. Thus, muddy water would serve equally as well as
+distilled water. However, as it cannot be readily ascertained that nothing
+is dissolved chemically when water is dirty, we are only correct when we
+employ pure water.</p>
+
+
+<p><br /><b>94. Precautions to ensure correct Graduation.</b>&mdash;Those who possess a
+boiling-point thermometer should satisfy themselves that it has been
+correctly graduated. To do this, it is advisable to verify it with the
+reading of a standard barometer reduced to 32&deg; F. The table of &#8220;Vapour
+Tension&#8221; (given at <a href="#Page_62">p. 62</a>) will furnish the means of comparison. Thus, if
+the reduced reading of the barometer, corrected also for latitude, be
+29&middot;922, the thermometer should show 212&deg; as the boiling-point of water at
+the same time and place; if 29&middot;745, the thermometer should read 211&middot;7; and
+so on as per table. In this way the error of the chief point of the scale
+can be obtained. Other parts of the scale may be checked with a standard
+thermometer, by subjecting both to the same temperature, and comparing
+their indications. The graduations as fixed by some makers are not always
+to be trusted; and this essential test should be conducted with the utmost
+nicety and care.</p>
+
+<p>Admiral FitzRoy writes, in his <i>Notes on Meteorology</i>:&mdash;&#8220;Each degree of
+the boiling-point thermometer is equivalent to about 550 <i>feet of ascent</i>,
+or one-tenth to<span class="pagenum"><a name="Page_97" id="Page_97">[Pg 97]</a></span> 55 feet; therefore, the smallest error in the graduation
+of the thermometer itself will affect the height deduced materially.</p>
+
+<p>&#8220;In the thermometer which is graduated from 212&deg; (the boiling-point) to
+180&deg;, similarly to those intended for the purpose of measuring heights,
+there must have been a starting point, or zero, from which to begin the
+graduation. I have asked an optician in London how he fixed that zero, the
+boiling-point. &#8216;By boiling water at my house,&#8217; he replied. &#8216;Where is your
+house?&#8217; In such a part of the town, he answered. I said: &#8216;What height is
+it above the sea?&#8217; to which he replied, &#8216;I do not know;&#8217; and when I asked
+the state of the barometer when he boiled the water, whether the mercury
+was high or low, he said that he had not looked at it! Now, as this
+instrument is intended to measure heights and to decide differences of
+some hundred, if not thousand feet upwards, at least one should endeavour
+to ascertain a reliable starting point. From inquiries made, I believe
+that the determination of the boiling-point of ordinary thermometers has
+been very vague, not only from the extreme difficulties of the process
+itself (which are well known to opticians), but from the radical errors of
+not allowing for the pressure of the atmosphere at the time of
+graduation&mdash;which may be much, even an inch higher or lower, than the
+mean, or any <i>given height</i>&mdash;while the elevation of the place above the
+level of the sea is also unnoticed. Then there is another source of error,
+a minor one, perhaps: the inner limit, the 180&deg; point, is fixed only by
+comparison with another thermometer; it may be right, or it may be very
+much out, as may be the intermediate divisions; for the difficulty of
+ascertaining degree by degree is great: and it must be remembered that the
+measurement of a very high mountain depends upon those inner degrees from
+200&deg; down to 180&deg;, thereabouts. Hence, the difficulty of making a reliable
+observation by boiling water seems to be greater than has been generally
+admitted.&#8221;</p>
+
+
+<p><br /><b>95. Method of Calculating Heights from Observations with the Mountain
+Thermometer.</b>&mdash;Having considered how to make observations with the proper
+care and accuracy, it becomes necessary to know how to deduce the height
+by calculation. That a constant intimate relation exists between the
+boiling temperature of water and the pressure of the air, we have already
+learned. This knowledge is the result of elaborate experiments made by
+several scientific experimentalists, who have likewise constructed formul&aelig;
+and tables for the conversion of the boiling temperatures into the
+corresponding pressures of vapour, or, which is equivalent, of the
+atmosphere, when the operation is performed in the open air. As might be
+expected, there is not a perfect accord in the results arrived at by
+different persons. Regnault is the most recent, and his experiments are
+considered the most reliable.</p>
+
+<p>From Regnault&#8217;s table of vapour tension, we can obtain the pressure in
+inches of mercury at 32&deg;, which corresponds to the observed boiling-point;
+or <i>vice versa</i>, if required. From the pressure, the height may be deduced
+by the method for finding heights by means of the barometer.</p>
+
+<p><span class="pagenum"><a name="Page_98" id="Page_98">[Pg 98]</a></span>The following table expresses very nearly the elevation in feet
+corresponding to a fall of 1&deg; in the temperature of boiling water:&mdash;</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td align="center">Boiling Temperatures<br />between.</td><td><span class="spacer">&nbsp;</span></td>
+    <td align="center">Elevation in Feet<br />for each Degree.</td></tr>
+<tr><td><span style="margin-left: 1em;">214&deg; and 210&mdash;</span></td><td>&nbsp;</td><td align="center">520</td></tr>
+<tr><td><span style="margin-left: 1em;">210 &nbsp; and 200&mdash;</span></td><td>&nbsp;</td><td align="center">530</td></tr>
+<tr><td><span style="margin-left: 1em;">200 &nbsp; and 190</span></td><td>&nbsp;</td><td align="center">550</td></tr>
+<tr><td><span style="margin-left: 1em;">190 &nbsp; and 180</span></td><td>&nbsp;</td><td align="center">570</td></tr></table>
+
+<p>These numbers agree very well with the results of theory and actual
+observation. The assumption is that the boiling-point will be diminished
+1&deg; for each 520 feet of ascent until the temperature becomes 210&deg;, then
+530 feet of elevation will lower it one degree until the water boils at
+200&deg;, and so on; the air being at 32&deg;.</p>
+
+<p>Let <i>H</i> represent the vertical height in feet between two stations; <i>B</i>
+and <i>b</i>, the boiling-points of water at the lower and upper stations
+respectively; <i>f</i>, the factor found in the above table. Then</p>
+
+<p class="center"><i>H</i> = <i>f</i> (<i>B</i> - <i>b</i>)</p>
+
+<p>Further, let <i>m</i> be the mean temperature of the stratum of air between the
+stations. Now, if the mean temperature is less than 32&deg;, the column of air
+will be shorter; and if greater, longer than at 32&deg;. According to
+Regnault, air expands <span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">491&middot;13</span>
+or &middot;002036 of its volume at 32&deg;, for each degree increase of heat. Calling the correction due to the mean
+temperature of air <i>C</i>, its value will be found from the equation,</p>
+
+<p class="center"><i>C</i> = <i>H</i> (<i>m</i> - 32) &middot;002036</p>
+
+<p>Calling the corrected height <i>H&prime;</i>, it will be found from the formula,</p>
+
+<p class="center"><i>H&prime;</i> = <i>H</i> + <i>H</i> (<i>m</i> - 32) &middot;002036</p>
+
+<p>that is,</p>
+
+<p class="center"><i>H&prime;</i> = <i>H</i> {1 + (<i>m</i> - 32) &middot;002036}</p>
+
+<p>and substituting the value of <i>H</i>,</p>
+
+<p class="center"><i>H&prime;</i> = <i>f</i> (<i>B</i> - <i>b</i>) {1 + (<i>m</i> - 32) &middot;002036}</p>
+
+<p>Strictly, according to theoretical considerations, there is a correction
+due to latitude, as in the determination of heights by the barometer; but
+its value is so small that it is practically of no importance.</p>
+
+<p>If a barometer be observed at one of the stations, the table of vapour
+tensions (<a href="#Page_62">p. 62</a>) will be useful in converting the pressure into the
+corresponding boiling-point, or <i>vice versa</i>; so that the difference of
+height may be found either by the methods employed for the boiling-point
+thermometer or the barometer.</p>
+
+<p>In conclusion, it may be remarked that observers who have good instruments
+at considerable elevations, as sites on mountains or plateaus, would
+confer a benefit to science, by registering for a length of time the
+barometer along with the boiling temperature of water, as accurately as
+possible. Such observations would serve to<span class="pagenum"><a name="Page_99" id="Page_99">[Pg 99]</a></span> verify the accuracy of
+theoretical deductions, and fix with certainty the theoretical scale with
+the barometer indications.</p>
+
+<p><i>Example, in calculating Heights from the Observations of the
+Boiling-point of Water.</i>&mdash;1. At Geneva the observed boiling-point of water
+was 209&deg;&middot;335; on the Great St. Bernard it was 197&deg;&middot;64; the mean
+temperature of the intermediate air was 63&deg;&middot;5; required the height of the
+Great St. Bernard above Geneva.</p>
+
+<p>Method by formula:&mdash;</p>
+
+<p class="center"><i>H&prime;</i> = <i>f</i> (<i>B</i> - <i>b</i>) {1 + (<i>m</i> - 32&deg;) &middot;002036}</p>
+
+<p>In this case <i>f</i> is between 530 and 550, or 540.</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td><i>B</i> =</td>
+    <td>209&middot;335</td>
+    <td>&nbsp;</td>
+    <td><span class="spacer">&nbsp;</span></td>
+    <td><i>m</i> =</td>
+    <td>63&middot;5</td></tr>
+<tr><td><i>b</i> =</td>
+    <td class="botbor">197&middot;64</td>
+    <td colspan="3">&nbsp;</td>
+    <td class="botbor">32</td></tr>
+<tr><td>&nbsp;</td>
+    <td><span style="margin-left: .5em;">11&middot;695</span></td>
+    <td colspan="3">&nbsp;</td>
+    <td>31&middot;5</td></tr>
+<tr><td><i>f</i> =</td>
+    <td class="botbor"><span style="margin-left: 1.75em;">540</span></td>
+    <td colspan="3">&nbsp;</td>
+    <td class="botbor"><span style="margin-left: 1em;">&middot;002036</span></td></tr>
+<tr><td>&nbsp;</td>
+    <td><span style="margin-left: .5em;">6315&middot;3</span></td>
+    <td colspan="3">&nbsp;</td>
+    <td><span style="margin-left: .5em;">0&middot;0641340</span></td></tr>
+<tr><td>&nbsp;</td>
+    <td class="botbor"><span style="margin-left: 1em;">1&middot;064</span></td>
+    <td colspan="3">&nbsp;</td>
+    <td class="botbor"><span style="margin-left: .5em;">1</span></td></tr>
+<tr><td><i>H&prime;</i> =</td>
+    <td class="botbor2"><span style="margin-left: .5em;">6719&middot;5</span></td><td>feet.</td>
+    <td colspan="2">&nbsp;</td>
+    <td><span style="margin-left: .5em;">1&middot;064</span></td></tr></table>
+
+<p>Method by Tables supplied with boiling-point apparatus made by Messrs.
+Negretti and Zambra:&mdash;</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td>209&middot;335</td>
+    <td>gives</td>
+    <td>1464</td>
+    <td>in Table I.</td></tr>
+<tr><td>197&middot;64</td>
+    <td align="center">"</td>
+    <td class="botbor">7736</td>
+    <td align="center">"</td></tr>
+<tr><td colspan="2">&nbsp;</td>
+    <td>6272</td></tr>
+<tr><td align="right">63&middot;5</td>
+    <td align="center">"</td>
+    <td class="botbor"><span style="margin-left: 1.5em;">1&middot;07</span></td>
+    <td>in Table II.</td></tr>
+<tr><td>Height</td>
+    <td>&nbsp;</td>
+    <td class="botbor2">6711</td></tr></table>
+
+<p><br /><b>96. Thermometers for Engineers.</b>&mdash;<i>1st. Salinometer.</i>&mdash;Under the
+circumstances at which fresh water boils at 212&deg;, sea water boils at
+213&deg;&middot;2. The boiling temperature is raised by the chemical solution of any
+substance in the water, and the more with the increase of matter
+dissolved.</p>
+
+<p>From a knowledge of this principle, marine engineers make use of the
+thermometer to determine the amount of salts held in solution by the water
+in the boilers of sea-going steamers. Common sea-water contains <span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">33</span> of
+its volume of salt and other earthy matters. As evaporation proceeds, the
+solution becomes proportionally stronger, and more heat is required to
+produce steam. The following table from the work of Messrs. Main and
+Brown, on the Marine Steam-Engine, shows the relation between the
+boiling-point under the mean pressure of the atmosphere, or 80 inches of
+mercury, and the proportion of matter dissolved in the water:&mdash;</p>
+
+<p><span class="pagenum"><a name="Page_100" id="Page_100">[Pg 100]</a></span></p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td>Proportion of Salt in 100 parts of water</td>
+    <td align="center">0</td>
+    <td>Boiling-point</td>
+    <td>212&deg;</td></tr>
+<tr><td><span style="margin-left: 4em;">"</span><span style="margin-left: 7em;">"</span></td>
+    <td align="center"><span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">33</span></td>
+    <td align="center">"</td>
+    <td>213&middot;2</td></tr>
+<tr><td><span style="margin-left: 4em;">"</span><span style="margin-left: 7em;">"</span></td>
+    <td align="center"><span style="font-size: 0.8em;"><sup>2</sup></span>&frasl;<span style="font-size: 0.6em;">33</span></td>
+    <td align="center">"</td>
+    <td>214&middot;4</td></tr>
+<tr><td><span style="margin-left: 4em;">"</span><span style="margin-left: 7em;">"</span></td>
+    <td align="center"><span style="font-size: 0.8em;"><sup>3</sup></span>&frasl;<span style="font-size: 0.6em;">33</span></td>
+    <td align="center">"</td>
+    <td>215&middot;5</td></tr>
+<tr><td><span style="margin-left: 4em;">"</span><span style="margin-left: 7em;">"</span></td>
+    <td align="center"><span style="font-size: 0.8em;"><sup>4</sup></span>&frasl;<span style="font-size: 0.6em;">33</span></td>
+    <td align="center">"</td>
+    <td>216&middot;6</td></tr>
+<tr><td><span style="margin-left: 4em;">"</span><span style="margin-left: 7em;">"</span></td>
+    <td align="center"><span style="font-size: 0.8em;"><sup>5</sup></span>&frasl;<span style="font-size: 0.6em;">33</span></td>
+    <td align="center">"</td>
+    <td>217&middot;9</td></tr>
+<tr><td><span style="margin-left: 4em;">"</span><span style="margin-left: 7em;">"</span></td>
+    <td align="center"><span style="font-size: 0.8em;"><sup>6</sup></span>&frasl;<span style="font-size: 0.6em;">33</span></td>
+    <td align="center">"</td>
+    <td>219&middot;0</td></tr>
+<tr><td><span style="margin-left: 4em;">"</span><span style="margin-left: 7em;">"</span></td>
+    <td align="center"><span style="font-size: 0.8em;"><sup>7</sup></span>&frasl;<span style="font-size: 0.6em;">33</span></td>
+    <td align="center">"</td>
+    <td>220&middot;2</td></tr>
+<tr><td><span style="margin-left: 4em;">"</span><span style="margin-left: 7em;">"</span></td>
+    <td align="center"><span style="font-size: 0.8em;"><sup>8</sup></span>&frasl;<span style="font-size: 0.6em;">33</span></td>
+    <td align="center">"</td>
+    <td>221&middot;4</td></tr>
+<tr><td><span style="margin-left: 4em;">"</span><span style="margin-left: 7em;">"</span></td>
+    <td align="center"><span style="font-size: 0.8em;"><sup>9</sup></span>&frasl;<span style="font-size: 0.6em;">33</span></td>
+    <td align="center">"</td>
+    <td>222&middot;5</td></tr>
+<tr><td><span style="margin-left: 4em;">"</span><span style="margin-left: 7em;">"</span></td>
+    <td align="center"><span style="font-size: 0.8em;"><sup>10</sup></span>&frasl;<span style="font-size: 0.6em;">33</span></td>
+    <td align="center">"</td>
+    <td>223&middot;7</td></tr>
+<tr><td><span style="margin-left: 4em;">"</span><span style="margin-left: 7em;">"</span></td>
+    <td align="center"><span style="font-size: 0.8em;"><sup>11</sup></span>&frasl;<span style="font-size: 0.6em;">33</span></td>
+    <td align="center">"</td>
+    <td>224&middot;9</td></tr>
+<tr><td><span style="margin-left: 4em;">"</span><span style="margin-left: 7em;">"</span></td>
+    <td align="center"><span style="font-size: 0.8em;"><sup>12</sup></span>&frasl;<span style="font-size: 0.6em;">33</span></td>
+    <td align="center">"</td>
+    <td>226&middot;0</td></tr></table>
+
+<div class="figleft">Fig. 74.<br /><img src="images/fig_74.jpg" alt="" /></div>
+
+<p>When the salts in solution amount to <span style="font-size: 0.8em;"><sup>12</sup></span>&frasl;<span style="font-size: 0.6em;">33</span>, the water is saturated. It has
+also been ascertained that, when a solution of <span style="font-size: 0.8em;"><sup>4</sup></span>&frasl;<span style="font-size: 0.6em;">33</span> is attained,
+incrustation of the substances commences on the boiler. Hence, it is a
+rule with engineers to expel some of the boiling water, when the
+thermometer indicates a temperature of 216&deg;, and introduce some more cold
+water, in order to prevent incrustation, which not only injures the
+boiler, but opposes the passage of heat to the water. The thermometer used
+for this purpose should be very accurately graduated, and the scale must
+be considerably higher than, though it need not read much below 212&deg;.</p>
+
+<p><i>2nd. Pressure Gauge.</i>&mdash;The elasticity of gases augments by increase of
+temperature, and <i>vice versa</i>; it follows, therefore, that when steam is
+generated in a closed boiler, its temperature rises beyond the boiling
+temperature of 212&deg;, owing to the increased pressure upon the water. The
+law connecting the pressure and the corresponding temperature of steam is
+the same as that upon which the boiling of fluids under diminished
+atmospheric pressure takes place. Hence, the indications of the
+thermometer become exponents of steam pressure. Engineers are furnished,
+in works on the steam-engine, with tables, from which the pressure
+corresponding to a given temperature, or the converse, can be obtained by
+mere inspection.</p>
+
+<p>Fig. 74 represents the thermometer employed as a steam-pressure gauge. It
+is fitted in a brass case, with screw-plug and washers for closing the
+boiler when the thermometer is not in use. The scale shows the pressure
+corresponding to the temperature, from 15 to 120 lbs., above the
+atmospheric pressure, which is usually taken as 15 lbs. on the square
+inch.</p>
+
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_101" id="Page_101">[Pg 101]</a></span></p>
+<h2><a name="CHAPTER_XI" id="CHAPTER_XI"></a>CHAPTER XI.</h2>
+<p class="center"><span class="large">INSTRUMENTS FOR ASCERTAINING THE HUMIDITY OF THE AIR.</span></p>
+
+<p><br /><b>97. Hygrometric Substances.</b>&mdash;The instruments devised for the purpose of
+ascertaining the humidity of the atmosphere are termed <i>hygrometers</i>. The
+earliest invented hygrometers were constructed of substances readily acted
+upon by the vapour in the air, such as hair, grass, seaweed, catgut, &amp;c.,
+which all absorb moisture, and thereby increase in length, and when
+deprived of it by drying they contract. Toy-like hygrometers, upon the
+principle of absorption, are still common as ornaments for mantel-pieces.
+A useful little instrument of this class, formed from the beard of the
+wild oat, is made to resemble a watch in external appearance, and is
+designed to prove the dampness or dryness of beds: a moveable hand points
+out on the dial the hygrometric condition of the clothes upon which the
+instrument is laid.</p>
+
+
+<div class="figright">Fig. 75.<br /><img src="images/fig_75.jpg" alt="" /></div>
+
+<p><br /><b>98. Saussure&#8217;s Hygrometer</b>, formerly used as a meteorologic instrument, but
+now regarded as an ornamental curiosity, is represented in fig. 75. Its
+action depends upon a prepared hair, fixed at one end to the frame of the
+instrument, and wound round a pulley at the other. The pulley carries a
+pointer which has a counterpoise sufficient to keep the hair stretched. By
+this means the shrinking and lengthening of the hair cause the pointer to
+traverse a graduated arc indicating the relative humidity.</p>
+
+<p>Such instruments, however ingenious, are not of scientific value; because
+they do not admit of rigid comparison, are liable to alter in their
+contractile and expansive properties, and cannot be made to indicate
+precisely alike.</p>
+
+
+<p><br /><b>99. Dew-Point.</b>&mdash;The amount of water which the air can sustain in an
+invisible form increases with the temperature; but for every definite
+temperature there is a limit to the amount of vapour which can be thus
+diffused. When the air is cooled, the vapour present may be more than it
+can sustain; part will then be condensed as dew, rain, hail or snow,
+according to the meteorologic circumstances. The temperature which the air
+has when it is so fully saturated with vapour that any excess will be
+deposited as dew, is called the <i>dew-point</i>.</p>
+
+
+<p><br /><b>100. Drosometer.</b>&mdash;&#8220;To measure the quantity of dew deposited each night, an
+instrument is used called a <i>Drosometer</i>. The most simple process consists
+in exposing to the open air bodies whose exact weight is known, and then
+weighing them afresh after they are covered with dew. According to Wells,
+locks of wool, weighing about eight grains, are to be preferred, which are
+to be divided [formed] into spherical masses of the diameter of about two
+inches.&#8221;&mdash;<i>K&oelig;mtz.</i></p>
+
+
+<p><br /><span class="pagenum"><a name="Page_102" id="Page_102">[Pg 102]</a></span><b>101. Humidity.</b>&mdash;The proportion existing between the amount of vapour
+actually present in the air at any time, and the quantity necessary to
+completely saturate it, is called <i>the degree of humidity</i>. It is usually
+expressed in a centesimal scale, 0 being perfect dryness, and 100 complete
+saturation.</p>
+
+<p>The pressure, or tension, of vapour at the dew-point temperature, divided
+by the tension of vapour at the air temperature and the quotient
+multiplied by 100, gives the degree of humidity. (Regnault&#8217;s Tables should
+be used.)</p>
+
+<p>Hence the utility of instruments for determining the dew-point.</p>
+
+
+<div class="figleft">Fig. 76.<br /><img src="images/fig_76.jpg" alt="" /></div>
+
+<p><br /><b>102. Leslie&#8217;s Hygrometer.</b>&mdash;This instrument consists of a glass syphon
+tube, terminated with a bulb or ball at each end, turned outwards from
+each other, as in fig. 76. The tube is partly filled with concentrated
+sulphuric acid, tinged by carmine. One of the balls is covered smoothly
+with fine muslin, and is kept continually moistened with pure water, drawn
+from a vase placed near it by the capillary attraction of a few strands of
+clean cotton-wick. The descent of the coloured liquid in the other stem
+will mark the diminution of temperature caused by the evaporation of the
+water from the humid surface. The drier the ambient air is, the more
+rapidly will the evaporation go on; and the cold produced will be greater.
+When the air is nearly saturated with moisture, the evaporation goes on
+slowly; the cold produced is moderate, because the ball regains a large
+portion of its lost heat from surrounding bodies; and the degree of
+refrigeration of the ball is an index of the dryness of the air.</p>
+
+<p>&#8220;Should the water become frozen on the ball, this hygrometer will still
+act; for evaporation goes on from the surface of ice in proportion to the
+dryness of the air. Leslie estimates, that when the ball is moist, air, at
+the temperature of the ball, will take up moisture equal to the
+sixteen-thousandth part of its weight, for each degree of his hygrometer;
+and as ice in melting requires one-seventh of the caloric consumed in
+converting water into vapour, when the ball is frozen, the hygrometer will
+sink more than when wet by 1&deg; in 7&deg;; and hence, in the frozen state, we
+must increase the value of the degrees one-seventh: so that each of them
+will correspond to an absorption of moisture equal to
+one-fourteen-thousandth part of the weight of the air.</p>
+
+<p>&#8220;When this hygrometer stands at 15&deg;, the air feels damp; from 30&deg; to 40&deg;,
+we reckon it dry; from 50&deg; to 60&deg;, very dry; and from 70&deg; upwards, we
+should call it intensely dry. A room would feel uncomfortable, and would
+probably be unwholesome, if the instrument in it did not reach 30&deg;.<small><a name="f8.1" id="f8.1" href="#f8">[8]</a></small> In
+thick fogs it keeps almost at the beginning of the scale. In winter, in
+our climate, it ranges from 5&deg; to<span class="pagenum"><a name="Page_103" id="Page_103">[Pg 103]</a></span> 15&deg;; in summer often from 15&deg; to 55&deg;;
+and sometimes attains 80&deg; or 90&deg;. The greatest degree of dryness ever
+noticed by Leslie was at Paris, in the month of September, when the
+hygrometer indicated 120&deg;.&#8221;&mdash;<i>Professor Trail, in &#8220;Library of Useful
+Knowledge.&#8221;</i></p>
+
+<p>In estimating the value of the indications of this hygrometer, it should
+be borne in mind that the scale adopted by Leslie was <i>millesimal</i>, that
+is to say, from the freezing to the boiling-point of water was divided
+into a thousand parts; ten millesimal degrees are therefore equal to one
+of the scale of Celsius.</p>
+
+
+<p class="center"><br />103. DANIEL&#8217;S HYGROMETER.</p>
+
+<div class="figright">Fig. 77.<br /><img src="images/fig_77.jpg" alt="" /></div>
+
+<p>This instrument was invented about the year 1820, by Professor Daniel, the
+distinguished author of <i>Meteorological Essays</i>; and it entirely
+superseded all hygrometers depending upon the absorption of moisture. The
+form of the instrument is shown in fig. 77.</p>
+
+<p>It consists of a glass tube, about one-eighth of an inch in diameter of
+bore, bent twice at right angles, and terminated, at each end, in a bulb
+about one inch and a quarter in diameter. In one limb of the tube is
+enclosed a delicate thermometer, which descends to the centre of the
+adjoining bulb, which is about three-parts filled with sulphuric ether.
+All the other parts of the tube are carefully freed from air, so that they
+are occupied by the vapour of the ether. This bulb is generally made of
+black glass; the other is transparent, but covered with a piece of fine
+muslin. The support for the tube has a thermometer attached, which shows
+the temperature of the external air. The tube can be removed from the
+stand, and the parts are made to pack, with a necessary phial of ether, in
+a small box, which can easily be got into the pocket.</p>
+
+<p><i>How to use the Hygrometer.</i>&mdash;This instrument gives the dew-point by
+direct observation, which must be made in the following manner:&mdash;Having
+fixed the tube upon the stand, with the bulbs vertically downward, the
+ether is all caused to flow into the lower ball by inclining the tube. The
+temperature of the air is noted by the exposed thermometer. Then some
+ether is poured, from a dropping tube fitting into the neck of the phial,
+upon the muslin-covered bulb. The rapid evaporation of this ether cools
+the bulb and causes condensation of the ethereal vapour in its interior.
+This gives rise to rapid evaporation of the ether in the lower bulb,
+whereby its temperature is greatly reduced. The air in the vicinity is
+deprived of its warmth by the cold bulb, and is soon cooled to the
+temperature at which it is perfectly<span class="pagenum"><a name="Page_104" id="Page_104">[Pg 104]</a></span> saturated with the vapour which it
+contains. Cooled ever so little below this temperature, some aqueous
+vapour will be condensed, and will form a dew upon the black-glass bulb.
+At the first indication of the deposit of dew the reading of the internal
+thermometer is taken: which is the dew-point.</p>
+
+<p>This hygrometer has undeniable disadvantages. The surface upon which the
+dew condenses is small, and requires a peculiar direction of light in
+which to see it well. The observer, having his attention on the bulb and
+the thermometer, cannot always fix with precision the dew-point; and hence
+he is recommended to note the temperature at the appearance and at the
+disappearance of the dew, in order that the chance of error may be
+diminished. Without doubt, the necessarily long continuance of the
+observer near the instrument influences, to some extent, the observed
+temperatures; and the difficulty of not being always able to procure pure
+ether for the experiments is not the least of the drawbacks to the use of
+the instrument. Some of these disadvantages are obviated in Regnault&#8217;s
+hygrometer.</p>
+
+
+<div class="figleft">Fig. 78.<br /><img src="images/fig_78.jpg" alt="" /></div>
+
+<p class="center"><br />104. REGNAULT&#8217;S CONDENSER HYGROMETER</p>
+
+<p>(Fig. 78) consists of a tube, <i>C</i>, made of silver, very thin, and
+perfectly polished; the tube is larger at one end than the other, the
+large part being 1&middot;8 inches in depth, by 0&middot;8 in diameter; this is fitted
+tightly to a brass stand, <i>B</i>, with a telescopic arrangement for adjusting
+when making an observation.</p>
+
+<p>The tube, <i>C</i>, has a small lateral tubulure, to which is attached an
+India-rubber tube, with ivory mouth-piece; this tubulure enters <i>C</i> at
+right angles near the top, and traverses it to the bottom of the largest
+part.</p>
+
+<p>A delicate thermometer, <i>D</i>, is inserted through a cork, or India-rubber
+washer, at the open end of the tube, <i>C</i>, the bulb of which descends to
+the centre of its largest part.</p>
+
+<p><i>G</i> is an attached thermometer for taking the temperature of the air, and
+<i>F</i> is a bottle containing ether.</p>
+
+<p><i>To use the Condenser Hygrometer</i>, a sufficient quantity of ether is
+poured into the silver tube to cover the thermometer bulb: on allowing air
+to pass bubble by bubble through the ether, by breathing in the tube, <i>E</i>,
+an uniform temperature will be obtained; if the ether continues to be
+agitated, by breathing briskly through the tube a rapid reduction of
+temperature will be the result; at the moment the ether is cooled down to
+the dew-point temperature, the external surface of that portion of the
+silver tube containing ether will become covered with a coating of
+moisture, and the degree shown by the thermometer at that instant will be
+the temperature of the dew-point.</p>
+
+<p>This form of hygrometer, for ascertaining by direct observation the
+dew-point, is so superior to Daniell&#8217;s, both from its being more certain
+in its indications and<span class="pagenum"><a name="Page_105" id="Page_105">[Pg 105]</a></span> economical in use, that Messrs. Negretti and
+Zambra have been induced to modify it, and reduce its price to little more
+than that of a good Daniell&#8217;s Hygrometer.</p>
+
+<div class="figright">Fig. 79.<br /><img src="images/fig_79.jpg" alt="" /></div>
+
+<p><br /><b>105. Temperature of Evaporation.</b>&mdash;When the air is not saturated with
+vapour, evaporation is going on with more or less activity, according as
+the temperature is high or low, rising or falling. Now vapour cannot be
+formed without an expenditure of heat; as we invariably find that the
+process of evaporation lowers the temperature of the liquid from which the
+vapour is produced, and, by communication, that of contiguous substances
+also. Thus the emigrant, crossing the line under the scorching influence
+of the vertical sun, wraps a wet towel round his can of water, swings it
+in the breeze, to evaporate the moisture of the towel, and obtains a glass
+of cool water. So also, European residents in India, during the hot
+season, spread out mats in their apartments, and keep them wet, in order
+that the evaporation may cool the air. This principle has been applied,
+for the purpose of ascertaining the hygrometric condition of the air, in
+the instrument known as Mason&#8217;s hygrometer, or psychrometer, which is now
+in general use, from its simplicity, accuracy, and ease of observing.</p>
+
+
+<p class="center"><br />106. MASON&#8217;S HYGROMETER.</p>
+
+<p><br /><b>The Dry and Wet Bulb Hygrometer, or Psychrometer</b>, known also as Mason&#8217;s
+hygrometer (fig. 79), consists of two parallel thermometers, as nearly
+identical as possible, mounted on a wooden bracket, one marked <i>dry</i>, the
+other <i>wet</i>. The bulb of the wet thermometer is covered with thin muslin,
+and round the neck is twisted a conducting thread of lamp-wick, which
+passes into a vessel of water, placed at such a distance as to allow a
+length of conducting thread, of about three inches; the cup or glass is
+placed on one side, and a little beneath, so that the water within may not
+affect the reading of the <i>dry bulb thermometer</i>. In observing, the eye
+should be placed on a level with the top of the mercury in the tube, and
+the observer should refrain from breathing whilst taking an observation.</p>
+
+<p>The <i>dry</i> bulb thermometer indicates the temperature of the air itself;
+while the wet bulb, cooled by evaporation, shows a lower temperature
+according to the rapidity of evaporation.</p>
+
+<p><span class="pagenum"><a name="Page_106" id="Page_106">[Pg 106]</a></span><i>To find the Dew-point.</i>&mdash;From the readings of the two thermometers, the
+dew-point can be deduced by formul&aelig; (that known as Apjohn&#8217;s is considered
+the most theoretically true), or from the valuable Hygrometric Tables by
+J. Glaisher, Esq., F.R.S.</p>
+
+<p>For practical purposes in estimating the comparative humidity, the annexed
+table, which is a reduction from Mr. Glaisher&#8217;s elaborate work, will be
+sufficient; it will at least serve to assist in familiarising the
+inexperienced in the value of the psychrometer&#8217;s indications:&mdash;</p>
+
+<table border="0" cellpadding="0" cellspacing="0" summary="table">
+<tr><td rowspan="3" class="btlr" align="center">Temperature<br />by the<br />Dry Bulb<br />Thermometer.</td>
+    <td class="btr" colspan="6" align="center">Difference between Dry-bulb and Wet-bulb Readings.</td></tr>
+<tr><td class="btr" align="center">2&deg;</td>
+    <td class="btr" align="center">4&deg;</td>
+    <td class="btr" align="center">6&deg;</td>
+    <td class="btr" align="center">8&deg;</td>
+    <td class="btr" align="center">10&deg;</td>
+    <td class="btr" align="center">12&deg;</td></tr>
+<tr><td class="btr" colspan="6" align="center">Degree of Humidity.</td></tr>
+<tr><td class="btlr" align="center"><span style="margin-left: .5em;">34&deg;</span></td>
+    <td class="btr" align="center">79</td>
+    <td class="btr" align="center">63</td>
+    <td class="btr" align="center">50</td>
+    <td class="btr" align="center">..</td>
+    <td class="btr" align="center">..</td>
+    <td class="btr" align="center">..</td></tr>
+<tr><td class="blr" align="center">36</td>
+    <td class="br" align="center">82</td>
+    <td class="br" align="center">66</td>
+    <td class="br" align="center">53</td>
+    <td class="br" align="center">..</td>
+    <td class="br" align="center">..</td>
+    <td class="br" align="center">..</td></tr>
+<tr><td class="blr" align="center">38</td>
+    <td class="br" align="center">83</td>
+    <td class="br" align="center">68</td>
+    <td class="br" align="center">56</td>
+    <td class="br" align="center">45</td>
+    <td class="br" align="center">..</td>
+    <td class="br" align="center">..</td></tr>
+<tr><td class="blr" align="center">40</td>
+    <td class="br" align="center">84</td>
+    <td class="br" align="center">70</td>
+    <td class="br" align="center">58</td>
+    <td class="br" align="center">47</td>
+    <td class="br" align="center">..</td>
+    <td class="br" align="center">..</td></tr>
+<tr><td class="blr" align="center">42</td>
+    <td class="br" align="center">84</td>
+    <td class="br" align="center">71</td>
+    <td class="br" align="center">59</td>
+    <td class="br" align="center">49</td>
+    <td class="br" align="center">..</td>
+    <td class="br" align="center">..</td></tr>
+<tr><td class="blr" align="center">44</td>
+    <td class="br" align="center">85</td>
+    <td class="br" align="center">72</td>
+    <td class="br" align="center">60</td>
+    <td class="br" align="center">50</td>
+    <td class="br" align="center">..</td>
+    <td class="br" align="center">..</td></tr>
+<tr><td class="blr" align="center">46</td>
+    <td class="br" align="center">86</td>
+    <td class="br" align="center">73</td>
+    <td class="br" align="center">61</td>
+    <td class="br" align="center">51</td>
+    <td class="br" align="center">..</td>
+    <td class="br" align="center">..</td></tr>
+<tr><td class="blr" align="center">48</td>
+    <td class="br" align="center">86</td>
+    <td class="br" align="center">73</td>
+    <td class="br" align="center">62</td>
+    <td class="br" align="center">52</td>
+    <td class="br" align="center">44</td>
+    <td class="br" align="center">..</td></tr>
+<tr><td class="blr" align="center">50</td>
+    <td class="br" align="center">86</td>
+    <td class="br" align="center">74</td>
+    <td class="br" align="center">63</td>
+    <td class="br" align="center">53</td>
+    <td class="br" align="center">45</td>
+    <td class="br" align="center">..</td></tr>
+<tr><td class="blr" align="center">52</td>
+    <td class="br" align="center">86</td>
+    <td class="br" align="center">74</td>
+    <td class="br" align="center">64</td>
+    <td class="br" align="center">54</td>
+    <td class="br" align="center">46</td>
+    <td class="br" align="center">..</td></tr>
+<tr><td class="blr" align="center">54</td>
+    <td class="br" align="center">86</td>
+    <td class="br" align="center">74</td>
+    <td class="br" align="center">64</td>
+    <td class="br" align="center">55</td>
+    <td class="br" align="center">47</td>
+    <td class="br" align="center">..</td></tr>
+<tr><td class="blr" align="center">56</td>
+    <td class="br" align="center">87</td>
+    <td class="br" align="center">75</td>
+    <td class="br" align="center">65</td>
+    <td class="br" align="center">56</td>
+    <td class="br" align="center">48</td>
+    <td class="br" align="center">..</td></tr>
+<tr><td class="blr" align="center">58</td>
+    <td class="br" align="center">87</td>
+    <td class="br" align="center">76</td>
+    <td class="br" align="center">66</td>
+    <td class="br" align="center">57</td>
+    <td class="br" align="center">49</td>
+    <td class="br" align="center">..</td></tr>
+<tr><td class="blr" align="center">60</td>
+    <td class="br" align="center">88</td>
+    <td class="br" align="center">76</td>
+    <td class="br" align="center">66</td>
+    <td class="br" align="center">58</td>
+    <td class="br" align="center">50</td>
+    <td class="br" align="center">43</td></tr>
+<tr><td class="blr" align="center">62</td>
+    <td class="br" align="center">88</td>
+    <td class="br" align="center">77</td>
+    <td class="br" align="center">67</td>
+    <td class="br" align="center">58</td>
+    <td class="br" align="center">50</td>
+    <td class="br" align="center">44</td></tr>
+<tr><td class="blr" align="center">64</td>
+    <td class="br" align="center">88</td>
+    <td class="br" align="center">77</td>
+    <td class="br" align="center">67</td>
+    <td class="br" align="center">59</td>
+    <td class="br" align="center">51</td>
+    <td class="br" align="center">45</td></tr>
+<tr><td class="blr" align="center">66</td>
+    <td class="br" align="center">88</td>
+    <td class="br" align="center">78</td>
+    <td class="br" align="center">68</td>
+    <td class="br" align="center">60</td>
+    <td class="br" align="center">52</td>
+    <td class="br" align="center">45</td></tr>
+<tr><td class="blr" align="center">68</td>
+    <td class="br" align="center">88</td>
+    <td class="br" align="center">78</td>
+    <td class="br" align="center">68</td>
+    <td class="br" align="center">60</td>
+    <td class="br" align="center">52</td>
+    <td class="br" align="center">46</td></tr>
+<tr><td class="blr" align="center">70</td>
+    <td class="br" align="center">88</td>
+    <td class="br" align="center">78</td>
+    <td class="br" align="center">69</td>
+    <td class="br" align="center">61</td>
+    <td class="br" align="center">53</td>
+    <td class="br" align="center">47</td></tr>
+<tr><td class="blr" align="center">72</td>
+    <td class="br" align="center">89</td>
+    <td class="br" align="center">79</td>
+    <td class="br" align="center">69</td>
+    <td class="br" align="center">61</td>
+    <td class="br" align="center">54</td>
+    <td class="br" align="center">48</td></tr>
+<tr><td class="blr" align="center">74</td>
+    <td class="br" align="center">89</td>
+    <td class="br" align="center">79</td>
+    <td class="br" align="center">70</td>
+    <td class="br" align="center">62</td>
+    <td class="br" align="center">55</td>
+    <td class="br" align="center">48</td></tr>
+<tr><td class="blr" align="center">76</td>
+    <td class="br" align="center">89</td>
+    <td class="br" align="center">79</td>
+    <td class="br" align="center">71</td>
+    <td class="br" align="center">63</td>
+    <td class="br" align="center">55</td>
+    <td class="br" align="center">49</td></tr>
+<tr><td class="blr" align="center">78</td>
+    <td class="br" align="center">89</td>
+    <td class="br" align="center">79</td>
+    <td class="br" align="center">71</td>
+    <td class="br" align="center">63</td>
+    <td class="br" align="center">56</td>
+    <td class="br" align="center">50</td></tr>
+<tr><td class="blr" align="center">80</td>
+    <td class="br" align="center">90</td>
+    <td class="br" align="center">80</td>
+    <td class="br" align="center">71</td>
+    <td class="br" align="center">63</td>
+    <td class="br" align="center">56</td>
+    <td class="br" align="center">50</td></tr>
+<tr><td class="blr" align="center">82</td>
+    <td class="br" align="center">90</td>
+    <td class="br" align="center">80</td>
+    <td class="br" align="center">72</td>
+    <td class="br" align="center">64</td>
+    <td class="br" align="center">57</td>
+    <td class="br" align="center">51</td></tr>
+<tr><td class="blr" align="center">84</td>
+    <td class="br" align="center">90</td>
+    <td class="br" align="center">80</td>
+    <td class="br" align="center">72</td>
+    <td class="br" align="center">64</td>
+    <td class="br" align="center">57</td>
+    <td class="br" align="center">51</td></tr>
+<tr><td class="bblr" align="center">86</td>
+    <td class="bbr" align="center">90</td>
+    <td class="bbr" align="center">80</td>
+    <td class="bbr" align="center">72</td>
+    <td class="bbr" align="center">64</td>
+    <td class="bbr" align="center">58</td>
+    <td class="bbr" align="center">52</td></tr></table>
+
+<p><span class="pagenum"><a name="Page_107" id="Page_107">[Pg 107]</a></span>The total quantity of aqueous vapour which at any temperature can be
+diffused in the air being represented by 100, the per-centage of vapour
+actually present will be found in the table opposite the temperature of
+the dry thermometer, and under the difference between the dry-bulb and
+wet-bulb temperatures. The degree of humidity for intermediate
+temperatures and differences to those given in the table can be easily
+estimated sufficiently accurately for most practical purposes.</p>
+
+<p>The difference between the two thermometer readings taken from the reading
+of the wet bulb, gives the dew-point very nearly, when the air is at any
+temperature between freezing and 80&deg;. This simple rule will be found
+serviceable to horticulturists, since it will enable them to estimate the
+chilling effect of dew or hoar-frost on tender plants.</p>
+
+<p><i>Use as an Indicator of Weather.</i>&mdash;In our climate, the usual difference
+between the thermometer readings,&mdash;in the open air, shaded from the sun,
+reflected heat, and currents of air,&mdash;ranges from one to twelve degrees.
+In hot and dry climates, as India and Australia, the range out of doors
+has been found as much as 30&deg;, occasionally.</p>
+
+<p>When the moisture is frozen, the bulb should be wetted afresh, and the
+reading taken just before it again freezes; but the observation then is of
+little value, and for general purposes need not be taken, as the air is
+known to be dry in frosty weather.</p>
+
+<p>The muslin or cotton rag should be washed once or twice a week by pouring
+water over the bulb; and it should be replaced by a fresh piece at least
+once a month. Accuracy depends very much upon keeping the wet bulb clean,
+and not <i>too</i> wet.</p>
+
+<p>In connection with the barometer, this hygrometer is very useful, not only
+on land, but especially at sea, where other kinds of hygrometers cannot be
+practically used. A fall in the barometer is indicative of coming wind or
+rain: if the hygrometer shows increasing dampness by the difference of the
+readings becoming smaller,&mdash;rain may therefore be anticipated. On the
+contrary, if the hygrometer shows continuing or increasing dryness, a
+stronger wind is probable, without rain.</p>
+
+<p><i>Domestic Uses.</i>&mdash;Mason&#8217;s hygrometer is useful in regulating the moisture
+of the air of apartments; a difference in the thermometer readings of from
+5&deg; to 8&deg; being considered healthy. Many complaints require that the
+temperature and humidity of the air which the invalid breathes should be
+carefully regulated. Hence it is a valuable household instrument. In a
+room, it should be placed away from the fire as much as possible, but not
+exposed to draughts of air.</p>
+
+<p>Figs. 80 and 81 show cheap arrangements of the instrument for domestic
+purposes. Other arrangements are given to the instrument to make it
+suitable for exhibiting the hygrometrical state of the air in hot-houses,
+conservatories, malting-houses, warehouses, manufactories, &amp;c.</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td align="center">Fig. 80.</td><td><span class="spacer2">&nbsp;</span></td>
+    <td align="center">Fig. 81.</td><td><span class="spacer2">&nbsp;</span></td>
+    <td align="center">Fig. 82.</td><td><span class="spacer2">&nbsp;</span></td></tr>
+<tr><td align="center"><img src="images/fig_80.jpg" alt="" /></td><td>&nbsp;</td>
+    <td align="center"><img src="images/fig_81.jpg" alt="" /></td><td>&nbsp;</td>
+    <td align="center"><img src="images/fig_82.jpg" alt="" /></td><td>&nbsp;</td></tr></table>
+
+<p>Fig. 82 shows the instrument arranged on brass tripod stand, with folding
+legs and metal cover, to render it portable.</p>
+
+
+<p><br /><span class="pagenum"><a name="Page_108" id="Page_108">[Pg 108]</a></span><b>107. Self-Registering Hygrometer.</b>&mdash;A maximum thermometer and a minimum
+thermometer, each fitted up as a wet-bulb thermometer, record the highest
+and lowest temperature of evaporation during the interval of observation.
+Negretti&#8217;s mercurial maximum, and an alcohol minimum, answer best.</p>
+
+
+<p><br /><b>108. Causes of Dew.</b>&mdash;&#8220;The aqueous vapour of our atmosphere is a powerful
+radiant; but it is diffused through air which usually exceeds its own mass
+more than one hundred times. Not only, then, its own heat, but the heat of
+the large quantity of air which surrounds it, must be discharged by the
+vapour, before it can sink to its point of condensation. The retardation
+of chilling due to this cause enables good solid radiators, at the earth&#8217;s
+surface, to outstrip the vapour in their speed of refrigeration; and
+hence, upon these bodies, aqueous vapour may be condensed to liquid, or
+even congealed to hoar-frost, while at a few feet above the surface it
+still maintains its gaseous state.&#8221;<small><a name="f9.1" id="f9.1" href="#f9">[9]</a></small> The amount of moisture so deposited
+will vary with different atmospheric conditions. If the sky be decidedly
+cloudy or misty, the heat radiated from the earth will be partly restored
+by counter-radiation from the visible vapour; the cooling of the earth&#8217;s
+surface will, therefore, take place slowly, and little dew will be
+deposited. On the other hand, if the air contain transparent vapour, and
+the sky appear clear, the counter-radiation will be less, the earth will
+cool rapidly, and the deposit of dew will be copious; provided the night
+be comparatively calm, for, when the wind blows, the circulating air
+supplies heat to the radiating substances, and prevents any considerable
+chilling.</p>
+
+<p>The dew which falls in tropical countries greatly exceeds in abundance
+what we experience in our climate; because the air is there, from the
+great heat, capable of<span class="pagenum"><a name="Page_109" id="Page_109">[Pg 109]</a></span> sustaining a large amount of vapour in the
+transparent state, and the conditions most favourable for a maximum
+reduction of temperature by radiation are present. At those places, or
+upon those substances which cool the lowest and most readily, the dew
+falls most copiously.</p>
+
+<p>&nbsp;</p>
+<p class="center">Fig. 83.</p>
+<div class="figcenter"><img src="images/fig_83.jpg" alt="" /></div>
+<p>&nbsp;</p>
+
+<p><br /><b>109. Plan of Exposing Thermometers</b>, &amp;c.&mdash;Figure 83 is an illustration of a
+convenient slab for supporting thermometers in an exposed position
+attached to a stand (such as <a href="#glaisher">Glaisher&#8217;s, described in Chapter XVI.</a>) for
+ordinary scientific observations. It has a projecting ledge, <i>B</i>, to carry
+off rain from the instruments, the slab, <i>A</i>, being erected vertically.
+The hygrometer is placed at <i>E</i>, with the vase of water at <i>F</i>. An alcohol
+minimum thermometer is represented at <i>C</i>, in the position most favourable
+to its certain action; and at <i>D</i> is shown one of Negretti &amp; Zambra&#8217;s
+maximum thermometers, the position of which may be more nearly horizontal
+than there exhibited, although a slight depression of the bulb-end of the
+frame is desirable, but not necessary, as this thermometer can be used in
+any position.</p>
+
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_110" id="Page_110">[Pg 110]</a></span></p>
+<h2><a name="CHAPTER_XII" id="CHAPTER_XII"></a>CHAPTER XII.</h2>
+<p class="center"><span class="large">INSTRUMENTS USED FOR MEASURING THE RAINFALL.</span></p>
+
+<p><br />The instruments in use for measuring the quantity of rain which falls on a
+given spot are of very simple construction. Perhaps the simplest is:&mdash;</p>
+
+
+<p><br /><b>110. Howard&#8217;s Rain-Gauge.</b>&mdash;It consists of a copper funnel, a stout glass
+or stone bottle, and a measuring glass. The bottle is to be placed upon
+the ground, with the funnel resting on its neck. A brass band or cylinder
+fixed upon the outer surface of the funnel envelops the neck of the
+bottle, and the pipe of the funnel extends nearly to the bottom of the
+bottle; so that loss by evaporation is avoided as much as possible. The
+receiving space of the funnel is formed by a brass ring, five inches in
+diameter, very accurately turned. The measuring vessel enables the
+observer to note the rainfall in inches, tenths, and hundredths of an
+inch.</p>
+
+<div class="figleft">Fig. 84.<br /><img src="images/fig_84.jpg" alt="" /></div>
+
+<p><br /><b>111. Glaisher&#8217;s Rain-Gauge.</b>&mdash;The rain-gauge designed by Mr. Glaisher, the
+well-known meteorologist, and used by most observers of the present day,
+is arranged for the reception of the water which falls upon its receiving
+surface only, and for the prevention of loss by evaporation. The rain is
+first collected in a funnel, <i>B</i>, (fig. 84,) the receiving surface of
+which is turned in a lathe. The conical surface of the funnel slopes to
+the pipe, <i>E</i>, at an angle of 60&deg; from the horizontal receiving surface.
+The tube, <i>E</i>, is of small aperture, and is bent up, in order to retain
+the last few drops of rain, so that the only opening for the escape of
+vapour may be closed as long as possible. The funnel, <i>B</i>, fits upon the
+cylinder, <i>A</i>, tightly in the groove, <i>D</i>. A copper can is placed inside
+the cylinder, <i>A</i>, to receive the rain from the funnel. Once or twice a
+day, or after a shower, this can should be taken out, and the water
+measured in the glass measure, <i>C</i>, which is graduated to hundredths of an
+inch, according to the calculated quantity of water, determined by the
+area of<span class="pagenum"><a name="Page_111" id="Page_111">[Pg 111]</a></span> the receiving space. In use, this gauge should be partly sunk in
+the ground, so that the top may be about five inches above it. Thus
+situated, there will be little or no evaporation from it during any month
+of the year; and the readings need not be taken daily, although desirable.</p>
+
+
+<p><br /><b>112. Rain-Gauge with Float.</b>&mdash;In this construction the graduated glass
+measure is dispensed with. The cylinder of the gauge is made less in
+diameter than the funnel, and a hollow, very flattened spheroid of copper
+forming a float, and carrying a vertical graduated boxwood scale which
+moves through the orifice of the funnel, is placed in it. As the rain
+accumulates the float rises, and the amount of rain in the gauge is read
+upon the scale from the top of the gauge, a bar, having a hole at the
+centre for the passage of the scale, being fixed diametrically across the
+receiving space of the funnel. The gauge is provided at the bottom with a
+brass cock, by which the water may be allowed to flow out of it whenever
+necessary.</p>
+
+<p>This form of gauge is not very suitable for the measurement of small
+quantities; but is admirably adapted for localities where the rainfall is
+excessive.</p>
+
+<div class="figright">Fig. 85.<br /><img src="images/fig_85.jpg" alt="" /></div>
+
+<p><br /><b>113. Rain-Gauge with Side-Tube.</b>&mdash;This instrument, as represented in fig.
+85, is a cylindrical vessel, mounted on a base shaped as a frustum of a
+cone. This base may be filled with sand or gravel to make the instrument
+stable, so that when placed upon a lawn or in a garden it may have an
+ornamental appearance. The funnel for collecting the rain is larger in
+diameter than the cylinder. Parallel to the cylinder, and communicating
+with the lowest part of the interior and extending to its top, is a
+graduated glass tube, open at both ends. The rain collected will rise as
+high in this tube as in the cylinder, and its amount can therefore be read
+off without any trouble. The gauge is emptied by the brass tap at the
+bottom of the cylinder.</p>
+
+
+<p><br /><b>114. Admiral FitzRoy&#8217;s Rain-Gauge.</b>&mdash;A form of rain-gauge, very well
+adapted for expeditious observation at any time, has been designed by
+Admiral FitzRoy, and extensively employed by his observers. It is
+cylindrical in shape, with the funnel let into the top; and the rainfall
+is collected in an inner and much smaller cylinder, so that a small fall
+is represented by a considerable depth of water in the gauge. The amount
+of rain which has fallen is ascertained by a dipping tube, similar in
+principle to the dipping syphon used by gaugers for taking out specimens
+of wines or spirits from casks by simply removing the bung. A short,
+vertical, tubular opening provided with a cap, which is attached to the
+instrument by a chain that it may not be lost, is formed in the funnel.
+The measuring tube, which has a small hole at each end, should be<span class="pagenum"><a name="Page_112" id="Page_112">[Pg 112]</a></span> placed
+upright in the gauge; then the thumb should be pressed over the upper
+aperture, while the tube is lifted gently out, holding in the lower part a
+quantity of water representing the depth of the rain in the gauge, the
+upper edge of which is at the mark to be read off. The glass tube is
+graduated to inches and tenths; hundredths of an inch can be readily
+estimated by the eye. The marks are fixed by actual trial with a standard
+gauge, and are artificial, not true, inches.</p>
+
+
+<p><br /><b>115. Self-Registering Rain-Gauge.</b>&mdash;The rain-gauge can be combined with
+clock-work and other mechanism so as to be self-recording of the amount of
+rain, the time, and duration of its fall. For the details of construction
+the reader is referred to the next chapter, where he will find the
+instrument described in connection with Osler&#8217;s anemometer, as the
+&#8220;pluviometer.&#8221; To observe and duly record the times of commencement and
+termination of rain is very desirable. Scarcely any observer can attempt
+to do this even approximately from personal observation. Hence the want of
+a cheap and simple self-recording rain-gauge is much felt, the present
+construction being too expensive for all but a few individuals.</p>
+
+<p>In 1862, Mr. R. Strachan estimated the duration and amount of rain in
+London (Gray&#8217;s Inn Road) as follows:&mdash;</p>
+
+<table border="0" cellpadding="0" cellspacing="0" summary="table">
+<tr><td class="btlr" align="center"><span class="smcap">Months.</span></td>
+    <td class="btr" align="center"><span class="smcap">Inches.</span></td>
+    <td class="btr" align="center"><span class="smcap">Days.</span></td>
+    <td class="btrdoub" align="center"><span class="smcap">Hours.</span></td>
+    <td class="btr" align="center"><span class="smcap">Months.</span></td>
+    <td class="btr" align="center"><span class="smcap">Inches.</span></td>
+    <td class="btr" align="center"><span class="smcap">Days.</span></td>
+    <td class="btr" align="center"><span class="smcap">Hours.</span></td></tr>
+<tr><td class="btlr">January.</td>
+    <td class="btr" align="center">1&middot;86</td>
+    <td class="btr" align="center">19</td>
+    <td class="btrdoub" align="center">88</td>
+    <td class="btr">July.</td>
+    <td class="btr" align="center">2&middot;27</td>
+    <td class="btr" align="center">17</td>
+    <td class="btr" align="center">68</td></tr>
+<tr><td class="blr">February.</td>
+    <td class="br" align="center">0&middot;37</td>
+    <td class="br" align="center"><span style="margin-left: .5em;">9</span></td>
+    <td class="brdoub" align="center">25</td>
+    <td class="br">August.</td>
+    <td class="br" align="center">2&middot;45</td>
+    <td class="br" align="center">12</td>
+    <td class="br" align="center">72</td></tr>
+<tr><td class="blr">March.</td>
+    <td class="br" align="center">3&middot;40</td>
+    <td class="br" align="center">22</td>
+    <td class="brdoub" align="center"><span style="margin-left: -.5em;">130</span></td>
+    <td class="br">September.</td>
+    <td class="br" align="center">1&middot;70</td>
+    <td class="br" align="center">12</td>
+    <td class="br" align="center">55</td></tr>
+<tr><td class="blr">April.</td>
+    <td class="br" align="center">2&middot;34</td>
+    <td class="br" align="center">14</td>
+    <td class="brdoub" align="center">80</td>
+    <td class="br">October.</td>
+    <td class="br" align="center">3&middot;23</td>
+    <td class="br" align="center">21</td>
+    <td class="br" align="center">94</td></tr>
+<tr><td class="blr">May.</td>
+    <td class="br" align="center">3&middot;04</td>
+    <td class="br" align="center">16</td>
+    <td class="brdoub" align="center">90</td>
+    <td class="br">November.</td>
+    <td class="br" align="center">1&middot;12</td>
+    <td class="br" align="center">10</td>
+    <td class="br" align="center">53</td></tr>
+<tr><td class="bblr">June.</td>
+    <td class="bbr" align="center">2&middot;45</td>
+    <td class="bbr" align="center">20</td>
+    <td class="bbrdoub" align="center"> 83</td>
+    <td class="bbr">December.</td>
+    <td class="bbr" align="center">1&middot;44</td>
+    <td class="bbr" align="center">17</td>
+    <td class="bbr" align="center">66</td></tr></table>
+
+<p>&#8220;During the year 1862, the rainfall amounted to 25&middot;67 inches. Rain fell on
+179 days, that is, on nearly every other day. The hours of rain were
+estimated at 904; therefore, if the rain had fallen continuously, it would
+have lasted nearly 38 days and nights.&#8221;<small><a name="f10.1" id="f10.1" href="#f10">[10]</a></small> The value of similar estimates
+of the rainfall by numerous observers would be very great to meteorology.</p>
+
+
+<p><br /><b>116. The principle of measurement</b> in all these gauges is the relation
+existing between the areas of the collecting and receiving surfaces; that
+is, between the area of the funnel into which the rain falls, and the area
+of the cylinder which receives it. In Howard&#8217;s and Glaisher&#8217;s gauges, this
+cylinder is virtually the measuring glass itself; in the others, above
+described, the measuring scales show the same depth of water as in the
+cylinder of the gauge.</p>
+
+<p>The cylinder being of less diameter than the funnel, and receiving all the
+rain collected by the funnel, it follows that its contents will have an
+increased depth. Now equal cylindrical volumes, having different
+diameters, are to each other in<span class="pagenum"><a name="Page_113" id="Page_113">[Pg 113]</a></span> length inversely as the squares of the
+diameters. Hence, if the funnel be 9 inches and the cylinder 3 inches in
+diameter, a fall of 1 inch of rain will be represented in the gauge by 9
+inches; for 3&sup2; : 9&sup2; : : 1 : <i>x</i> = 9. In this case, therefore, a length of
+nine inches of the measuring glass, tube, or scale, would represent an
+inch of rainfall, and be divided into tenths and hundredths of the
+artificial inch.</p>
+
+
+<p><br /><b>117. Position for Rain-Gauge, &amp;c.</b>&mdash;Rain gauges should be placed on the
+ground, in any position exposed to a free fall of rain, snow, or hail,
+where neither walls, buildings, nor trees shelter or cause eddies of wind.
+They should be supported by a frame, or other means, to prevent them being
+blown down by the wind, but so that they can be readily emptied.</p>
+
+<p>During snow or frost, the gauge must be watched, and its contents melted
+by placing it in a warm room, either when the amount is to be measured, or
+the funnel is filled up with snow. A tin vessel of equal area to the
+funnel may at such times be useful as a substitute.</p>
+
+<p>Rain gauges are constructed of metal, usually copper, which, besides being
+readily workable, is little affected by atmospheric influences. If made of
+iron or zinc, they should be well japanned; if of copper, this is not so
+essential. The capacity of a gauge should be sufficient to contain at
+least the probable maximum fall of rain in a day at the locality. Those
+required for rainy districts must be of large size.</p>
+
+
+<p><br /><b>118. Causes of Rain.</b>&mdash;When the invisible vapour which is diffused in the
+atmosphere becomes sufficiently cooled, it appears visible as mist or
+cloud, and a further reduction of temperature causes its precipitation as
+rain, hail, or snow. The cooling of the higher regions of the atmosphere
+is doubtless the chief cause of this condensation; but the property which
+aqueous vapour possesses of radiating heat may also contribute to the
+result. Moreover, the law which regulates the amount of vapour which air
+at any particular temperature can sustain in a transparent state,
+determines that when two bodies of air at different temperatures,
+saturated with vapour, intermix, some moisture must be rendered visible;
+and hence, it is not only possible, but highly probable, that rain may
+result from the conflict of different winds. Let us imagine two cubic
+yards of air, both saturated with moisture, but having the respective
+temperatures of 50 and 70 degrees, to come into contact. There will be a
+tendency to equalize the temperature to a mean, which is 60&deg;; and during
+this process, some of the vapour will be condensed.</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td>For</td>
+    <td>in the air at</td>
+    <td>50&deg;</td>
+    <td>there is</td>
+    <td><span class="spacer">&nbsp;</span></td>
+    <td>110&middot;7</td>
+    <td>grains of</td>
+    <td>vapour<small><a name="f11.1" id="f11.1" href="#f11">[11]</a></small></td></tr>
+<tr><td>and</td>
+    <td class="center">"</td>
+    <td>70</td>
+    <td align="center">"</td>
+    <td>&nbsp;</td>
+    <td class="botbor">216&middot;0</td>
+    <td colspan="2" align="center">"</td></tr>
+<tr><td colspan="5">Total amount of vapour</td>
+    <td>326&middot;7</td>
+    <td colspan="2" align="center">"</td></tr>
+<tr><td colspan="5">But two cubic yards of air at 60&deg; can only sustain</td>
+    <td class="botbor">313&middot;2</td>
+    <td colspan="2" align="center">"</td></tr>
+<tr><td colspan="5">Hence there will be deposited</td>
+    <td class="botbor2">13&middot;5</td>
+    <td align="center"><span style="margin-left: 1em;">"</span></td>
+    <td>rain.</td></tr></table>
+
+<p><span class="pagenum"><a name="Page_114" id="Page_114">[Pg 114]</a></span>It may be conceded, therefore, that when a warm and moist current of air
+encounters a body of cold air which may not be extremely dry, the mixture
+is unable to retain the whole of the vapour in an invisible state; so that
+the excess becomes visible as mist or fog, and, when the temperature has
+become sufficiently lowered, rain. The British Isles are more or less
+enveloped in fog, or mist, at the commencement of easterly winds, which,
+with a sudden change of wind, is exhibited even in summer; while the
+south-westerly winds, warm, and arriving from the ocean, deposit large
+quantities of rain by the cooling effect of the land, colder by reason of
+its latitude. When rain occurs with a northerly wind, it is probably due
+to the deposition from an upper south-westerly current, often apparently
+proved by the movements of the upper clouds.</p>
+
+
+<p><br /><b>119. Laws of Rain-fall.</b>&mdash;Tropical countries have a dry and a wet season
+during the year: <i>dry</i>, when the sun is at the opposite side of the
+equator; <i>wet</i>, when the sun is overhead. With reference to the British
+Isles, the statistics collected by Mr. G. J. Symons indicate that: 1st.
+The stations of least rain are inland, or on the east or south-east
+coasts; the stations of greatest rain are on the western coasts. 2nd. The
+rain-fall is very large in the vicinity of mountain chains or groups,
+unless the station happens to be some miles to the north-eastward.</p>
+
+<p>It may be well to illustrate these remarks by quoting<small><a name="f12.1" id="f12.1" href="#f12">[12]</a></small> the average fall
+at a few places, grouping them as&mdash;</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td colspan="2" align="center">Westerly.</td><td><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span></td>
+    <td colspan="2" align="center">Central.</td><td><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span></td>
+    <td colspan="2" align="center">Easterly.</td></tr>
+<tr><td>&nbsp;</td><td><small>Inches.</small></td><td>&nbsp;</td>
+    <td>&nbsp;</td><td><small>Inches.</small></td><td>&nbsp;</td>
+    <td>&nbsp;</td><td><small>Inches.</small></td></tr>
+<tr><td>Bodmin</td><td align="right">43</td><td>&nbsp;</td>
+    <td>Enfield</td><td align="right">23</td><td>&nbsp;</td>
+    <td>Witham (Essex)</td><td align="right">21</td></tr>
+<tr><td>Bolton (Lancashire)</td><td align="right">44</td><td>&nbsp;</td>
+    <td>Epping</td><td align="right">23</td><td>&nbsp;</td>
+    <td>Patrington (Hull)</td><td align="right">21</td></tr>
+<tr><td>Coniston (Windermere)</td><td align="right">71</td><td>&nbsp;</td>
+    <td>Derby</td><td align="right">24</td><td>&nbsp;</td>
+    <td>Sunderland</td><td align="right">17</td></tr>
+<tr><td>Seathwaite</td><td align="right">127</td><td>&nbsp;</td>
+    <td>York</td><td align="right">22</td><td>&nbsp;</td>
+    <td>Inveresk (Edinburgh)</td><td align="right">25</td></tr>
+<tr><td>Torosay (I. of Mull)</td><td align="right">75</td><td>&nbsp;</td>
+    <td>Stirling</td><td align="right">39</td><td>&nbsp;</td>
+    <td>Pittenweem (Fife)</td><td align="right">24</td></tr>
+<tr><td>Killaloe (Limerick)</td><td align="right">38</td><td>&nbsp;</td>
+    <td>Perth</td><td align="right">29</td><td>&nbsp;</td>
+    <td>Dublin</td><td align="right">22</td></tr></table>
+
+<p>Mr. Green, the celebrated aeronaut, has asserted from his experience,
+&#8220;that whenever a fall of rain happens, and the sky is entirely overcast,
+there will invariably be found to exist another stratum of cloud at a
+certain elevation above the former;&#8221; and the recent scientific balloon
+ascents by Mr. Glaisher have tended to confirm this theory. Mr. Glaisher
+says, &#8220;It would seem to be an established fact, that whenever rain is
+falling from an overcast sky, there is a second stratum above.&#8221; &#8220;It would
+also seem that when the sky is overcast without rain, that there is no
+stratum of cloud above, but that the sun is shining on the upper surface.
+In every instance in which I have been up under these circumstances, I
+have found such to be the case, agreeing in this respect also with Mr.
+Green&#8217;s observations.&#8221;</p>
+
+<p><span class="pagenum"><a name="Page_115" id="Page_115">[Pg 115]</a></span>The amount of rain collected in a gauge placed near the surface of the
+earth is larger than in any gauge placed above it; and the higher the
+gauge is placed, the less water is collected. Mr. Glaisher contends that
+his balloon experiments corroborate this law.</p>
+
+
+<p><br /><b>120. Utility of Statistics of Rain-fall.</b>&mdash;The utility of knowing the
+rain-fall of any locality is sufficiently obvious, and little need be said
+upon the subject. The rain-gauge should be in the hands of every gardener
+and farmer. In the management of out-door plants and crops, as well as in
+the construction of cisterns and tanks for the supply of water, a rain
+gauge is a valuable assistant. By its use, the gardener will be guided in
+judging how far the supply of moisture to the earth is needed; and he will
+also see how beneficial is even a hasty shower to growing plants, when he
+considers that a fall of rain measuring the tenth of an inch in depth,
+corresponds to the deposit of about forty hogsheads per acre. The study of
+the rain-fall of a country is of considerable interest to agriculturists.
+The health and increase of domestic animals, the development of the
+productions of the land, as well as the daily labours of the farmer, are
+dependent upon the excess or deficiency of rain. &#8220;It must be a subject of
+great satisfaction and confidence to the husbandman to know at the
+beginning of a summer, by the certain evidence of meteorological results
+on record, that the season, in the ordinary course of things, may be
+expected to be a dry and warm one; or to find, in a certain period of it,
+that the average quantity of rain to be expected for the month has fallen.
+On the other hand, when there is reason, from the same source of
+information, to expect much rain, the man who has courage to begin his
+operations under an unfavourable sky, but with good ground to conclude,
+from the state of his instruments and his collateral knowledge, that a
+fair interval is approaching, may often be profiting by his observations;
+while his cautious neighbour who waited &#8216;for the weather to settle&#8217; may
+find that he has let the opportunity go by. This superiority, however, is
+attainable by a very moderate share of application to the subject; and by
+the keeping of a plain diary of the barometer and rain-gauge, with the
+hygrometer and vane, under his daily notice.&#8221;<small><a name="f13.1" id="f13.1" href="#f13">[13]</a></small> The statistics of
+rain-fall are not only valuable and interesting in a meteorological point
+of view, and for agricultural purposes, but are also highly important in
+connection with sanitary arrangements for towns, and engineering
+operations. This is especially evident to the hydraulic engineer. As rain
+is an important source of water-supply to rivers, canals, and reservoirs,
+it is evident that a knowledge of the probable fall for any season or
+month, at a given place, as furnished by averages of the observations of
+former years, will be the data upon which the engineer will base his plans
+for providing for floods or droughts; while the measurement of the actual
+quantity which has just fallen, as gathered from the indications of a
+series of gauges, will suggest to him the precautions to adopt either to
+economise or conduct away the in-pouring waters.</p>
+
+<p><span class="pagenum"><a name="Page_116" id="Page_116">[Pg 116]</a></span>&#8220;When a canal is conducted across an undulating country, its course is
+necessarily governed by the accidents of the ground, and it alternately
+rises and falls. In this case, rising by a succession of levels, it
+necessarily arrives at a certain highest level, which is called by
+engineers the <i>summit level</i>. From this it again descends by a
+corresponding series of levels. Now, it is evident that, supposing the
+locks to be all equal in magnitude, the ascent of a vessel will require
+the descent of as much water from the summit to the lowest level as would
+fill a single lock; for this quantity of water must be discharged from
+each lock of the series when the vessel passes through it.</p>
+
+<p>&#8220;The same may be said of the process by which the vessel descends along
+the series of locks on the other side of the summit. It appears,
+therefore, that a supply of water must always be maintained on the summit
+level sufficient to fill a single lock twice for each vessel which crosses
+the summit.</p>
+
+<p>&#8220;It happens, fortunately, that by the laws of natural evaporation, rain is
+precipitated in greater quantities on elevated summits than on the
+intermediate valleys, so that the moving power, in this case, accommodates
+itself to the exigencies of intercommunication.&#8221;&mdash;<i>Dr. Lardner&#8217;s &#8220;Handbook
+of Natural Philosophy.&#8221;</i></p>
+
+
+<p><br /><b>121. New Form of Rain-Gauge.</b>&mdash;Since the foregoing pages were in type, a
+modification of Howard&#8217;s rain-gauge has been arranged by Mr. Symons, which
+is compact in design, convenient in use, and low in price. It combines the
+advantages of most gauges; having solidity, and facility of measurement.
+The bottle is placed in a tin case, to the bottom of which are attached
+stout spikes, which, when forced into the earth, prevent its being upset
+either by wind or accident. The bottle being transparent, and slits made
+in the case, the fall of rain is seen at a glance, or with a race-glass,
+from a window. The funnel being attached to the cover of the case is
+thereby kept strictly horizontal, and the depth of rain can be accurately
+measured by lifting the bottle from its case and emptying it into a
+graduated glass jar.</p>
+
+<p>The funnel of this gauge is a very deep cone, to prevent the rain drops
+outsplashing. When properly placed, the receiving surface will be twelve
+inches above the ground, which experience has shown to be the most
+advantageous height.</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_117" id="Page_117">[Pg 117]</a></span></p>
+<h2><a name="CHAPTER_XIII" id="CHAPTER_XIII"></a>CHAPTER XIII.</h2>
+<p class="center"><span class="large">APPARATUS EMPLOYED FOR REGISTERING THE DIRECTION, PRESSURE, AND VELOCITY OF THE WIND.</span></p>
+
+<div class="figright">Fig. 86.<br /><img src="images/fig_86.jpg" alt="" /></div>
+
+<p><br /><b>122. The Vane.</b>&mdash;The instrument by which the wind&#8217;s direction is most
+generally noted, is the vane, or weather-cock, and all that need be said
+of it here is that the points north, east, south and west, usually
+attached to it, should indicate the <i>true</i> and not the <i>magnetic</i>
+directions; and that care should be taken to prevent its setting fast.
+Very complicated instruments are required for ascertaining the pressure
+and velocity of the wind, and these are called <i>Anemometers</i>. The simplest
+is <i>Lind&#8217;s</i>.</p>
+
+<p><br /><b>123. Lind&#8217;s Anemometer, or Wind-Gauge</b> (fig. 86), invented so late as 1775,
+for showing the pressure of the wind, consists of a glass syphon, the
+limbs parallel to each other, and each limb the same diameter. One end of
+the syphon is bent at right angles to the limb, so as to present a
+horizontal opening to the wind. A graduated scale, divided to inches and
+tenths, is attached to the syphon tube, reading either way from a zero
+point in the centre of the scale. The whole instrument is mounted on a
+spindle, surmounted by a vane, and is moved freely in any direction by the
+wind, always presenting the open end towards the quarter from which the
+wind blows. To use the instrument, it is simply filled up to the zero
+point with water, and then exposed to the wind; the difference in the
+level of the water gives the force of the wind in inches and tenths, by
+adding together the amount of depression in one limb, and elevation in the
+other, the <i>sum of the two</i> being the height of a column of water which
+the wind is capable of sustaining at that time.</p>
+
+<p class="center"><br /><span class="pagenum"><a name="Page_118" id="Page_118">[Pg 118]</a></span><span class="smcap">Table</span>,</p>
+
+<p>Showing the Force of Wind on a square foot, for different heights of the
+column of Water in Lind&#8217;s Wind-Gauge.</p>
+
+<table border="0" cellpadding="0" cellspacing="0" summary="table">
+<tr><td class="btlr" align="center">Inches.</td>
+    <td class="btr" align="center">Force in lbs.</td>
+    <td class="btr" align="center">Common designation<br />of such Wind.</td></tr>
+<tr><td class="btlr" align="center">6</td>
+    <td class="btr" align="center">31&middot;75</td>
+    <td class="btr">A Hurricane.</td></tr>
+<tr><td class="blr" align="center">5</td>
+    <td class="br" align="center">26&middot;04</td>
+    <td class="br">A violent Storm.</td></tr>
+<tr><td class="blr" align="center">4</td>
+    <td class="br" align="center">20&middot;83</td>
+    <td class="br">A great Storm.</td></tr>
+<tr><td class="blr" align="center">3</td>
+    <td class="br" align="center">15&middot;62</td>
+    <td class="br">A Storm.</td></tr>
+<tr><td class="blr" align="center">2</td>
+    <td class="br" align="center">10&middot;42</td>
+    <td class="br">A strong Wind.</td></tr>
+<tr><td class="blr" align="center">1</td>
+    <td class="br" align="center"><span style="margin-left: .5em;">5&middot;21</span></td>
+    <td class="br">A high Wind.</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;5</span></td>
+    <td class="br" align="center"><span style="margin-left: .5em;">2&middot;60</span></td>
+    <td class="br">A brisk Wind.</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1em;">&middot;1</span></td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;52</span></td>
+    <td class="br">A fresh Breeze.</td></tr>
+<tr><td class="blr" align="center"><span style="margin-left: 1.5em;">&middot;05</span></td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;26</span></td>
+    <td class="br">A gentle Breeze.</td></tr>
+<tr><td class="bblr" align="center">0.</td>
+    <td class="bbr" align="center"><span style="margin-left: -.5em;">0.</span></td>
+    <td class="bbr">A Calm.</td></tr></table>
+
+
+<p><br /><b>124. Modification of Lind&#8217;s Gauge.</b>&mdash;<i>Sir W. Snow Harris</i> has effected a
+modification of Lind&#8217;s anemometer, with a view of obtaining a hand
+instrument for use at sea more especially. At present the force of the
+wind is estimated at sea by an arbitrary scale, suggested by Sir F.
+Beaufort, the late hydrographer; 0 being calm, 12 the strongest hurricane,
+and the intermediate numerals giving the varying strength of the wind.
+There has been a long-felt want of instrumental means for obtaining this
+data at sea, if merely for the sake of checking occasionally personal
+estimations, which may vary considerably among different observers.
+Harris&#8217;s wind gauge is intended to be held by hand, while facing the wind,
+and keeping it in proper position by attending to a spirit-level attached.
+When in position, and held firmly, the tube has to be opened to the wind
+by pressure of the thumb acting upon jointed levers, controlled by
+springs. The pressure of the wind moves the enclosed liquid; and by
+withdrawing the thumb, the tube is closed so as to keep the liquid in its
+position; the reading is then taken from its scale, either in pounds on
+the square foot, miles per hour, or the ordinary designations of wind, as
+light, fresh, strong, &amp;c.</p>
+
+<div class="figright">Fig. 87.<br /><img src="images/fig_87.jpg" alt="" /></div>
+
+<p><br /><b>125. Robinson&#8217;s Anemometer.</b>&mdash;<i>Dr. Robinson</i>, of Armagh, is the inventor of
+a very successful anemometer, which determines the horizontal velocity of
+the wind. It was first used in 1850, in the meteorological and tidal
+observations made on the coast of Ireland under the direction of the Rev.
+Dr. Lloyd. No meteorological observatory should be without this valuable
+instrument, which is essential in determining the average velocity of the
+wind of a locality as distinguished from<span class="pagenum"><a name="Page_119" id="Page_119">[Pg 119]</a></span> the most frequent wind of the
+same place. It is represented in fig. 87. Four hollow hemispherical cups,
+<i>A A</i>, are extended upon conjugate diameters, or arms, with their
+diametrical planes placed vertically, and facing the same way upon a
+vertical axis, <i>B</i>, which has at its lower extremity an endless screw,
+<i>D</i>. The axis is supported at <i>C</i> so as to turn with as little friction as
+possible. The endless screw is placed in gear with a train of wheels and
+pinions. Each wheel carries an index over a stationary dial in front; or
+the index is fixed, and the graduations are placed upon the wheels
+themselves.</p>
+
+<p>Dr. Robinson has proved, both by theory and experiment, that the centre of
+any one of the cups so mounted and set in motion by the wind, revolves
+with one-third of the wind&#8217;s velocity. If, therefore, the diametrical
+distance between the centres of the cups be one foot, the circle described
+by the centres in one revolution is 3&middot;1416 feet, and the velocity of the
+wind will be three times this, or 9&middot;42 feet, which must be referred to
+time for the absolute rate. The instrument is sometimes made with the
+centres of the cups 1&middot;12 feet apart, so that the circle described is
+<span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">1500</span> of a mile in circumference. Hence, to produce one revolution of the
+cups, the wind must travel three times as fast, or <span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">500</span> of a mile.
+Therefore, 500 revolutions will be produced by one mile of wind; so that
+the dials may be graduated to register the velocity in miles and tenths of
+miles. The simplest arrangement is with five dials, recording respectively
+10, 100, 1,000, 10,000 and 100,000 revolutions.</p>
+
+<p><i>Directions for using Robinson&#8217;s Anemometer.</i>&mdash;The dials read off in the
+same manner as the register of a gas meter, commencing with the dial
+farthest from the endless screw.</p>
+
+<p>&#8220;The figures on the first dial indicate so many hundreds of thousands of
+revolutions; those on the second dial so many tens of thousands; those on
+the third, thousands; those on the fourth, hundreds; and those on the
+fifth so many tens.</p>
+
+<p>&#8220;The instrument should be read every morning at 9 o&#8217;clock; and, usually,
+it will only be necessary to read the first three dials. The figures can
+be entered as they are read off. Should the index point <i>between</i> two
+figures, the less of the two is to be taken.</p>
+
+<p>&#8220;For example, if the first dial points to 7, or between 7 and 8; while the
+second dial indicates 4; and the third, 5; the entry to be made is 745
+(indicative of 745 <i>thousand</i> revolutions).</p>
+
+<p>&#8220;Every time the index of the first dial is found to have passed zero (0),
+a cross or star is to be prefixed to the next (a lower) reading.</p>
+
+<p><span class="pagenum"><a name="Page_120" id="Page_120">[Pg 120]</a></span>&#8220;To ascertain how many <i>thousands</i> of revolutions have been made during
+the month, it will simply be necessary to subtract the first reading from
+the last, and prefix to the three figures thus obtained a figure
+corresponding to the number of stars in the column. For every <i>thousand</i>
+revolutions there are two miles of wind: we have therefore only to
+multiply by 2 to find how many miles of wind have passed during the month.</p>
+
+<p>&#8220;Two entries must be made for the last day of each month (the one being
+written under the other), so as to bring the readings down to 9 <span class="smcaplc">A.M.</span> on
+the 1st of the following month. The same entry which ends one month, will
+therefore begin the next. This repetition of one entry is necessary, in
+order to prevent losing a day&#8217;s wind.</p>
+
+<table border="0" cellpadding="0" cellspacing="0" summary="table">
+<tr><td><span style="margin-left: 1em;">&#8220;The accompanying example of the</span></td><td><span class="spacer">&nbsp;</span></td><td align="right">687</td></tr>
+<tr><td>readings of an Anemometer for 13 days</td><td>&nbsp;</td><td align="right">773</td></tr>
+<tr><td>will illustrate the method of making</td><td>&nbsp;</td><td align="right">822</td></tr>
+<tr><td>the entries, &amp;c.</td><td>&nbsp;</td><td align="right">855</td></tr>
+<tr><td><span style="margin-left: 1em;">&#8220;In this instance, the first read-</span></td><td>&nbsp;</td><td align="right">900</td></tr>
+<tr><td>ing (687) is less than the last (793).</td><td>&nbsp;</td><td align="right">953</td></tr>
+<tr><td>When the first reading is greater than</td><td>&nbsp;</td><td align="right">990</td></tr>
+<tr><td>the last, it will be necessary to borrow</td><td>&nbsp;</td><td align="right">*066</td></tr>
+<tr><td>1,000 in making the subtractions,</td><td>&nbsp;</td><td align="right">197</td></tr>
+<tr><td>and then deduct one from the number</td><td>&nbsp;</td><td align="right">323</td></tr>
+<tr><td>of stars. Thus, if the first reading</td><td>&nbsp;</td><td align="right">414</td></tr>
+<tr><td>of the series on the margin had</td><td>&nbsp;</td><td align="right">597</td></tr>
+<tr><td>been 887, the result would have been</td><td>&nbsp;</td><td align="right">712</td></tr>
+<tr><td>906 instead of 1106.</td><td>&nbsp;</td><td align="right" class="botbor">793</td></tr>
+<tr><td colspan="2">&nbsp;</td><td align="right">1106</td><td>&nbsp; thousands of revolutions.</td></tr>
+<tr><td colspan="2">&nbsp;</td><td class="botbor" align="right">2</td></tr>
+<tr><td>&nbsp;</td><td class="br" align="right">13</td><td class="botbor" align="right">&nbsp; 2212</td><td>&nbsp; miles of wind in period.</td></tr>
+<tr><td colspan="2">&nbsp;</td><td align="right">170</td><td>&nbsp; miles of wind per day, on an average.</td></tr></table>
+
+<p>&#8220;The foregoing directions are all which require to be regularly attended
+to. But it may be interesting at times to find the velocity of the wind
+during a period of a few minutes. This may be ascertained by observing the
+difference of two readings of all the dials, with an interval of some
+minutes between them, when a very brief calculation will suffice; but
+perhaps the simplest method is the following:&mdash;</p>
+
+<div class="figright">Fig. 88.<br /><img src="images/fig_88.jpg" alt="" /></div>
+
+<p>&#8220;Take two readings, with an interval of 12 minutes between them. The
+difference of these readings, divided by 10, is the velocity of the wind
+in miles per hour. Thus&mdash;if the reading of the five dials (from left to
+right) at noon is 15206, and at 12 minutes past 12 is 15348, the velocity
+of the wind is 14&middot;2 miles per hour.&#8221;&mdash;<i>Admiral FitzRoy, F.R.S.</i></p>
+
+<p><span class="pagenum"><a name="Page_121" id="Page_121">[Pg 121]</a></span>A lever and clutch are sometimes fitted to this anemometer, as in fig. 88,
+for throwing the train out of gear when not required to register. It may
+also be connected with clock-work so as to be self-recording, by causing
+the mechanism to impress a mark upon prepared paper moved by the
+apparatus, at certain intervals of time.</p>
+
+<p>This anemometer should be fixed in an exposed situation, as high above
+ground as may be convenient for reading. It may be made very portable, by
+the arms which carry the cups being fitted to unscrew or to fold down.
+When fitted in gimbals, it can be used at sea with much advantage.</p>
+
+<p>The pressure of the wind has been experimentally proved to vary as the
+square of the velocity; the relation being <i>V&sup2;</i> = 200 &times; <i>P</i>. From this
+formula, therefore, the pressure can be calculated corresponding to the
+observed velocity.</p>
+
+
+<p><br /><b>126. Whewell&#8217;s Anemometer.</b>&mdash;This apparatus, the invention of the
+celebrated Dr. W. Whewell, registers the horizontal motion of the air with
+the direction. Its mechanism may be described in general terms, as
+follows:&mdash;</p>
+
+<p>A horizontal brass plate is attached to a vertical spindle, which passes
+through the axis of a fixed cylinder, being supported by a bearing at the
+lower end, and working in a collar at the upper. A vane is attached, by
+which the plate is moved about according to the direction of the wind. A
+fly, having eight fans, each fixed at an angle of 45&deg; with the axle, is
+placed upon the plate so that the axle is in the line of direction of the
+vane. An endless screw on the axle turns a vertical wheel having one
+hundred teeth, the axle to which has also an endless screw working into a
+horizontal wheel, having a like number of teeth, and which communicates
+motion to a vertical screw fifteen inches long. On this screw is placed a
+moveable nut, which carries a pencil. Round the cylinder is wrapped daily
+a paper divided for the points of the compass. The wind acting upon the
+vane will cause the plate to turn; and the screw which carries the pencil
+will travel with it, so that the pencil will mark upon the paper the
+direction of the wind. The fly will also be set in motion, and thereby the
+nut upon the screw will descend, so that the attached pencil will trace a
+vertical line upon the paper. When the fans on the axle are 2&middot;3 inches
+from axis to end, and 1&middot;9 inches wide, and the thread of the screw such
+that forty-five revolutions will cause the nut to descend two inches,
+75&middot;85 miles of wind will cause the pencil to descend through a vertical
+space of two inches; but the actual trace upon the paper will be longer in
+proportion to the magnitude of change of azimuth, or direction, of the
+wind.</p>
+
+
+<p><br /><span class="pagenum"><a name="Page_122" id="Page_122">[Pg 122]</a></span><b>127. Osler&#8217;s Anemometer, and Pluviometer.</b>&mdash;Mr. Follet Osler is the
+inventor of a self-recording apparatus which registers the direction and
+pressure of the wind, and the amount and duration of rain, upon the same
+sheet of paper. His apparatus has met with very much approbation, and has
+been erected in many observatories. The mechanism may be modified in
+various ways, and the following is a description of the simplest and most
+recent arrangement.</p>
+
+<p>&nbsp;</p>
+<p class="center">Fig. 89.</p>
+<div class="figcenter"><img src="images/fig_89.jpg" alt="" /></div>
+<p>&nbsp;</p>
+
+<p>The instrument, of which fig. 89 is a diagram rather than a picture,
+consists, first, of a vane, <i>V</i>, of a wedge-shape form, which is found to
+answer better than a flat vane; for the latter is always in a neutral
+line, and therefore is not sufficiently<span class="pagenum"><a name="Page_123" id="Page_123">[Pg 123]</a></span> sensitive. A wind-mill governor
+has been substituted for the vane to get the direction of the wind, with
+advantage. At the lower end of the tube, <i>T T</i>, is a small pinion, working
+in a rack, <i>r</i>, which moves backwards and forwards as the wind presses the
+vane. To this rack a pencil, <i>x</i>, is attached, which marks the direction
+of the wind on a properly ruled paper, placed horizontally beneath, and so
+adjusted as to progress at the rate of half an inch per hour, by means of
+a simple contrivance connecting it with a good clock. The paper is shown
+in the illustration upon the table of the instrument.</p>
+
+<p>The pressure plate, <i>F</i>, for ascertaining the force of the wind, is one
+foot square, placed immediately beneath, and at right angles with the
+vane; it is supported by light bars, running horizontally on friction
+rollers, and communicating with flattened springs, 1, 2, 3, so that the
+plate, when affected by the pressure of the wind, acts upon them, and they
+transfer such action to a copper chain passing down the interior of the
+direction tube, and over a pulley at the bottom. A light copper wire
+connects this chain with the spring lever, <i>y y</i>, carrying a pencil which
+records the pressure upon the paper below. Mr. Osler much prefers a spring
+to any other means for ascertaining the force of the wind, because it is
+of the highest importance to have as little matter in motion as possible,
+otherwise the momentum acquired will cause the pressure plate to give very
+erroneous indications. The pressure plate is as light as is consistent
+with strength. It is kept before the wind by the vane, and is urged out by
+three or more springs, so that with light winds one only is compressed,
+and two, or more, according to the strength of the wind.</p>
+
+<p>The <i>pluviometer</i> is placed on the right in the figure, <i>P P</i> being the
+plane of the roof of the building. The rain funnel, <i>R</i>, exposes an area
+of about 200 square inches. The water collected in it is conveyed by a
+tube through the roof of the building into a glass vessel, <i>G</i>, so
+adjusted and graduated as to indicate a quarter of an inch of rain for
+every 200 square inches of surface, <i>i. e.</i> 50 cubic inches. <i>G</i> is
+supported by spiral springs, <i>b b</i>, which are compressed by the
+accumulating rain. A glass tube, open at both ends, is cemented into the
+bottom of <i>G</i>, and over it is placed a larger one closed at the top like a
+bell glass. The smaller tube thus forms the long leg of a syphon, and the
+larger tube acts as the short leg. The water, having risen to the level of
+the top of the inner tube, drops over into a little copper tilt, <i>t</i>, in
+the globe, <i>S</i>, beneath the reservoir. This tilt is divided into two equal
+partitions by a slip of copper, and placed upon an axis not exactly
+balanced, but so that one end or the other preponderates. The water then
+drops into the end of the tilt which happens to be uppermost, and when
+quite full it falls over, throwing the water into the globe, <i>S</i>, from
+which it flows away by the waste pipe. In this way an imperfect vacuum is
+produced in the globe, quite sufficient to produce a draught in the small
+tube of the syphon, or the long leg; and the whole contents of the
+reservoir, <i>G</i>, immediately run off, and the spiral springs, <i>b b</i>,
+elevate the reservoir to its original position. To produce this action, a
+quarter of an inch of rain must have fallen. The registration is easily
+understood. A spring lever, <i>z</i>,<span class="pagenum"><a name="Page_124" id="Page_124">[Pg 124]</a></span> carrying a pencil, is attached by a
+cord, <i>c</i>, to <i>S</i>. This spring always keeps the cord tight, so that as the
+apparatus descends during the fall of rain, the spring advances the pencil
+more and more from the zero of the scale upon the paper beneath, until a
+quarter of an inch has fallen, when the pencil is drawn back to zero by
+the ascent of the reservoir.</p>
+
+<p>The clock movement carries the registering paper forward by one of the
+wheels working into a rack attached to the frame.</p>
+
+<p>The adjustment of the instrument should be carefully made at its first
+erection. The scale for pressure should be established experimentally, by
+applying weights of 2, 4, 6, &amp;c., lbs., to move the pressure plate.</p>
+
+<p>The registration trace for twenty-four hours is readily understood. The
+direction is recorded on the centre part; the pressure on one side, and
+the rain on the other. Lines parallel to the length of the paper show no
+rain, steady wind, and constant pressure. On the rain trace, a line
+parallel to the width of the paper shows that the pencil had been drawn
+back to zero, a quarter of an inch of rain having fallen. The hour lines
+are in the direction of the width of the paper.</p>
+
+<p>At the International Exhibition 1862, Messrs. Negretti and Zambra
+exhibited an improved Osler&#8217;s anemometer, having combined with it
+Robinson&#8217;s cups, so that the pressure and velocity appear on the same
+sheet, on which a line an inch in length is recorded at every ten miles;
+thus the complete instrument shows continuously the direction, pressure,
+and velocity of the wind.</p>
+
+
+<p><br /><b>128. Beckley&#8217;s Anemometer.</b>&mdash;Mr. R. Beckley, of the Kew Observatory, has
+devised a self-registering anemometer, which consists of three principal
+parts: Robinson&#8217;s cups for the determination of velocity; a double fan, or
+wind-mill governor, for obtaining the direction; and a clock to move a
+cylinder, around which registration paper is wrapped. The paper records
+the time, velocity, and direction of the wind for twenty-four hours, when
+it must be replaced. It has a cast-iron tubular support, or pedestal to
+carry the external parts&mdash;the cups and the fans,&mdash;which must be erected
+upon the roof of the building upon which it is desired to mount the
+instrument.</p>
+
+<p>The fans keep their axis at right angles to the wind; and with any change
+of direction they move, carrying with them an outer brass tube, which
+rests upon friction balls on the top of the pedestal, and is attached to a
+tubular shaft passing through the interior of the pedestal, and
+terminating with a mitre wheel. The mitre wheel, working with other cogged
+wheels, communicates the motion of the direction shaft to a cylinder
+carrying a pencil, to record the direction.</p>
+
+<p>The shaft carrying the cups is supported upon friction balls, placed in a
+groove formed on the top of the direction shaft, and passing through the
+interior of that shaft, comes out below the mitre wheel, where it is
+terminated in an endless screw, or worm.</p>
+
+<p>Upon the wind moving the cups, motion is given to the innermost shaft,<span class="pagenum"><a name="Page_125" id="Page_125">[Pg 125]</a></span>
+thence to the worm-wheel, whence motion is given to a pencil which
+registers the velocity.</p>
+
+<p>De la Rue&#8217;s metallic paper is used in registration, it having the property
+of receiving a trace from a brass pencil. The pencils can, therefore, be
+made in the most convenient form. Mr. Beckley forms each pencil of a strip
+of brass wrapped round a cylinder, making a very thin threaded screw, so
+that the contact of the pencil cylinder and the clock cylinder is a mere
+point of the metallic thread. The pencil cylinders are placed side by side
+upon the cylinder turned by the clock, and require no spring or other
+appliance to keep them to their work, but always make contact with the
+registration paper by their own gravity. They therefore require no
+attention, and being as long as the trace which they make, they will last
+a long time.</p>
+
+<p>The velocity pencil has only one turn on the cylinder, and its pitch is
+equal to a scale of fifty miles upon the paper. The direction pencil has
+likewise one turn on its cylinder, its pitch being equal to a scale of the
+cardinal points of the compass upon the paper.</p>
+
+<p>The clock gives a uniform motion of half an inch per hour to the cylinder
+upon which the paper is fastened.</p>
+
+<p>The registering mechanism of the instrument is very compact, requiring
+only a space of about 18 inches by 8 inches.</p>
+
+<p>In the Report of the British Association for 1858, Mr. Beckley has given a
+detailed description of his anemometer, with drawings of all the parts.</p>
+
+
+<p><br /><b>129. Self-Registering Lind&#8217;s Anemometer.</b>&mdash;A Lind&#8217;s wind-gauge, designed to
+register the maximum pressure, was exhibited at the International
+Exhibition 1862, by Mr. E. G. Wood. The bend of the syphon is contracted
+to obtain steadiness. On the leeward limb a hole is drilled corresponding
+in size with the contracted portion of the tube. The edge of the hole
+corresponds with the zero of the scale. On the pressure of the wind
+increasing, as much of the water as would have risen above the aperture
+flows away, and therefore the quantity left indicates the greatest
+pressure of the wind since the last setting of the instrument, which is
+done by filling it with water up to the zero point.</p>
+
+
+<p><br /><b>130. Anemometric Observations.</b>&mdash;To illustrate the value of anemometric
+observations, we quote from a paper by Mr. Hartnup, on the results
+obtained from Osler&#8217;s Anemometer, at the Liverpool Observatory. The six
+years&#8217; observations, ending 1857, gave for the yearly average of the
+winds: North-easterly, on 60 days, at 7&middot;8 miles per hour; North-westerly,
+on 112 days, at 15&middot;4 miles per hour; South-easterly, on 115 days, at 11&middot;0
+miles per hour; South-westerly, on 77 days, at 13&middot;8 miles per hour; and
+one day calm. From the same observations, the average variation in the
+strength of the wind during the 24 hours is:&mdash;11 miles per hour, the
+minimum force, occurring at 1&#189; a.m.; until 6 a.m. it remains much the
+same, being then 11&middot;3 miles per hour; at 10 a.m. it is 13&middot;4 miles per
+hour; at 1&#189; p.m.<span class="pagenum"><a name="Page_126" id="Page_126">[Pg 126]</a></span> the wind is at its maximum strength, being 14&middot;8 miles
+per hour; at 5 p.m. it is again 13&middot;4 miles per hour, and at 9 p.m. 11&middot;3
+miles per hour. Hence it appears that the wind falls to its minimum force
+much more gradually than it rises to its maximum; that the decrease and
+increase are equal and contrary, so that the curve is symmetrical; and
+that generally the force of wind is less at night than during the day.</p>
+
+<p>&#8220;There is evidence,&#8221; says Admiral FitzRoy, &#8220;in Mr. Hartnup&#8217;s very valuable
+anemometrical results, which seems to prove that to his observatory, in a
+valley, with buildings and hills to the north-eastward, the real polar
+current does not blow from N.E., but nearer S.E. By his reliable digest of
+winds experienced there, it appears that those most prevalent were from
+W.N.W. and S.S.E. But in England, generally, the <i>prevailing</i> winds are
+<i>believed</i> to be westerly, inclining to south-westerly, and
+north-easterly; while of all winds, the south-easterly is about the
+rarest.</p>
+
+<p>&#8220;At Lord Wrottesley&#8217;s observatory, in Staffordshire, about 530 feet above
+the sea, there appears to be considerably less strength of wind at any
+given time, when a gale is blowing <i>generally</i>, than occurs simultaneously
+at places along the sea-coast: whence the inference is, that undulations
+of the land&#8217;s surface and hills, diminish the strength of wind materially
+by frictional resistance.</p>
+
+<p>&#8220;All the synoptic charts hitherto advanced at the Board of Trade exhibit a
+marked diminution of force inland compared with that on the sea-coast.
+Indeed, the coast itself offers similar evidence, in its stunted, sloping
+trees, and comparative barrenness.&#8221;<small><a name="f14.1" id="f14.1" href="#f14">[14]</a></small></p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_127" id="Page_127">[Pg 127]</a></span></p>
+<h2><a name="CHAPTER_XIV" id="CHAPTER_XIV"></a>CHAPTER XIV.</h2>
+<p class="center"><span class="large">INSTRUMENTS FOR INVESTIGATING ATMOSPHERIC ELECTRICITY.</span></p>
+
+<div class="figright">Fig. 90.<br /><img src="images/fig_90.jpg" alt="" /></div>
+
+<p><br /><b>131. Atmospheric Electroscope.</b>&mdash;The simplest instrument for ascertaining
+at any time the electric condition of the atmosphere is an electroscope
+composed of two equal pieces of gold leaf, suspended from a brass support,
+and insulated, as well as protected from the movement of the air, by a
+glass covering. Fig. 90 represents such an instrument. The cap of the
+brass support is fitted for the reception, in the vertical direction, of a
+metallic rod, not less than two or three feet in length. The top of the
+rod carries a clip. The instrument acts according to the law, that bodies
+similarly electrified repel each other; but when dissimilarly electrified,
+they attract each other. To make an observation, the instrument is placed
+in the open air, and a lighted piece of cigar fusee, or touch-paper, is
+fixed in the clip. The electricity of the air is collected by the
+substance undergoing combustion, and conducted by the rod to the gold
+leaf; and the pieces, being similarly electrified, separate more or less
+according to the amount of electricity present. The kind is determined by
+the effect of either an excited stick of sealing-wax or rod of glass upon
+the electrified gold leaf. A rod of glass, when rubbed briskly with a silk
+handkerchief or piece of woollen cloth, becomes positively electrified, or
+excited, as it is termed. A stick of sealing-wax, similarly treated,
+acquires the negative state. If, therefore, an excited glass rod be
+presented to the cap of the instrument, and it cause the pieces of gold
+leaf to diverge still further, the electric state of the air must be
+analogous to that of the glass, that is, <i>positive</i>; if they approach, it
+is <i>negative</i>. On the contrary, if a stick of sealing-wax be used, the
+pieces will be repelled more apart if they have acquired negative
+electricity from the air; and they will converge if they have a positive
+charge.</p>
+
+<p>By means of this very simple instrument, meteorological observers can
+readily ascertain the electric condition of the lower air at any time.</p>
+
+<p><span class="pagenum"><a name="Page_128" id="Page_128">[Pg 128]</a></span><span class="smcap">Note.</span>&mdash;A book containing
+strips of gold leaf is sent with the Electrometer to replace the gold leaves when torn or broken in use.</p>
+
+<p>To mount fresh gold leaves, unscrew the brass plate to which is attached
+the rod supporting the leaves; then moisten with the breath the flat piece
+of brass, and press it gently down on one strip of gold, whilst the book
+is only partly opened; the second leaf is attached in the same manner.</p>
+
+
+<p><br /><b>132. Volta&#8217;s Electrometer</b> is similar to the instrument just described,
+except that instead of gold leaf two light pieces of straw, or two pith
+balls, are freely suspended from the conductor; the amount of the electric
+charge being estimated from the degrees of divergence, shown by a
+graduated arc.</p>
+
+
+<p><br /><b>133. Peltier&#8217;s Electrometer</b> is a much superior instrument in point of
+sensibility. A tall glass tube an inch or more in diameter, is connected
+to a glass receiver, mounted on a base fitted with levelling screws. At
+the top of the tube is formed a globe from four to five inches in
+diameter, which is thickly gilt on the exterior, so as to form a good
+conducting surface. A wire passes from the ball down the tube into the
+receiver, where it is bent up, and ends in a steel point over the centre
+of the base. A bent wire, carrying a small magnetic needle, is balanced on
+the steel point, so that the magnet, with the fine wire, arranges itself
+horizontally in the direction of the magnetic meridian. If any cloud or
+portion of air in the neighbourhood be in an electrical state, it will act
+by induction upon the gilt ball, and the needle will be deflected from its
+north and south direction.</p>
+
+<p>A graduated circle indicates the number of degrees of the deflection,
+which will be greater or less according to the tension of the electricity.
+To ascertain whether the electricity is positive or negative, a stick of
+shellac or glass must be employed, as already described.</p>
+
+<div class="figleft">Fig. 91.<br /><img src="images/fig_91.jpg" alt="" /></div>
+
+<p><br /><b>134. Bohnenberger&#8217;s Electroscope</b> may be fitted with a metallic conductor,
+and used with great advantage for observing atmospheric electricity. &#8220;The
+principal parts of the instrument, as improved by Becqu&eacute;rel, are the
+following:&mdash;<i>A B</i>, fig. 91, is a small dry galvanic pile of from 500 to
+800 pairs, about a quarter of an inch in diameter; when the plates are
+pressed together, such a pile will be from 2 to 2&#189; inches in length.
+The wires, which are bent so as to stand above the pile, terminate in two
+plates, <i>P</i> and <i>M</i>, which are the poles of the pile. These plates, which
+are 2 inches by &#189; an inch, are parallel and opposite to each other. It
+is convenient for their opposite sides to be slightly convex, for them to
+be gilded or coated with platinum, and for them to run on the polar wires,
+by the latter being made to pass through a small hole in them. One of
+these plates will always be in a state of positive,<span class="pagenum"><a name="Page_129" id="Page_129">[Pg 129]</a></span> and the other of
+negative, electricity; between them suspend the very fine gold leaf, <i>D
+G</i>, which is attached to the conductor, <i>C D</i>, of copper wire. If the leaf
+hang exactly between the two plates, it is equally attracted by each, and
+will therefore be in a state of repose. The apparatus should be protected
+by a bell-glass, fitting exactly, and having an opening at the top through
+which the copper wire, <i>C D</i>, passes; the wire, however, is insulated by
+its being contained in a glass tube, which is made to adhere to the
+bell-glass by means of a small portion of shellac or gum-lac. Screw on a
+metal ball or plate, to impart to it the electricity you wish to test,
+which will be conveyed by the copper wire to the gold leaf, and the latter
+will immediately move towards the plate which has the opposite polarity.
+This electroscope is, beyond doubt, one of the most delicate ever
+constructed, and is well adapted to show small quantities of positive and
+negative electricity.</p>
+
+<p>&#8220;To ensure the susceptibility of electroscopes and electrometers placed
+under bell-glasses, precautions should be taken to render the air they
+contain as dry as possible, which may be effected by enclosing in a
+suitable vessel a little melted chloride of calcium beneath the glass.&#8221;</p>
+
+<p>The galvanic pile employed in this electroscope is that invented by
+Zamboni. &#8220;It differs from the common hydro-electric batteries principally
+in this, that the presence of the electromotive liquid is dispensed with,
+and that in its place is substituted some moist substance of low
+conducting power, generally paper. The electromotors in these piles are
+composed for the most part of Dutch gold (copper) and silver (zinc) paper
+pressed one on the other, with their paper sides together, out of which
+discs are cut with a diameter of from a quarter of an inch to an inch.
+More powerful pairs of plates may be obtained by using only the silver
+paper and smearing its paper side with a thin coat of honey, on which some
+finely pulverized peroxide of manganese has been sprinkled, and all the
+sides similarly coated are presented one way. Powerful pairs of plates may
+also be made by pasting pure gold leaf on the paper side of zinc-paper.
+These plates are then to be arranged, just as in the ordinary voltaic
+pile, one above the other, so that the similar metallic surfaces may all
+lie one way; press them tightly together; tie them with pretty stout silk
+threads, and press them into a glass tube of convenient size. The metal
+rims of the tubes, which must be well connected with the outermost pairs
+of plates, form the poles of the pile, the negative pole being in the
+extreme zinc surface, and the positive in the extreme copper or manganese
+surface.</p>
+
+<p>&#8220;The electromotive energy called into action in these dry piles is less
+than that excited in the moist or hydro-electric piles, principally on
+account of the imperfect conduction of the paper. The accumulation of
+electricity at their poles also goes on less rapidly, and consequently the
+electrical tension continues for a long while unaltered; whereas, in all
+moist piles, even in the most constant of them, the tension is maintained,
+comparatively speaking, for but a short time, on account of<span class="pagenum"><a name="Page_130" id="Page_130">[Pg 130]</a></span> the chemical
+action and decomposition of the electromotive fluid&mdash;causes of disturbance
+which do not exist in the dry pile.&#8221;<small><a name="f15.1" id="f15.1" href="#f15">[15]</a></small></p>
+
+
+<p><br /><b>135. Thomson&#8217;s Electrometer.</b>&mdash;Professor W. Thomson, of Glasgow, has
+devised an atmospheric electrometer, which is likely to become eminently
+successful, in the hands of skilful observers. It is mainly a torsion
+balance combined with a Leyden-jar. The index is an aluminium needle
+strung on a fine platinum wire, passing through its centre of gravity, and
+stretched firmly between two points. The needle and wire are carefully
+insulated from the greater part of the instrument, but are in metallic
+communication with two small plates fixed beside the two ends of the
+needle, and termed the repelling plates. A second pair of larger plates
+face the repelling plates, on the opposite side of the needle, but
+considerably farther from it. These plates are in connection with the
+inner coating of a Leyden-jar, and are termed the attracting plates. The
+whole instrument is enclosed in a metal cage, to protect the glass
+Leyden-jar and the delicate needle.</p>
+
+<p>The Leyden-jar should be charged when the instrument is used. Its effect
+is two-fold: it increases greatly the sensibility of the instrument, and
+enables the observer to distinguish between positive and negative
+electrification.</p>
+
+<p>The air inside the jar is kept dry by pumice-stone, slightly moistened
+with sulphuric acid; by which means very perfect insulation is maintained.</p>
+
+<p>Electrodes, or terminals, are brought outside the instrument, by which the
+Leyden-jar can be charged, and the needle system connected with the body,
+the electric state of which is to be tested.</p>
+
+<p>For the purpose of testing the electric state of the atmosphere, the
+instrument is provided with a conductor and support for a burning match,
+or, preferably, with an arrangement termed a water-dropping collector; by
+either of which means the electricity of the air is conveyed to the needle
+system.</p>
+
+<p>The needle abuts upon the repelling plates when not influenced by
+electricity, in which position it is at zero. It can always be brought
+back to zero by a torsion-head, turning one end of the platinum wire, but
+insulated from it, and provided with a graduated circle, so that the
+magnitude of the arc, that the torsion-head is moved through to bring the
+needle to zero, measures the force tending to deflect it.</p>
+
+<p>The action of the instrument is as follows:&mdash;The Leyden-jar is to be
+highly charged, say negatively; and the repelling plates are to be
+connected with the earth. The needle will then be deflected against a
+stop, under the combined influence of attraction from the Leyden-jar, or
+attracting plates, and repulsion from the repelling plates due to the
+positive charge induced on the needle and its plates by the Leyden-jar
+plates. The platinum wire must then be turned round by the torsion-head so
+as to bring back the needle to zero; and the number of degrees of torsion
+required will measure the force with which the needle is attracted. Next,
+let the needle<span class="pagenum"><a name="Page_131" id="Page_131">[Pg 131]</a></span> plates be disconnected from the earth, and connected with
+the insulated body, the electric state of which is to be tested. In
+testing the atmosphere, the conductor and lighted match, or water-dropping
+apparatus, must be applied.</p>
+
+<p>If the electricity of the body be positive, it will augment the positive
+charge in the needle plates, induced by the Leyden-jar plates; and
+consequently the needle will be more deflected than by the action of the
+jar alone. If the electricity of the body be negative, it will tend to
+neutralize the positive charge; and the needle will be less deflected.
+Hence the kind of electricity present in the air becomes at once apparent,
+without the necessity of an experimental test. The platinum wire must then
+be turned till the needle is brought to zero, and the number of degrees
+observed; which is a measure of the intensity of the electrification.</p>
+
+<p>Any loss of charge from the Leyden-jar which may from time to time occur,
+reducing the sensibility inconveniently, may be made good by additions
+from a small electrophorus which accompanies the instrument.<small><a name="f16.1" id="f16.1" href="#f16">[16]</a></small></p>
+
+<p>The instrument may be made self-recording by the aid of clockwork and
+photography. To effect this, a clock gives motion to a cylinder, upon
+which photographic paper is mounted. The needle of the electrometer is
+made to carry a small reflector; and rays from a properly adjusted source
+of light are thrown by the reflector, through a small opening, upon the
+photographic paper. It is evident, that as the cylinder revolves, a trace
+will be left upon the paper, showing the magnitude of, and variations in,
+the deflection of the needle.</p>
+
+
+<p><br /><b>136. Fundamental Facts regarding Atmospheric Electricity.</b>&mdash;The <i>general</i>
+electrical condition of the atmosphere is <i>positive</i> in relation to the
+surface of the earth and ocean, becoming more and more positive as the
+altitude increases. When the sky is overcast, and the clouds are moving in
+different directions, it is subject to great and sudden variations,
+changing rapidly from positive to negative, and the reverse. During fog,
+rain, hail, sleet, snow, and thunderstorms, the electrical state of the
+air undergoes many variations. The intensity of the electricity increases
+with hot weather following a series of wet days, or of wet weather coming
+after a continuance of dry days. The atmospheric electricity, in fact,
+seems to depend for its intensity and kind upon the direction and
+character of the prevailing wind, under ordinary circumstances. It has an
+annual and a diurnal variation. There is a greater diurnal change of
+tension in winter than in summer. By comparing observations from month to
+month, a gradual increase of tension is perceived from July to February,
+and a decrease from February to July. The intensity seems to vary with the
+temperature. The diurnal variation exhibits two periods of greatest and
+two of least intensity. In summer, the <i>maxima</i> occur about 10 a.m. and 10
+p.m.; the <i>minima</i> about 2 a.m. and noon. In winter, the <i>maxima</i> take
+place near 10 a.m. and 8 p.m.; the <i>minima</i> near 4 a.m. and 4 p.m.</p>
+
+<p>The researches of Saussure, Beccaria, Crosse, Qu&eacute;t&egrave;let, Thompson, and<span class="pagenum"><a name="Page_132" id="Page_132">[Pg 132]</a></span>
+FitzRoy have tended to show that during the prevalence of polar currents
+of air positive electricity is developed, and becomes more or less active
+according to the greater or less coldness and strength of wind; but with
+winds from the equatorial direction there is little evidence of sensitive
+electricity, and when observable, it is of the negative kind. Storms and
+gales of wind are generally attended, in places, with lightning and
+thunder; and as the former are very often attributed to the conflict of
+polar and equatorial winds, the difference of the electric tension of
+these winds may account for the latter phenomena. It is not our intention
+to enter upon the general consideration of thunderstorms; the facts which
+we have given may be of service to the young observer; and finally, as it
+is interesting to be able to judge of the locality of a thunderstorm, the
+following simple rule will be of service, and sufficiently accurate:&mdash;Note
+by a second&#8217;s watch the number of seconds which elapse from the sight of
+the lightning to the commencement of the thunder; divide them by five, and
+the quotient will be the distance in miles. Thus, if thunder is heard ten
+seconds after the lightning was seen, the distance from the seat of the
+storm will be about two miles. The interval between the flash and the roll
+has seldom been observed greater than seventy-two seconds.</p>
+
+
+<p><br /><b>137. Lightning Conductors.</b>&mdash;&#8220;The line of danger, whether from the burning
+or lifting power of lightning, is the line of strong and obstructed
+currents of air, of the greatest aerial friction.&#8221;<small><a name="f17.1" id="f17.1" href="#f17">[17]</a></small> Trees, church
+spires, wind-mills and other tall structures, obstruct the aerial
+currents, and hence their exposure to danger. The highest objects of the
+landscape, especially those that are nearest the thunder cloud, will
+receive the lightning stroke. The more elevated the object, the more
+likely is it to be struck. Of two or more objects, equally tall and near,
+the lightning is invariably found to select the best conductor of
+electricity, and even to make a circuitous path to get to it. Hence the
+application and evident advantage of metallic rods, called <i>lightning
+conductors</i>, attached to buildings and ships. A lightning conductor should
+be pointed at top, and extend some feet above the highest part of the
+edifice, or mast. It should be made of copper, which is a better
+conducting medium than iron, and more durable, being less corrosive. It
+must be unbroken throughout its length, and extend to the bottom of the
+building, and even some distance into the ground, so as to conduct the
+electricity into a well or moist soil. If it be connected with the lead
+and iron work in the structure of the house, it will be all the better, as
+affording a larger surface, and a readier means of exit for the fluid. In
+a ship, the lower end of the conductor should be led into communication
+with the hull, if of iron, and with the copper sheathing, if a wooden
+vessel; so that, spread over a large surface, it may escape more readily
+to the water.</p>
+
+
+<p><br /><b>138. Precautions against Lightning.</b>&mdash;Experience seems to warrant the
+assumption that any building or ship, fitted with a substantial lightning
+conductor,<span class="pagenum"><a name="Page_133" id="Page_133">[Pg 133]</a></span> is safe from danger during a thunderstorm. Should a house or
+vessel be undefended by a conductor, it may be advisable to adopt a few
+precautions against danger. In a house, the fire-place should be avoided,
+because the lightning may enter by the chimney, its sooty lining being a
+good conductor. &#8220;Through chimneys, lightning has a way into most houses;
+and therefore, it is wise, by opening doors or windows, to give it a way
+out. Wherever the aerial current is fiercest, there the danger is
+greatest; and if we kept out of the way of currents or draughts, we keep
+out of the way of the lightning.&#8221;<small><a name="f18.1" id="f18.1" href="#f18">[18]</a></small> Lightning evinces as it were a
+preference for metallic substances, and will fly from place to place, even
+out of the direct line of its passage to the earth, to enter such bodies.
+It is therefore well to avoid, as much as possible, gildings, silvered
+mirrors, and articles of metal. The best place is perhaps the middle of
+the room, unless a draught passes, or a metallic lamp or chandelier should
+be hanging from the ceiling. The neighbourhood of bad conductors, such as
+glass windows, not being open, and on a thick bed of mattrasses, are safe
+places. The quality of trees as lightning conductors is considered to
+depend upon their height and moisture, those which are taller and
+relatively more humid being struck in preference to their fellows;
+therefore, it is unwise to seek shelter under tall and wet trees during a
+thunderstorm. In the absence of any other shelter, it would be better to
+lie down on the ground.</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_134" id="Page_134">[Pg 134]</a></span></p>
+<h2><a name="CHAPTER_XV" id="CHAPTER_XV"></a>CHAPTER XV.</h2>
+<p class="center"><span class="large">OZONE AND ITS INDICATORS.</span></p>
+
+<p><br /><b>139. Nature of Ozone.</b>&mdash;During the action of a powerful electric machine,
+and in the decomposition of water by the voltaic battery, a peculiar odour
+is perceptible, which is considered to arise from the generation of a
+substance to which the term ozone has been given, on account of its having
+been first detected by smell, which, for a long time after its discovery,
+was its only known characteristic. A similar odour is evolved by the
+influence of phosphorus on moist air, and in other cases of slow
+combustion. It is also traceable, by the smell, in air,&mdash;where a flash of
+lightning has passed immediately before. Afterwards it was established
+that the same element possessed an oxidising property. It was found to be
+liberated at the oxygen electrode when water was decomposed by an electric
+current; and has been regarded by some chemists as what is termed an
+<i>allotropic</i> form of oxygen, while others speak of it as oxygen in the
+<i>nascent</i> state, and some even regard it as intimately related to
+chlorine. So various are the existing notions of the nature of this
+obscure agent.</p>
+
+<p>Its oxidising property affords a ready means for its detection, even when
+the sense of smell completely fails. The methods of noting the presence
+and measuring the amount of ozone present in the air, are very simple;
+being the free exposure to the air, defended from rain and the direct rays
+of the sun, of prepared test-papers. There are two kinds of test-papers.
+One kind was invented by Dr. Schonbein, the original discoverer of ozone;
+and the other, which is more generally approved, by Dr. Moffat.</p>
+
+
+<p><br /><b>140. Schonbein&#8217;s Ozonometer</b> consists of strips of paper, previously
+saturated with a solution of starch and iodide of potassium, and dried.
+The papers are suspended in a box, or otherwise properly exposed to the
+air, for a given interval, as twenty-four hours. The presence of ozone is
+shown by the test-paper acquiring a purple tint when momentarily immersed
+in water. The amount is estimated by the depth of the tint, according to a
+scale of ten tints furnished for the purpose, which are distinguished by
+numbers from 1 to 10. The ozone decomposes the compound which iodine forms
+with hydrogen, and, it is presumed, combines as oxygen with hydrogen,
+while the iodine unites with the starch, giving the blue colour when
+moist.</p>
+
+
+<p><br /><b>141. Dr. Moffat&#8217;s Ozonometer</b> consists of papers prepared in a somewhat
+similar manner to Schonbein&#8217;s; but they do not require immersion in water.
+The <span class="pagenum"><a name="Page_135" id="Page_135">[Pg 135]</a></span>presence of ozone is shown by a brown tint, and the amount by the
+depth of tint according to a scale of ten tints, which is furnished with
+each box of the papers.</p>
+
+<p>Moffat&#8217;s have the advantage of preserving their tint for years, if kept in
+the dark, or between the leaves of a book; and are simpler to use.</p>
+
+<div class="figright">Fig. 92.<br /><img src="images/fig_92.jpg" alt="" /></div>
+
+<p><br /><b>142. Sir James Clark&#8217;s Ozone Cage</b> (fig. 92), consists of two cylinders of
+very fine wire gauze, one fitting into the other; the wire gauze being of
+such a fineness as to permit the free ingress of air, at the same time
+that it shuts out all light that would act injuriously on the test-paper,
+which is suspended by a clip or hook attached to the upper part of the
+inner cylinder.</p>
+
+
+<p><br /><b>143. Distribution and Effects of Ozone.</b>&mdash;Mr. Glaisher has found that &#8220;the
+amount of ozone at stations of low elevation is small; at stations of high
+elevation, it is almost always present; and at other and intermediate
+stations, it is generally so. The presence and amount of ozone would seem
+to be graduated by the elevation, and to increase from the lowest to the
+highest ground. The amount of ozone is less in towns than in the open
+country at the same elevation; and less at inland than at sea-side
+stations.&#8221; It seems to abound most with winds from the sea, and to be most
+prevalent where the air is considered the purest and most salubrious. This
+may seem, says Admiral FitzRoy, in <i>The Weather Book</i>, to point to a
+connection between ozone and chlorine gas, which is in and over sea-water,
+and which <i>must</i> be brought by any wind that blows from the sea. It
+prevails more over the ocean and near it than over land, especially land
+remote from the sea; and, says the Admiral, it affects the gastric juice,
+improves digestion, and has a tanning effect. Dr. Daubeny, in his
+<i>Lectures on Climate</i>, writes: &#8220;Its presence must have a sensible
+influence upon the purity of the air, by removing from it f&oelig;tid and
+injurious organic effluvia. It is also quite possible that ozone may play
+an important part in regulating the functions of the vegetable kingdom
+likewise; and although it would be premature at present to speculate upon
+its specific office, yet, for this reason alone, it may be well to note
+the fact of its frequency, in conjunction with the different phases which
+vegetation assumes, persuaded that no principle can be generally diffused
+throughout nature, as appears to be the case, with this, without having
+some important and appropriate use assigned for it to fulfil.&#8221;</p>
+
+
+<p><br /><b>144. Registering Ozonometer.</b>&mdash;Dr. E. Lancaster has contrived an
+ozonometer, the object of which is to secure the constant registration of
+ozone, so that the varying quantities present in the atmosphere may be
+detected and registered. For this purpose, an inch of ozone paper passes
+in each hour, by clock-work, beneath an opening in the cover of the
+instrument.</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_136" id="Page_136">[Pg 136]</a></span></p>
+<h2><a name="CHAPTER_XVI" id="CHAPTER_XVI"></a>CHAPTER XVI.</h2>
+<p class="center"><span class="large">INSTRUMENTS NOT DESCRIBED IN THE PRECEDING CHAPTERS.</span></p>
+
+<p><br /><b>145. Chemical Weather Glass.</b>&mdash;This curious instrument appears to have been
+invented more than a hundred years ago, but the original maker is not
+known. It is simply a glass vial about ten inches long and three quarters
+of an inch in diameter, which is nearly filled, and hermetically sealed,
+with the following mixture:&mdash;Two drachms of camphor, half a drachm of
+nitrate of potassium, half a drachm of chlorate of ammonium, dissolved in
+about two fluid ounces of absolute alcohol mixed with two ounces of
+distilled water. All the ingredients should be as pure as possible, and
+each vial filled separately. When the instruments are made in numbers and
+filled from a common mixture, some get more than the due proportion of the
+solid ingredients, and consequently such glasses do not exhibit that
+uniformity of appearance and changes, that undoubtedly should accompany
+similar influencing circumstances. It is in consequence of a want of
+precision and fixed principle of manufacture, that these interesting
+instruments are not properly appreciated, and more generally used.</p>
+
+<p>The glass should be kept quite undisturbed, exposed to the north, and
+shaded from the sun. Camphor is soluble in alcohol, but not in water,
+while both water and alcohol have different solvent powers, according to
+the temperature; hence, the solid ingredients being in excess for certain
+conditions of solution, depending upon temperature chiefly, and perhaps
+electricity and the action of light also, appear as crystals and disappear
+with the various changes that occur in the weather.</p>
+
+<p>The various appearances thus presented in the menstruum have been inferred
+to prognosticate atmospheric changes. The following rules have been
+deduced from careful study of the glass and weather:&mdash;</p>
+
+<p>1. During cold weather, beautiful fern-like or feathery crystallization is
+developed at the top, and sometimes even throughout the liquid. This is
+the normal state of the glass during winter. The crystallization increases
+with the coldness; and if the structure grows downward, the cold will
+continue.</p>
+
+<p>2. During warm and serene weather, the crystals dissolve, the upper and
+greater part of the liquid becoming perfectly clear. This is the normal
+state of the glass during summer. The less amount of crystallization, that
+is, the greater the clear portion of the liquid (for there is always some
+of the composition visible at the bottom), the greater the probability of
+continued fine dry weather.</p>
+
+<p>3. When the upper portion is clear, and flakes of the composition rise to
+the top and aggregate, it is a sign of increasing wind and stormy
+weather.</p>
+
+<p><span class="pagenum"><a name="Page_137" id="Page_137">[Pg 137]</a></span>4. In cold weather, if the top of the liquid becomes thick and cloudy, it
+denotes approaching rain.</p>
+
+<p>5. In warm weather, if small crystals rise in the liquid, which still
+maintains its clearness, rain may be expected.</p>
+
+<p>6. Sharpness in the points and features of the fern-like structure of the
+crystals, is a sign of fine weather; but when they begin to break up, and
+are badly defined, unsettled weather may be expected.</p>
+
+<p>Admiral FitzRoy, in <i>The Weather Book</i>, writes of this instrument as
+follows:&mdash;&#8220;Since 1825, we have generally had some of these glasses, as
+curiosities rather than otherwise; for nothing certain could be made of
+their variations until lately, when it was fairly demonstrated that if
+fixed undisturbed in free air, not exposed to radiation, fire, or sun, but
+in the ordinary light of a well-ventilated room, or, <i>preferably</i>, in the
+outer air, the chemical mixture in a so-called storm-glass varies in
+character with the <i>direction</i> of the wind&mdash;not its force, <i>specially</i>
+(though it <i>may</i> so vary in <i>appearance</i>, only from another cause,
+<i>electrical tension</i>).</p>
+
+<p>&#8220;As the atmospheric current veers toward, comes from, or is only
+<i>approaching</i> from the polar direction, this chemical mixture&mdash;if closely,
+even microscopically watched&mdash;is found to grow like <i>fir</i>, <i>yew</i>, fern
+leaves, or hoar-frost&mdash;or like crystallizations.</p>
+
+<p>&#8220;As the wind, or great body of air, tends more from the <i>opposite</i>
+quarter, the lines or spikes&mdash;all regular, hard, or crisp
+features&mdash;gradually diminish, till they vanish.</p>
+
+<p>&#8220;Before, and in a continued southerly wind, the mixture sinks slowly
+downward in the vial, till it becomes shapeless, like melting white sugar.</p>
+
+<p>&#8220;Before, or during the continuance of a northerly wind (polar current),
+the crystallizations are beautiful (if the mixture is correct, the glass a
+<i>fixture</i>, and duly <i>placed</i>); but the least motion of the liquid disturbs
+them.</p>
+
+<p>&#8220;When the main currents meet, and turn <i>toward the west</i>, making
+<i>easterly</i> winds, stars are more or less numerous, and the liquid dull, or
+less clear. When, and while they <i>combine by the west</i>, making westerly
+winds, the liquid is clear, and the crystallization well-defined, without
+loose stars.</p>
+
+<p>&#8220;While <i>any hard</i> or <i>crisp</i> features are visible below, above, or at the
+top of the liquid (where they form for polar winds), there is <i>plus</i>
+electricity in the air; a <i>mixture</i> of polar current co-existing <i>in that
+locality</i> with the opposite, or southerly.</p>
+
+<p>&#8220;When nothing but soft, melting, sugary substance is seen, the atmospheric
+current (feeble or strong as it may be) is southerly with <i>minus</i>
+electricity, unmixed with, and <i>uninfluenced</i> by, the contrary wind.</p>
+
+<p>&#8220;Repeated trials with a delicate galvanometer, applied to measure electric
+tension in the air, have proved these facts, which are now found useful
+for aiding, with the barometer and thermometer, in forecasting weather.</p>
+
+<p>&#8220;Temperature affects the mixture much, but not solely; as many comparisons
+of winter with summer changes of temperature have fully proved.</p>
+
+<p><span class="pagenum"><a name="Page_138" id="Page_138">[Pg 138]</a></span>&#8220;A confused appearance of the mixture, with flaky spots, or stars, in
+motion, and less clearness of the liquid, indicates south-easterly wind,
+probably strong to a gale.</p>
+
+<p>&#8220;Clearness of the liquid, with more or less perfect crystallizations,
+accompanies a combination, or a contest, of the main currents, by the
+<i>west</i>, and very remarkable these differences are,&mdash;the results of these
+air currents acting on each other <i>from</i> eastward, or from an entirely
+opposite direction, the <i>west</i>.</p>
+
+<p>&#8220;The glass should be wiped clean now and then,&mdash;and once or twice a year
+the mixture should be disturbed, by inverting and gently shaking the glass
+vial.&#8221;</p>
+
+<div class="figleft">Fig. 93.<br /><img src="images/fig_93.jpg" alt="" /></div>
+
+<p><br /><b>146. Leslie&#8217;s Differential Thermometer.</b>&mdash;A glass tube having a large bulb
+at each extremity, and bent twice at right angles, as represented in
+figure 93, containing strong sulphuric acid tinged with carmine, and
+supported at the centre by a wooden stand, constitutes the differential
+thermometer as invented by Professor Leslie. The instrument is designed to
+exhibit and measure small differences of temperature. Each leg of the
+instrument is usually from three to six inches long, and the balls are
+about four inches apart. The calibre of the legs is about <span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">50</span> inch, not
+more; the other part of the tube may be wider. The tube is filled with the
+liquid, the bulbs contain air. When both bulbs are heated alike, each
+scale indicates zero. The scale is divided so that the space between the
+freezing and the boiling-points of water is equal to 1,000 parts. When one
+bulb is heated more than the other, the difference of temperature is
+delicately shown by the descent of the coloured fluid from the heated
+ball. It is uninfluenced by changes in the temperature of the atmosphere;
+hence it is admirably adapted for experiments of radiant heat. The theory
+of the instrument is that gases expand equally for uniform increments of
+heat.</p>
+
+
+<p><br /><b>147. Rumford&#8217;s Differential Thermometer</b> differs from that just described
+in simply containing only a small bubble of liquid, which lies in the
+centre of the tube, when both bulbs are similarly influenced. The bulbs
+and other parts of the tube contain air. When one bulb is more heated than
+the other, the bubble moves towards the one less heated; and the scale
+attached to the horizontal part of the tube affords a measurement of the
+difference of temperature.</p>
+
+<div class="figright">Fig. 94.<br /><img src="images/fig_94.jpg" alt="" /></div>
+
+<p><a name="glaisher" id="glaisher"></a><br /><b>148. Glaisher&#8217;s Thermometer Stand.</b>&mdash;The thermometer stand consists of a
+horizontal board as a base, of a vertical board projecting upwards from
+one edge of the horizontal one, and of two parallel inclined boards,
+separated from each other by blocks of three inches in thickness,
+connected at the top with the vertical,<span class="pagenum"><a name="Page_139" id="Page_139">[Pg 139]</a></span> and at the bottom with the
+horizontal board, and the air passes freely about and between them all. To
+the top of the inclined boards is connected a small projecting roof to
+prevent the rain falling on the bulbs of the instrument, which are carried
+on the face of the vertical board, with their bulbs projecting below it,
+so that the air plays freely on the bulbs from all sides. The whole frame
+revolves on an upright post firmly fixed to the ground, as shown in the
+engraving, fig. 94; and in use, the inclined side is always turned towards
+the sun.</p>
+
+
+<p><br /><b>149. Thermometer Screen, for use at Sea.</b>&mdash;This screen, or shade, was
+designed by Admiral FitzRoy, and has been in use for several years on
+board H.M. vessels and many merchant-ships. It is about twenty-four inches
+long by twelve wide and eight deep; having lattice-work sides, door, and
+bottom; with perforation also at top, so contrived that the air has free
+access to the interior, while the direct rays of the sun, rain, and sea
+spray are effectually excluded from the thermometers mounted inside. There
+is ample space for two thermometers placed side by side on brackets, at
+least three inches from each other or any part of the exterior of the
+screen. One thermometer should be fitted up as a &#8220;wet bulb&#8221; (see <a href="#Page_105">p. 105</a>).
+A small vessel of water can easily be fixed inside the screen so as to
+retain its position and contents under the usual motions of the ship; and
+by means of a piece of cotton-wick, or muslin rag tied round the bulb of
+the thermometer and trailing into the cup of water, keep the bulb
+constantly moist.</p>
+
+<p>Self-registering thermometers should be protected by a similar screen. It
+has been found that thermometric observations made at sea are not valuable
+for scientific purposes unless the instruments have been duly protected by
+such a screen.</p>
+
+<div class="figright">Fig. 95.<br /><img src="images/fig_95.jpg" alt="" /></div>
+
+<p><br /><b>150. Anemoscope</b>, or Portable Wind Vane for travellers, with compass, bar
+needle, &amp;c., shows the direct course of the wind to half a point of the
+compass.</p>
+
+
+<p><br /><b>151. Evaporating Dish, or Gauge</b> (fig. 95), for showing the amount of<span class="pagenum"><a name="Page_140" id="Page_140">[Pg 140]</a></span>
+evaporation from the earth&#8217;s surface. This gauge consists of a brass
+vessel, the area or evaporating surface of which is accurately determined;
+and also a glass cylindrical measure, graduated into inches, tenths, and
+hundredths of inches. In use, the evaporating gauge is nearly filled with
+water, the quantity having been previously measured by means of the glass
+cylinder; it is then placed out of doors, freely exposed to the action of
+the atmosphere; after exposure, the water is again measured, and the
+difference between the first and second measurement shows the amount of
+evaporation that has taken place. If rain has fallen during the exposure
+of the gauge, the quantity collected by it must be deducted from the
+measured quantity; the amount is shown by the quantity of rain collected
+in the rain gauge. The wire cage round the gauge is to prevent animals,
+birds, &amp;c., from drinking the water.</p>
+
+
+<p><br /><b>152. Dr. Babington&#8217;s Atmidometer</b>, or instrument for measuring the
+evaporation from water, <i>ice or snow</i>, consists of an oblong hollow bulb
+of glass or copper, beneath which and communicating with it by a
+contracted neck is a second globular bulb, duly weighted with mercury or
+shot. The upper bulb is surmounted by a small glass or metal stem, having
+a scale graduated to grains and half-grains; on the top of which is fixed
+horizontally a shallow metal pan. The bulbs are immersed in a vessel of
+water having a circular hole in the cover through which the stem rises.
+Distilled water is then gradually poured into the pan above, until the
+zero of the stem sinks to a level with the cover of the vessel. Thus
+adjusted, as the water in the pan evaporates, the stem ascends, and the
+amount of evaporation is indicated in grains. This instrument affords a
+means of measuring evaporation from <i>ice or snow</i>. An adjustment for
+temperature is necessary.</p>
+
+
+<p><br /><b>153. Cloud Reflector.</b>&mdash;At the International Exhibition 1862, Mr. J. T.
+Goddard exhibited a cloud mirror, for ascertaining the direction in which
+the clouds are moving.</p>
+
+<p>The mirror is laid on a horizontal support near a window, and fastened so
+that the point marked north may coincide with the south point of the
+horizon,&mdash;the several points will consequently be reversed. The edge of a
+conspicuous cloud is brought to the centre of the mirror, and the observer
+keeps perfectly still until it passes off at the margin, where the true
+point of the horizon <i>from which</i> the clouds are coming can be read off.</p>
+
+
+<p><br /><b>154. Sunshine Recorder.</b>&mdash;Mr. Goddard also exhibited an instrument which he
+calls by this name. It works by letting the sun&#8217;s rays pass through a
+narrow slit, and fall on photographic paper wound round a barrel moved by
+clock-work; the paper being changed daily, and the photographic impression
+developed and fixed in the usual manner.<small><a name="f19.1" id="f19.1" href="#f19">[19]</a></small></p>
+
+
+<p class="center"><br /><span class="pagenum"><a name="Page_141" id="Page_141">[Pg 141]</a></span>155. SET OF PORTABLE INSTRUMENTS.</p>
+
+<p>In a small box, 8 in. by 8 in. by 4 in., a complete set of meteorological
+instruments have been packed. The lid of the box, by an ingenious
+arrangement, is made to take off and hang up; on it are permanently fixed
+for observation, a maximum and minimum, and a pair of dry and wet bulb
+thermometers. The interior of the box contains a maximum thermometer in
+vacuo for solar radiation, and a minimum for terrestrial purposes, one of
+Negretti and Zambra&#8217;s small pocket aneroid barometers, pedometer for
+measuring distances, pocket compass, clinometer, and lastly a rain gauge.
+This latter instrument consists of an accurately turned brass ring having
+an india rubber body fastened to it to receive the rain, which is measured
+off by a small graduated glass, also contained in the box. Gentlemen
+travelling will find this compact observatory all that can be desired for
+meteorological observations.</p>
+
+
+<p class="center"><br />156. IMPLEMENTS.</p>
+
+<p>The practical meteorologist will find the following articles very useful,
+if not necessary. They scarcely require description; an enumeration will
+therefore suffice:&mdash;<i>Weather Diagrams</i>, or prepared printed and ruled
+forms, whereon to exhibit graphically the readings of the various
+instruments to render their indications useful in foretelling weather,
+&amp;c.;&mdash;<i>Meteorological Registers</i>, or Record Books, for recording all
+observations, and the deductions;&mdash;<i>Cloud Pictures</i>, by which the clouds
+can be readily referred to their particular classification, very necessary
+to the inexperienced and learners;&mdash;Cyclone Glasses, or Horns, outline
+Maps with Wind-markers, are also useful, especially in forecasting
+weather.</p>
+
+
+<div class="figleft">Fig. 96.<br /><img src="images/fig_96.jpg" alt="" /></div>
+
+<div class="figright">Fig. 97.<br /><img src="images/fig_97.jpg" alt="" /></div>
+
+<p class="center"><br />157. HYDROMETER.</p>
+
+<p>A simple kind of hydrometer is very much used at sea, as &#8220;a sea-water
+test;&#8221; and as the observations are usually recorded in a meteorological
+register or the ship&#8217;s log-book, it may not be altogether out of place to
+give a description of it here.</p>
+
+<p>It is constructed of glass. If made of brass, the corrosive action of
+salt-water soon renders the instrument erroneous in its indications. The
+shapes usually given to the instruments are shown in figs. 96 and 97. A
+globular bulb is blown, and partly filled with mercury or small shot, to
+make the instrument float steadily in a vertical position. From the neck
+of the bulb the glass is expanded into an oval or a cylindrical shape, to
+give the instrument sufficient volume for flotation; finally, it is
+tapered off to a narrow upright stem which encloses an ivory scale, and is
+closed at the top. The divisions on the scale read downward, so as to
+measure the length of the stem which stands above the surface of any
+liquid in which the hydrometer is floated. The denser the fluid, the
+higher will the instrument rise; the rarer, the lower it will sink.</p>
+
+<p><span class="pagenum"><a name="Page_142" id="Page_142">[Pg 142]</a></span>The indications depend upon the hydrostatic principle, that floating
+bodies displace a quantity of the fluid which sustains them equal to their
+own weight. According, therefore, as the specific gravities of fluids
+differ from each other, so will vary the quantities of the fluids
+displaced by the same body when floated successively in each.</p>
+
+<p>The specific gravity of distilled water, at the temperature of 62&deg; <i>F</i>,
+being taken as unity, the depth to which the instrument sinks when gently
+immersed in such water is the zero of the scale. The graduations extend
+from 0 to 40; the latter being the mark which will be level with the
+surface when the instrument is placed in water, the specific gravity of
+which is 1&middot;040. In recording observations, the last two figures
+only&mdash;being the figures on the scale&mdash;are written down. Sea-water usually
+ranges from 1&middot;020 to 1&middot;036.</p>
+
+<p>A small tin, copper, or glass cylinder is useful for containing the water
+to be tested. It should be wider than the hydrometer, and always filled to
+the brim. If fitted to a stand, which is supported by gimbals, it will be
+very convenient. Water in a bucket, basin, or other wide vessel, acquires
+motion at sea, and the eye cannot be brought low enough (on account of the
+edges) to read off the scale accurately.</p>
+
+<p>Errors of observation may occur with the hydrometer, if it be put into
+water without being clean, or without being carefully wiped. The
+instrument is extremely accurate if correctly used. It should be kept free
+from contact with the sides of the vessel; and all dust, smears, or
+greasiness, should be scrupulously avoided, by carefully wiping it with a
+clean cloth before and after use.</p>
+
+<p>Whenever the temperature of the water tested differs from 62&deg;, a
+correction to the reading is necessary, for the expansion or contraction
+of the glass, as well as the water itself, in order to reduce all
+observations to one generally adopted standard.</p>
+
+<p>Negretti and Zambra&#8217;s hydrometer, with thermometer in the stem, shows the
+density and temperature in one instrument.</p>
+
+<p>For the following Tables we are indebted to the kindness of Admiral
+FitzRoy:&mdash;</p>
+
+<p><span class="smcap">Table</span> for reducing observations made with a <span class="smcap">Brass Hydrometer</span>, assuming the
+linear expansion of brass to be 0&middot;000009555 for 1&deg; F. The correction is
+additive for all temperatures above 62&deg;, and subtractive for temperatures
+below 62&deg;.</p>
+
+<table border="0" cellpadding="0" cellspacing="0" summary="table">
+<tr><td class="btlr" align="center"><i>t&deg;</i></td>
+    <td class="btrdoub" align="center">Correction.</td>
+    <td class="btr" align="center"><i>t&deg;</i></td>
+    <td class="btrdoub" align="center">Correction.</td>
+    <td class="btr" align="center"><i>t&deg;</i></td>
+    <td class="btrdoub" align="center">Correction.</td>
+    <td class="btr" align="center"><i>t&deg;</i></td>
+    <td class="btr" align="center">Correction.</td></tr>
+<tr><td class="btlr" align="center">32</td>
+    <td class="btrdoub" align="center">-0&middot;0014</td>
+    <td class="btr" align="center">48</td>
+    <td class="btrdoub" align="center">-0&middot;0010</td>
+    <td class="btr" align="center">64</td>
+    <td class="btrdoub" align="center">+0&middot;0002</td>
+    <td class="btr" align="center">80</td>
+    <td class="btr" align="center">+0&middot;0020</td></tr>
+<tr><td class="blr" align="center">33</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0014</span></td>
+    <td class="br" align="center">49</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0009</span></td>
+    <td class="br" align="center">65</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0003</span></td>
+    <td class="br" align="center">81</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;0021</span></td></tr>
+<tr><td class="blr" align="center">34</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0014</span></td>
+    <td class="br" align="center">50</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0009</span></td>
+    <td class="br" align="center">66</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0004</span></td>
+    <td class="br" align="center">82</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;0023</span></td></tr>
+<tr><td class="blr" align="center">35</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0014</span></td>
+    <td class="br" align="center">51</td>
+    <td class="brdoub" align="center">-0&middot;0008</td>
+    <td class="br" align="center">67</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0005</span></td>
+    <td class="br" align="center">83</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;0024</span></td></tr>
+<tr><td class="blr" align="center">36</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0014</span></td>
+    <td class="br" align="center">52</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0008</span></td>
+    <td class="br" align="center">68</td>
+    <td class="brdoub" align="center">+0&middot;0006</td>
+    <td class="br" align="center">84</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;0026</span></td></tr>
+<tr><td class="blr" align="center">37</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0014</span></td>
+    <td class="br" align="center">53</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0007</span></td>
+    <td class="br" align="center">69</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0007</span></td>
+    <td class="br" align="center">85</td>
+    <td class="br" align="center">+0&middot;0027</td></tr>
+<tr><td class="blr" align="center">38</td>
+    <td class="brdoub" align="center">-0&middot;0014</td>
+    <td class="br" align="center">54</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0006</span></td>
+    <td class="br" align="center">70</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0008</span></td>
+    <td class="br" align="center">86</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;0029</span></td></tr>
+<tr><td class="blr" align="center">39</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0013</span></td>
+    <td class="br" align="center">55</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0006</span></td>
+    <td class="br" align="center">71</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0009</span></td>
+    <td class="br" align="center">87</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;0030</span></td></tr>
+<tr><td class="blr" align="center">40</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0013</span></td>
+    <td class="br" align="center">56</td>
+    <td class="brdoub" align="center">-0&middot;0005</td>
+    <td class="br" align="center">72</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0010</span></td>
+    <td class="br" align="center">88</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;0032</span></td></tr>
+<tr><td class="blr" align="center">41</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0013</span></td>
+    <td class="br" align="center">57</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0004</span></td>
+    <td class="br" align="center">73</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0011</span></td>
+    <td class="br" align="center">89</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;0033</span></td></tr>
+<tr><td class="blr" align="center">42</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0013</span></td>
+    <td class="br" align="center">58</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0003</span></td>
+    <td class="br" align="center">74</td>
+    <td class="brdoub" align="center">+0&middot;0013</td>
+    <td class="br" align="center">90</td>
+    <td class="br" align="center">+0&middot;0035</td></tr>
+<tr><td class="blr" align="center">43</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0012</span></td>
+    <td class="br" align="center">59</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0003</span></td>
+    <td class="br" align="center">75</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0014</span></td>
+    <td class="br" align="center">91</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;0036</span></td></tr>
+<tr><td class="blr" align="center">44</td>
+    <td class="brdoub" align="center">-0&middot;0012</td>
+    <td class="br" align="center">60</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0002</span></td>
+    <td class="br" align="center">76</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0015</span></td>
+    <td class="br" align="center">92</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;0038</span></td></tr>
+<tr><td class="blr" align="center">45</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0011</span></td>
+    <td class="br" align="center">61</td>
+    <td class="brdoub" align="center">-0&middot;0001</td>
+    <td class="br" align="center">77</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0016</span></td>
+    <td class="br" align="center">93</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;0040</span></td></tr>
+<tr><td class="blr" align="center">46</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0011</span></td>
+    <td class="br" align="center">62</td>
+    <td class="brdoub" align="center"><span style="margin-left: .25em;">0&middot;0000</span></td>
+    <td class="br" align="center">78</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0018</span></td>
+    <td class="br" align="center">94</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;0041</span></td></tr>
+<tr><td class="bblr" align="center">47</td>
+    <td class="bbrdoub" align="center">-0&middot;0010</td>
+    <td class="bbr" align="center">63</td>
+    <td class="bbrdoub" align="center">+0&middot;0001</td>
+    <td class="bbr" align="center">79</td>
+    <td class="bbrdoub" align="center">+0&middot;0019</td>
+    <td class="bbr" align="center">95</td>
+    <td class="bbr" align="center">+0&middot;0043</td></tr></table>
+
+<p><span class="pagenum"><a name="Page_143" id="Page_143">[Pg 143]</a></span><span class="smcap">Table</span> for reducing
+observations made with a <span class="smcap">Glass Hydrometer</span>, assuming the
+linear expansion of glass to be 0&middot;00000463 for 1&deg; F. The correction is
+additive for temperatures above 62&deg;, and subtractive for temperatures below 62&deg;.</p>
+
+<table border="0" cellpadding="0" cellspacing="0" summary="table">
+<tr><td class="btlr" align="center"><i>t&deg;</i></td>
+    <td class="btrdoub" align="center">Correction.</td>
+    <td class="btr" align="center"><i>t&deg;</i></td>
+    <td class="btrdoub" align="center">Correction.</td>
+    <td class="btr" align="center"><i>t&deg;</i></td>
+    <td class="btrdoub" align="center">Correction.</td>
+    <td class="btr" align="center"><i>t&deg;</i></td>
+    <td class="btr" align="center">Correction.</td></tr>
+<tr><td class="btlr" align="center">32</td>
+    <td class="btrdoub" align="center">-0&middot;0019</td>
+    <td class="btr" align="center">48</td>
+    <td class="btrdoub" align="center">-0&middot;0012</td>
+    <td class="btr" align="center">64</td>
+    <td class="btrdoub" align="center">+0&middot;0002</td>
+    <td class="btr" align="center">80</td>
+    <td class="btr" align="center">+0&middot;0023</td></tr>
+<tr><td class="blr" align="center">33</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0019</span></td>
+    <td class="br" align="center">49</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0011</span></td>
+    <td class="br" align="center">65</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0003</span></td>
+    <td class="br" align="center">81</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;0024</span></td></tr>
+<tr><td class="blr" align="center">34</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0018</span></td>
+    <td class="br" align="center">50</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0011</span></td>
+    <td class="br" align="center">66</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0004</span></td>
+    <td class="br" align="center">82</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;0026</span></td></tr>
+<tr><td class="blr" align="center">35</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0018</span></td>
+    <td class="br" align="center">51</td>
+    <td class="brdoub" align="center">-0&middot;0010</td>
+    <td class="br" align="center">67</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0005</span></td>
+    <td class="br" align="center">83</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;0027</span></td></tr>
+<tr><td class="blr" align="center">36</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0018</span></td>
+    <td class="br" align="center">52</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0009</span></td>
+    <td class="br" align="center">68</td>
+    <td class="brdoub" align="center">+0&middot;0007</td>
+    <td class="br" align="center">84</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;0029</span></td></tr>
+<tr><td class="blr" align="center">37</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0017</span></td>
+    <td class="br" align="center">53</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0008</span></td>
+    <td class="br" align="center">69</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0008</span></td>
+    <td class="br" align="center">85</td>
+    <td class="br" align="center">+0&middot;0031</td></tr>
+<tr><td class="blr" align="center">38</td>
+    <td class="brdoub" align="center">-0&middot;0017</td>
+    <td class="br" align="center">54</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0008</span></td>
+    <td class="br" align="center">70</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0009</span></td>
+    <td class="br" align="center">86</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;0032</span></td></tr>
+<tr><td class="blr" align="center">39</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0017</span></td>
+    <td class="br" align="center">55</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0007</span></td>
+    <td class="br" align="center">71</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0010</span></td>
+    <td class="br" align="center">87</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;0034</span></td></tr>
+<tr><td class="blr" align="center">40</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0016</span></td>
+    <td class="br" align="center">56</td>
+    <td class="brdoub" align="center">-0&middot;0006</td>
+    <td class="br" align="center">72</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0012</span></td>
+    <td class="br" align="center">88</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;0036</span></td></tr>
+<tr><td class="blr" align="center">41</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0016</span></td>
+    <td class="br" align="center">57</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0005</span></td>
+    <td class="br" align="center">73</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0013</span></td>
+    <td class="br" align="center">89</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;0037</span></td></tr>
+<tr><td class="blr" align="center">42</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0015</span></td>
+    <td class="br" align="center">58</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0004</span></td>
+    <td class="br" align="center">74</td>
+    <td class="brdoub" align="center">+0&middot;0014</td>
+    <td class="br" align="center">90</td>
+    <td class="br" align="center">+0&middot;0039</td></tr>
+<tr><td class="blr" align="center">43</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0015</span></td>
+    <td class="br" align="center">59</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0003</span></td>
+    <td class="br" align="center">75</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0016</span></td>
+    <td class="br" align="center">91</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;0041</span></td></tr>
+<tr><td class="blr" align="center">44</td>
+    <td class="brdoub" align="center">-0&middot;0014</td>
+    <td class="br" align="center">60</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0002</span></td>
+    <td class="br" align="center">76</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0017</span></td>
+    <td class="br" align="center">92</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;0042</span></td></tr>
+<tr><td class="blr" align="center">45</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0014</span></td>
+    <td class="br" align="center">61</td>
+    <td class="brdoub" align="center">-0&middot;0001</td>
+    <td class="br" align="center">77</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0018</span></td>
+    <td class="br" align="center">93</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;0044</span></td></tr>
+<tr><td class="blr" align="center">46</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0013</span></td>
+    <td class="br" align="center">62</td>
+    <td class="brdoub" align="center"><span style="margin-left: .25em;">0&middot;0000</span></td>
+    <td class="br" align="center">78</td>
+    <td class="brdoub" align="center"><span style="margin-left: 1em;">&middot;0020</span></td>
+    <td class="br" align="center">94</td>
+    <td class="br" align="center"><span style="margin-left: 1em;">&middot;0046</span></td></tr>
+<tr><td class="bblr" align="center">47</td>
+    <td class="bbrdoub" align="center">-0&middot;0013</td>
+    <td class="bbr" align="center">63</td>
+    <td class="bbrdoub" align="center">+0&middot;0001</td>
+    <td class="bbr" align="center">79</td>
+    <td class="bbrdoub" align="center">+0&middot;0021</td>
+    <td class="bbr" align="center">95</td>
+    <td class="bbr" align="center">+0&middot;0048</td></tr></table>
+
+
+<p class="center"><br />158. NEWMAN&#8217;S SELF-REGISTERING TIDE-GAUGE.</p>
+
+<p>At places where the phenomena of the tides are of much maritime
+importance, a continuous series of observations upon the rise and fall,
+and times of change, is essentially necessary as a basis for the
+construction of good tide tables; and as such observations should also be
+accompanied with the registration of atmospheric phenomena, we have no
+hesitation in inserting a description of an accurate self-registering
+tide-gauge.</p>
+
+<p>The tide-gauge, as shown in the illustration, consists of a cylinder, <i>A</i>,
+which is made to revolve on its axis once in twenty-four hours by the
+action of the clock, <i>B</i>. A chain, to which is attached the float, <i>D</i>,
+passes over the wheel, <i>C</i>, and on the axis of this wheel, <i>C</i> (in about
+the middle of it) is a small toothed wheel, placed so as to be in contact
+with a larger toothed wheel carrying a cylinder, <i>E</i>, over which passes
+another smaller chain. This chain, passing along the upper surface of the
+cylinder, <i>A</i>, and round a second cylinder, <i>F</i>, at its further end, is
+acted on by a spring so as to be kept in a constant state of tension. In
+the middle of this chain a small tube is fixed for carrying a pencil,
+which, being gently pressed down by means of a small weight on the top of
+it, performs the duty of marking on paper placed round the cylinder the
+progress of the rise or fall of the tide as the cylinder revolves, and as
+it is drawn by the chain forward or backward by the rise or fall of the
+float. The paper is prepared with lines equidistant from each other, to
+correspond with the hours of the clock, crossed by others showing the
+number of feet of rise and fall.</p>
+
+<p><span class="pagenum"><a name="Page_144" id="Page_144">[Pg 144]</a></span></p>
+<div class="figcenter"><img src="images/page_144tmb.jpg" alt="" /><br />
+<a href="images/page_144.jpg"><small>Larger Image</small></a></div>
+
+<p>The cylinder while in action revolves from left to right to a spectator
+facing the clock, and the pencil is carried horizontally along the top of
+the cylinder; and the large wheel being made to revolve by the rise and
+fall of the float, turns the wheel<span class="pagenum"><a name="Page_145" id="Page_145">[Pg 145]</a></span> with the small cylinder, <i>E</i>,
+attached to it. If the tide is <i>falling</i>, the small chain is wound round
+the cylinder, <i>E</i>, and the pencil is drawn towards the large wheel; but if
+the tide is <i>rising</i>, the small chain is wound on to the cylinder, <i>F</i>, by
+means of the spring contained in it, which constantly keeps it in a state
+of tension. Thus, by means of the rise and fall of the tide, a lateral
+progress is given to the pencil, while the cylinder is made to revolve on
+its axis by the clock, so that a line is traced on the paper showing the
+exact state of the tide continuously, without further attention than is
+necessary to change the paper once every day, and to keep the pencil
+carefully pointed; or a metallic pencil may be used, which will require
+little, if any, attention.</p>
+
+<p>A good self-registering tide-gauge is a valuable and important acquisition
+wherever tidal observations are required, and the only perfectly efficient
+instrument of this kind is that invented by the late Mr. John Newman, of
+Regent Street, London. It is now in action in several parts of the world,
+silently and <i>faithfully</i> performing its duty, requiring no other kind of
+attention than that of a few minutes daily, and thus admitting the
+employment of the person on any other service whose duty it would
+otherwise have been to have registered the tide. It has done much by its
+faithful records in contributing to the construction of good tide tables
+for many places; for those unavoidable defects dependent on merely
+watching the surface on a divided scale are set aside by it, all erroneous
+conclusions excluded, and a true delineation of Nature&#8217;s own making is
+preserved by it for the theorist.</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_146" id="Page_146">[Pg 146]</a></span></p>
+<h2>ADDENDA.</h2>
+
+
+<p>1. French barometers are graduated to millimetres. An English inch is
+equal to 25&middot;39954 millimetres. Hence, 30 inches on the English barometer
+scales correspond to 762 millimetres on the French barometer scales.
+Conversions from one scale to another can be effected by the following
+formul&aelig;:&mdash;</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td>(1) Inches = millimetres divided by</td><td><span class="spacer2">&nbsp;</span></td><td>25&middot;39954</td></tr>
+<tr><td>(2) Millimetres = inches multiplied by</td><td>&nbsp;</td><td>25&middot;39954</td></tr></table>
+
+<p>Of course, a table of equivalent values should be drawn up and employed,
+when a large number of observations are to be converted from one scale to
+the other.</p>
+
+
+<p><br />2. In Germany, barometers are sometimes graduated with old French inches
+and lines,&mdash;the vernier generally indicating the tenth of a line.</p>
+
+<p class="center"><span class="smcap">Old French Lineal Measure.</span></p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td colspan="4">&nbsp;</td>
+    <td colspan="2" align="center"><small>English Inches.</small></td></tr>
+<tr><td align="right">1</td>
+    <td>douzi&egrave;me, or point</td>
+    <td colspan="3">&nbsp;</td>
+    <td><span style="margin-left: .5em;">0&middot;0074</span></td></tr>
+<tr><td align="right">12</td>
+    <td>points</td>
+    <td>=</td>
+    <td>1 ligne</td>
+    <td>=</td>
+    <td><span style="margin-left: .5em;">0&middot;0888</span></td></tr>
+<tr><td align="right">12</td>
+    <td>lignes</td>
+    <td>=</td>
+    <td>1 pouce</td>
+    <td>=</td>
+    <td><span style="margin-left: .5em;">1&middot;065765</span></td></tr>
+<tr><td align="right">12</td>
+    <td>pouces</td>
+    <td>=</td>
+    <td>1 pied</td>
+    <td>=</td>
+    <td>12&middot;7892</td></tr>
+<tr><td align="right">1</td>
+    <td>pied</td>
+    <td>=</td>
+    <td>324&middot;7 millimetres.</td></tr></table>
+
+<p>&#8220;The Germans indicate inches by putting two accents after the number;
+lines, by putting three accents; 27&Prime; 3&prime;&Prime;&middot;85, means 27 inches 3 lines 85
+hundredths of a line; more frequently, they give the height in lines, and
+the preceding number becomes 327&prime;&Prime;&middot;85.&#8221;&mdash;<i>Kaemtz.</i></p>
+
+
+<p><br />3. <i>Rule for finding Diameter of Bore of a Barometer Tube.</i></p>
+
+<p>&#8220;If the maker has not taken care to measure the interior diameter
+directly, it may be deduced from the exterior diameter. The exterior
+diameter is first measured by calipers, and, by deducting from this
+diameter 0&middot;1 of an inch for tubes from &middot;3 to &middot;5 of an inch in external
+diameter, we have an approximation to the interior diameter of the
+tube.&#8221;&mdash;<i>Kaemtz.</i></p>
+
+
+<p><br /><span class="pagenum"><a name="Page_147" id="Page_147">[Pg 147]</a></span>4. <span class="smcap">Wind Scales.</span></p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td colspan="3" align="center" class="botbor">Sea Scale.</td><td><span class="spacer">&nbsp;</span></td><td>&nbsp;</td><td><span class="spacer">&nbsp;</span></td>
+    <td align="center" class="botbor">Wind.</td><td><span class="spacer">&nbsp;</span></td><td>&nbsp;</td><td><span class="spacer">&nbsp;</span></td>
+    <td colspan="3" align="center" class="botbor">Land Scale.</td></tr>
+<tr><td><span style="margin-left: .5em;">0</span></td><td align="center">to</td><td><span style="margin-left: .5em;">3</span></td><td>&nbsp;</td>
+    <td>=</td><td>&nbsp;</td>
+    <td align="center">Light</td><td>&nbsp;</td>
+    <td>=</td><td>&nbsp;</td>
+    <td align="right">0</td><td align="center">to</td><td>1</td></tr>
+<tr><td><span style="margin-left: .5em;">3</span></td><td align="center">"</td><td><span style="margin-left: .5em;">5</span></td><td>&nbsp;</td>
+    <td>=</td><td>&nbsp;</td>
+    <td align="center">Moderate</td><td>&nbsp;</td>
+    <td>=</td><td>&nbsp;</td>
+    <td align="right">1</td><td align="center">"</td><td>2</td></tr>
+<tr><td><span style="margin-left: .5em;">5</span></td><td align="center">"</td><td><span style="margin-left: .5em;">7</span></td><td>&nbsp;</td>
+    <td>=</td><td>&nbsp;</td>
+    <td align="center">Fresh</td><td>&nbsp;</td>
+    <td>=</td><td>&nbsp;</td>
+    <td align="right">2</td><td align="center">"</td><td>3</td></tr>
+<tr><td><span style="margin-left: .5em;">7</span></td><td align="center">"</td><td><span style="margin-left: .5em;">8</span></td><td>&nbsp;</td>
+    <td>=</td><td>&nbsp;</td>
+    <td align="center">Strong</td><td>&nbsp;</td>
+    <td>=</td><td>&nbsp;</td>
+    <td align="right">3</td><td align="center">"</td><td>4</td></tr>
+<tr><td><span style="margin-left: .5em;">8</span></td><td align="center">"</td><td>10</td><td>&nbsp;</td>
+    <td>=</td><td>&nbsp;</td>
+    <td align="center">Heavy</td><td>&nbsp;</td>
+    <td>=</td><td>&nbsp;</td>
+    <td align="right">4</td><td align="center">"</td><td>5</td></tr>
+<tr><td>10</td><td align="center">"</td><td>12</td><td>&nbsp;</td>
+    <td>=</td><td>&nbsp;</td>
+    <td align="center">Violent</td><td>&nbsp;</td>
+    <td>=</td><td>&nbsp;</td>
+    <td align="right">5</td><td align="center">"</td><td>6</td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td colspan="3" align="center" class="botbor">Pressure<br />in Pounds<br />(Avoirdupois)</td><td colspan="3">&nbsp;</td>
+    <td align="center" class="botbor">(Land Scale).</td><td colspan="3">&nbsp;</td>
+    <td colspan="3" align="center" class="botbor">Velocity in<br />Miles<br />(Hourly).</td></tr>
+<tr><td colspan="3" align="center"><span style="margin-left: .5em;">&#189;</span></td><td>&nbsp;</td>
+    <td>=</td><td>&nbsp;</td>
+    <td align="center">1</td><td>&nbsp;</td>
+    <td>=</td><td>&nbsp;</td>
+    <td colspan="3" align="center">10</td></tr>
+<tr><td colspan="3" align="center"><span style="margin-left: .5em;">5</span></td><td>&nbsp;</td>
+    <td>=</td><td>&nbsp;</td>
+    <td align="center">2</td><td>&nbsp;</td>
+    <td>=</td><td>&nbsp;</td>
+    <td colspan="3" align="center">32</td></tr>
+<tr><td colspan="3" align="center">10</td><td>&nbsp;</td>
+    <td>=</td><td>&nbsp;</td>
+    <td align="center">3</td><td>&nbsp;</td>
+    <td>=</td><td>&nbsp;</td>
+    <td colspan="3" align="center">45</td></tr>
+<tr><td colspan="3" align="center">21</td><td>&nbsp;</td>
+    <td>=</td><td>&nbsp;</td>
+    <td align="center">4</td><td>&nbsp;</td>
+    <td>=</td><td>&nbsp;</td>
+    <td colspan="3" align="center">65</td></tr>
+<tr><td colspan="3" align="center">26</td><td>&nbsp;</td>
+    <td>=</td><td>&nbsp;</td>
+    <td align="center">5</td><td>&nbsp;</td>
+    <td>=</td><td>&nbsp;</td>
+    <td colspan="3" align="center">72</td></tr>
+<tr><td colspan="3" align="center">32</td><td>&nbsp;</td>
+    <td>=</td><td>&nbsp;</td>
+    <td align="center">6</td><td>&nbsp;</td>
+    <td>=</td><td>&nbsp;</td>
+    <td colspan="3" align="center">80</td></tr></table>
+
+
+<p><br />5. Letters to Denote the State of the Weather.</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td><i>b</i></td><td>denotes</td><td>blue sky, whether with clear or hazy atmosphere.</td></tr>
+<tr><td><i>c</i></td><td align="center">"</td><td>cloudy, that is detached opening clouds.</td></tr>
+<tr><td><i>d</i></td><td align="center">"</td><td>drizzling rain.</td></tr>
+<tr><td><i>f</i></td><td align="center">"</td><td>fog.</td></tr>
+<tr><td><i>h</i></td><td align="center">"</td><td>hail.</td></tr>
+<tr><td><i>l</i></td><td align="center">"</td><td>lightning.</td></tr>
+<tr><td><i>m</i></td><td align="center">"</td><td>misty, or hazy so as to interrupt the view.</td></tr>
+<tr><td><i>o</i></td><td align="center">"</td><td>overcast, gloomy, dull.</td></tr>
+<tr><td><i>p</i></td><td align="center">"</td><td>passing showers.</td></tr>
+<tr><td><i>q</i></td><td align="center">"</td><td>squally.</td></tr>
+<tr><td><i>r</i></td><td align="center">"</td><td>rain.</td></tr>
+<tr><td><i>s</i></td><td align="center">"</td><td>snow.</td></tr>
+<tr><td><i>t</i></td><td align="center">"</td><td>thunder.</td></tr>
+<tr><td><i>u</i></td><td align="center">"</td><td>ugly, threatening appearance of sky.</td></tr>
+<tr><td><i>v</i></td><td align="center">"</td><td>unusual visibility of distant objects.</td></tr>
+<tr><td><i>w</i></td><td align="center">"</td><td>wet, that is dew.</td></tr></table>
+
+<p>A letter repeated denotes much, as <i>r r</i>, heavy rain; <i>f f</i>, dense fog;
+
+and a figure attached denotes duration in hours, as 14 <i>r</i>, 14 hours rain.</p>
+
+<p>By the combination of these letters, all the ordinary phenomena of the
+weather may be recorded with certainty and brevity.</p>
+
+<p><span class="smcap">Examples.</span>&mdash;<i>b c</i>, blue sky with less proportion of cloud. 2 <i>r r l l t</i>,
+heavy rain for two hours, with much lightning, and some thunder.</p>
+
+<p>The above methods of recording the force of wind and state of weather
+were<span class="pagenum"><a name="Page_148" id="Page_148">[Pg 148]</a></span> originally proposed by Admiral Sir Francis Beaufort. They are now in
+general use at sea, and by many observers on land.</p>
+
+
+<p><br />6. Table of Expansion by Heat from 32&deg; to 212&deg; F.</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td>Platinum</td><td><span class="spacer">&nbsp;</span></td>
+    <td>0&middot;0008842</td>
+    <td>of the length.</td></tr>
+<tr><td>Glass, Flint</td><td>&nbsp;</td>
+    <td>0&middot;0008117</td>
+    <td><span style="margin-left: 2em;">"</span></td></tr>
+<tr><td><span style="margin-left: 1em;">"</span><span style="margin-left: 1.3em;">with Lead</span></td><td>&nbsp;</td>
+    <td>0&middot;0008622</td>
+    <td><span style="margin-left: 2em;">"</span></td></tr>
+<tr><td>Brass</td><td>&nbsp;</td>
+    <td>0&middot;0018708</td>
+    <td><span style="margin-left: 2em;">"</span></td></tr>
+<tr><td>Mercury</td><td>&nbsp;</td>
+    <td>0&middot;0180180</td>
+    <td><span style="margin-left: 2em;">"</span></td></tr>
+<tr><td>Water</td><td>&nbsp;</td>
+    <td>0&middot;0433200,</td>
+    <td>from 39&deg; to 212&deg;</td></tr>
+<tr><td>Alcohol</td><td>&nbsp;</td>
+    <td>0&middot;1100</td>
+    <td><span style="margin-left: 1em;">"</span><span style="margin-left: .8em;">32&deg; to 174&deg;</span></td></tr>
+<tr><td>Nitric Acid</td><td>&nbsp;</td>
+    <td>0&middot;1100</td></tr>
+<tr><td>Sulphuric Acid</td><td>&nbsp;</td>
+    <td>0&middot;0600</td></tr></table>
+
+
+<p><br />7. Table of Specific Gravity of Bodies at 32&deg; F. except water, which is
+taken at 39&deg;&middot;4.</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td>Water</td><td><span class="spacer">&nbsp;</span></td>
+    <td colspan="2">&nbsp;</td>
+    <td><span style="margin-left: .5em;">1&middot;000</span></td></tr>
+<tr><td>Alcohol, pure</td><td>&nbsp;</td>
+    <td colspan="2">&nbsp;</td>
+    <td><span style="margin-left: .5em;">0&middot;791</span></td></tr>
+<tr><td><span style="margin-left: 1.5em;">"</span><span style="margin-left: 1.5em;">proof</span></td><td>&nbsp;</td>
+    <td colspan="2">&nbsp;</td>
+    <td><span style="margin-left: .5em;">0&middot;916</span></td></tr>
+<tr><td>Mercury</td><td>&nbsp;</td>
+    <td colspan="2">&nbsp;</td>
+    <td>13&middot;596</td></tr>
+<tr><td>Glass</td><td>&nbsp;</td>
+    <td><span style="margin-left: 1em;">3</span></td><td>to</td><td><span style="margin-left: .5em;">2&middot;7</span></td></tr>
+<tr><td>Brass</td><td>&nbsp;</td>
+    <td>7&middot;8</td><td>to</td><td><span style="margin-left: .5em;">8&middot;54</span></td></tr>
+<tr><td>Platinum</td><td>&nbsp;</td>
+    <td>21</td><td>to</td><td>22&middot;00</td></tr></table>
+
+<p>Weight of a cubic foot of water, at the temperature of comparison, 62&middot;425
+lbs. avoirdupois.</p>
+
+<p>The pound avoirdupois contains 7,000 grains.</p>
+
+<p>Air is 813&middot;67 times lighter than water.</p>
+
+<p>The linear expansions are the mean values of the results of various
+experimentalists. The specific gravities are as given in Professor
+Rankine&#8217;s <i>Applied Mechanics</i>.</p>
+
+
+<p><br />8. Important Temperatures. Under the circumstances of&mdash;</p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td colspan="4">&nbsp;</td><td align="center">&deg;</td></tr>
+<tr><td>Water</td><td><span class="spacer2">&nbsp;</span></td>
+    <td align="center">boiling at</td><td><span class="spacer">&nbsp;</span></td>
+    <td align="right">212</td></tr>
+<tr><td>Mercury</td><td>&nbsp;</td>
+    <td align="center">boils at</td><td>&nbsp;</td>
+    <td align="right">660</td></tr>
+<tr><td>Sulphuric Acid</td><td>&nbsp;</td>
+    <td align="center">"</td><td>&nbsp;</td>
+    <td align="right">590</td></tr>
+<tr><td>Oil of Turpentine</td><td>&nbsp;</td>
+    <td align="center">"</td><td>&nbsp;</td>
+    <td align="right">560</td></tr>
+<tr><td>Nitric Acid</td><td>&nbsp;</td>
+    <td align="center">"</td><td>&nbsp;</td>
+    <td align="right">242</td></tr>
+<tr><td>Alcohol</td><td>&nbsp;</td>
+    <td align="center">"</td><td>&nbsp;</td>
+    <td align="right">174</td></tr>
+<tr><td>A Saturated Solution of Salt</td><td>&nbsp;</td>
+    <td align="center">"</td><td>&nbsp;</td>
+    <td align="right">218</td></tr>
+<tr><td>Vital Heat</td>
+    <td colspan="3">&nbsp;</td>
+    <td align="right">96</td></tr>
+<tr><td>Olive Oil begins to solidify</td>
+    <td colspan="3">&nbsp;</td>
+    <td align="right">36</td></tr>
+<tr><td>Fresh Water freezes</td>
+    <td colspan="3">&nbsp;</td>
+    <td align="right">32</td></tr>
+<tr><td>Sea Water freezes</td>
+    <td colspan="3">&nbsp;</td>
+    <td align="right">28</td></tr>
+<tr><td>Mercury freezes</td>
+    <td colspan="3">&nbsp;</td>
+    <td align="right">-39</td></tr></table>
+
+
+<p class="center"><br /><span class="pagenum"><a name="Page_149" id="Page_149">[Pg 149]</a></span>
+9. TABLE OF METEOROLOGICAL ELEMENTS, FORMING EXPONENTS OF THE CLIMATE OF LONDON.</p>
+
+<table border="0" cellpadding="0" cellspacing="0" summary="table">
+<tr><td align="center" class="btlr">1841<br />to<br />1861.<br /><br />Months.</td>
+    <td align="center" class="btr">Mean<br />Height of<br />Barometer,<br />reduced<br />to 32&deg; F.,<br />at the mean<br />sea-level.</td>
+    <td align="center" class="btr">Mean<br />Monthly<br />Range of<br />Barometer.</td>
+    <td align="center" class="btr">Mean<br />of all the<br />Highest<br />Temp&#8217;s.</td>
+    <td align="center" class="btr">Mean<br />of all the<br />Lowest<br />Temp&#8217;s.</td>
+    <td align="center" class="btr">Mean<br />Temp.</td>
+    <td align="center" class="btr">Mean<br />Temp.<br />of Dew-point.</td>
+    <td align="center" class="btr">Mean<br />Degree of<br />Humidity.</td>
+    <td align="center" class="btr">Mean<br />Number of<br />Rainy Days.</td>
+    <td align="center" class="btr">Average<br />Rainfall.</td>
+    <td align="center" class="btr">Average<br />Amount<br />of Cloud<br />(10=<br />overcast).</td>
+    <td align="center" class="btr">Prevalent<br />Winds.</td>
+    <td align="center" class="btr">Sun<br />above the<br />Horizon on Middle<br />Day.</td>
+    <td align="center" class="btr"><span class="smcap">Remarks.</span></td></tr>
+<tr><td class="btlr">&nbsp;</td>
+    <td align="center" class="btr">Inches.</td>
+    <td align="center" class="btr">Inches.</td>
+    <td align="center" class="btr">&deg;</td>
+    <td align="center" class="btr">&deg;</td>
+    <td align="center" class="btr">&deg;</td>
+    <td align="center" class="btr">&deg;</td>
+    <td class="btr">&nbsp;</td>
+    <td class="btr">&nbsp;</td>
+    <td align="center" class="btr">Inches.</td>
+    <td class="btr">&nbsp;</td>
+    <td class="btr">&nbsp;</td>
+    <td align="center" class="btr">Hours.</td>
+    <td class="btr">&nbsp;</td></tr>
+<tr><td class="blr">Jan.</td>
+    <td class="br" align="center">29&middot;932</td>
+    <td class="br" align="center">1&middot;44</td>
+    <td class="br" align="center">43&middot;2</td>
+    <td class="br" align="center">33&middot;7</td>
+    <td class="br" align="center">38&middot;3</td>
+    <td class="br" align="center">35&middot;4</td>
+    <td class="br" align="center">89</td>
+    <td class="br" align="center">11</td>
+    <td class="br" align="center">1&middot;8</td>
+    <td class="br" align="center">7&middot;7</td>
+    <td class="br" align="center">W. to N.</td>
+    <td class="br" align="center">8&#189;</td>
+    <td class="br">The majority of the nights are frosty.</td></tr>
+<tr><td class="blr">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td></tr>
+<tr><td class="blr">Feb.</td>
+    <td class="br" align="center">29&middot;962</td>
+    <td class="br" align="center">1&middot;22</td>
+    <td class="br" align="center">44&middot;7</td>
+    <td class="br" align="center">33&middot;2</td>
+    <td class="br" align="center">38&middot;4</td>
+    <td class="br" align="center">34&middot;4</td>
+    <td class="br" align="center">85</td>
+    <td class="br" align="center">10</td>
+    <td class="br" align="center">1&middot;6</td>
+    <td class="br" align="center">7&middot;4</td>
+    <td class="br" align="center">S. to W.</td>
+    <td class="br" align="center">10</td>
+    <td class="br">10 frosty nights on the average.</td></tr>
+<tr><td class="blr">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td></tr>
+<tr><td class="blr">Mar.</td>
+    <td class="br" align="center">29&middot;967</td>
+    <td class="br" align="center">1&middot;23</td>
+    <td class="br" align="center">50&middot;0</td>
+    <td class="br" align="center">35&middot;3</td>
+    <td class="br" align="center">41&middot;7</td>
+    <td class="br" align="center">36&middot;4</td>
+    <td class="br" align="center">82</td>
+    <td class="br" align="center">10</td>
+    <td class="br" align="center">1&middot;5</td>
+    <td class="br" align="center">6&middot;6</td>
+    <td class="br" align="center">N. to E.</td>
+    <td class="br" align="center">12</td>
+    <td class="br">12 frosty nights on the average.<br />Strong winds.</td></tr>
+<tr><td class="blr">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td></tr>
+<tr><td class="blr">Apr.</td>
+    <td class="br" align="center">29&middot;907</td>
+    <td class="br" align="center">1&middot;06</td>
+    <td class="br" align="center">56&middot;8</td>
+    <td class="br" align="center">38&middot;6</td>
+    <td class="br" align="center">46&middot;3</td>
+    <td class="br" align="center">39&middot;9</td>
+    <td class="br" align="center">79</td>
+    <td class="br" align="center">11</td>
+    <td class="br" align="center">1&middot;8</td>
+    <td class="br" align="center">6&middot;1</td>
+    <td class="br" align="center">N. to E.</td>
+    <td class="br" align="center">14</td>
+    <td class="br">6 frosty nights on the average.</td></tr>
+<tr><td class="blr">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td></tr>
+<tr><td class="blr">May</td>
+    <td class="br" align="center">29&middot;931</td>
+    <td class="br" align="center">1&middot;02</td>
+    <td class="br" align="center">64&middot;4</td>
+    <td class="br" align="center">44&middot;2</td>
+    <td class="br" align="center">52&middot;8</td>
+    <td class="br" align="center">45&middot;5</td>
+    <td class="br" align="center">76</td>
+    <td class="br" align="center">11</td>
+    <td class="br" align="center">2&middot;1</td>
+    <td class="br" align="center">6&middot;1</td>
+    <td class="br" align="center">S. to W.</td>
+    <td class="br" align="center">15&#189;</td>
+    <td class="br">Very rarely frost.</td></tr>
+<tr><td class="blr">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td></tr>
+<tr><td class="blr">June</td>
+    <td class="br" align="center">29&middot;960</td>
+    <td class="br" align="center">0&middot;89</td>
+    <td class="br" align="center">71&middot;2</td>
+    <td class="br" align="center">50&middot;2</td>
+    <td class="br" align="center">59&middot;2</td>
+    <td class="br" align="center">50&middot;8</td>
+    <td class="br" align="center">74</td>
+    <td class="br" align="center">11</td>
+    <td class="br" align="center">1&middot;9</td>
+    <td class="br" align="center">6&middot;1</td>
+    <td class="br" align="center">W. to N.</td>
+    <td class="br" align="center">16&#189;</td>
+    <td class="br">Sun attains greatest North Declination, 21st.</td></tr>
+<tr><td class="blr">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td></tr>
+<tr><td class="blr">July</td>
+    <td class="br" align="center">29&middot;970</td>
+    <td class="br" align="center">0&middot;79</td>
+    <td class="br" align="center">73&middot;8</td>
+    <td class="br" align="center">53&middot;2</td>
+    <td class="br" align="center">61&middot;9</td>
+    <td class="br" align="center">53&middot;9</td>
+    <td class="br" align="center">76</td>
+    <td class="br" align="center">11</td>
+    <td class="br" align="center">2&middot;7</td>
+    <td class="br" align="center">6&middot;9</td>
+    <td class="br" align="center">W. to N.</td>
+    <td class="br" align="center">16</td>
+    <td class="br">&nbsp;</td></tr>
+<tr><td class="blr">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td></tr>
+<tr><td class="blr">Aug.</td>
+    <td class="br" align="center">29&middot;954</td>
+    <td class="br" align="center">0&middot;97</td>
+    <td class="br" align="center">72&middot;8</td>
+    <td class="br" align="center">53&middot;4</td>
+    <td class="br" align="center">61&middot;3</td>
+    <td class="br" align="center">54&middot;1</td>
+    <td class="br" align="center">77</td>
+    <td class="br" align="center">11</td>
+    <td class="br" align="center">2&middot;4</td>
+    <td class="br" align="center">6&middot;5</td>
+    <td class="br" align="center">W. to N.</td>
+    <td class="br" align="center">14&#189;</td>
+    <td class="br">&nbsp;</td></tr>
+<tr><td class="blr">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td></tr>
+<tr><td class="blr">Sept.</td>
+    <td class="br" align="center">29&middot;997</td>
+    <td class="br" align="center">0&middot;95</td>
+    <td class="br" align="center">67&middot;4</td>
+    <td class="br" align="center">48&middot;9</td>
+    <td class="br" align="center">56&middot;9</td>
+    <td class="br" align="center">51&middot;1</td>
+    <td class="br" align="center">81</td>
+    <td class="br" align="center">12</td>
+    <td class="br" align="center">2&middot;4</td>
+    <td class="br" align="center">5&middot;9</td>
+    <td class="br" align="center">S. to W.</td>
+    <td class="br" align="center">12&#189;</td>
+    <td class="br">&nbsp;</td></tr>
+<tr><td class="blr">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td></tr>
+<tr><td class="blr">Oct.</td>
+    <td class="br" align="center">29&middot;860</td>
+    <td class="br" align="center">1&middot;33</td>
+    <td class="br" align="center">58&middot;3</td>
+    <td class="br" align="center">43&middot;7</td>
+    <td class="br" align="center">50&middot;2</td>
+    <td class="br" align="center">46&middot;0</td>
+    <td class="br" align="center">87</td>
+    <td class="br" align="center">13</td>
+    <td class="br" align="center">2&middot;8</td>
+    <td class="br" align="center">6&middot;9</td>
+    <td class="br" align="center">S. to W.</td>
+    <td class="br" align="center">10&#189;</td>
+    <td class="br">A few frosty nights.<br />Heavy gales.</td></tr>
+<tr><td class="blr">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td></tr>
+<tr><td class="blr">Nov.</td>
+    <td class="br" align="center">29&middot;929</td>
+    <td class="br" align="center">1&middot;53</td>
+    <td class="br" align="center">49&middot;3</td>
+    <td class="br" align="center">37&middot;7</td>
+    <td class="br" align="center">43&middot;4</td>
+    <td class="br" align="center">40&middot;1</td>
+    <td class="br" align="center">89</td>
+    <td class="br" align="center">12</td>
+    <td class="br" align="center">2&middot;4</td>
+    <td class="br" align="center">7&middot;2</td>
+    <td class="br" align="center">S.W.</td>
+    <td class="br" align="center">9</td>
+    <td class="br">11 nights frosty.</td></tr>
+<tr><td class="blr">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td>
+    <td class="br">&nbsp;</td></tr>
+<tr><td class="blr">Dec.</td>
+    <td class="br" align="center">29&middot;979</td>
+    <td class="br" align="center">1&middot;52</td>
+    <td class="br" align="center">45&middot;0</td>
+    <td class="br" align="center">35&middot;5</td>
+    <td class="br" align="center">40&middot;1</td>
+    <td class="br" align="center">36&middot;9</td>
+    <td class="br" align="center">89</td>
+    <td class="br" align="center">12</td>
+    <td class="br" align="center">1&middot;9</td>
+    <td class="br" align="center">7&middot;4</td>
+    <td class="br" align="center">W.</td>
+    <td class="br" align="center">8</td>
+    <td class="br">Sun attains greatest South Declination, 21st.</td></tr>
+<tr><td class="btlr">Year</td>
+    <td class="btr" align="center">29&middot;946</td>
+    <td class="btr" align="center">1&middot;16</td>
+    <td class="btr" align="center">58&middot;0</td>
+    <td class="btr" align="center">42&middot;3</td>
+    <td class="btr" align="center">49&middot;2</td>
+    <td class="btr" align="center">43&middot;7</td>
+    <td class="btr" align="center">82</td>
+    <td class="btr" align="center">133</td>
+    <td class="btr" align="center">25&middot;3</td>
+    <td class="btr" align="center">6&middot;7</td>
+    <td class="btr" align="center">...</td>
+    <td class="btr" align="center">...</td>
+    <td class="btr">&nbsp;</td></tr>
+<tr><td class="bbtl">&nbsp;</td>
+    <td class="bbt" align="center">1</td>
+    <td class="bbt" align="center">2</td>
+    <td class="bbt" align="center">3</td>
+    <td class="bbt" align="center">4</td>
+    <td class="bbt" align="center">5</td>
+    <td class="bbt" align="center">6</td>
+    <td class="bbt" align="center">7</td>
+    <td class="bbt" align="center">8</td>
+    <td class="bbt" align="center">9</td>
+    <td class="bbt" align="center">10</td>
+    <td class="bbt" align="center">11</td>
+    <td class="bbt" align="center">12</td>
+    <td class="bbtr" align="center">13</td></tr></table>
+
+<p><span class="pagenum"><a name="Page_150" id="Page_150">[Pg 150]</a></span>In the above Table, columns 1 to 10 are results obtained at the Royal
+Observatory, Greenwich, by J. Glaisher, Esq., F.R.S. The data contained in
+columns 2 and 10, are deduced from observations extending over the years
+1841 to 1855 inclusive, and are copied from Edward Hughes&#8217; <i>Third Reading
+Book</i>; the other columns are results of observations made during the
+twenty years ending 1861. The rest of the information is from Luke
+Howard&#8217;s <i>Climate of London</i>.</p>
+
+<p>These valuable data indicate the characteristics of the weather in each
+month in the suburbs of London, and will be found tolerably accurate as
+indications of weather, and serviceable as standards for comparisons of
+observed results, at most places in England.</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_151" id="Page_151">[Pg 151]</a></span></p>
+<p class="center"><span class="huge">STANDARD WORKS ON METEOROLOGY</span></p>
+<p class="center">SUPPLIED BY NEGRETTI &amp; ZAMBRA.</p>
+<p>&nbsp;</p>
+
+<p class="center">THE WEATHER BOOK:<br />
+A MANUAL OF PRACTICAL METEOROLOGY.<br />
+By Vice-Admiral <span class="smcap">FitzRoy</span>, F.R.S., M.I.F., &amp;c.<br />
+<i>Price</i>, &pound;0 15 6</p>
+
+<p class="center">THE LAW OF STORMS,<br />
+By <span class="smcap">H. W. Dove</span>, F.R.S.<br />
+Translated by <span class="smcap">R. H. Scott</span>, M.A.<br />
+<i>Price</i>, &pound;0 10 6</p>
+
+<p class="center">L. F. K&AElig;MTZ&#8217;S &#8220;COMPLETE COURSE OF METEOROLOGY,&#8221;<br />
+Translated by <span class="smcap">C. V. Walker</span>, Esq.<br />
+<i>Price</i>, &pound;0 12 6</p>
+
+<p class="center">PRACTICAL METEOROLOGY,<br />
+By <span class="smcap">John Drew</span>, Ph.D., F.R.A.S.<br />
+<i>Price</i>, &pound;0 5 0</p>
+
+<p class="center">HYGROMETRICAL TABLES,<br />
+Adapted to the use of the Wet and Dry Bulb Thermometer,<br />
+By <span class="smcap">James Glaisher</span>, Esq., F.R.S.<br />
+<i>Price</i>, &pound;0 2 6</p>
+
+<p class="center">TABLES OF THE CORRECTIONS FOR TEMPERATURES,<br />
+To reduce observations to the 32&deg; Fahrenheit, for Barometers with brass scales<br />
+extending from the cistern to the top of the mercurial column,<br />
+By <span class="smcap">James Glaisher</span>, Esq., F.R.S.<br />
+<i>Price</i>, &pound;0 1 0</p>
+
+<p class="center"><span class="pagenum"><a name="Page_152" id="Page_152">[Pg 152]</a></span>TABLE OF THE DIURNAL RANGE OF THE BAROMETER,<br />
+By <span class="smcap">James Glaisher</span>, Esq., F.R.S.<br />
+<i>Price</i>, &pound;0 0 6</p>
+
+<p class="center">TABLES FOR CALCULATION OF HEIGHTS FROM OBSERVATIONS<br />
+ON THE BOILING-POINT OF WATER,<br />
+Adapted to the use of Negretti and Zambra&#8217;s Boiling-point Apparatus.<br />
+<i>Price</i>, &pound;0 1 0</p>
+
+<p class="center">A THERMOMETRICAL TABLE,<br />
+ON THE SCALES OF FAHRENHEIT, REAUMUR, AND CENTIGRADE,<br />
+By <span class="smcap">Alfred S. Taylor</span>, Esq., M.D., &amp;c.<br />
+<i>Price</i>, in Sheet, with explanatory Pamphlet, &pound;0 1 6</p>
+
+<p class="center">METEOROLOGICAL TABLES,<br />
+For the reduction of Barometrical and Hygrometrical Observations, Determination<br />
+of Heights by the Barometer and Boiling-point Thermometer, &amp;c.<br />
+By <span class="smcap">G. Harvey Simmonds</span>, M.B.M.S.<br />
+<i>Price</i>, &pound;0 2 6</p>
+
+<p class="center">BAROMETER MANUAL,<br />
+Compiled by Vice-Admiral <span class="smcap">FitzRoy</span>, F.R.S.,<br />
+For the Board of Trade.<br />
+<i>Price</i>, &pound;0 0 6</p>
+
+<p class="center">POCKET METEOROLOGICAL REGISTER AND NOTE-BOOK,<br />
+With Diagrams for exhibiting the Fluctuations of Barometer, &amp;c.<br />
+Printed on metallic paper. <i>Price</i>, with Pencil, &pound;0 3 0</p>
+
+<p class="center">LONDON:<br />
+PRINTED BY STRAHAN AND WILLIAMS,<br />
+7 LAWRENCE LAND, CHEAPSIDE, E.C.</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_153" id="Page_153">[Pg 153]</a></span></p>
+<p class="center"><span class="large">NEGRETTI &amp; ZAMBRA&#8217;S</span><br />
+<span class="huge">PATENT RECORDING AND DEEP-SEA THERMOMETER.</span><small><a name="f20.1" id="f20.1" href="#f20">[20]</a></small></p>
+
+
+<p>This Thermometer differs from all other Registering or Recording
+Thermometers in the following important particulars:&mdash;</p>
+
+<p>I. The Thermometer contains only Mercury without any admixture of Alcohol
+or other fluid.</p>
+
+<p>II. It has no indices or springs, and its indications are by the column of
+Mercury only.</p>
+
+<p>III. It can be carried in any position, and cannot possibly be put out of
+order except by actual breakage of the instrument.</p>
+
+<p>And lastly, it will indicate and record the exact temperature at any hour
+of the day or night, or the exact temperature at any depth of the sea,
+irrespective of either warm or cold currents, or stratum through which the
+Thermometer may have to pass in its descent or ascent, this last very
+special quality renders this Thermometer superior for deep-sea
+temperatures to any others; for those now being used in the &#8220;Challenger&#8221;
+sounding expedition are liable to give erroneous indications owing to
+their indices slipping, and otherwise getting deranged&mdash;(This was proved
+by Messrs. Negretti and Zambra at a Meeting of the British Meteorological
+Society,) and <i>under certain conditions of temperature</i> it is not possible
+by the old Thermometers to obtain true temperatures at certain depths
+which might be required. <i>Annexed is a copy of a report to the Admiralty
+from Captain G. S. Nares, of H.M.S. &#8220;Challenger,&#8221; dated Melbourne, March
+25th, 1874, which we have taken from</i> <span class="smcap">Nature</span>, <i>July 30th, 1874, proving
+the assertion.</i></p>
+
+<p>&#8220;In the report to the Admiralty of Capt. G. S. Nares, of H.M.S.
+<i>Challenger</i> dated Melbourne, March 25, 1874, Capt. Nares, speaking of the
+temperature of the ocean, especially near the pack edge of the ice,
+says:&mdash;&#8216;At a short distance from the pack, the surface water rose to 32&deg;,
+but at a depth of 40 fathoms we always found the temperature to be 29&deg;;
+this continued to 300 fathoms, the depth in which most of the icebergs
+float, after which there is a stratum of slightly warmer water of 33&deg; or
+34&deg;. As the thermometers had to pass through these two belts of water
+before reaching the bottom, the indices registered those temperatures, and
+it was impossible to obtain the exact temperature of the bottom whilst
+near the ice, but the observations made in lower latitudes show that it is
+about 31&deg;. More exact results could not have been obtained even had Mr.
+Siemens&#8217;s apparatus been on board.&#8217; It seems to us that the difficulty
+mentioned is one which would certainly have been surmounted by Messrs.
+Negretti and Zambra&#8217;s new Recording Thermometers, a description of which
+appeared in <span class="smcap">Nature</span>, vol. ix. p. 387; this being exactly one of the cases
+to which this instrument is peculiarly adapted. We believe the inventors
+and makers have greatly improved their Thermometer since our description
+appeared, and no doubt means will be taken by the Admiralty to transmit
+one to the <i>Challenger</i>.&#8221;</p>
+
+<div class="figright"><img src="images/ad_fig_1.jpg" alt="" /><br />Fig. 1.</div>
+
+<p class="center"><br /><span class="large">DESCRIPTION OF THE DEEP-SEA RECORDING THERMOMETER.</span></p>
+
+<p>In the first place, it must be observed that the bulb of the Thermometer
+is protected so as to resist the pressure of the ocean, which varies
+according to depth that of three thousand fathoms being something like
+three tons pressure on the square inch. The manner of protecting the bulb
+was invented by Messrs. Negretti and Zambra in 1857, and has been latterly
+copied by other persons and brought out as a new invention. The manner of
+protecting the bulb has been described by<span class="pagenum"><a name="Page_154" id="Page_154">[Pg 154]</a></span> the late Admiral R. FitzRoy, in
+the first number of Meteorological Papers, page 55, published July 5th,
+1857, as follows:</p>
+
+<p class="blockquot">&#8220;Referring to the erroneous readings of all thermometers, consequent
+on their delicate bulbs being compressed by the great pressure of the
+ocean, he says:&mdash;&#8216;With a view to obviate this failing, Messrs.
+Negretti and Zambra undertook to make a case for the weak bulbs, which
+should transmit temperature, but resist pressure. Accordingly a tube
+of thick glass is sealed outside the delicate bulb, between which and
+the casing is a space all round, which is nearly filled with mercury.
+The small space not so filled is a vacuum, into which the mercury can
+be expanded, or forced by heat or mechanical compression, without
+doing injury to or even compressing the inner or much more delicate
+bulb.&#8217;&#8221;</p>
+
+<p>The construction of this instrument for deep-sea temperatures is as
+follows:&mdash;</p>
+
+<div class="figleft"><img src="images/ad_fig_2.jpg" alt="" /><br />Fig. 2.</div>
+
+<p class="blockquot">In shape it is like a syphon with parallel legs, all in one piece and
+having a continuous communication, as in the annexed figure. The scale
+of the Thermometer is pivoted on a centre and being attached in a
+perpendicular position to a simple apparatus (which will be presently
+described), is lowered to any depth that may be desired. In its
+descent the Thermometer acts as an ordinary instrument, the mercury
+rising or falling according to the temperature of the stratum through
+which it passes; but so soon as the descent ceases, and a reverse
+motion is given to the line, so as to pull the Thermometer towards the
+surface, the instrument turns once on its centre, first bulb
+uppermost, and afterwards bulb downwards. This causes the mercury,
+which was in the left-hand column, first to pass into the dilated
+syphon bend at the top, and thence into the right-hand tube, where it
+remains, indicating on a graduated scale the exact temperature at the
+time it was turned over. The woodcut, Fig. 1, shows the position of
+the mercury <i>after</i> the instrument has been thus turned on its centre.
+A is the bulb; B the outer coating or protecting cylinder; C is the
+space of rarefied air, which is reduced if the outer casing be
+compressed; D is a small glass plug on the principle of Negretti and
+Zambra&#8217;s Patent Maximum Thermometer, which cuts off, in the moment of
+turning, the mercury in the tube from that of the bulb, thereby
+ensuring that none but the mercury in the tube can be transferred into
+the indicating column; E is an enlargement made in the bend so as to
+enable the mercury to pass quickly from one tube to another in
+revolving; and F is the indicating tube or Thermometer proper. In its
+action, as soon as the Thermometer is put in motion, and immediately
+the tube has acquired a slightly oblique position, the mercury breaks
+off at the point D, runs into the curved and enlarged portion E, and
+eventually falls into the tube F when this tube resumes its original
+perpendicular position.</p>
+
+<div class="figright"><img src="images/ad_fig_3.jpg" alt="" /><br />Fig. 3.</div>
+
+<p><span class="pagenum"><a name="Page_155" id="Page_155">[Pg 155]</a></span>The contrivance for turning the Thermometer over may be described as a
+frame with a vertical propeller; to this frame the instrument is pivoted.
+On its descent through the water the propeller is lifted out of gear and
+revolves freely on its axis; but so soon as the instrument is pulled
+towards the surface the propeller falls into gear and revolves in the
+contrary direction, turning the Thermometer over once, and then becoming
+locked and immovable.</p>
+
+<div class="blockquot">
+<p><i>Directions for adjusting the Thermometer previous to its being lowered in
+the Sea.</i></p>
+
+<p>I. The mercury must all be in the left-hand column.</p>
+
+<p>II. The short peg at the back of the thermometer must be in front of
+the stop plate S +; in order to effect this, pull the knob which stops
+the Thermometer, and slightly turn the propeller, to make the
+Thermometer advance sufficiently to escape the stop plate.</p></div>
+
+<p>Negretti &amp; Zambra&#8217;s Patent Atmospheric Recording Thermometer, Fig. 3,
+differs from the Deep-sea Thermometer by its not having the double or
+protected bulb, it not being required for resisting pressures. In this
+case the instrument is turned over by a simple clock movement, which can
+be set to any hour it may be desirable; the Thermometer is fixed on the
+clock, and when the hand arrives at the hour determined upon, and to which
+the clock is set as in setting an alarum clock, a spring is released and
+the Thermometer turns over as before described.</p>
+
+<p>Messrs. Negretti and Zambra have arranged a Wet and Dry Bulb Hygrometer
+upon the same plan.</p>
+
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_156" id="Page_156">[Pg 156]</a></span></p>
+
+<p class="center"><span class="huge">NEGRETTI &amp; ZAMBRA&#8217;S</span></p>
+<p class="center"><span class="large">PRICE LIST</span></p>
+<p class="center">OF</p>
+<p class="center"><span class="giant">STANDARD METEOROLOGICAL</span></p>
+<p class="center">AND OTHER</p>
+<p class="center"><span class="huge">PHILOSOPHICAL INSTRUMENTS.</span></p>
+
+<p><i>The marginal figures in this List and the numbers of the wood engravings
+refer to paragraphs in &#8220;Negretti &amp; Zambra&#8217;s Treatise on Meteorological
+Instruments.&#8221;</i></p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td>&nbsp;</td>
+    <td>&nbsp;</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="center">&pound;</td>
+    <td align="center">s.</td>
+    <td align="center">d.</td></tr>
+<tr><td align="right">4</td>
+    <td><b>Standard Barometers</b>, Fortin&#8217;s arrangement, as Fig. 3 with mahogany board</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">8</td>
+    <td align="right">8</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto ditto with Millemetre and English scales</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">9</td>
+    <td align="right">9</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto ditto with tube, 0&middot;45 internal diameter and millimetre scale</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">10</td>
+    <td align="right">10</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td><b>Observatory Standard Barometers</b>, extra large tubes and cisterns</td>
+    <td align="right">&pound;25</td>
+    <td align="right">0</td>
+    <td align="right">0</td>
+    <td><span class="spacer2">&nbsp;</span></td>
+    <td align="right">35</td>
+    <td align="right">0</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto ditto arranged for observations being taken by the Cathetometer</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">18</td>
+    <td align="right">18</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td><b>Cathetometer</b>, for use with above</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">21</td>
+    <td align="right">0</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">9</td>
+    <td><b>Self Compensating Standard Barometer</b>, Fig. 6</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">20</td>
+    <td align="right">0</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">10</td>
+    <td><b>Standard Barometer</b>, with electrical adjustment</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">15</td>
+    <td align="right">15</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">11</td>
+    <td><b>Pediment Barometers</b>, Fig 7</td>
+    <td align="right">&pound;1</td>
+    <td align="right">1</td>
+    <td align="right">0</td>
+    <td>&nbsp;</td>
+    <td align="right">2</td>
+    <td align="right">2</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto<span class="spacer">&nbsp; &nbsp;</span>ditto<span class="spacer">&nbsp; &nbsp; &nbsp;</span>Fig. 8</td>
+    <td align="right">&pound;3</td>
+    <td align="right">3</td>
+    <td align="right">0</td>
+    <td>&nbsp;</td>
+    <td align="right">3</td>
+    <td align="right">10</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto<span class="spacer">&nbsp; &nbsp;</span>ditto<span class="spacer">&nbsp; &nbsp; &nbsp;</span>Fig. 9</td>
+    <td align="right">&pound;4</td>
+    <td align="right">10</td>
+    <td align="right">0</td>
+    <td>&nbsp;</td>
+    <td align="right">5</td>
+    <td align="right">10</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto<span class="spacer">&nbsp; &nbsp;</span>ditto<span class="spacer">&nbsp; &nbsp; &nbsp;</span>Fig. 10</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">8</td>
+    <td align="right">10</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto ditto ditto handsome carved mountings, in mahogany, oak, or walnut wood<span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span>&pound;8 &nbsp; 8 &nbsp; 0</td>
+    <td align="right">&pound;10</td>
+    <td align="right">10</td>
+    <td align="right">0</td>
+    <td>&nbsp;</td>
+    <td align="right">12</td>
+    <td align="right">12</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">14</td>
+    <td><b>FitzRoy&#8217;s Storm or Sea Coast Fishery Barometer</b>, Fig. 12</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">5</td>
+    <td align="right">5</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto<span class="spacer">&nbsp; &nbsp;</span>ditto with two verniers</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">6</td>
+    <td align="right">6</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto<span class="spacer">&nbsp; &nbsp;</span>ditto mounted in ornamental carved frames, oak, walnut, or mahogany</td>
+    <td align="right">&pound;6</td>
+    <td align="right">10</td>
+    <td align="right">0</td>
+    <td>&nbsp;</td>
+    <td align="right">8</td>
+    <td align="right">8</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">19</td>
+    <td><b>Marine Barometers</b>, ordinary forms, Figs. 13 and 14<span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span><span class="spacer">&nbsp; &nbsp;</span><span class="spacer">&nbsp;</span>&pound;2 &nbsp; 2s.</td>
+    <td align="right">&pound;2</td>
+    <td colspan="2"><span style="margin-left: .2em;">10s.</span></td>
+    <td>&nbsp;</td>
+    <td align="right">3</td>
+    <td align="right">3</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto<span class="spacer">&nbsp; &nbsp;</span>ditto<span class="spacer">&nbsp; &nbsp;</span>Best mounted</td>
+    <td align="right">&pound;5</td>
+    <td colspan="2"><span style="margin-left: .7em;">5s.</span></td>
+    <td>&nbsp;</td>
+    <td align="right">6</td>
+    <td align="right">6</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">20</td>
+    <td><b>The Board of Trade or Kew Marine Barometer</b>, Fig. 15,<span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span><span class="spacer">&nbsp; &nbsp;</span>&pound;4 &nbsp; 4s.</td>
+    <td align="right">&pound;5</td>
+    <td colspan="2"><span style="margin-left: .7em;">5s.</span></td>
+    <td>&nbsp;</td>
+    <td align="right">6</td>
+    <td align="right">6</td>
+    <td align="right">0</td></tr>
+<tr><td valign="top" align="right">22</td>
+    <td><b>Negretti and Zambra&#8217;s FitzRoy Marine or Gun Barometer</b>, Fig. 16, with N. and Z.&#8217;s<br />
+    Patent Porcelain Scales, as used in Her Majesty&#8217;s Navy</td>
+    <td colspan="4">&nbsp;</td>
+    <td valign="bottom" align="right">5</td>
+    <td valign="bottom" align="right">10</td>
+    <td valign="bottom" align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Extra Tube for ditto</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">1</td>
+    <td align="right">15</td>
+    <td align="right">0</td></tr>
+<tr><td align="right"><span class="pagenum"><a name="Page_157" id="Page_157">[Pg 157]</a></span>25</td>
+    <td><b>Negretti and Zambra&#8217;s Farmer&#8217;s Barometer or Domestic Weather Glass</b>, Fig. 17</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">2</td>
+    <td align="right">10</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">28</td>
+    <td><b>Negretti and Zambra&#8217;s Miner&#8217;s Barometers</b><span class="spacer">&nbsp;</span><span class="spacer">&nbsp; &nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span><span class="spacer">&nbsp; &nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>&pound;1 &nbsp; 1s.</td>
+    <td align="right">&pound;2</td>
+    <td colspan="2"><span style="margin-left: .7em;">2s.</span></td>
+    <td>&nbsp;</td>
+    <td align="right">3</td>
+    <td align="right">3</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">31</td>
+    <td><b>Dial or Wheel Barometers</b>, Figs. 18, 19, 20, 21<span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>&pound;3 &nbsp; 3s.</td>
+    <td align="right">&pound;4</td>
+    <td colspan="2"><span style="margin-left: .7em;">4s.</span></td>
+    <td>&nbsp;</td>
+    <td align="right">5</td>
+    <td align="right">5</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto &nbsp; ditto in carved ornamental mountings<span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>&pound;5 &nbsp; 10s.
+    <span class="spacer2">&nbsp;&nbsp;</span>&pound;6 &nbsp; 6s.</td>
+    <td align="right">&pound;8</td>
+    <td colspan="2"><span style="margin-left: .7em;">8s.</span></td>
+    <td>&nbsp;</td>
+    <td align="right">10</td>
+    <td align="right">10</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto &nbsp; ditto rosewood, inlaid with pearl or metal.<span class="spacer2">&nbsp;</span>Made to order, Figs. 22 and 23.<br />
+    <span style="margin-left: 1em;">Price varying with size, &amp;c.</span></td></tr>
+<tr><td align="right">37</td>
+    <td><b>Gay Lussac&#8217;s Syphon Tube Mountain Barometer</b></td>
+    <td align="right">&pound;6</td>
+    <td align="right">6</td>
+    <td align="right">0</td>
+    <td>&nbsp;</td>
+    <td align="right">8</td>
+    <td align="right">8</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">32</td>
+    <td><b>Standard Syphon Barometer</b>, Gay Lussac&#8217;s arrangement, Fig. 24</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">5</td>
+    <td align="right">5</td>
+    <td align="right">0</td></tr>
+<tr><td valign="top" align="right">38</td>
+    <td><b>Negretti and Zambra&#8217;s Standard Mountain Barometer</b>, with Fortin&#8217;s cistern,<br />with tripod stand and travelling case, Fig. 30</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">10</td>
+    <td align="right">10</td>
+    <td align="right">0</td></tr>
+<tr><td valign="top" align="right">34</td>
+    <td><b>Barograph, or Self-registering Barometer</b>, with syphon mercurial tube.<br />Negretti and Zambra&#8217;s improved arrangement, Fig. 26</td>
+    <td align="right">18</td>
+    <td align="right">18</td>
+    <td align="right">0</td>
+    <td>&nbsp;</td>
+    <td align="right">25</td>
+    <td align="right">0</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td><b>Negretti and Zambra&#8217;s Self-recording Aneroid Barometer</b>, with <b>Clock</b></td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">22</td>
+    <td align="right">0</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td>&nbsp;</td><td colspan="5" align="center">48. <b>ANEROID BAROMETERS.</b></td></tr>
+<tr><td>&nbsp;</td>
+    <td><b>Aneroid Barometers,</b> with card dials 4&#189; inches diameter, best quality.</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">2</td>
+    <td align="right">10</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto<span class="spacer">&nbsp;</span>ditto<span class="spacer">&nbsp; &nbsp; &nbsp;</span>with silvered metal dial</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">3</td>
+    <td align="right">0</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto<span class="spacer">&nbsp;</span>ditto<span class="spacer">&nbsp; &nbsp; &nbsp;</span>with ditto &nbsp; &nbsp; and thermometer</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">3</td>
+    <td align="right">10</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td colspan="3">Ditto<span class="spacer">&nbsp;</span>ditto<span class="spacer">&nbsp; &nbsp; &nbsp;</span>ditto with corrected scale,
+    as supplied by Negretti and Zambra to the Royal Navy</td>
+    <td colspan="2">&nbsp;</td>
+    <td align="right">5</td>
+    <td align="right">5</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td><b>Aneroid Barometers</b>, with elegantly-chased dials</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">4</td>
+    <td align="right">4</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto<span class="spacer">&nbsp;</span>ditto<span class="spacer">&nbsp; &nbsp; &nbsp;</span>with raised ring on dial</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">5</td>
+    <td align="right">5</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto<span class="spacer">&nbsp;</span>ditto<span class="spacer">&nbsp; &nbsp; &nbsp;</span>ditto<span class="spacer">&nbsp;</span>with thermometer</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">6</td>
+    <td align="right">6</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td colspan="3"><b>Aneroid Barometer</b>, for altitude measurements with revolving ring, carrying index, range of scale<br />
+    <span style="margin-left: 1em;">20,000 feet 4&#189; inches diameter, with magnifier</span></td>
+    <td colspan="2">&nbsp;</td>
+    <td align="right">8</td>
+    <td align="right">8</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td>&nbsp;</td><td colspan="5" align="center"><b>POCKET ANEROID BAROMETERS.</b> Fig. 34.</td></tr>
+<tr><td align="right">49</td>
+    <td><b>Pocket Aneroid Barometer</b>, 2&#190; inches diameter, with silvered metal scale</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">3</td>
+    <td align="right">3</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td colspan="3">Ditto<span class="spacer">&nbsp;</span>ditto &nbsp; for measuring altitudes to 10,000 feet compensated for temperature, in leather case</td>
+    <td colspan="2">&nbsp;</td>
+    <td align="right">5</td>
+    <td align="right">5</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto<span class="spacer">&nbsp;</span>ditto<span class="spacer">&nbsp; &nbsp;</span>ditto<span class="spacer">&nbsp;&nbsp;</span>
+    <span class="spacer">&nbsp;</span>to 20,000 feet, with magnifier</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">6</td>
+    <td align="right">6</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td align="right">50</td>
+    <td><b>WATCH-SIZE ANEROID BAROMETERS</b> in gilt metal cases (see figure 35.)</td></tr>
+<tr><td>&nbsp;</td>
+    <td><b>Watch-Size Aneroid Barometer</b>, weather range</td>
+    <td align="right">&pound;3</td>
+    <td align="right">3</td>
+    <td align="right">0</td>
+    <td>&nbsp;</td>
+    <td align="right">4</td>
+    <td align="right">4</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td colspan="3">Ditto<span class="spacer">&nbsp;</span>ditto<span class="spacer">&nbsp;</span> of best construction, extra thin, for meteorological observations<br />
+    <span style="margin-left: 1em;">or altitude measurements to 10,000 feet</span></td>
+    <td colspan="2">&nbsp;</td>
+    <td align="right">5</td>
+    <td align="right">5</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td colspan="3">Ditto<span class="spacer">&nbsp;</span>ditto<span class="spacer">&nbsp;</span> ditto to 20,000 feet, compensated for temperature</td>
+    <td colspan="2">&nbsp;</td>
+    <td align="right">6</td>
+    <td align="right">6</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td colspan="5" align="center">Either of the above Watch-size Barometers may be had in Stout Silver Cases at a cost of &pound;2 2s. extra</td></tr>
+<tr><td>&nbsp;</td>
+    <td colspan="5" align="center"><i>Watch-size Aneroid Barometers in Solid Gold, highly-finished cases. &pound;15 15s. to &pound;21.</i></td></tr>
+<tr><td>&nbsp;</td>
+    <td colspan="5" align="center">Table Stands for Aneroid Barometers of Carved Oak or other woods,<br />10s. 6d., 25s., 35s., to &pound;5 5s.</td></tr>
+<tr><td>&nbsp;</td>
+    <td colspan="5" align="center"><b>Ships&#8217; Aneroid Barometers</b>, in suitable mountings &nbsp;&nbsp; &pound;2 10s.
+    &nbsp;&nbsp; &pound;3 3s. &nbsp;&nbsp; &pound;5 5s. &nbsp;&nbsp; &pound;6 6s.</td></tr>
+<tr><td>&nbsp;<span class="pagenum"><a name="Page_158" id="Page_158">[Pg 158]</a></span></td></tr>
+<tr><td align="right">47</td>
+    <td><b>Sympiesometer</b>, for Ship use</td>
+    <td align="right">&pound;3</td>
+    <td align="right">3</td>
+    <td align="right">0</td>
+    <td>&nbsp;</td>
+    <td align="right">4</td>
+    <td align="right">10</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto<span class="spacer">&nbsp;</span>ditto<span class="spacer">&nbsp;</span>Pocket form, Fig. 32</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">4</td>
+    <td align="right">4</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td colspan="5" align="center">The Sympiesometer is now rarely used, the Aneroid Barometer being found<br />equally sensitive and less liable to derangement.</td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td align="right">56</td>
+    <td><b>Independent Standard Thermometers</b>, Fig. 36</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">5</td>
+    <td align="right">5</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">57</td>
+    <td><b>Standard Thermometers</b>, for Boiling Point Apparatus</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">1</td>
+    <td align="right">10</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td><b>Chemists&#8217; or Brewers&#8217; Standard Reference Thermometers</b></td>
+    <td align="right">&pound;1</td>
+    <td colspan="2"><span style="margin-left: .5em;">1s.</span></td>
+    <td>&nbsp;</td>
+    <td align="right">2</td>
+    <td align="right">2</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">47</td>
+    <td><b>Chemical Thermometers</b>, divided on the stem. Fig. 48.</td>
+    <td>&nbsp;</td>
+    <td>10s.</td>
+    <td>6d.</td>
+    <td>&nbsp;</td>
+    <td align="right">0</td>
+    <td align="right">15</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td><b>Chemical Thermometers</b>, Boxwood Scale<span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span>7s. 6d.</td>
+    <td>&nbsp;</td>
+    <td>10s.</td>
+    <td>6d.</td>
+    <td>&nbsp;</td>
+    <td align="right">0</td>
+    <td align="right">12</td>
+    <td align="right">6</td></tr>
+<tr><td align="right">64</td>
+    <td><b>Thermometers on Boxwood Scales</b>, Fig. 37.<span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span><span class="spacer">&nbsp;&nbsp;</span>1s. &nbsp;&nbsp; 1s. 6d. &nbsp;&nbsp; 2s. 6d.</td>
+    <td>&nbsp;</td>
+    <td><span style="margin-left: .5em;">3s.</span></td>
+    <td>6d.</td>
+    <td>&nbsp;</td>
+    <td align="right">0</td>
+    <td align="right">4</td>
+    <td align="right">6</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto<span class="spacer">&nbsp;</span>ditto<span class="spacer">&nbsp;</span>larger sizes</td>
+    <td>&nbsp;</td>
+    <td><span style="margin-left: .5em;">7s.</span></td>
+    <td>6d.</td>
+    <td>&nbsp;</td>
+    <td align="right">0</td>
+    <td align="right">12</td>
+    <td align="right">6</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto<span class="spacer">&nbsp;</span>ditto<span class="spacer">&nbsp;</span>Engraved glass scales, Fig. 39<span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span>15s. &nbsp;&nbsp; &pound;1 1s.</td>
+    <td align="right">&pound;1</td>
+    <td colspan="2"><span style="margin-left: .5em;">5s.</span></td>
+    <td>&nbsp;</td>
+    <td align="right">1</td>
+    <td align="right">10</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td>&nbsp;</td><td colspan="5" align="center"><b>POCKET THERMOMETERS, IN VARIOUS MOUNTINGS.</b></td></tr>
+<tr><td>&nbsp;</td>
+    <td colspan="5" align="center">Fig. 38 10s. 6d.<span class="spacer">&nbsp;</span>Fig. 40 10s. &nbsp;&nbsp; 6d. 15s.<span class="spacer">&nbsp;</span>
+    Fig. 41 5s. 6d. &nbsp;&nbsp;  8s. 6d. &nbsp;&nbsp; 12s. 6d.</td></tr>
+<tr><td align="right">63</td>
+    <td><b>Thermometers of extreme Sensitiveness</b>,<span class="spacer">&nbsp; &nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp; &nbsp;</span><span class="spacer">&nbsp; &nbsp;</span><span class="spacer">&nbsp; &nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp; &nbsp;</span>15s.</td>
+    <td align="right">&pound;1</td>
+    <td colspan="2"><span style="margin-left: .3em;">10s.</span></td>
+    <td>&nbsp;</td>
+    <td align="right">2</td>
+    <td align="right">2</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td>&nbsp;</td>
+    <td><b>Drawing Room or Mantel Thermometers</b>, various mountings, Figs. 42 and 43.<br />
+    <span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp; &nbsp;</span>12s. 6d. &nbsp;&nbsp; 15s. &nbsp;&nbsp; &pound;1 1s. &nbsp;&nbsp; &pound;1 10s.</td>
+    <td valign="bottom" align="right">&pound;2</td>
+    <td valign="bottom" colspan="2"><span style="margin-left: .8em;">2s.</span></td>
+    <td>&nbsp;</td>
+    <td valign="bottom" align="right">2</td>
+    <td valign="bottom" align="right">10</td>
+    <td valign="bottom" align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td><b>Bracket Window Thermometers</b>, Fig. 46<span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span>12s. 6d. &nbsp; &nbsp; 15s.</td>
+    <td align="right">&pound;1</td>
+    <td colspan="2"><span style="margin-left: .8em;">1s.</span></td>
+    <td>&nbsp;</td>
+    <td align="right">1</td>
+    <td align="right">10</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td><b>Bath Thermometers</b>, Figs. 44</td>
+    <td>&nbsp;</td>
+    <td><span style="margin-left: 1em;">7s.</span></td>
+    <td colspan="2">6d.</td>
+    <td align="right">0</td>
+    <td align="right">15</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">66</td>
+    <td><b>Sugar Boiling Thermometers</b><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span><span class="spacer">&nbsp; &nbsp;</span>&pound;1 12s.</td>
+    <td align="right">&pound;2</td>
+    <td colspan="2"><span style="margin-left: .5em;">2s.</span></td>
+    <td>&nbsp;</td>
+    <td align="right">3</td>
+    <td align="right">3</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">67</td>
+    <td><b>Earth Thermometers</b>, Fig. 51</td>
+    <td align="right">&pound;1</td>
+    <td align="right">10s.</td>
+    <td colspan="2">&nbsp;</td>
+    <td align="right">2</td>
+    <td align="right">2</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td><span style="margin-left: 1em;">Hot Bed</span><span class="spacer">&nbsp;</span>Ditto<span class="spacer">&nbsp;</span><span class="spacer">&nbsp; &nbsp; &nbsp;</span>
+    <span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp; </span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    12s. 6d.</td>
+    <td align="right">&pound;1</td>
+    <td align="right">5s.</td>
+    <td colspan="2">&nbsp;</td>
+    <td align="right">1</td>
+    <td align="right">10</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">68</td>
+    <td><b>Marine Thermometer</b>, Fig. 52<span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp; &nbsp;</span>7s. 6d.</td>
+    <td>&nbsp;</td>
+    <td align="right">8s.</td>
+    <td align="right">6d.</td>
+    <td>&nbsp;</td>
+    <td align="right">0</td>
+    <td align="right">10</td>
+    <td align="right">6</td></tr>
+<tr><td align="right">65</td>
+    <td><b>Super Heated or Steam Pressure Thermometers</b>, Fig. 74, Figs. 49 and 50<span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    &pound;1 5s. &nbsp;&nbsp; &pound;1 10s.</td>
+    <td align="right">&pound;2</td>
+    <td align="right">2s.</td>
+    <td colspan="2">&nbsp;</td>
+    <td align="right">2</td>
+    <td align="right">10</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td>&nbsp;</td><td colspan="5" align="center"><b>SELF-REGISTERING THERMOMETERS FOR HEAT.</b></td></tr>
+<tr><td align="right">72</td>
+    <td><b>Negretti and Zambra&#8217;s Patent Standard Maximum Self-Registering Thermometer</b>,<small><a name="f21.1" id="f21.1" href="#f21">[21]</a></small> Fig. 54</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">1</td>
+    <td align="right">1</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">72</td>
+    <td><b>Negretti and Zambra&#8217;s Patent Maximum Thermometer</b>, on boxwood scale</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">0</td>
+    <td align="right">10</td>
+    <td align="right">6</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto,<span class="spacer2">&nbsp;</span>ditto,<span class="spacer2">&nbsp;</span>
+    on Negretti and Zambra&#8217;s Patent Solid Porcelain or Metal Scales on oak mounting</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">0</td>
+    <td align="right">12</td>
+    <td align="right">6</td></tr>
+<tr><td align="right">70</td>
+    <td><b>Rutherford&#8217;s Maximum Thermometer</b>, on boxwood or metal scale, with steel index<span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span>5s. 6d.</td>
+    <td>&nbsp;</td>
+    <td align="right">7s.</td>
+    <td align="right">6d.</td>
+    <td>&nbsp;</td>
+    <td align="right">0</td>
+    <td align="right">10</td>
+    <td align="right">6</td></tr>
+<tr><td align="right">71</td>
+    <td><b>Phillip&#8217;s Maximum Thermometer</b>, on boxwood or metal scale, with air index<span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span><span class="spacer">&nbsp; &nbsp;</span>7s. 6d.</td>
+    <td>&nbsp;</td>
+    <td align="right">10</td>
+    <td align="right">6</td>
+    <td>&nbsp;</td>
+    <td align="right">0</td>
+    <td align="right">12</td>
+    <td align="right">6</td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td>&nbsp;</td><td colspan="5" align="center"><b>SELF-REGISTERING THERMOMETERS FOR COLD.</b></td></tr>
+<tr><td align="right">73</td>
+    <td><b>Negretti and Zambra&#8217;s Standard Minimum Self-Registering Thermometer</b>, Fig. 55</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">1</td>
+    <td align="right">1</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">73</td>
+    <td><b>Rutherford&#8217;s Minimum Thermometer</b>, on boxwood or metal scale<span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span><span class="spacer">&nbsp; &nbsp;</span>3s. 6d. &nbsp;&nbsp; 5s. 6d.</td>
+    <td>&nbsp;</td>
+    <td align="right">7s.</td>
+    <td align="right">6d.</td>
+    <td>&nbsp;</td>
+    <td align="right">0</td>
+    <td align="right">10</td>
+    <td align="right">6</td></tr>
+<tr><td align="right"><span class="pagenum"><a name="Page_159" id="Page_159">[Pg 159]</a></span>73</td>
+    <td><b>Rutherford&#8217;s Minimum Thermometer</b>, on Negretti and Zambra&#8217;s Patent solid porcelain scale</td>
+    <td>&nbsp;</td>
+    <td align="right">10s.</td>
+    <td align="right">6d.</td>
+    <td>&nbsp;</td>
+    <td align="right">0</td>
+    <td align="right">12</td>
+    <td align="right">6</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto, on Negretti and Zambra&#8217;s porcelain or metal scales and oak mounting</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">0</td>
+    <td align="right">12</td>
+    <td align="right">6</td></tr>
+<tr><td valign="top" align="right">74</td>
+    <td><b>Negretti and Zambra&#8217;s Horticultural Self-Registering Thermometer.</b> The scale is made of<br />
+    stout zinc, enclosing the tube; the figures and divisions are boldly marked for quickly and easily<br />reading the indications, Fig. 56</td>
+    <td colspan="4">&nbsp;</td>
+    <td valign="bottom" align="right">0</td>
+    <td valign="bottom" align="right">3</td>
+    <td valign="bottom" align="right">6</td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td align="right">83</td>
+    <td><b>Negretti and Zambra&#8217;s Patent Solar Radiation Thermometer</b>, Fig. 63</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">1</td>
+    <td align="right">5</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">84</td>
+    <td>Ditto,<span class="spacer">&nbsp;</span>ditto,<span class="spacer">&nbsp;</span>ditto, in vacuo, Fig. 64</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">1</td>
+    <td align="right">10</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto,<span class="spacer">&nbsp;</span>ditto,<span class="spacer">&nbsp;</span>ditto, improved form, with test gauge</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">2</td>
+    <td align="right">2</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">85</td>
+    <td><b>Negretti and Zambra&#8217;s Terrestrial Radiation Thermometer</b></td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">1</td>
+    <td align="right">5</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Brass Stands for above, Fig. 65</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">0</td>
+    <td align="right">5</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">76 and 77</td>
+    <td><b>Negretti and Zambra&#8217;s Patent Mercurial Minimum Thermometers</b></td>
+    <td align="right">&pound;2</td>
+    <td align="right">10s.</td>
+    <td colspan="2">&nbsp;</td>
+    <td align="right">2</td>
+    <td align="right">2</td>
+    <td align="right">0</td></tr>
+<tr><td valign="top" align="right">81</td>
+    <td><b>Maxima and Minima Thermometers</b>, on Sixe&#8217;s arrangement, Fig. 62, various forms of mounting<br /><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span>12s. 6d. &nbsp;&nbsp; 14s. &nbsp;&nbsp; 21s.</td>
+    <td>&nbsp;</td>
+    <td valign="bottom" align="right">30s.</td>
+    <td colspan="2">&nbsp;</td>
+    <td valign="bottom" align="right">2</td>
+    <td valign="bottom" align="right">2</td>
+    <td valign="bottom" align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td><b>Pocket Maxima and Minima Thermometers</b>, Negretti and Zambra&#8217;s Patent, in convenient cases</td>
+    <td align="right">&pound;2</td>
+    <td align="right">2s.</td>
+    <td colspan="2">&nbsp;</td>
+    <td align="right">2</td>
+    <td align="right">10</td>
+    <td align="right">0</td></tr>
+<tr><td valign="top" align="right">89</td>
+    <td colspan="4"><b>Deep Sea Registering Thermometer</b>, with Negretti and Zambra&#8217;s improved protected bulb,<br />in copper cylinder, Fig. 69.</td>
+    <td>&nbsp;</td>
+    <td valign="bottom" align="right">2</td>
+    <td valign="bottom" align="right">10</td>
+    <td valign="bottom" align="right">0</td></tr>
+<tr><td valign="top" align="right">89<small><a href="#f21">[21]</a></small></td>
+    <td><b>Negretti and Zambra&#8217;s Improved Deep Sea Thermometer</b>, with vulcanite mountings,<br />in copper cylinder, with door, small size</td>
+    <td colspan="4">&nbsp;</td>
+    <td valign="bottom" align="right">2</td>
+    <td valign="bottom" align="right">5</td>
+    <td valign="bottom" align="right">0</td></tr>
+<tr><td align="right">90</td>
+    <td><b>Negretti and Zambra&#8217;s Patent Recording Deep Sea Thermometer</b></td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">10</td>
+    <td align="right">10</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">91</td>
+    <td>Ditto,<span class="spacer">&nbsp;</span>ditto,<span class="spacer">&nbsp;</span>ditto<span class="spacer">&nbsp;</span><b>Recording Thermometer</b></td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">4</td>
+    <td align="right">4</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">92</td>
+    <td>Ditto,<span class="spacer">&nbsp;</span>ditto,<span class="spacer">&nbsp;</span>ditto<span class="spacer">&nbsp;</span><b>Hygrometer</b></td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">6</td>
+    <td align="right">6</td>
+    <td align="right">0</td></tr>
+<tr><td valign="top" align="right">93</td>
+    <td><b>Improved Boiling Point Mountain Thermometer</b>, or Hypsometric Apparatus, with Tables, Figs. 72 and 73,<br />in leather case with strap</td>
+    <td colspan="4">&nbsp;</td>
+    <td valign="bottom" align="right">5</td>
+    <td valign="bottom" align="right">5</td>
+    <td valign="bottom" align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Extra Thermometer for Ditto</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">1</td>
+    <td align="right">10</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">106</td>
+    <td><b>Negretti and Zambra&#8217;s Standard Wet and Dry Bulb Hygrometer</b>, Fig. 79</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">2</td>
+    <td align="right">2</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Wet and Dry Bulb Hygrometers, various mountings<span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    <span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span><span class="spacer">&nbsp;</span>
+    30s. &nbsp;&nbsp; 25s. &nbsp;&nbsp; 21s.</td>
+    <td>&nbsp;</td>
+    <td align="right">14s.</td>
+    <td colspan="2">&nbsp;</td>
+    <td align="right">0</td>
+    <td align="right">10</td>
+    <td align="right">6</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Pocket Hygrometers, in box</td>
+    <td align="right">&pound;2</td>
+    <td align="right">2s.</td>
+    <td colspan="2">&nbsp;</td>
+    <td align="right">2</td>
+    <td align="right">10</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">103</td>
+    <td><b>Daniell&#8217;s Hygrometer</b>, Fig. 77</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">3</td>
+    <td align="right">3</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">104</td>
+    <td><b>Regnault&#8217;s Hygrometer</b>, Fig. 78</td>
+    <td align="right">&pound;3</td>
+    <td align="right">10s.</td>
+    <td colspan="2">&nbsp;</td>
+    <td align="right">5</td>
+    <td align="right">5</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Aspirator for Ditto</td>
+    <td align="right">&pound;1</td>
+    <td align="right">15s.</td>
+    <td colspan="2">&nbsp;</td>
+    <td align="right">2</td>
+    <td align="right">15</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td valign="top" align="right">110</td>
+    <td><b>Howard&#8217;s Rain Gauge</b>, has a 5-inch copper Funnel, with turned brass rim fitted to a stout stone-ware or<br />
+    glass bottle, with a graduated glass measure, divided to 100ths of an inch</td>
+    <td colspan="4">&nbsp;</td>
+    <td valign="bottom" align="right">0</td>
+    <td valign="bottom" align="right">10</td>
+    <td valign="bottom" align="right">6</td></tr>
+<tr><td>&nbsp;</td>
+    <td><b>Symons&#8217; Portable Rain Gauge</b>, (5-inch) with graduated glass measure, japanned tin</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">0</td>
+    <td align="right">10</td>
+    <td align="right">6</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto<span class="spacer">&nbsp;</span>ditto<span class="spacer">&nbsp;</span>in stout copper</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">0</td>
+    <td align="right">15</td>
+    <td align="right">0</td></tr>
+<tr><td valign="top" align="right">111</td>
+    <td><b>Glaisher&#8217;s Rain Gauge</b>, the receiving surface is 8-inches diameter, of stout japanned metal,<br />with graduated glass measure, Fig. 84</td>
+    <td colspan="4">&nbsp;</td>
+    <td valign="bottom" align="right">1</td>
+    <td valign="bottom" align="right">1</td>
+    <td valign="bottom" align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto<span class="spacer">&nbsp;</span>ditto,<span class="spacer">&nbsp;</span>of stout copper</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">1</td>
+    <td align="right">10</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Receiving Pots for ditto, extra 2s. and 3s. 6d.</td></tr>
+<tr><td valign="top" align="right"><span class="pagenum"><a name="Page_160" id="Page_160">[Pg 160]</a></span>113</td>
+    <td><b>Rain Gauge</b>, having a receiving surface of 12 inches diameter, and graduated glass gauge tube, divided to<br />
+    hundredths of an inch, in japanned metal, with brass tap</td>
+    <td colspan="4">&nbsp;</td>
+    <td valign="bottom" align="right">2</td>
+    <td valign="bottom" align="right">10</td>
+    <td valign="bottom" align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto<span class="spacer">&nbsp;</span>ditto, Fig. 85, in copper</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">3</td>
+    <td align="right">10</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto<span class="spacer">&nbsp;</span>ditto, with sliding rod instead of graduated tube, japanned tin</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">2</td>
+    <td align="right">2</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td><b>Rain Gauges</b>, of any form or area made to order, with suitable measuring glasses.</td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td align="right">123</td>
+    <td><b>Lind&#8217;s Anemometer</b>, Fig. 86</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">2</td>
+    <td align="right">2</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">125</td>
+    <td><b>Robinson&#8217;s Anemometer</b>, Fig. 87</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">3</td>
+    <td align="right">3</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto<span class="spacer">&nbsp;</span>ditto,<span class="spacer">&nbsp;</span>Improved arrangement</td>
+    <td align="right">&pound;4</td>
+    <td align="right">10s.</td>
+    <td colspan="2">&nbsp;</td>
+    <td align="right">5</td>
+    <td align="right">15</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto<span class="spacer">&nbsp;</span>ditto,<span class="spacer">&nbsp;</span>with clutch movement, Fig. 88</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">6</td>
+    <td align="right">15</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td><b>Negretti and Zambra&#8217;s Improved Air Meter</b>, of extreme sensitiveness, very portable</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">4</td>
+    <td align="right">4</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Large Air Meters made to order.</td></tr>
+<tr><td align="right">127</td>
+    <td><b>Osler&#8217;s Self-Registering Anemometer and Rain Gauge</b>, Fig. 89</td>
+    <td colspan="3">&pound;84 to</td>
+    <td>&nbsp;</td>
+    <td align="right">150</td>
+    <td align="right">0</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">128</td>
+    <td><b>Berkley&#8217;s Anemometers</b> fitted up to order, <i>to suit the Observatory</i>.</td></tr>
+<tr><td align="right">131</td>
+    <td><b>Gold Leaf Electrometer</b>, Fig. 90</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">1</td>
+    <td align="right">1</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">133</td>
+    <td><b>Peltier&#8217;s Electrometer</b></td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">4</td>
+    <td align="right">4</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">134</td>
+    <td><b>Bohnenberger&#8217;s Electroscope</b>, Fig. 91</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">8</td>
+    <td align="right">8</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">135</td>
+    <td><b>Thompson&#8217;s Electrometer</b>, to order</td></tr>
+<tr><td>&nbsp;</td>
+    <td><b>Lightning Conductors</b> fitted up to order.</td></tr>
+<tr><td align="right">142</td>
+    <td><b>Ozone Cage</b>, Fig. 92</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">0</td>
+    <td align="right">18</td>
+    <td align="right">0</td></tr>
+<tr><td>&nbsp;</td>
+    <td>Ditto<span class="spacer">&nbsp;</span>ditto, copper</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">1</td>
+    <td align="right">5</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">146</td>
+    <td><b>Leslie&#8217;s Differential Thermometer</b>, Fig. 93</td>
+    <td align="right">&pound;1</td>
+    <td align="right">10s.</td>
+    <td colspan="2">&nbsp;</td>
+    <td align="right">2</td>
+    <td align="right">2</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">148</td>
+    <td><b>Thermometer Stand (Glaisher&#8217;s)</b></td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">3</td>
+    <td align="right">3</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">149</td>
+    <td><b>Thermometer Screen</b> for Sea use</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">3</td>
+    <td align="right">3</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">150</td>
+    <td><b>Anemoscope</b>, or Portable Vane, Fig. 94</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">2</td>
+    <td align="right">5</td>
+    <td align="right">0</td></tr>
+<tr><td align="right">151</td>
+    <td><b>Evaporating Dish</b>, Fig. 95</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">1</td>
+    <td align="right">2</td>
+    <td align="right">6</td></tr>
+<tr><td align="right">157</td>
+    <td><b>Sea Water Hydrometers</b>, Board of Trade Marine, Figs. 96 and 97</td>
+    <td colspan="4">&nbsp;</td>
+    <td align="right">0</td>
+    <td align="right">5</td>
+    <td align="right">6</td></tr>
+<tr><td align="right">158</td>
+    <td><b>Newman&#8217;s Self-Registering Tide Gauge</b>, Fig. 158, fitted to the Building to order</td>
+    <td colspan="2">&nbsp;</td>
+    <td colspan="2" align="right">From</td>
+    <td align="right">50</td>
+    <td align="right">0</td>
+    <td align="right">0</td></tr></table>
+
+
+<p>&nbsp;</p>
+<p class="center"><i>Further Information as to Price, &amp;c., will be found in</i><br />
+NEGRETTI &amp; ZAMBRA&#8217;S<br />
+ENCYCLOP&AElig;DIC CATALOGUE<br />
+OF<br />
+MATHEMATICAL, PHILOSOPHICAL, OPTICAL, PHOTOGRAPHIC,<br />
+AND<br />
+STANDARD<br />
+METEOROLOGICAL INSTRUMENTS,<br />
+<br />
+<i>Containing very numerous Comparative Tables of Reference,<br />
+and Illustrated by upwards of</i><br />
+<br />
+ELEVEN HUNDRED ENGRAVINGS.<br />
+<br />
+Royal 8vo. Cloth, Gilt Lettered&mdash;Price 5s. 6d.</p>
+
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><strong>Footnotes:</strong></p>
+
+<p><a name="f1" id="f1" href="#f1.1">[1]</a> Second Number of &#8220;Meteorological Papers,&#8221; issued by the Board of
+Trade.</p>
+
+<p><a name="f2" id="f2" href="#f2.1">[2]</a> With reference to these barometers, we have received the subjoined
+testimonial, with permission to use it as we please.</p>
+
+<div class="blockquot">
+<p class="right">&#8220;<i>Meteorologic Office, 12th June, 1863.</i></p>
+
+<p>&#8220;<span class="smcap">Messrs. Negretti &amp; Zambra</span>,</p>
+
+<p>&#8220;The barometers which you have lately supplied to Her Majesty&#8217;s ships
+through this Office are much approved, being good for general service,
+afloat or on land.</p>
+
+<p>&#8220;(Signed)<span class="spacer">&nbsp;</span>R. FITZROY.&#8221;</p></div>
+
+<p><a name="f3" id="f3" href="#f3.1">[3]</a> <i>Vide</i> C. Daubeny, F.R.S., &#8220;On Climate.&#8221;</p>
+
+<p><a name="f4" id="f4" href="#f4.1">[4]</a> <i>Vide</i> Report of the British Association, 1862.</p>
+
+<p><a name="f5" id="f5" href="#f5.1">[5]</a> See <a href="#Page_42">page 42</a> for the Tables.</p>
+
+<p><a name="f6" id="f6" href="#f6.1">[6]</a> The quotations in this section are from Tyndall&#8217;s <i>Heat considered as
+a Mode of Motion</i>.</p>
+
+<p><a name="f7" id="f7" href="#f7.1">[7]</a> Dr. Daubeny, F.R.S., <i>On Climate</i>.</p>
+
+<p><a name="f8" id="f8" href="#f8.1">[8]</a> Leslie <i>On the Relations of Air, Heat, and Moisture</i>.</p>
+
+<p><a name="f9" id="f9" href="#f9.1">[9]</a> Tyndall&#8217;s <i>Heat considered as a Mode of Motion</i>.</p>
+
+<p><a name="f10" id="f10" href="#f10.1">[10]</a> Vide <i>Horological Journal</i>, Vol. V.</p>
+
+<p><a name="f11" id="f11" href="#f11.1">[11]</a> <i>Hygrometrical Tables</i>, by J. Glaisher, Esq., F.R.S.</p>
+
+<p><a name="f12" id="f12" href="#f12.1">[12]</a> Vide <i>Report of the British Association</i>, 1862. It may be added, for
+the information of those who are about to commence observing, that Mr.
+Symons, of Camden Road Villas, London, is desirous of securing returns of
+rain-fall from as many stations as possible, in order to render more
+complete his annual reports to the British Association.</p>
+
+<p><a name="f13" id="f13" href="#f13.1">[13]</a> Luke Howard&#8217;s <i>Climate of London</i>.</p>
+
+<p><a name="f14" id="f14" href="#f14.1">[14]</a> Vide <i>Third Number of Meteorological Papers</i>, issued by the Board of
+Trade.</p>
+
+<p><a name="f15" id="f15" href="#f15.1">[15]</a> <i>Elements of Physics</i>, by C. F. Peschel.</p>
+
+<p><a name="f16" id="f16" href="#f16.1">[16]</a> This description is modified from that in Report of the Jurors for
+Class XIII. International Exhibition, 1862.</p>
+
+<p><a name="f17" id="f17" href="#f17.1">[17]</a> <i>All the Year Round</i>, No. 224.</p>
+
+<p><a name="f18" id="f18" href="#f18.1">[18]</a> <i>All the Year Round</i>, No. 224.</p>
+
+<p><a name="f19" id="f19" href="#f19.1">[19]</a> Vide <i>Jurors&#8217; Reports</i>.</p>
+
+<p><a name="f20" id="f20" href="#f20.1">[20]</a> See also <a href="#Page_90">page 90</a> of this Treatise.</p>
+
+<p><a name="f21" id="f21" href="#f21.1">[21]</a> These Instruments are the only Maximum Thermometers that can be
+recommended, as unless they be broken, they cannot be put out of
+adjustment. Fully described under the head of Standard Maximum
+Thermometers in our large Catalogue, and <a href="#Page_72">page 72</a> of our <i>Treatise on
+Meteorological Instruments</i>.</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><strong>Transcriber&#8217;s Note:</strong> In an effort to make the wide table on page 149 easier to read, the months were
+abbreviated by the transcriber. In addition, &#8220;Temperature&#8221; was changed
+to &#8220;Temp.&#8221; and &#8220;Temperatures&#8221; to &#8220;Temp&#8217;s.&#8221;</p>
+
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
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+The Project Gutenberg EBook of A Treatise on Meteorological Instruments, by
+Henry Negretti and Joseph Zambra
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: A Treatise on Meteorological Instruments
+       Explanatory of Their Scientific Principles, Method of
+       Construction, and Practical Utility
+
+Author: Henry Negretti
+        Joseph Zambra
+
+Release Date: June 18, 2011 [EBook #36457]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK TREATISE ON METEOROLOGICAL INSTRUMENTS ***
+
+
+
+
+Produced by The Online Distributed Proofreading Team at
+https://www.pgdp.net (This file was produced from images
+generously made available by The Internet Archive.)
+
+
+
+
+
+
+
+
+
+A TREATISE ON METEOROLOGICAL INSTRUMENTS.
+
+
+
+
+  LONDON:
+  PRINTED BY WILLIAMS AND STRAHAN,
+  7 LAWRENCE LANE, CHEAPSIDE, E.C.
+
+
+
+
+  A TREATISE ON METEOROLOGICAL INSTRUMENTS:
+
+  EXPLANATORY OF
+
+  THEIR SCIENTIFIC PRINCIPLES,
+  METHOD OF CONSTRUCTION, AND PRACTICAL UTILITY.
+
+
+  BY NEGRETTI & ZAMBRA,
+
+  METEOROLOGICAL INSTRUMENT MAKERS TO THE QUEEN, THE ROYAL OBSERVATORY,
+  GREENWICH, THE BRITISH METEOROLOGICAL SOCIETY, THE BRITISH AND FOREIGN
+  GOVERNMENTS, ETC. ETC. ETC.
+
+
+  LONDON:
+  PUBLISHED AND SOLD AT NEGRETTI & ZAMBRA'S ESTABLISHMENTS:
+  1 HATTON GARDEN, E.C., 59 CORNHILL, E.C., 122 REGENT STREET W.,
+  AND 153 FLEET STREET, E.C.
+
+  1864.
+
+  _Price Five Shillings._
+
+
+
+
+PREFACE.
+
+
+The national utilisation of Meteorology in forewarning of storms, and the
+increasing employment of instruments as weather indicators, render a
+knowledge of their construction, principles, and practical uses necessary
+to every well-informed person. Impressed with the idea that we shall be
+supplying an existing want, and aiding materially the cause of
+Meteorological Science, in giving a plain description of the various
+instruments now in use, we have endeavoured, in the present volume, to
+condense such information as is generally required regarding the
+instruments used in Meteorology; the description of many of which could
+only be found in elaborate scientific works, and then only briefly touched
+upon. Every Meteorological Instrument now in use being fully described,
+with adequate directions for using, the uninitiated will be enabled to
+select those which seem to them best adapted to their requirements. With
+accounts of old or obsolete instruments we have avoided troubling the
+reader; on the other hand, we were unwilling to neglect those which,
+though of no great practical importance, are still deserving of notice
+from their being either novel or ingenious, or which, without being
+strictly scientific, are in great demand as simple weather-glasses and
+articles of trade.
+
+We trust, therefore, that the work (however imperfect), bearing in mind
+the importance of the subject, will be acceptable to general readers, as
+well as to those for whose requirements it has been prepared.
+
+The rapid progress made in the introduction of new apparatus of
+acknowledged superiority has rendered the publication of some description
+absolutely necessary. The Report of the Jurors for Class XIII. of the
+International Exhibition, 1862, on Meteorological Instruments, fully bears
+out our assertion, as shown by the following extract:--
+
+"The progress in the English department has been very great;--in
+barometers, thermometers, anemometers, and in every class of instruments.
+At the close of the Exhibition of 1851, there seemed to have arisen a
+general anxiety among the majority of makers to pay every attention to all
+the essentials necessary for philosophical instruments, not only in their
+old forms, but also with the view of obtaining other and better forms.
+This desire has never ceased; and no better idea can be given of the
+continued activity in these respects, than the number of patents taken out
+for improvements in meteorological instruments in the interval between the
+recent and preceding exhibitions, which amount to no less than forty-two."
+* * * "In addition to numerous improvements patented by Messrs. Negretti
+and Zambra, there is another of great importance, which they did not
+patent, viz. enamelling the tubes of thermometers, enabling the makers to
+use finer threads of mercury in the construction of all thermometers; for
+the contrast between the opaque mercury and the enamel back of the tubes
+is so great, that the finest bore or thread of mercury, which at one time
+could not be seen without the greatest difficulty, is now seen with
+facility; and throughout the British and Foreign departments, the makers
+have availed themselves of this invention, the tubes of all being made
+with enamelled backs. It is to be hoped that the recent exhibition will
+give a fresh stimulus to the desire of improvement, and that the same rate
+of progress will be continued."
+
+To fulfil the desire of the International Jury in the latter portion of
+the above extract will be the constant study of
+
+NEGRETTI & ZAMBRA.
+
+_1st January, 1864._
+
+
+
+
+TABLE OF CONTENTS.
+
+
+  CHAPTER I. INSTRUMENTS FOR ASCERTAINING THE ATMOSPHERIC PRESSURE.
+
+  SECTION
+
+  1. Principle of the Barometer.
+
+  2. Construction of Barometers.
+
+  3. Fortin's Barometer Cistern.
+
+  4. STANDARD BAROMETER.
+
+  5. Correction due to Capillarity.
+
+  6.      "        "   Temperature.
+
+  7.      "        "   Height.
+
+  8. The Barometer Vernier.
+
+  9. SELF-COMPENSATING STANDARD BAROMETER.
+
+  10. BAROMETER WITH ELECTRICAL ADJUSTMENT.
+
+  11. PEDIMENT BAROMETERS.
+
+  12. The Words on the Scale.
+
+  13. Correction due to Capacity of Cistern.
+
+  14. PUBLIC BAROMETERS.
+
+  15. FISHERY OR SEA-COAST BAROMETERS.
+
+  16. Admiral FitzRoy's Words for the Scale.
+
+  17. Instructions for Sea-coast Barometer.
+
+  18. French Sea-coast Barometer.
+
+  19. COMMON MARINE BAROMETER.
+
+  20. THE KEW MARINE BAROMETER.
+
+  21. Method of verifying Barometers.
+
+  22. FITZROY'S MARINE BAROMETER.
+
+  23. Words for its Scale.
+
+  24. Trials of this Barometer under Gun-fire.
+
+  25. NEGRETTI AND ZAMBRA'S FARMER'S BAROMETER AND DOMESTIC WEATHER-GLASS.
+
+  26. Rules for Foretelling the Weather.
+
+  27. Causes which may bring about a Fall or a Rise in the Barometer.
+
+  28. Use of the Barometer in the Management of Mines.
+
+  29. Use of the Barometer in estimating the Height of Tides.
+
+
+  CHAPTER II. SYPHON TUBE BAROMETERS.
+
+  30. Principle of.
+
+  31. DIAL, OR WHEEL, BAROMETERS.
+
+  32. STANDARD SYPHON BAROMETER.
+
+
+  CHAPTER III. BAROGRAPHS, OR SELF-REGISTERING BAROMETERS.
+
+  SECTION
+
+  33. MILNE'S SELF-REGISTERING BAROMETER.
+
+  34. MODIFICATION OF MILNE'S BAROMETER.
+
+  35. KING'S SELF-REGISTERING BAROMETER.
+
+  36. SYPHON, WITH PHOTOGRAPHIC REGISTRATION.
+
+
+  CHAPTER IV. MOUNTAIN BAROMETERS.
+
+  37. GAY LUSSAC'S MOUNTAIN BAROMETER.
+
+  38. FORTIN'S MOUNTAIN BAROMETER.
+
+  39. NEWMAN'S MOUNTAIN BAROMETER.
+
+  40. NEGRETTI AND ZAMBRA'S PATENT MOUNTAIN AND OTHER BAROMETERS.
+
+  41. Short Tube Barometer.
+
+  42. Method of Calculating Heights by the Barometer; Tables and Examples.
+
+
+  CHAPTER V. SECONDARY BAROMETERS.
+
+  43. Desirability of Magnifying the Barometer Range.
+
+  44. HOWSON'S LONG-RANGE BAROMETER.
+
+  45. MCNEIL'S LONG-RANGE BAROMETER.
+
+  46. The Water-glass Barometer.
+
+  47. SYMPIESOMETERS.
+
+  48. ANEROIDS.
+
+  49. SMALL SIZE ANEROIDS.
+
+  50. WATCH ANEROID.
+
+  51. Measurement of Heights by the Aneroid; Example.
+
+  52. METALLIC BAROMETER.
+
+
+  CHAPTER VI. INSTRUMENTS FOR ASCERTAINING TEMPERATURE.
+
+  53. Temperature.
+
+  54. Thermometric Substances.
+
+  55. Description of the Thermometer.
+
+  56. STANDARD THERMOMETER.
+
+  57. Method of ascertaining the exact Boiling Temperature; Tables, &c.
+
+  58. Displacement of the Freezing Point.
+
+  59. The Scale.
+
+  60. The method of testing Thermometers.
+
+  61. Porcelain Scale-Plates.
+
+  62. Enamelled Tubes.
+
+  63. THERMOMETERS OF EXTREME SENSITIVENESS.
+
+  64. VARIETIES OF THERMOMETERS.
+
+  65. SUPERHEATED STEAM THERMOMETER.
+
+  66. THERMOMETER FOR SUGAR BOILING.
+
+  67. EARTH THERMOMETER.
+
+  68. MARINE THERMOMETER.
+
+
+  CHAPTER VII. SELF-REGISTERING THERMOMETERS.
+
+  69. Importance of.
+
+  70. RUTHERFORD'S MAXIMUM THERMOMETER.
+
+  71. PHILLIPS'S    DITTO    DITTO.
+
+  72. NEGRETTI AND ZAMBRA'S PATENT MAXIMUM THERMOMETER.
+
+  73. RUTHERFORD'S ALCOHOL MINIMUM THERMOMETER.
+
+  74. HORTICULTURAL MINIMUM THERMOMETER.
+
+  75. BAUDIN'S ALCOHOL MINIMUM THERMOMETER.
+
+  76. Mercurial Minima Thermometers desirable.
+
+  77. NEGRETTI AND ZAMBRA'S PATENT MERCURIAL MINIMUM THERMOMETER.
+
+  78. NEGRETTI AND ZAMBRA'S SECOND PATENT MERCURIAL MINIMUM THERMOMETER.
+
+  79. CASELLA'S PATENT MERCURIAL MINIMUM THERMOMETER.
+
+  80. Day and Night Thermometer.
+
+  81. SIXE'S SELF-REGISTERING THERMOMETER.
+
+
+  CHAPTER VIII. RADIATION THERMOMETERS.
+
+  82. Solar and Terrestrial Radiation considered.
+
+  83. SOLAR RADIATION THERMOMETER.
+
+  84. VACUUM SOLAR RADIATION THERMOMETER.
+
+  85. TERRESTRIAL RADIATION THERMOMETER.
+
+  86. AETHRIOSCOPE.
+
+  87. PYRHELIOMETER.
+
+  88. ACTINOMETER.
+
+
+  CHAPTER IX. DEEP-SEA THERMOMETERS.
+
+  89. ON SIXE'S PRINCIPLE.
+
+  90. JOHNSON'S METALLIC THERMOMETER.
+
+
+  CHAPTER X. BOILING-POINT THERMOMETERS.
+
+  91. Ebullition.
+
+  92. Relation between Boiling-Point and Elevation.
+
+  93. HYPSOMETRIC APPARATUS.
+
+  94. Precautions to ensure Correct Graduation.
+
+  95. Method of Calculating Heights from Observations with the Mountain
+      Thermometer; Example.
+
+  96. THERMOMETERS FOR ENGINEERS.
+
+
+  CHAPTER XI. INSTRUMENTS FOR ASCERTAINING THE HUMIDITY OF THE AIR.
+
+  97. Hygrometric Substances.
+
+  98. SAUSSURE'S HYGROMETER.
+
+  99. Dew-Point.
+
+  100. DROSOMETER.
+
+  101. Humidity.
+
+  102. LESLIE'S HYGROMETER.
+
+  103. DANIEL'S HYGROMETER.
+
+  104. REGNAULT'S CONDENSER HYGROMETER.
+
+  105. Temperature of Evaporation.
+
+  106. MASON'S HYGROMETER.
+
+  107. SELF-REGISTERING HYGROMETER.
+
+  108. Causes of Dew.
+
+  109. Plan of Exposing Thermometers.
+
+
+  CHAPTER XII. INSTRUMENTS USED FOR MEASURING THE RAINFALL.
+
+  110. HOWARD'S RAIN-GAUGE.
+
+  111. GLAISHER'S RAIN-GAUGE.
+
+  112. RAIN-GAUGE WITH FLOAT.
+
+  113. RAIN-GAUGE WITH SIDE TUBE.
+
+  114. FITZROY'S RAIN-GAUGE.
+
+  115. SELF-REGISTERING RAIN-GAUGE.
+
+  116. The principle of Measurement.
+
+  117. Position for Rain-gauge, &c.
+
+  118. Cause of Rain.
+
+  119. Laws of Rainfall.
+
+  120. Utility of Statistics of Rainfall.
+
+  121. NEW FORM OF RAIN-GAUGE.
+
+
+  CHAPTER XIII. APPARATUS EMPLOYED FOR REGISTERING THE DIRECTION,
+  PRESSURE, AND VELOCITY OF THE WIND.
+
+  122. THE VANE.
+
+  123. LIND'S WIND-GAUGE.
+
+  124. HARRIS'S WIND-GAUGE.
+
+  125. ROBINSON'S ANEMOMETER.
+
+  126. WHEWELL'S ANEMOMETER.
+
+  127. OSLER'S ANEMOMETER AND PLUVIOMETER.
+
+  128. BECKLEY'S ANEMOMETER.
+
+  129. SELF-REGISTERING WIND-GAUGE.
+
+  130. Anemometric Observations.
+
+
+  CHAPTER XIV. INSTRUMENTS FOR INVESTIGATING ATMOSPHERIC ELECTRICITY.
+
+  131. ATMOSPHERIC ELECTROSCOPE.
+
+  132. VOLTA'S ELECTROMETER.
+
+  133. PELTIER'S ELECTROMETER.
+
+  134. BOHNENBERGER'S ELECTROSCOPE.
+
+  135. THOMSON'S ELECTROMETER.
+
+  136. Fundamental Facts.
+
+  137. Lightning Conductors.
+
+  138. Precautions against Lightning.
+
+
+  CHAPTER XV. OZONE AND ITS INDICATORS.
+
+  139. Nature of Ozone.
+
+  140. SCHONBEIN'S OZONOMETER.
+
+  141. MOFFAT'S OZONOMETER.
+
+  142. CLARK'S OZONE CAGE.
+
+  143. Distribution and Effects of Ozone.
+
+  144. LANCASTER'S REGISTERING OZONOMETER.
+
+
+  CHAPTER XVI. MISCELLANEOUS INSTRUMENTS.
+
+  145. CHEMICAL WEATHER GLASS.
+
+  146. LESLIE'S DIFFERENTIAL THERMOMETER.
+
+  147. ROMFORD'S DIFFERENTIAL THERMOMETER.
+
+  148. GLAISHER'S THERMOMETER STAND.
+
+  149. THERMOMETER SCREEN, FOR USE AT SEA.
+
+  150. ANEMOSCOPE.
+
+  151. EVAPORATING DISH, OR GAUGE.
+
+  152. ADMIDOMETER.
+
+  153. CLOUD REFLECTOR.
+
+  154. SUNSHINE RECORDER.
+
+  155. SET OF PORTABLE INSTRUMENTS.
+
+  156. IMPLEMENTS.
+
+  157. HYDROMETER.
+
+  158. NEWMAN'S SELF-REGISTERING TIDE-GAUGE.
+
+
+
+
+TABLES.
+
+
+                                                                      PAGE
+
+  Table of Corrections, for Capillary Depression of the Mercury
+  in Boiled and in Unboiled Barometer-Tubes                              6
+
+  Tables for Deducing Heights by means of the Barometer:--
+
+    No. 1. Approximate Height due to Barometric Pressure                42
+
+    No. 2. Correction for Mean Temperature of Air                       44
+
+    No. 3. Correction due to Latitude                                   44
+
+    No. 4. Correction due to Approximate Elevation                      45
+
+  Tables for Determining the Temperature of the Vapour of Boiling
+  Water at any Place:--
+
+    No. 5. Factor due to Latitude                                       62
+
+    No. 6. Temperature and Tension                                      62
+
+  Table of Temperature of the Soil                                      69
+
+  Table of Difference of Elevation corresponding to a fall of 1 deg.
+  in the Boiling-point of Water                                         98
+
+  Table showing Proportion of Salt for various Boiling Temperatures
+  of Sea-Water                                                         100
+
+  Table for finding the Degree of Humidity from Observations with
+  Mason's Hygrometer                                                   108
+
+  Table showing Amount and Duration of Rain at London, in 1862         112
+
+  Table of Average British Rainfall in Westerly, Central, and
+  Easterly districts                                                   114
+
+  Table showing Force of Wind, for use with Lind's Wind-Gauge          118
+
+  Tables for Correcting Observations made with--
+
+    Brass Hydrometers                                                  142
+
+    Glass Hydrometers                                                  143
+
+
+
+
+ADDENDA.
+
+
+                                                                      PAGE
+
+  1. Rule for converting Millimetres into Inches, et vice versa        146
+
+  2. Old French Lineal Measure, with English Equivalents               146
+
+  3. Rule for finding Diameter of Bore of Barometer Tube               146
+
+  4. Wind Scales                                                       147
+
+  5. Letters to denote the State of the Weather                        147
+
+  6. Table of Expansion of Bodies                                      148
+
+  7. Table of Specific Gravity of Bodies                               148
+
+  8. Important Temperatures                                            148
+
+  9. Table of Meteorological Elements, forming Exponents of the
+  Climate of London                                                    149
+
+  10. List of Works on Meteorology                                     151
+
+
+
+
+METEOROLOGICAL INSTRUMENTS.
+
+
+In the pursuits and investigations of the science of Meteorology, which is
+essentially a science of observation and experiment, instruments are
+required for ascertaining, 1. the pressure of the atmosphere at any time
+or place; 2. the temperature of the air; 3. the absorption and radiation
+of the sun's heat by the earth's surface; 4. the humidity of the air; 5.
+the amount and duration of rainfall; 6. the direction, the horizontal
+pressure, and the velocity of winds; 7. the electric condition of the
+atmosphere, and the prevalence and activity of ozone.
+
+
+
+
+CHAPTER I.
+
+INSTRUMENTS FOR ASCERTAINING THE ATMOSPHERIC PRESSURE.
+
+
+[Illustration: Fig. 1.]
+
+=1. Principle of the Barometer.=--The first instrument which gave the
+exact measure of the pressure of the atmosphere was invented by
+Torricelli, in 1643. It is constructed as follows:--A glass tube, CD (fig.
+1), about 34 inches long, and from two to four-tenths of an inch in
+diameter of bore, having one end closed, is filled with mercury. In a cup,
+B, a quantity of mercury is also poured. Then, placing a finger securely
+over the open end, C, invert the tube vertically over the cup, and remove
+the finger when the end of the tube dips into the mercury. The mercury in
+the tube then partly falls out, but a column, AB, about 30 inches in
+height, remains supported. This column is a weight of mercury, the
+pressure of which upon the surface of that in the cup is precisely
+equivalent to the corresponding pressure of the atmosphere which would be
+exerted in its place if the tube were removed. As the atmospheric pressure
+varies, the length of this mercurial column also changes. It is by no
+means constant in its height; in fact, it is very seldom stationary, but
+is constantly rising or falling through a certain extent of the tube, at
+the level of the sea, near which the above experiment is supposed to be
+performed. It is, therefore, an instrument by which the fluctuations
+taking place in the pressure of the atmosphere, arising from changes in
+its weight and elasticity, can be shown and measured. It has obtained the
+name _Barometer_, or measurer of heaviness,--a word certainly not happily
+expressive of the utility of the invention. If the bore of the barometer
+tube be uniform throughout its length, and have its sectional area equal
+to a square inch, it is evident that the length of the column, which is
+supported by the pressure of the air, expresses the number of cubic inches
+of mercury which compose it. The weight of this mercury, therefore,
+represents the statical pressure of the atmosphere upon a square inch of
+surface. In England the annual mean height of the barometric column,
+reduced to the sea-level and to the temperature of 32 deg. Fahrenheit, is
+about 29.95 inches. A cubic inch of mercury at this temperature has been
+ascertained to weigh 0.48967 lbs. avoirdupois. Hence, 29.95 x 0.48967=
+14.67 lbs., is the mean value of the pressure of the atmosphere on each
+square inch of surface, near the sea-level, about the latitude of 50
+degrees. Nearer the equator this mean pressure is somewhat greater; nearer
+the poles, somewhat less. For common practical calculations it is assumed
+to be 15 lbs. on the square inch. When it became apparent that the
+movements of the barometric column furnished indications of the probable
+coming changes in the weather, an attempt was made to deduce from recorded
+observations the barometric height corresponding to the most notable
+characteristics of weather. It was found that for fine dry weather the
+mercury in the barometer at the sea-level generally stood above 30 inches;
+changeable weather happened when it ranged from 30 to 29 inches, and when
+rainy or stormy weather occurred it was even lower. Hence, it became the
+practice to place upon barometer scales words indicatory of the weather
+likely to accompany, or follow, the movements of the mercury; whence the
+instruments bearing them obtained the name "Weather Glasses."
+
+
+=2. Construction of Barometers.=--In order that the instrument may be
+portable, it must be made a fixture and mounted on a support; and,
+further, to render it scientifically or even practically useful, many
+precautions are required in its construction. The following remarks apply
+to the construction of all barometers:--Mercury is universally employed,
+because it is the heaviest of fluids, and therefore measures the
+atmospheric pressure by the shortest column. Water barometers have been
+constructed, and they require to be at least 34 feet long. Oil, or other
+fluids, might be used. Mercury, however, has other advantages: it has
+feeble volatility, and does not adhere to glass, if pure. Oxidised, or
+otherwise impure mercury, may adhere to glass; moreover, such mercury
+would not have the density of the pure metal, and therefore the barometric
+column would be either greater or less than it should be. The mercury of
+commerce generally contains lead; sometimes traces of iron and sulphur. It
+is necessary, therefore, for the manufacturer to purify the mercury; and
+this is done by washing it with diluted acetic, or sulphuric acid, which
+dissolves the impurities. No better test can be found for ascertaining if
+the mercury be pure than that of filling a delicate thermometer tube; if,
+on exhausting the air from this thermometer, the mercury will freely run
+up and down the bore, which is probably one thousandth of an inch in
+diameter, the mercury from which this thermometer was made will be found
+fit for any purpose, and with it a tube may be filled and boiled, not only
+of one inch, but even of two inches diameter. In all barometers it is
+requisite that the space above the mercurial column should be completely
+void of air and aqueous vapour, because these gases, by virtue of their
+elasticity, would depress the column. To exclude these the mercury is
+introduced, and boiled in the tube, over a charcoal fire, kept up for the
+purpose. In this manner the air and vapour which adhere to the glass are
+expanded, and escape away. One can tell whether a barometer has been
+properly "boiled," as it is termed, by simply holding the tube in a
+slanting direction and allowing the mercury to strike the top. If the
+boiling has been well performed, the mercury will give a clear, metallic
+sound; if not, a dull, flat sound, showing some air to be present.
+
+When the mercury in a barometer tube rises or falls, the level of the
+mercury in the cup, or _cistern_, as it is generally termed, falls or
+rises by a proportionate quantity, which depends upon the relative areas
+of the interior of the tube and of the cistern. It is necessary that this
+should be taken into consideration in ascertaining the exact height of the
+column. If a fixed scale is applied to the tube, the correct height may be
+obtained by applying a correction for capacity. A certain height of the
+mercury is ascertained to be accurately measured by the scale, and should
+be marked on the instrument as the _neutral point_. Above this point the
+heights measured are all less, and below, all more, than they should be.
+The ratio between the internal diameters of the tube and cistern (which
+should also be stated on the instrument, as, for instance, capac. 1/50)
+supplies the data for finding the correction to be applied. This
+correction is obviated by constructing the cistern so as to allow of the
+surface of the mercury in it being adjustable to the commencement of the
+fixed scale, as by Fortin's or Negretti's plan. It is also unnecessary in
+barometers constructed on what is now called the "Kew method." These will
+all be detailed in their proper place. The tube, being fixed to the
+cistern, may have a moveable scale applied to it. But such an arrangement
+requires the utmost care and skill in observing, and is seldom seen except
+in first-class Observatories.
+
+
+[Illustration: Fig. 2.]
+
+=3. Fortin's Barometer.=--Fortin's plan of constructing a barometer
+cistern is shown in fig. 2. The cistern is formed of a glass cylinder,
+which allows of the level of the mercury within being seen. The bottom of
+the cylinder is made of sheep-skin or leather, like a bag, so as to allow
+of being pushed up or lowered by means of a screw, D B, worked from
+beneath. This screw moves through the bottom of a brass cylinder, C C,
+which is fixed outside, and protects the glass cylinder containing the
+mercury. At the top of the interior of the cistern is fixed a small piece
+of ivory, A, the point of which exactly coincides with the zero of the
+scale. This screw and moveable cistern-bottom serve also to render the
+barometer portable, by confining the mercury in the tube, and preventing
+its coming into the cistern, which is thus made too small to receive it.
+
+
+4. STANDARD BAROMETER.
+
+Fig. 3 represents a Standard Barometer on Fortin's principle. The
+barometer tube is enclosed and protected by a tube of brass extending
+throughout its whole length; the upper portion of the brass tube has two
+longitudinal openings opposite each other; on one side of the front
+opening is the barometrical scale of English inches, divided to show, by
+means of a vernier, 1/500th of an inch; on the opposite side is sometimes
+divided a scale of French millimetres, reading also by a vernier to 1/10th
+of a millimetre (see directions for reading the vernier, page 7). A
+thermometer, C, is attached to the frame, and divided to degrees, which
+can be read to tenths; it is necessary for ascertaining the temperature of
+the instrument, in order to correct the observed height of the barometer.
+
+[Illustration: Fig. 3.]
+
+As received by the observer, the barometer will consist of two parts,
+packed separately for safety in carriage,--1st, the barometer tube and
+cistern, filled with mercury, the brass tube, with its divided scale and
+thermometer; and 2nd, a mahogany board, with bracket at top, and brass
+ring with three adjusting screws at bottom.
+
+_Directions for fixing the Barometer._--In selecting a position for a
+barometer, care should be taken to place it so that the sun cannot shine
+upon it, and that it is not affected by direct heat from a fire. The
+cistern should be from two to three feet above the ground, which will give
+a height for observing convenient to most persons. A standard barometer
+should be compared with an observatory standard of acknowledged accuracy,
+to determine its index error; which, as such instruments are graduated by
+micrometrical apparatus of great exactitude, will be constant for all
+parts of the scale. It should be capable of turning on its axis by a
+movement of the hand, so that little difficulty can ever be experienced in
+obtaining a good light for observation. Having determined upon the
+position in which to place the instrument, fix the mahogany board as
+nearly vertical as possible, and ascertain if the barometer is perfect and
+free from air, in the following manner:--lower the screw at the bottom of
+the cistern several turns, so that the mercury in the tube, when held
+upright, may fall two or three inches from the top; then slightly incline
+the instrument from the vertical position, and if the mercury in striking
+the top elicit a sharp tap, the instrument is perfect. Supposing the
+barometer to be in perfect condition, as it is almost sure to be, it is
+next suspended on the brass bracket, its cistern passing through the ring
+at bottom, and allowed to find its vertical position, after which it is
+firmly clamped by means of the three thumb-screws.
+
+_To Remove the Instrument when fixed to another Position._--If it should
+be necessary to remove the barometer,--first, by means of the adjusting
+screw, drive the mercury to the top of the tube, turning it gently when it
+is approaching the top, and cease directly any resistance is experienced;
+next, remove from the upper bracket or socket; lift the instrument and
+invert it, carrying it with its lower end upwards.
+
+_Directions for taking an Observation._--Before making an observation, the
+mercury in the cistern must be raised or lowered by means of the
+thumb-screw, F, until the ivory point, E, and its reflected image in the
+mercury, D, are just in contact; the vernier is then moved by means of the
+milled head, until its lower termination just excludes the light from the
+top of the mercurial column; the reading is then taken by means of the
+scale on the limb and the vernier. The vernier should be made to read
+upward in all barometers, unless for a special object, as this arrangement
+admits of the most exact setting. In observing, the eye should be placed
+in a right line with the fore and back edges of the lower termination of
+the vernier; and this line should be made to form a tangent to the apex of
+the mercurial column. A small reflector placed behind the vernier and
+moving with it, so as to assist in throwing the light through the back
+slit of the brass frame on to the glass tube, is advantageous; and the
+observer's vision may be further assisted by the aid of a reading lens.
+The object is, in these Standard Barometers, to obtain an exact reading,
+which can only be done by having the eye, the fore part of the zero edge
+of the vernier, the top of the mercurial column, and the back of the
+vernier, in the same horizontal plane.
+
+_Uniformity of Calibre._--The diameter of that part of the tube through
+which the oscillations of the mercury will take place is very carefully
+examined to insure uniformity of calibre, and only those tubes are used
+which are as nearly as possible of the same diameter throughout. The size
+of the bore should be marked on the frame of the barometer in tenths and
+hundredths of an inch. A correction due to capillary action, and depending
+on the size of the tube, must be applied to the readings.
+
+
+=5. Correction due to Capillarity.=--When an open tube of small bore is
+plunged into mercury, the fluid will not rise to the same level inside as
+it has outside. Hence, the effect of capillary action is to depress the
+mercurial column; and the more so the smaller the tube. The following
+table gives the correction for tubes in ordinary use:--
+
+  Diameter     Depression, in     Depression, in
+  of tube.     boiled tubes.      unboiled tubes.
+   INCH.          INCH.               INCH.
+
+   0.60           0.002               0.004
+   0.55           0.003               0.005
+   0.50           0.003               0.007
+   0.45           0.005               0.010
+   0.40           0.007               0.015
+   0.35           0.010               0.021
+   0.15           0.044               0.029
+   0.10           0.070               0.041
+   0.30           0.014               0.058
+   0.25           0.020               0.086
+   0.20           0.029               0.140
+
+This correction is always additive to the observed reading of the
+barometer.
+
+
+=6. Correction due to Temperature.=--In all kinds of mercurial barometers
+attention must be given to the temperature of the mercury. As this metal
+expands and contracts very much for variations of temperature, its density
+alters correspondingly, and in consequence the height of the barometric
+column also varies. To ascertain the temperature of the mercury, a
+thermometer is placed near the tube, and is sometimes made to dip into the
+mercury in the cistern. The freezing point of water, 32 deg.F., is the
+temperature to which all readings of barometers must be reduced, in order
+to make them fairly comparable. The reduction may be effected by
+calculation, but the practical method is by tables for the purpose; and
+for these tables we refer the reader to the works mentioned at the end of
+this book.
+
+
+=7. Correction due to Height above the Half-tide Level.=--Further, in
+order that barometrical observations generally may be made under similar
+circumstances, the readings, corrected for capacity, capillarity, and
+temperature, should be reduced to what they would be at the sea-level, by
+adding a correction corresponding to the height above the mean level of
+the sea, or of half-tide. For practical purposes of comparison with
+barometric pressure at other localities, add one-tenth of an inch to the
+reading for each hundred feet of elevation above the sea. For scientific
+accuracy this will not suffice, but a correction must be obtained by means
+of Schuckburg's formula, or tables computed therefrom.
+
+
+=8. The Barometer Vernier.=--The _vernier_, an invaluable contrivance for
+measuring small spaces, was invented by Peter Vernier, about the year
+1630. The barometer scale is divided into inches and tenths. The vernier
+enables us to accurately subdivide the tenths into hundredths, and, in
+first-class instruments, even to thousandths of an inch. It consists of a
+short scale made to pass along the graduated fixed scale by a sliding
+motion, or preferably by a rack-and-pinion motion, the vernier being fixed
+on the rack, which is moved by turning the milled head of the pinion. The
+principle of the vernier, to whatever instrumental scale applied, is that
+the divisions of the moveable scale are to those in an equal length of the
+fixed scale in the proportion of two numbers which differ from each other
+by unity.
+
+[Illustration: Fig. 4.]
+
+[Illustration: Fig. 5.]
+
+The scales of standard barometers are usually divided into half-tenths, or
+.05, of an inch, as represented, in fig. 5, by AB. The vernier, CD, is
+made equal in length to twenty-four of these divisions, and divided into
+twenty-five equal parts; consequently one space on the scale is larger
+than one on the vernier by the twenty-fifth part of .05, which is .002
+inch, so that such a vernier shows differences of .002 inch. The vernier
+of the figure reading upwards, the lower edge, D, will denote the top of
+the barometer column; and is the zero of the vernier scale. In fig. 4, the
+zero being in line exactly with 29 inches and five-tenths of the fixed
+scale, the barometer reading would be 29.500 inches. It will be seen that
+the vernier line, _a_, falls short of a division of the scale by, as we
+have explained, .002 inch; _b_, by .004; _c_, by .006; _d_, by .008; and
+the next line by one hundredth. If, then, the vernier be moved so as to
+make _a_ coincide with _z_, on the scale, it will have moved through .002
+inch; and if 1 on the vernier be moved into line with _y_ on the scale,
+the space measured will be .010. Hence, the figures 1, 2, 3, 4, 5 on the
+vernier measure hundredths, and the intermediate lines even thousandths of
+an inch. In fig. 5, the zero of the vernier is intermediate 29.65 and
+29.70 on the scale. Passing the eye up the vernier and scale, the second
+line above 3 is perceived to lie evenly with a line of the scale. This
+gives .03 and .004 to add to 29.65, so that the actual reading is 29.684
+inches. It may happen that no line on the vernier _accurately_ lies in the
+same straight line with one on the scale; in such a case a doubt will
+arise as to the selection of one from two equally coincident, and the
+intermediate thousandth of an inch should be taken.
+
+For the ordinary purposes of the barometer as a "weather-glass," such
+minute measurement is not required. Hence, in household and marine
+barometers the scale need only be divided to tenths, and the vernier
+constructed to measure hundredths of an inch. This is done by making the
+vernier either 9 or 11-10ths of an inch long, and dividing it into ten
+equal parts. The lines above the zero line are then numbered from 1 to 10;
+sometimes the alternate divisions only are numbered, the intermediate
+numbers being very readily inferred. Hence, if the first line of the
+vernier agrees with one on the scale, the next must be out one-tenth of a
+tenth, or .01 of an inch from agreement with the next _scale_ line; the
+following vernier line must be .02 out, and so on. Consequently, when the
+vernier is set to the mercurial column, the difference shown by the
+vernier from the tenth on the scale is the hundredths to be added to the
+inches and tenths of the scale.
+
+A little practice will accustom a person to set and read any barometer
+quickly; an important matter where accuracy is required, as the heat of
+the body, or the hand, is very rapidly communicated to the instrument, and
+may vitiate, to some extent, the observation.
+
+
+9. SELF-COMPENSATING STANDARD BAROMETER.
+
+This barometer has been suggested to Messrs. Negretti and Zambra by
+Wentworth Erk, Esq. It consists of a regular barometer; but attached to
+the vernier is a double rack worked with one pinion, so that in setting or
+adjusting the vernier in one position, the second rack moves in directly
+the opposite direction, carrying along with it a plug or plunger the exact
+size of the internal diameter of the tube dipping in the cistern, so that
+whatever the displacement that has taken place in the cistern, owing to
+the rise or fall of the mercury, it is exactly compensated by the plug
+being more or less immersed in the mercury, so that no capacity correction
+is required.
+
+A barometer on this principle is, however, no novelty, for at the Royal
+Society's room a very old instrument may be seen reading somewhat after
+the same manner.
+
+[Illustration: Fig. 6.]
+
+Fig. 6 is an illustration of the appearance of this instrument. The
+cistern is so constructed that the greatest amount of light is admitted to
+the surface of the mercury.
+
+
+10. BAROMETER WITH ELECTRICAL ADJUSTMENT.
+
+This barometer is useful to persons whose eyesight may be defective; and
+is capable of being read off to greater accuracy than ordinary barometers,
+as will be seen by the following description:--The barometer consists of
+an upright tube dipping into a cistern, so contrived, that an up-and-down
+movement, by means of a screw, can be imparted to it. In the top of the
+tube a piece of platina wire is hermetically sealed. The cistern also has
+a metallic connection, so that by means of covered copper wires (in the
+back of the frame) a circuit is established; another connection also
+exists by means of a metallic point dipping into the cistern. The circuit,
+however, can be cut off from this by means of a switch placed about midway
+up the frame; on one side of the tube is placed a scale of inches; a small
+circular vernier, divided into 100 parts, is connected with the dipping
+point, and works at right angles with this scale.
+
+To set the instrument in action for taking an observation, a small battery
+is connected by means of two small binding screws at the bottom of the
+frame. The switch is turned upwards, thereby disconnecting the dipping
+point; the cistern is then screwed up, so that the mercury in the tube is
+brought into contact with the platina wire at the top; the instant this is
+effected the magnetic needle seen on the barometer will be deflected. The
+switch is now turned down; by so doing the connection with the upper wire
+or platina is cut off, and established instead only between the dipping
+point carrying the circular vernier and the bottom of the cistern; the
+point is now screwed by means of the milled head until the needle is again
+deflected. We may now be sure that the line on the circular vernier that
+cuts the division on the scale is the exact height of the barometer.
+Although the description here given may seem somewhat lengthy, the
+operation itself is performed in less time than would be taken in reading
+off an ordinary instrument.
+
+
+11. PEDIMENT BAROMETERS.
+
+[Illustration: Fig. 7.]
+
+[Illustration: Fig. 8.]
+
+[Illustration: Fig. 9.]
+
+[Illustration: Fig. 10.]
+
+[Illustration: Fig. 11.]
+
+These Barometers, generally for household purposes, are illustrated by
+figs. 7 to 11. They are intended chiefly for "weather glasses," and are
+manufactured to serve not only a useful, but an ornamental purpose as
+well. They are usually framed in wood, such as mahogany, rosewood, ebony,
+oak or walnut, and can be obtained either plain or handsomely and
+elaborately carved and embellished, in a variety of designs, so as to be
+suitable for private rooms, large halls, or public buildings. The scales
+to the barometer and its attached thermometer may be ivory, porcelain, or
+silvered metal. It is not desirable that the vernier should read nearer
+than one-hundredth of an inch. Two verniers and scales may be fitted one
+on either side of the mercurial column, so that one can denote the last
+reading, and thus show at a glance the extent of rise or fall in the
+interval. The scale and thermometer should be covered with plate glass. A
+cheap instrument has an open face and plain frame, with sliding vernier
+instead of rack-and-pinion motion. The barometer may or may not have a
+moveable bottom to the cistern, with screw for the purpose of securing the
+mercury for portability. The cistern should not, however, require
+adjustment to a zero or fiducial point. It should be large enough to
+contain the mercury, which falls from 31 to 27 inches, without any
+appreciable error on the height read off on the scale.
+
+
+=12. The Words on the Scale.=--The following words are usually engraved on
+the scales of these barometers, although they are not now considered of so
+much importance as formerly:--
+
+  At 31  inches         Very dry.
+  "  30.5  "            Settled fair.
+  "  30    "            Fair.
+  "  29.5  "            Changeable.
+  "  29    "            Rain.
+  "  28.5  "            Much rain.
+  "  28    "            Stormy.
+
+The French place upon their barometers a similar formula:--
+
+  At 785 millimetres    Tres-sec.
+  "  776      "         Beau-fixe.
+  "  767      "         Beau temps.
+  "  758      "         Variable.
+  "  749      "         Pluie ou vent.
+  "  740      "         Grande pluie.
+  "  731      "         Tempete.
+
+Manufacturers of barometers have uniformly adopted these indications for
+all countries, without regard to the elevation above the sea, or the
+different geographical conditions; and as it can readily be shown that the
+height and variations of the barometer are dependent on these, it follows
+that barometers have furnished indications which, under many
+circumstances, have been completely false. Even in this country, and near
+the sea-level, storms are frequent with the barometer not below 29; rain
+is not uncommon with the glass at 30; even fine weather sometimes occurs
+with a low pressure; while it is evident that at an elevation of a few
+thousand feet the mercury would never rise to 30 inches; hence, according
+to the scale, there should never be fair weather there. If tempests
+happened as seldom in our latitude as the barometer gets down to 28
+inches, the maritime portion of the community at least would be happy
+indeed. These words have long been ridiculed by persons acquainted with
+the causes of the barometric fluctuations; nevertheless opticians continue
+to place them on the scales, evidently only because they appear to add to
+the importance of the instrument in the eyes of those who have not learned
+their general inutility. In different regions of the world, the
+indications of the barometer are modified by the conditions peculiar to
+the geographical position and elevation above the sea, and it is necessary
+to take account of these in any attempt to found rules of general utility
+in connection with the barometer as a weather guide. All that can be said
+in favour of these words is, that within a few hundred feet of the
+sea-level, when the column rises or falls gradually during two or three
+days towards "Fair" or "Rain," the indications they afford of the coming
+weather are generally extremely probable; but when the variations are
+quick, upward or downward, they presage unsettled or stormy weather.
+
+Admiral FitzRoy writes:--"The words on the scales of barometers should not
+be so much regarded, for weather indications, as the rising or falling of
+the mercury; for if it stands at _Changeable_, and then rises a little
+towards _Fair_, it presages a change of wind or weather, though not so
+great as if the mercury had risen higher; and, on the contrary, if the
+mercury stands above _Fair_ and falls, it presages a change, though not to
+so great a degree as if it had stood lower; besides which, the direction
+and force of wind are not in any way noticed. It is not from the point at
+which the mercury stands that we are alone to form a judgment of the state
+of the weather, but from its _rising_ or _falling_; and from the movements
+of immediately preceding days as well as hours, keeping in mind effects of
+change of _direction_ and dryness, or moisture, as well as alteration of
+force or strength of wind."[1]
+
+
+=13. Correction due to Capacity of Cistern.=--These barometers, having no
+adjustment for the zero of the scale, require a correction for the varying
+level of the mercury in the cistern, when the observations are required
+for strict comparison with other barometric observations, or when they are
+registered for scientific purposes; but for the common purpose of
+predicting the weather, this correction is unnecessary. The neutral point,
+and the ratio of the bore of the tube to the diameter of the cistern, must
+be known (see p. 3). Then the capacity correction, as it is termed, is
+found as follows:--Take the fractional part, expressed by the capacity
+ratio, of the difference between the observed reading and the height of
+the neutral point; then, if the mercury stand _below_ the neutral point,
+_subtract_ this result from the reading; if it stand _above_, _add_ it to
+the reading.
+
+For example, suppose the neutral point to be 29.95 inches, and the
+capacity ratio 1/50, required the correction when the barometer reads
+30.78.
+
+  Here                  30.78 - 29.95 = 0.83
+  Correction                = 0.83/50 = +0.02 nearly.
+  Scale reading                         30.78
+                                        -----
+  Correct reading                       30.80
+                                        =====
+
+Of course the correction could as easily be found to three decimal places,
+if desirable. It is evident that the correction is more important the
+greater the distance of the top of the mercury from the neutral point.
+
+
+14. PUBLIC BAROMETERS.
+
+Since the increased attention paid to the signs of forthcoming weather of
+late years, and the good which has resulted therefrom to farmers,
+gardeners, civil engineers, miners, fishermen, and mariners generally, by
+forewarning of impending wet or stormy weather, the desirability of having
+good barometers exposed in public localities has become evident.
+
+Barometers may now be seen attached to drinking fountains, properly
+protected, and are frequently consulted by the passers-by. But it is among
+those whose lives are endangered by sudden changes in the weather,
+fishermen especially, that the warning monitor is most urgently required.
+Many poor fishing villages and towns have therefore been provided by the
+Board of Trade, at the public expense, and through the humane effort of
+Admiral FitzRoy, with first-class barometers, each fixed in a conspicuous
+position, so as to be easily accessible to all who desire to consult it.
+Following this example, the Royal National Life Boat Institution has
+supplied each of its stations with a similar storm warner; the Duke of
+Northumberland and the British Meteorological Society have erected several
+on the coast of Northumberland; and many other individuals have presented
+barometers to maritime places with which they are connected.
+
+These barometers have all been manufactured by Messrs. Negretti and
+Zambra. The form given to the instrument seems well adapted for public
+purposes.
+
+
+[Illustration: Fig. 12.]
+
+=15. Fishery or Sea-coast Barometers.=--Fig. 12 gives a representation of
+these coast and fishery barometers. The frame is of solid oak, firmly
+screwed together. The scales are very legibly engraved on porcelain by
+Negretti and Zambra's patent process. The thermometer is large, and easily
+read; and as this instrument is exposed, it will indicate the actual
+temperature sufficiently for practical purposes. The barometer tube is
+three-tenths of an inch in diameter of bore, exhibiting a good column of
+mercury; and the cistern is of such capacity, in relation to the tube,
+that the change of height in the surface of the mercury in the cistern
+corresponding to a change of height of three inches of mercury in the
+tube, is less than one-hundredth of an inch, and therefore, as the
+readings are only to be made to this degree of accuracy, this small error
+is of no importance. The cistern is made of boxwood, which is sufficiently
+porous to allow the atmosphere to influence the mercurial column; but the
+top is plugged with porous cane, to admit of free and certain play.
+
+
+=16. Admiral FitzRoy's Scale Words.=--The directions given on the scales
+of these barometers were drawn up by Admiral FitzRoy, F.R.S. They appear
+to be founded on the following considerations:--
+
+Supposing a compass diagram, with the principal points laid down, the N.E.
+is the wind for which the barometer stands highest; for the S.W. wind it
+is lowest. This is found to be so in the great majority of cases; but
+there are exceptions to this, as to all rules. The N.E. and S.W. may
+therefore be regarded as the poles of the winds, being opposite each
+other. When the wind veers from the S.W. through W. and N. to N.E., the
+barometer gradually rises; on the contrary, when the wind veers from N.E.
+and E. to S.E., S. and S.W., the mercury falls. A similar curious law
+exists in relation to the veering of the wind, and the action of the
+thermometer. As the wind veers from the S.W. to W. and N., the thermometer
+falls; as it veers from N.E. to E. and S., it rises, because the wind gets
+from a colder to a warmer quarter. The polar winds are cold, dry, and
+heavy. Those from the equatorial regions are warm, moist, and
+comparatively light.
+
+These laws have been clearly developed and expressed by Professor Dove in
+his work on the "Law of Storms." The warm winds of Europe are those which
+bring the greatest quantity of rain, as they blow from the ocean, and come
+heavily laden with moisture. The cold winds, besides containing less
+moisture, blow more from the land. The weight of the vapour of the warm
+winds tends to raise the barometric column; but, at the same time, the
+increased dilatation of the air tends to lower it. This latter influence
+being the stronger, the barometer always falls for these winds; and in
+regions where they traverse a large extent of land, retain their heat, and
+become necessarily very dry, the fall in the barometer will be greater.
+Admiral FitzRoy's words for the scales of barometers for use in northern
+latitudes, then, are as follows:--
+
+    _RISE._                      _FALL._
+      FOR                          FOR
+    N. ELY.                      S. WLY.
+  NW.--N.--E.                  SE.--S.--W.
+      DRY                          WET
+      OR                           OR
+     LESS                         MORE
+     WIND.                        WIND.
+   -------                      -------
+    EXCEPT                       EXCEPT
+   WET FROM                     WET FROM
+    N. ED.                       N. ED.
+   -------                      -------
+  Long foretold, long last;    First rise after low,
+  Short notice, soon past.     Foretells stronger blow.
+
+It will be perceived that the exception in each case applies to N.E.
+winds. The barometer may fall with north-easterly winds, but they will be
+violent and accompanied with rain, hail, or snow; again, it will rise with
+these winds accompanied with rain, when they are light, and bring only
+little rain. It rises, however, highest with the dry and light N.E. winds.
+
+These directions are very practically useful; they provide for
+geographical position--also for elevation above the sea--since they are
+not appended to any particular height of the column. They are suited to
+the northern hemisphere generally, as well as around the British Isles.
+The same directions are adapted to the southern hemisphere, by simply
+substituting for the letter N the letter S, reading south for north, and
+_vice versa_. South of the equator the cold winds come from the south; the
+warm, from the north. The S.E. wind in the southern hemisphere corresponds
+to the N.E. in the northern. The laws there are, while the wind veers from
+S.E. through E. to N. and N.W., the barometer falls and the thermometer
+rises. As the wind veers from N.W. through W. and S. to S.E., the
+barometer rises and the thermometer falls.
+
+
+=17. Instructions for the Sea-coast Barometer.=--The directions for fixing
+the barometer, and making it portable when it has to be removed, should be
+attended to carefully. The barometer should be suspended against a frame
+or piece of wood, so that light may be seen _through_ the tube. Otherwise
+a piece of paper, or a _white place_, should be behind the upper or _scale
+part_ of the _tube_.
+
+When suspended on a hook, or stout nail, apply the milled-head key (which
+will be found just below the scales) to the square brass pin at the lower
+end of the instrument, and turn _gently_ toward the left hand till the
+screw stops; then take off the key and replace it for use, near the scale,
+as it was before. The cistern bottom being thus _let down_, the mercury
+will sink to its proper level quickly.
+
+In removing this barometer it is necessary to _slope it gradually_, till
+the mercury is at the top of the tube, and then, with the instrument
+reversed, to screw up the cistern bottom, or bag, by the key, used
+_gently_, till it stops. It will then be portable, and may be carried with
+the _cistern_ end _upwards_, or lying flat; but it must not be jarred, or
+receive a concussion.
+
+
+=18. French Sea-coast Barometer.=--The French have imitated this form of
+barometer for coast service, and have translated Admiral FitzRoy's
+indications for the scale as follows:--
+
+          LA                            LA
+        HAUSSE                        BAISSE
+       INDIQUE.                      INDIQUE.
+       ---------                     ---------
+    DES VENTS DE LA               DES VENTS DE LA
+       PARTIE DU                     PARTIE DU
+          N.E.                          S.O.
+    (DU N.O. a l'E)               (DU S.E. a l'O.)
+    (PAR LE NORD. )               (PAR LE SUD.   )
+        DE LA                            DE
+      SECHERESSE.                   L'HUMIDITE.
+       ---------                     ---------
+        UN VENT                       UN VENT
+      PLUS FAIBLE                    PLUS FORT
+   EXCEPTE S'IL PLEUT            EXCEPTE S'IL PLEUT
+  AVEC DE FORTES BRISES        AVEC DE PETITES BRISES
+        DU N.E.                       DU N.E.
+       ---------                     ---------
+    Mouvements lents,             Le commencement
+     Temps durable.                de la hausse,
+       ---------                  apres une grande
+   Mouvements rapides,             baisse presage
+    Temps variable.               un Vent violent.
+
+
+MARINE BAROMETERS.
+
+=19. The Common Form.=--The barometer is of great use to the mariner, who,
+by using it as a "weather glass," is enabled to foresee and prepare for
+sudden changes in the weather. For marine purposes, the lower portion of
+the glass tube of the barometer must be contracted to a fine bore, to
+prevent oscillation in the mercurial column, which would otherwise be
+occasioned by the movements of the ship. This tube is cemented to the
+cistern, which is made of boxwood, and has a moveable leathern bottom, for
+the purpose of rendering the instrument portable, by screwing up the
+mercury compactly in the tube. The tube is enclosed in a mahogany frame,
+which admits of a variety of style in shape, finish, and display, to meet
+the different fancies and means of purchasers. The frame is generally
+enlarged at the upper part to receive the scales and the attached
+thermometer, which are covered by plate glass. The cistern is encased in
+brass for protection, the bottom portion unscrewing to give access to the
+portable screw beneath the cistern. Figs. 13 and 14 illustrate this form
+of barometer. Marine barometers require to be suspended, so that they may
+remain in a vertical position under the changeable positions of a vessel
+at sea. To effect this they are suspended in gimbals by a brass arm. The
+gimbals consist of a loose ring fastened by thumb-screws to the middle
+part of the frame of the barometer, in front and back. The forked end of
+the arm supports this ring at the sides, also by the aid of thumb-screws.
+Hence the superior weight of the cistern end is always sufficient to cause
+the instrument to move on its bearing screws, so as always to maintain a
+perpendicular position; in fact, it is so delicately held that it yields
+to the slightest disturbance in any direction. The other end of the arm is
+attached to a stout plate, having holes for screws, or fitted to slip into
+a staple or bracket, by which it may be fixed to any part of the cabin of
+a ship; the arm is hinged to the plate, for the purpose of turning the arm
+and barometer up whenever it is desirable.
+
+[Illustration: Fig. 13.]
+
+[Illustration: Fig. 14.]
+
+Other forms of barometer (to be immediately described) have superseded
+this in the British Marine, but the French still give the preference to
+the wooden frames. They think the barometer can be more securely mounted
+in wood, is more portable, and less liable to be broken by a sudden
+concussion than if fitted in a metal frame. The English deem the ordinary
+wooden barometers not sufficiently accurate, owing to the irregular
+expansion of wood, arising from its hygrometric properties. Some of the
+English opticians have shown that very portable, and really accurate
+barometers can be made in brass frames, and therefore the preference is
+now given to this latter material.
+
+
+=20. The Kew Marine Barometer.=--The form of barometer so-called, is that
+recommended by the Congress of Brussels, held in 1853, for the purpose of
+devising a systematic plan of promoting meteorological observations at
+sea.
+
+The materials employed in its construction are mercury, glass, iron, and
+brass. The upper part of the tube is carefully calibrated to ensure
+uniformity of bore, as this is a point upon which the accuracy of the
+instrument to some extent depends. At sea, the barometer has never been
+known to stand above 31 inches, nor below 27. These extremes have been
+attained with instruments of undoubted accuracy, but they are quite
+exceptional. It is not necessary, therefore, to carry the scales of marine
+barometers beyond these limits, but they should not be made shorter. If
+the vernier is adjusted to read upward, the scale should extend to 32
+inches, to allow room for the vernier to be set to 31 inches at least.
+Cases have occurred in which this could not be done, and rare, but
+valuable observations have been lost in consequence. If the scale part of
+the tube be not uniform in bore, the index error will be irregular
+throughout the scale. Whether the bore of the rest of the tube varies in
+diameter, is of no moment. From two to three inches below the measured
+part, the bore is contracted very much, to prevent the pulsations in the
+mercurial column--called "pumping"--which, otherwise, would occur at sea
+from the motion of the ship. In ordinary marine barometers, this
+contraction extends to the end of the tube. Below the contracted part is
+inserted a pipette--or Gay Lussac air-trap--which is a little elongated
+funnel with the point downwards. Its object is to arrest any air that may
+work in between the glass and the mercury. The bubble of air lodges at the
+shoulder, and can go up no farther. It is one of those simple contrivances
+which turn out remarkably useful. If any air gets into the tube, it does
+not get to the top, and therefore does not vitiate the performance of the
+barometer; for the mercury itself works up and down through the funnel.
+Below this, the tube should not be unnecessarily contracted.
+
+[Illustration: Fig. 15.]
+
+The open end of the tube is fixed into an iron cylinder, which forms the
+cistern of the barometer. Iron has no action upon mercury, and is
+therefore used instead of any other metal. One or two holes are made in
+the top of the cistern, which are covered on the inside with strong
+sheep-skin leather, so as to be impervious to mercury, but sufficiently
+porous for the outer air to act upon the column. The cistern is of
+capacity sufficient to receive the mercury which falls out of the tube
+until the column stands lower than the scale reads; and when the tube is
+completely full, there is enough mercury to cover the extremity so as to
+prevent access of air. There is no screw required for screwing up the
+mercury.
+
+The glass tube thus secured to the cistern is protected by a brass tubular
+frame, into which the iron cistern fits and screws compactly. Cork is used
+to form bearings for the tube. A few inches above the cistern is placed
+the attached thermometer. Its bulb is enclosed in the frame, so as to be
+equally affected by heat with the barometric column. The upper end of the
+frame is fitted with a cap which screws on, and embraces a glass shield
+which rests in a gallery formed on the frame below the scale, and serves
+to protect the silvered scale, as well as the inner tube, from dust and
+damp. A ring, moveable in a collar fixed on the frame above the centre of
+gravity of the instrument, is attached to gimbals, and the whole is
+supported by a brass arm in the usual manner; so that the instrument can
+be moved round its axis to bring any source of light upon it, and will
+remain vertical in all positions of the ship. The vernier reads to
+five-hundredths of an inch. No words are placed upon the scale, as the old
+formulary was deemed misleading. The vernier can be set with great
+exactness, as light is admitted to the top of the mercury by a front and
+a back slit in the frame. The lower edge of the vernier should be brought
+to the top of the mercury, so as just to shut out the light.
+
+It is evident that this form of barometer must be more reliable in its
+indications than those in wooden frames. The graduations can be accurately
+made, and they will be affected only by well-known alterations due to
+temperature. Some think the tube is too firmly held, and therefore liable
+to be broken by concussion more readily than that of an inferior
+instrument. This, however, appears a necessary consequence of greater
+exactness. It is an exceedingly good portable instrument, and can be put
+up and taken down very readily. These barometers are preferred to marine
+barometers in wood, wherever they have been used. In merchant ships, and
+under careful treatment, they have been found very durable. They may be
+sent with safety by railway, packed carefully in a wooden box.
+
+_Directions for Packing._--In removing this barometer it is necessary to
+slope it gradually till the mercury reaches the top of the tube. It is
+then portable, if carried cistern end upwards or lying flat. If carried
+otherwise, it will very probably be broken by the jerking motion of the
+heavy mercury in the glass tube. Of course it must not be jarred, or
+receive concussion.
+
+_Position for Marine Barometer._--Admiral FitzRoy, to whose valuable
+papers we are much indebted, writes in his "Barometer Manual":--"It is
+desirable to place the barometer in such a position as not to be in danger
+of a side blow, and also sufficiently far from the deck above to allow for
+the spring of the metal arm in cases of sudden movements of the ship.
+
+"If there is risk of the instrument striking anywhere when the vessel is
+much heeled, it will be desirable to put some soft padding on that place,
+or to check movement in that direction by a light elastic cord; in fixing
+which, attention must be paid to have it acting only where risk of a blow
+begins, not interfering otherwise with the free swing of the instrument: a
+very light cord attached above, when possible, will be least likely to
+interfere injuriously."
+
+
+=21. Method of verifying Marine and other Barometers.=--"In nearly all the
+barometers which had been employed at sea till recently the index
+correction varied through the range of scale readings, in proportion to
+the difference of capacity between the cistern and the tube. To find the
+index correction for a land barometer, comparison with a standard, at any
+part of the scale at which the mercury may happen to be, is generally
+considered sufficient. To test the marine barometer is a work of much more
+time, since it is necessary to find the correction for scale readings at
+about each half inch throughout the range of atmospheric pressure to which
+it may be exposed; and it becomes necessary to have recourse to artificial
+means of changing the pressure of the atmosphere on the surface of the
+mercury in the cistern.
+
+"The barometers to be thus tested are placed, together with a standard, in
+an air-tight chamber, to which an air-pump is applied, so that, by
+partially exhausting the air, the standard can be made to read much lower
+than the lowest pressure to which marine barometers are likely to be
+exposed; and by compressing the air it can be made to read higher than the
+mercury ever stands at the level of the sea. The tube of the standard is
+contracted similarly to that of the marine barometer, but a provision is
+made for adjusting the mercury in its cistern to the zero point. Glass
+windows are inserted in the upper part of the iron air-chamber, through
+which the scales of the barometers may be seen; but as the verniers cannot
+be moved in the usual way from outside the chamber, a provision is made
+for reading the height of the mercury independent of the verniers attached
+to the scales of the respective barometers. At a distance of some five or
+six feet from the air-tight chamber a vertical scale is fixed. The
+divisions on this scale correspond exactly with those on the tube of the
+standard barometer. A vernier and telescope are made to slide on the scale
+by means of a rack and pinion. The telescope has two horizontal wires, one
+fixed and the other moveable by a micrometer screw, so that the difference
+between the height of the column of mercury and the nearest division on
+the scale of the standard, and also of all the other barometers placed by
+the side of it for comparison, can be measured either with the vertical
+scale and vernier or the micrometer wire. The means are thus possessed of
+testing barometers for index error in any part of the scale, through the
+whole range of atmospheric pressure to which they are likely to be
+exposed; and the usual practice is to test them at every half inch from
+27.5 to 31 inches.
+
+"In this way barometers of various other descriptions have been tested,
+and some errors found to be so large that a few barometers read half an
+inch and upwards too high, while others read as much too low. In some
+cases those which were correct in one part of the scale were found to be
+from half an inch to an inch wrong in other parts. These barometers were
+of an old and ordinary, not to say inferior, construction. In some the
+mercury would not descend lower than about 29 inches, owing to a fault
+very general in the construction of many common barometers till lately in
+frequent use:--the _cistern was not large enough_ to hold the mercury
+which descended from the tube in a _low atmospheric pressure_.
+
+"When used on shore, this contraction of the tube causes the marine
+barometer to be _sometimes_ a little behind an ordinary land barometer,
+the tube of which is not contracted. The amount varies according to the
+rate at which the mercury is rising or falling, and ranges from 0.00 to
+0.02 of an inch. As the motion of the ship at sea causes the mercury to
+pass more rapidly through the contracted tube, the readings are almost the
+same there as they would be if the tube were not contracted, and in no
+case do they differ enough to be of importance in maritime use."
+
+The cistern of this marine barometer is generally made an inch and a
+quarter in diameter, and the scale part of the tube a quarter of an inch
+in bore. The inches on the scale, instead of being true, are shortened by
+.04 of an inch, in order to avoid the necessity of applying a correction
+due to the difference of capacity of the tube and cistern. This is done
+with much perfection, and the errors of the instruments, when compared
+with a standard by the apparatus used at Kew and Liverpool Observatories,
+are determined to the thousandth of an inch, and are invariably very
+uniform and small. The error so determined includes the correction due to
+capillarity, capacity, and error of graduation, and forms a constant
+correction, so that only one variable correction, that due to temperature,
+need be applied, when the barometer is suspended near the water line of
+the ship, to make the observations comparable with others. With all the
+advantages of this barometer, however, it has recently been superseded, to
+some extent, because it was found to require more care than could
+ordinarily be expected to be given to it by the commander of a ship.
+Seamen do not exactly understand the value of such nice accuracy as the
+thousandth part of an inch, but prefer an instrument that reads only to a
+hundredth part.
+
+
+22. THE FITZROY MARINE BAROMETER.
+
+Admiral FitzRoy deemed it desirable to construct a form of barometer as
+practically useful as possible for marine purposes. One that should be
+less delicate in structure than the Kew barometer, and not so finely
+graduated. One that could be set at a glance and read easily; that would
+be more likely to bear the common shocks unavoidable in a ship of war.
+Accordingly, the Admiral has devised a barometer, which he has thus
+described:--
+
+"This marine barometer, for Her Majesty's service, is adapted to _general_
+purposes.
+
+"It differs from barometers hitherto made in points of detail, rather than
+principle:--1. The glass tube is packed with vulcanised india-rubber,
+which checks vibration from concussion; but does not hold it rigidly, or
+prevent expansion. 2. It does not oscillate (or pump), though extremely
+sensitive. 3. The scale is porcelain, _very legible_, and not liable to
+change. 4. There is no iron anywhere (_to rust_). 5. Every part can be
+unscrewed, examined, or cleaned, by any careful person. 6. There is a
+_spare_ tube, fixed in a cistern, filled with boiled mercury, and _marked_
+for adjustment in this, or _any similar_ instrument.
+
+"These barometers are graduated to hundredths, and they will be found
+accurate to _that_ degree, namely the second decimal of an inch.
+
+"They are packed with vulcanised caoutchouc, in order that (by this, and
+by a peculiar strength of glass tube) guns may be fired near these
+instruments without causing injury to them by ordinary concussion.
+
+"It is hoped that all such instruments, for the public service at sea,
+will be quite similar, so that any spare tube will fit _any_ barometer.
+
+"_To Shift a Tube._--Incline the barometer slowly, and then take it down,
+after allowing the mercury to fill the upper part. Lay the instrument on a
+table, unscrew the outer cap at the joining just below the cistern swell,
+then unscrew the tube _and_ cistern, by turning the cistern gently,
+against the sun, or to _the left_, and draw out the tube very carefully
+_without bending it in the least_, _turning_ it a little, if required, as
+moved. Then insert the new tube very cautiously, screw in, and adjust to
+the diamond-cut mark for 27 inches. Attach the cap, and suspend the
+barometer for use.
+
+"If the mercury does not immediately quit the top of the tube, tap the
+cistern end rather sharply. In a well-boiled tube, with a good vacuum, the
+mercury hangs, at times, so adhesively as to deceive, by causing a
+supposition of some defect.
+
+"In about ten minutes the mercurial column should be nearly right; but as
+local temperature affects the brass, as well as the mercury, slowly and
+unequally, it may be well to defer any _exact comparisons with other
+instruments_ for some few hours."
+
+Messrs. Negretti and Zambra are the makers of these barometers for the
+Royal Navy. Fig. 16 is an illustration.
+
+[Illustration: Fig. 16.]
+
+The tube is fixed to a boxwood cistern, which is plugged with very porous
+cane at the top, to allow of the ready influence of a variation in
+atmospheric pressure upon the mercury. Round the neck of the cistern is
+formed a brass ring, with a screw thread on its circumference. This screws
+into the frame, and a mark on the tube is to be adjusted to 27 inches on
+the scale, the cistern covering screwed on, and the instrument is ready to
+suspend. The frame and all the fittings are brass, without any iron
+whatever; because the contact of the two metals produces a galvanic
+action, which is objectionable. The spare tube is fitted with
+india-rubber, and ready at any time to replace the one in the frame. The
+ease with which a tube can be replaced when broken is an excellent feature
+of the instrument. The spare tube is carefully stowed in a box, which can
+also receive the complete instrument when not in use. All the parts are
+made to a definite gauge; the frames are, therefore, all as nearly as
+possible similar to each other, and the tubes--like rifle bullets--are
+adjustible to any frame. If, then, the tube in use gets broken, the
+captain can replace it by the other; but, as it is securely packed with
+india-rubber, there is very little liability of its being broken by fair
+usage. Every person who knows the importance of the barometer on board
+ship, will acknowledge that the supplementary tube is a decided
+improvement. Many instruments of this description are afloat in the Royal
+Navy, and in a short time it may be expected that all the frames and tubes
+of barometers in the public service at sea will be similar in size and
+character; so that should a captain have the misfortune to get both his
+tubes broken, he would be able to borrow another from any ship he fell in
+with that had one to spare, which would be perfectly accurate, because it
+would have been verified before it was sent out.
+
+
+=23. Admiral FitzRoy's Words for the Scale.=--The graduation of inches and
+decimals are placed in this barometer on the right-hand side of the tube;
+and on a similar piece of porcelain, on the left-hand, are engraved, as
+legibly as they are expressed succinctly, the following words, of
+universal application in the interpretation of the barometer movements:--
+
+     _RISE_            _FALL_
+      FOR               FOR
+      COLD              WARM
+      DRY               WET
+       OR                OR
+      LESS              MORE
+      WIND.             WIND.
+    --------          --------
+     EXCEPT            EXCEPT
+    WET FROM          WET FROM
+   COOLER SIDE.      COOLER SIDE.
+
+Reverting to the explanation of the words on the "Coast" barometers (at
+page 14), and comparing and considering them as given for northern
+latitudes, and as they must be altered for southern latitudes, it will be
+perceived, that for all _cold_ winds the barometer rises; and falls for
+_warm_ winds. The mercury also falls for _increased_ strength of wind; and
+rises as the wind _lulls_. Likewise before or with rain the column of
+mercury falls; but it rises with fine dry weather. Putting these facts
+together, and substituting for the points of the compass the terms "cold"
+and "warm," the appropriateness of the words on the scale of this
+barometer is readily perceived. These concise and practical indications of
+the movements in the barometer are applicable for instruments intended for
+use in any region of the world, and are in perfect accordance with the
+laws of winds and weather deduced by Dove and other meteorologists. There
+is nothing objectionable in them, and being founded upon experience and
+the deductions made from numerous recorded observations of the weather in
+all parts of the world, as well as confirmed by the theories of science,
+they may consequently be considered as generally reliable. They involve no
+conjecture, but express succinctly scientific principles.
+
+
+=24. Trials of the FitzRoy Marine Barometer under Fire of Guns.=--Some of
+the first barometers made by Messrs. Negretti and Zambra on Admiral
+FitzRoy's principle were severely tried under the heaviest naval gun
+firing, on board H.M.S. _Excellent_; and under all the circumstances, they
+withstood the concussion. The purpose of the trials was "to ascertain
+whether the _vulcanized india-rubber packing_ round the glass tube of a
+_new marine barometer_ did check the vibration caused by firing, and
+whether guns might be fired close to these instruments without causing
+injury to them." In the first and second series of experiments, a marine
+barometer on Admiral FitzRoy's plan was tried against a marine barometer
+on the Kew principle, both instruments being new, and treated in all
+respects similarly. They were "hung over the gun, under the gun, and by
+the side of the gun, the latter both inside and outside a bulkhead,--in
+fact, in all ways that they would be tried in action with the bulkheads
+cleared away." The result was that the Kew barometer was broken and
+rendered useless, while the new pattern barometer was not injured in the
+least. In a third series of experiments, Mr. Negretti being present, five
+of the new pattern barometers were subjected to the concussion produced by
+firing a 68-pounder gun with shot, and 16 lbs. charge of powder. They were
+suspended from a beam immediately under the gun, then from a beam
+immediately over the gun, and finally they were suspended by the arm to a
+bulkhead, at a distance of only 3 ft. 6 in. from the axis of the gun; and
+the result was, according to the official report, "that all these
+barometers, however suspended, would stand, without the slightest injury,
+the most severe concussion that they would ever be likely to experience in
+any sea-going man-of-war." These trials were conducted under the
+superintendence of Captain Hewlett, C.B., and the guns were fired in the
+course of his _usual_ instructions. His reports to Admiral FitzRoy, giving
+all the particulars of the trials, are published in the "Ninth Number of
+Meteorological Papers," issued by the Board of Trade.[2]
+
+
+25. NEGRETTI AND ZAMBRA'S FARMER'S BAROMETER AND DOMESTIC WEATHER-GLASS.
+
+It is a well-known fact that the barometer is as much, or even more
+affected by a change of wind as it is by rain; and the objection raised
+against a simple barometer reading, as leaving the observer in doubt
+whether to expect wind or rain, is removed by the addition of the
+Hygrometer, an instrument indicating the comparative degree of dryness or
+dampness of the air;--a most important item in the determination of the
+coming weather.
+
+The farmer should not be content to let his crops lie at the mercy, so to
+speak, of the weather, when he has within his command instruments which
+may be the means of preventing damage to, and in cases total loss of, his
+crops.
+
+The farmer hitherto has had to depend for his prognostication of the
+weather on his own unassisted "Weather Wisdom;" and it is perfectly
+marvellous how expert he has become in its use. Science now steps in, not
+to ignore this experience, but on the contrary, to give it most valuable
+assistance by extending it, and enabling it to predict, with an accuracy
+hitherto unknown, the various changes that take place in this most
+variable of climates.
+
+To the invalid, the importance of predicting with tolerable accuracy the
+changes that are likely to occur in the weather, cannot be over-rated.
+Many colds would be prevented, if we could know that the morning so balmy
+and bright, would subside into a cold and cheerless afternoon. Even to the
+robust, much inconvenience may be prevented by a due respect to the
+indications of the hygrometer and the barometer, and the delicate in
+health will do well to regard its warnings.
+
+[Illustration: Fig. 17.]
+
+_Description of the Instrument._--The farmer's barometer, as figured in
+the margin, consists of an upright tube of mercury inverted in a cistern
+of the same fluid; this is secured against a strong frame of wood, at the
+upper end of which is fixed the scale, divided into inches and tenths of
+an inch. On either side of the barometer, or centre tube, are two
+thermometers--that on the left hand has its bulb uncovered and freely
+exposed, and indicates the temperature of the air at the place of
+observation; that on the right hand has its bulb covered with a piece of
+muslin, from which depend a few threads of soft lamp cotton; this cotton
+is immersed in the small cup situated just under the thermometer, this
+vessel being full of water; the water rises by capillary attraction to the
+muslin-covered bulb, and keeps it in a constantly moist state.
+
+These two thermometers, which we distinguish by the names "Wet Bulb" and
+"Dry Bulb," form the Hygrometer; and it is by the simultaneous reading of
+these two thermometers, and noting the difference that exists between
+their indications, that the humidity in the atmosphere is determined.
+
+Admiral FitzRoy's words (see p. 22) are placed upon the scale of the
+barometer, as the value of a reading depends, not so much on the actual
+height of the mercury in the tube, as it does on whether the column is
+rising, steady, or falling.
+
+The moveable screw at the bottom of the cistern is for the purpose of
+forcing the mercury to the top of the tube when the instrument is being
+carried from place to place, and it must always be unscrewed to its utmost
+limit when the barometer is hung in its proper place. After this it should
+never be touched.
+
+The manner in which the Hygrometer acts is as follows: It is a pretty
+well-known fact that water or wine is often cooled by a wet cloth being
+tied round the bottle, and then being placed in a current of air. The
+evaporation that takes place in the progressive drying of the cloth causes
+the temperature to fall considerably below that of the surrounding
+atmosphere, and the contents of the bottle are thus cooled. In the same
+manner, then, the covered wet bulb thermometer will be found _invariably_
+to read lower than the uncovered one; and the greater the dryness of the
+air, the greater will be the difference between the indications of the
+two thermometers; and the more moisture that exists in the air, the more
+nearly they will read alike.
+
+The cup must be kept filled with pure water, and occasionally cleaned out,
+to remove any dirt. The muslin, or cotton-wick, should also be renewed
+every few weeks. The hygrometer may be had separate from the barometer, if
+the combined instruments cannot be sufficiently exposed to the external
+air, this being essential for the successful use of the hygrometer.
+
+This farmer's weather-glass, then, consists of three distinct instruments:
+the barometer, the thermometer, and the hygrometer. He has thus at command
+the three instrumental data necessary for the prediction of the weather.
+And now to describe--
+
+_How to Use the Instrument._--The observations should be taken twice a
+day, say at 9 A.M. and 3 P.M.; and should be entered on a slip of paper,
+or a slate hung up by the barometer. The observer will then be able to see
+the different values of the readings from time to time, and to draw his
+conclusions therefrom.
+
+The thermometer on the left hand should first be read, and a note made of
+its indication, which is the temperature of the air. The wet bulb
+thermometer should now be read, and also noted; and the difference should
+be taken of these two readings. Next read the barometer by moving the
+small index at the side of the tube until it is on a level with the top of
+the mercury. Having noted the number of inches at which the column stands,
+compare with the last observation, and see immediately whether the
+barometer is rising, steady, or falling.
+
+Now, having taken the observations as above, we naturally ask the
+question, _What are we to predict from them?_
+
+And, probably, the best way of answering this query will be by giving an
+example. We will suppose that our readings yesterday were as
+follows:--Temperature, 70 deg.; Wet Bulb, 69 deg.; Difference, 1 deg.; =very moist
+air. Barometer, 29.5, and that rain has fallen.
+
+To-day, we read:--Temperature, 60 deg.; Wet Bulb, 55 deg.; Difference, 5 deg.; =dryer
+air. Barometer, 30. We may safely predict that the rain will cease, and
+probably we may have wind from the northward.
+
+In spring or autumn, if the barometric height be steady any where between
+29.5 and 30 inches, with the temperature about 60 deg., fresh to moderate
+south-westerly winds, with cloudy sky, will probably characterize the
+weather; the indications of the hygrometer being then specially
+serviceable in enabling us to foretell rain; but if the mercury become
+steady at about 30.5 inches, with temperature about 40 deg., north-easterly
+winds, dry air, and clear sky, may be confidently expected.
+
+Many cases will doubtless suggest themselves to the observer where these
+figures do not occur, and where he might find a difficulty in interpreting
+the indications of his instruments. We have, therefore, drawn up some
+concise rules for his guidance; and although they will not prove
+absolutely infallible guides to this acknowledged most difficult problem,
+still, they will be found of much service in foretelling the weather, when
+added to an intelligent observation of ordinary atmospheric phenomena, as
+force and direction of wind, nature of any particular season, and the time
+of year.
+
+
+26. RULES FOR FORETELLING THE WEATHER.
+
+A RISING BAROMETER.
+
+    A "Rapid" rise indicates unsettled weather.
+
+    A "Gradual" rise indicates settled weather.
+
+    A "Rise," with dry air, and cold increasing in summer, indicates wind
+    from northward; and if rain has fallen, better weather is to be
+    expected.
+
+    A "Rise," with moist air and a low temperature, indicates wind and
+    rain from northward.
+
+    A "Rise," with southerly wind, indicates fine weather.
+
+A STEADY BAROMETER,
+
+    With dry air and a seasonable temperature, indicates a continuance of
+    very fine weather.
+
+A FALLING BAROMETER.
+
+    A "Rapid" fall indicates stormy weather.
+
+    A "Rapid" fall, with westerly wind, indicates stormy weather from
+    northward.
+
+    A "Fall," with a northerly wind, indicates storm, with rain and hail
+    in summer, and snow in winter.
+
+    A "Fall," with increased moisture in the air, and the heat increasing,
+    indicates wind and rain from southward.
+
+    A "Fall," with dry air, and cold increasing (in winter), indicates
+    snow.
+
+    A "Fall," after very calm and warm weather, indicates rain with
+    squally weather.
+
+
+=27. Causes which may bring about a Fall or a Rise in the
+Barometer.=[3]--As heat produces rarefaction, a sudden rise of temperature
+in a distant quarter may affect the weight of the atmosphere over our
+heads, by producing an aerial current outwards, to supply the place of the
+lighter air which has moved from its former position; in which case the
+barometer will fall. Now such a movement in the atmosphere is likely to
+bring about an intermixture of currents of air of different temperatures,
+and from this intermixture rain is likely to result.
+
+On the other hand, as cold produces condensation, any sudden fall of
+temperature causes the column of air over the locality to contract and
+sink to a lower level, whilst other air rushes in from above to supply the
+void; and, accordingly, the barometer rises. Should this air, as often
+happens, proceed from the north, it will contain in general but little
+moisture; and hence, on reaching a warmer latitude, will take up the
+vapour of the air, so that dry weather will result.
+
+It is generally observed, that wind causes a fall in the instrument; and,
+indeed, in those greater movements of the atmosphere which we denominate
+storms or hurricanes, the depression is so considerable as to forewarn
+the navigator of his impending danger. It is evident, that a draught of
+air in any direction must diminish the weight of the column overhead, and
+consequently cause the mercury in the barometer to sink.
+
+The connection, therefore, of a sinking of the barometric column with rain
+is frequently owing to the wind causing an intermixture of the aerial
+currents which, by their motion, diminish the weight of the atmosphere
+over our heads; whilst a steady rise in the column indicates the absence
+of any great atmospheric changes in the neighbourhood, and a general
+exemption from those causes which are apt to bring about a precipitation
+of vapour.
+
+
+=28. Use of the Barometer in the management of Mines.=--The inflammable
+and suffocating gases, known to coal-miners as fire-damp and choke-damp,
+are specifically heavier than air; and as they issue from the fissures of
+the mine, or are released from the coal, the atmospheric pressure tends to
+drive them into the lowest and least ventilated galleries. Consequently a
+greatly reduced atmospheric pressure will favour a sudden outflow or
+advance of gas; whence may result cases of explosion or suffocation. It
+has been found that these accidents occur for the most part about the time
+of a low barometric column. A reliable barometer should, therefore, be
+systematically consulted by those entrusted with the management or control
+of coal-mines, so that greater vigilance and caution may be enjoined on
+the miners whenever the mercury falls low, especially after it has been
+unusually high for some days.
+
+
+=29. Use of the Barometer in estimating the Height of Tides.=--The
+pressure of the atmosphere affects the height of the tide, the water being
+in general higher as the barometer is lower. The expressions of seamen,
+that "frost nips the tide," and "fog nips the tide," are explained by the
+high barometer which usually accompanies frost and fog. M. Daussy, Sir J.
+C. Ross, and others, have established that a rise of one inch in the
+barometer will have a corresponding fall in the tide of about one foot.
+Therefore navigators and pilots will appreciate the following suggestion
+of Admiral FitzRoy:--
+
+"Vessels sometimes enter docks, or even harbours, where they have scarcely
+a foot of water more than their draught; and as docking, as well as
+launching large ships, requires a close calculation of height of water,
+the state of the barometer becomes of additional importance on such
+occasions."
+
+
+
+
+CHAPTER II.
+
+SYPHON TUBE BAROMETERS.
+
+
+=30. Principle of.=--If some mercury, or any other fluid, be poured into a
+tube of glass, bent in the form of =U=, and open at both ends, it will
+rise to the same height in both limbs, the tube being held vertically. If
+mercury be poured in first, and then water upon it at one end, these
+liquids will not come to the same level; the water will stand much higher
+than the mercury. If the height of the mercury, above the line of meeting
+of the fluids, be one inch, that of the water will be about
+thirteen-and-a-half inches. The explanation of this is, that the two
+columns balance each other. The pressure of the atmosphere in each limb is
+precisely similar; but the one column stands so much higher than the
+other, because the fluid of which it is composed is so much lighter, bulk
+for bulk, than the other. If one end of the tube be hermetically closed,
+the other limb be cut off within a few inches of the bend, and the tube
+carefully filled with mercury; by placing it in a vertical position, the
+mercury will fall, if the closed limb be long enough, until it is about
+thirty inches higher than that in the exposed limb, where it will remain.
+Here the atmosphere presses upon the short column; but not upon the long
+one. It is this pressure, therefore, which maintains the difference of
+level. In fact, it forms a barometer without a cistern, the short limb
+answering the purpose of a cistern. The first barometers on this principle
+were devised by the celebrated philosopher, Dr. Hook, as described in the
+next section.
+
+
+31. DIAL, OR WHEEL BAROMETERS.
+
+The familiar household "Weather Glasses" are barometers on the syphon
+principle. The portions of the two limbs through which the mercury will
+rise and fall with the varying pressure of the atmosphere are made of
+precisely the same diameter; while the part between them is contracted. On
+the mercury, in the exposed limb, rests a round float of ivory or glass;
+to this a string is attached and passed over and around a brass pulley,
+the other end carrying another lighter weight. The weight resting on the
+mercury rises and falls with it. On the spindle of the pulley, which
+passes through the frame and centre of the dial-plate, is fixed a light
+steel hand, which revolves as the pulley turns round. When the mercury
+falls for a decrease of atmospheric pressure, it rises by the same
+quantity in the short tube, and pushes up the float, the counterpoise
+falls, and thus moves the hand or pointer to the left. When the pressure
+increases, the pointer is drawn in a similar manner to the right.
+
+[Illustration: Fig. 18.]
+
+[Illustration: Fig. 19.]
+
+[Illustration: Fig. 20.]
+
+The dials are generally made of metal silvered over or enamelled, but
+porcelain may be used. If the circumference of the pulley, or "wheel," be
+two inches, it will revolve once for an alteration of level amounting to
+two inches in each tube, or four inches in the height of the barometric
+column; and as the dial may be from twenty to thirty-six inches in
+circumference, five to nine inches on the graduated scale corresponds to
+one inch of the column; and hence the sub-divisions are distinctly
+perceptible, and a vernier is not necessary.
+
+The motion of the pointer alone is visible; and a mahogany, or rosewood,
+frame, supports, covers, and renders the instrument ornamental and
+portable. In the back of the frame is a hinged door, which covers the
+cavity containing the tube and fixtures. The dial is covered by a glass in
+a brass rim, similar to a clock face. A brass index, working over the
+dial, moveable by a key or button, may be applied, and will serve to
+register the position of the hand when last observed. These instruments
+are usually fitted with a thermometer, and a spirit level; the latter for
+the purpose of getting the instrument perfectly vertical. They sometimes
+have, in addition, a hygrometer, a sympiesometer, an aneroid, a mirror, or
+a clock, &c., singly or combined. The frame admits of much variety of
+style and decoration. It may be carved or inlaid. The usual adjustment of
+scale is suited for localities at no considerable elevation above the
+sea. Accordingly, being commercial articles, they have been found
+frequently quite out of place. When intended for use at high elevations,
+they should have a special adjustment of scale. As household instruments
+they are serviceable, and ornamental. But the supply-and-demand principle
+upon which they are sold, has entailed upon those issued by inferior
+makers a generally bad adjustment of scale. The illustrations are those of
+ordinary designs.
+
+[Illustration: Fig. 21.]
+
+[Illustration: Fig. 22.]
+
+[Illustration: Fig. 23.]
+
+Dial barometers required for transmission to distant parts, as India and
+the Colonies, are furnished with a steel stop-cock, to render them
+portable more effectually than can be done by the method of _plugging_ the
+tube.
+
+
+32. STANDARD SYPHON BAROMETER.
+
+Fig. 24 represents the most accurate form of the Gay Lussac barometer. The
+short limb is closed at the top, after the mercury is introduced, and a
+small lateral puncture is made at _a_, which is covered over with a
+substance which permits the access of air, but prevents the escape of any
+mercury when the instrument is packed for travelling. The bent part of the
+tube is contracted to a capillary bore; and just above this, in the long
+limb, is placed the air-trap, already described (see p. 17), and here
+illustrated (fig. 25). When reversed, as it must be for portability, the
+capillary attraction keeps the mercury in the long branch. Should the
+mercury of the short column get detached, some small quantity of air _may_
+pass; but it will be arrested at the pipette, and will not vitiate the
+length of the barometric column. It can be easily expelled by gently
+shaking or tapping the instrument before suspending it for observation. In
+the illustration, the zero of the scale is placed at Z, near the middle of
+the tube; and the graduations extend above and below. In making an
+observation, it is necessary to take the reading ZA on the long branch,
+and ZB on the short one. The sum of the two gives the height of the
+barometer. The zero of the scale in some instruments is placed low down,
+so as to require the difference of the two readings to be taken. A
+thermometer is attached to the frame as usual.
+
+[Illustration: Fig. 24.]
+
+[Illustration: Fig. 25.]
+
+These instruments can be very accurately graduated, and are very exact in
+their indications, provided great care has been exercised in selecting the
+tubes, which must be of the same calibre throughout the parts destined to
+measure the variations of atmospheric pressure. They should be suspended
+so as to insure their hanging vertically.
+
+The syphon barometer does not require correction for capillarity nor for
+capacity, as each surface of the mercury is equally depressed by capillary
+attraction, and the quantity of mercury which falls from the long limb of
+the tube occupies the same length in the short one. The barometric height
+must, however, be corrected for temperature, as in the cistern barometer.
+Tables containing the temperature corrections to be applied to barometer
+readings for scales engraved on the glass tube, or on brass or wood
+frames, are published.
+
+
+
+
+CHAPTER III.
+
+BAROGRAPHS, OR SELF-REGISTERING BAROMETERS.
+
+
+=33. Milne's Self-Registering Barometer.=--For a long time a good and
+accurate self-recording barometer was much desired. This want is now
+satisfactorily supplied, not by one, but by several descriptions of
+apparatus. The one first to be described was the design of Admiral Sir A.
+Milne, who himself constructed, in 1857, we believe, the original
+instrument, which he used with much success. Since that time several of
+these instruments have been made, and have performed satisfactorily. The
+barometer tube is a syphon of large calibre, provided with a Gay Lussac
+pipette, or air-trap; and fitted with a float, a wheel, and a pointer, as
+in the "Dial" barometer. The float is attached to a delicate watch-chain,
+which passes over the wheel and is adequately counterpoised. Behind the
+indicating extremity of the pointer or hand is a projecting point, which
+faces the frame of the instrument, and is just within contact with the
+registering paper. A clock is applied, and fitted with auxiliary
+mechanism, so as to be able to move the mounted paper with regularity
+behind the pointer, and at designed equal intervals of time to release a
+system of levers and springs, so as to cause the marker to impress a dot
+on the paper, either by puncture or pencil-mark. The paper is ruled with
+horizontal lines for the range of the mercurial column, and parallel arcs
+of circles for the hours. Thus the barometer is rendered self-recording,
+by night or day, for a week or more; hence the great value of the
+instrument. The clock, index, and registering mechanism are protected from
+dust and interference by a glass front, hinged on and locked. As the
+temperature of the mercury is not registered, there is fixed to the frame
+a Sixe's thermometer to record the maximum and minimum temperatures, which
+should be noted at least every twenty-four hours.
+
+Admiral FitzRoy has suggested the name "Atmoscope" for Admiral Milne's
+barometer; and he has also termed it a "Barograph." This latter word
+appears to be applicable to all kinds of self-registering barometers
+hitherto designed. Of the arrangement under consideration Admiral FitzRoy
+writes:--"It shows the alterations in tension, or the pulsations, so to
+speak, of atmosphere, on a large scale, by hourly marks; and the diagram
+expresses, to a practised observer, what the 'indicator card' of a
+steam-cylinder shows to a skilful engineer, or a stethescope to a
+physician."
+
+
+[Illustration: Fig. 26.]
+
+=34. Modification of Milne's Barometer.=--The great difficulty to be
+overcome in Milne's barometer, is to adjust the mechanism for obtaining
+registration so that the action of the striker upon the pointer should not
+in the slightest degree move it from its true position. A different mode
+of registration, capable of recording accurately the least appreciable
+movement of the mercurial column, has been effected. In this instrument
+the registering paper is carried upon a cylinder or drum. By reference to
+the illustration, Fig. 26, the details of construction will be readily
+understood. It should, however, be mentioned, that it is not a picture of
+the outward appearance of the instrument. The position of the barometer
+should be behind the clock; it is represented on one side merely for the
+purpose of clearly illustrating the arrangement and principles. The
+instrument has a large syphon barometer tube, in which the mercurial
+column is represented. On the mercury at _A_, in its open end, rests a
+glass float, attached to a watch-chain, or suitable silken cord, the other
+end of which is connected to the top of the arched head on the short arm
+of a lever-beam. The long arm of the beam is twice the length of the short
+arm, for the following reason. As the mercury falls in the long limb, it
+rises through an equal space in the short limb of the tube, and _vice
+versa_. But the barometric column is the difference of height of the
+mercury in the two limbs; hence the rise or fall of the float through
+half-an-inch will correspond to a decrease or an increase of the
+barometric column of one inch. In order, then, to record the movements of
+the barometric column, and not those of the float, the arm of the beam
+connected with the float is only half the radius of the other arm. Both
+arms of the beam carry circular-arched heads, which are similar portions
+of the complete circles, the centre of curvature being the fulcrum, or
+axis. This contrivance maintains the leverage on each extremity of the
+beam always at the same distance from the fulcrum. From the top of the
+large arched head a piece of watch-chain descends, and is attached to the
+marker, _B_, which properly counterpoises the float, _A_, and is capable
+of easy movement along a groove in a brass bar, so as to indicate the
+barometric height on an ivory scale, _C_, fixed on the same vertical
+framing. On the opposite side of the marker, _B_, is formed a metallic
+point, which faces the registration sheet and is nearly in contact with
+it. The framing, which carries the scale and marker, is an arrangement of
+brass bars, delicately adjusted and controlled by springs, so as to permit
+of a quick horizontal motion, in a small arc, being communicated to it by
+the action of the hammer, _E_, of the clock, whereby the point of the
+marker is caused to impress a dot upon the paper. The same clock gives
+rotation to the hollow wooden cylinder, _D_, upon which is mounted the
+registering paper. The clock must be rewound when a fresh paper is
+attached to the cylinder, which may be daily, weekly, or monthly,
+according to construction; and the series of dots impressed upon the paper
+shows the height of the barometric column every hour by day and night. The
+space traversed by the marker is precisely equal to the range of the
+barometric column.
+
+
+=35. King's Self-Registering Barometer.=--Mr. Alfred King, Engineer of the
+Liverpool Gas-light Company, designed, so long ago as 1854, a barometer to
+register, by a continuous pencil-tracing, the variations in the weight of
+the atmosphere; and a highly-satisfactory self-recording barometer, on his
+principle and constructed under his immediate superintendence, has quite
+recently been erected at the Liverpool Observatory.
+
+[Illustration: Fig. 27.]
+
+Fig. 27 is the front elevation of this instrument. _A_, the barometer
+tube, is three inches in internal diameter, and it floats freely (not
+being fixed as usual) in the fixed cistern, _B_, guided by
+friction-wheels, _W_. The top end of the tube is fastened to a peculiar
+chain, which passes over a grooved wheel turning on finely-adjusted
+friction rollers. The other end of the chain supports the frame, _D_,
+which carries the tracing pencil. The frame is suitably weighted and
+guided, and faces the cylinder, _C_, around which the tracing paper is
+wrapped, and which rotates once in twenty-four hours by the movement of a
+clock. Mr. Hartnup, Director of the Liverpool Observatory, in his Annual
+Report, 1868, says:--"For one inch change in the mercurial column the
+pencil is moved through five inches, so that the horizontal lines on the
+tracing, which are half an inch apart, represent one-tenth of an inch
+change in the barometer. The vertical lines are hour lines, and being
+nearly three-quarters of an inch apart, it will be seen that the smallest
+appreciable change in the barometer, and the time of its occurrence, are
+recorded."
+
+"It has been remarked by persons in the habit of reading barometers with
+large tubes, that, in squally weather, sudden and frequent oscillations
+of the mercurial column are sometimes seen. Now, to register these small
+oscillations must be a very delicate test of the sensitiveness of a
+self-registering barometer, as the time occupied by the rise and fall of
+the mercury in the tube in some cases does not exceed one minute." Mr.
+Hartnup affirms that the tracing of this instrument exhibits such
+oscillations whenever the wind blows strong and in squalls.
+
+As the barometer in this instrument is precisely similar to the "Long
+Range Barometer" invented by Mr. McNeild (and which will be found
+described at page 48), it may be desirable to quote the following, from
+Mr. Hartnup's Report:--"Mr. King constructed a small model instrument to
+illustrate the principle. This instrument was entrusted to my care for
+examination, and it was exhibited to the scientific gentlemen who visited
+the Observatory in 1854, during the meeting of the British Association for
+the Advancement of Science."
+
+
+=36. Syphon, with Photographic Registration.=--A continuous
+self-registering barometer has been constructed, in which photography is
+employed. Those who may wish to adopt a similar apparatus, or thoroughly
+to understand the arrangements and mode of observation, should consult the
+detailed description given in the _Greenwich Magnetical and Meteorological
+Observations_, 1847. As the principles are applicable to photographic
+registration of magnetic and electric as well as meteorologic variations
+in instrumental indications, it would be beside our purpose to describe
+fully the apparatus.
+
+The barometer is a large syphon tube; the bore of the upper and lower
+extremities, through which the surfaces of the mercury rise and fall, is
+1-1/10 inch in diameter. The glass float in the open limb is attached to a
+wire, which moves a delicately-supported light lever as it alters its
+elevation. The fulcrum of the lever is on one side of the wire; the
+extremity on the other side, at four times this distance from the fulcrum,
+carries a vertical plate of opaque mica, having a small aperture. Through
+this hole the light of a gas-jet shines upon photographic paper wrapped
+round a cylinder placed vertically, and moved round its axis by a clock
+fixed with its face horizontal. The cylinder is delicately supported, and
+revolves in friction rollers. A bent wire on the axis is embraced by a
+prong on the hour hand of the time-piece; therefore the cylinder is
+carried round once in twelve hours. It might be arranged for a different
+period of rotation.
+
+As the cylinder rotates, the paper receives the action of the light, and a
+photographic trace is left of the movements of the barometer four times
+the extent of the oscillations of the float, or twice the length of the
+variations in the barometric column. Certain chemical processes are
+required in the preparation of the paper, and in developing the trace. The
+diagram which we give on the next page, with the explanation, taken from
+Drew's _Practical Meteorology_, will enable the above description to be
+better understood:
+
+[Illustration: Fig. 28.]
+
+"_Q e_ is a lever whose fulcrum is _e_, the counterpoise _f_ nearly
+supporting it; _s_ is an opaque plate of mica, with a small aperture at
+_p_, through which the light passes, having before been refracted by a
+cylindrical lens into a long ray, the portion only of which opposite the
+aperture _p_ impinges on the paper; _d_ is a wire supported by a float on
+the surface of the mercury; _G H_ is the barometer; _p_, the vertical
+cylinder charged with photographic paper; _r_, the photographic trace;
+_I_, the timepiece, carrying round the cylinder by the projecting arm _t_.
+It is evident that the respective distances of the float and the aperture
+_p_ from the fulcrum may be regulated so that the rise and fall of the
+float may be multiplied to any extent required." When _only_ the lower
+surface of the mercury in a syphon barometer is read, as in the instrument
+just described, a correction for temperature is strictly due to the height
+of the quicksilver in the _short_ tube; but this in so short a column will
+rarely be sensible.
+
+
+
+
+CHAPTER IV.
+
+MOUNTAIN BAROMETERS.
+
+
+=37. The Syphon Tube Mountain Barometer, on Gay Lussac's principle=,
+constructed as described at page 31, and fixed in a metallic tubular
+frame, forms a simple and light travelling instrument. The graduations are
+made upon the frame, and it is suspended for reading by a ring at the top,
+from beneath an iron tripod stand, which is usually supplied with it.
+Considerable care is requisite in adjusting the verniers, so as to keep
+the instrument steady and vertical. A drawback to the convenience of this
+barometer is the movement of the mercury in the short limb, which is
+generally not confined, and hence has every facility for becoming quickly
+oxidised in travelling. To remedy this, Messrs. Negretti and Zambra so
+construct the Mountain Syphon Barometer that by a simple half turn of a
+screw the mercury can be confined for portability, while the lower limb
+can be taken out for cleaning whenever found requisite.
+
+
+=38. Mountain Barometer on Fortin's principle.=--This barometer, with
+Fortin's cistern, as arranged by Messrs. Negretti and Zambra, is an
+elegant, manageable, and very accurate instrument for travelling purposes,
+and well adapted for careful measurement of heights. The cistern is made
+large enough to receive all the mercury that will fall from the tube at
+the highest attainable elevation. The screw at the bottom confines the
+mercury securely for carriage, and serves to adjust the surface of the
+mercury to the zero of the scale when making an observation. The vernier
+reads to .002 of an inch, and slides easily on the brass frame, which is
+made as small in diameter as is compatible with the size of the tube. The
+tube in this barometer should be altogether without contractions, so that
+the mercury will readily fall when it is set up for observation. It must
+be carefully calibrated, and its internal diameter ascertained, in order
+that correction may be made for capillarity. This correction, however,
+should be combined with the error of graduation, and form a permanent
+index error, ascertainable at any time by comparison with an acknowledged
+standard barometer.
+
+The barometer is supported in the tripod stand (furnished as part of the
+instrument) when used for observation. It is suspended by placing two
+studs, in the ring on the frame, in slots formed on the top of the stand,
+so that it hangs freely and vertically in gimbals. To the metal top of
+the stand, mahogany legs are hinged. To make the barometer portable, it
+must be lifted out of the stand, sloped gently until the mercury reaches
+the top, turning the screw at the bottom meanwhile; then invert and screw
+until the mercury is made tight. The inverted instrument packs in the
+stand, the legs being formed to fit round the frame; and receptacles are
+scooped out for the cistern, thermometer, gimbals, and vernier; so that
+the instrument is firmly surrounded by the wooden legs, which are held
+fast together by brass rings passed over them.
+
+
+[Illustration: Fig. 29.]
+
+=39. Newman's Mountain Barometer.=--Fig. 29 is an illustration of the
+mountain barometer known as Newman's. The cistern consists of two separate
+compartments;--the top of the lower and the bottom of the upper, being
+perfectly flat, are pivoted closely together at the centres, so that the
+lower can move through a small arc, when turned by the hand. This movement
+is limited by two stops. The top of the lower compartment and the bottom
+of the upper have each a circular hole, through which the mercury
+communicates. When the instrument is required for observation, the cistern
+is turned close up to the stop marked "_open_" or "_not portable_." When
+it is necessary to pack it for travelling, the mercurial column must be
+allowed to fill the tube by sloping the barometer gently; then invert it,
+and move the cistern to the stop marked "_shut_" or "_portable_." In this
+condition, the upper compartment is completely filled with mercury, and
+consequently that in the tube cannot move about, so as to admit air or
+endanger the tube. Nor can the mercury pass back to the lower compartment,
+as the holes are not now coincident, and the contact is made too perfect
+to allow the mercury to creep between the surfaces. The tube does not
+enter the lower compartment, which is completely full of mercury when the
+instrument is arranged for observation. The spare capacity of the upper
+cistern is sufficient to receive the mercury which descends from the tube
+to the limit of the engraved scale, which in these barometers generally
+extends only to about 20 inches. A lower limit could of course be given by
+increasing the size of the cisterns, which it is not advisable to do
+unless for a special purpose. This barometer may be had mounted in wood,
+or in brass frame. If in wood, it has a brass shield, which slides round
+the scale part of the frame, so as to be easily brought in front of the
+tube and scale as a protection in travelling; the vernier screw, in this
+case, being placed at the top of the instrument. When the scale is
+graduated with true inches, the neutral point, the capacity and
+capillarity corrections should be marked on the frame. The graduated
+scales, however, placed on these barometers in brass frames, are usually
+artificial inches, like the Kew plan of graduation; the advantage being
+that one simple correction only is required, viz. one for index error and
+capillarity combined, which can always be readily determined by comparison
+with a standard barometer; moreover, as no adjustment of cistern is
+required in reading, the instrument can be verified by artificial pressure
+throughout the scale, by the plan practised at Kew, Liverpool, &c., and
+already described (see p. 18).
+
+
+40. NEGRETTI & ZAMBRA'S PATENT MOUNTAIN AND OTHER BAROMETERS.
+
+This invention is intended to make mountain and other barometers of
+standard accuracy stronger, more portable, and less liable to derangement,
+when being carried about, than heretofore, by dispensing with the ordinary
+flexible cistern containing the mercury at the bottom of the instrument,
+and adapting in lieu thereof a rigid cistern constructed of glass and
+iron. The cistern is composed of a glass cylinder, which is secured in a
+metallic tube or frame. In order to render the cistern mercury-tight at
+top and bottom, metal caps are screwed into the tube or frame, and bear
+against leather washers placed between them and the edges of the glass
+cylinder. The upper cap of the cistern is tapped with a fine threaded
+screw to receive the iron plug or socket, into which the barometer tube is
+securely fixed. The whole length of this plug has a fine screw cut upon it
+by which the cistern can be screwed up or down. At the side of this plug
+or socket, extending from the lower end to within half an inch of the top,
+is cut a groove for admitting the air to the surface of the mercury within
+the cistern when the barometer is in use. An ivory point is screwed into
+the under surface of the plug, carrying the barometer tube. This ivory
+point is very carefully adjusted by measurement to be the zero point of
+the instrument, from which the barometer scale of inches is divided. The
+surface of the mercury in the cistern is adjusted to the zero point by
+screwing the cistern up or down until the ivory point and its reflected
+image are in contact.
+
+[Illustration: Fig. 30.]
+
+The instrument (fig. 30) is shown in a state of adjustment, ready to take
+an observation; but _when it is desired to render it portable, it must be
+inclined, until mercury from the cistern fills the tube; the cistern must
+then be screwed up on the socket_, so as to bring the face of the upper
+cap against the under side of the shoulder of the cover immediately above
+it; the instrument may then be carried without being liable to
+derangement.
+
+_Precautions necessary in using the Mountain Barometer._--On removing the
+barometer from its case after a journey, allow it to remain with its scale
+end downward, whilst the cistern is unscrewed to the extent of _one turn
+of the screw_, after which slightly shake the cistern; the mercury in it
+will then completely fill the end of the barometer tube, should any
+portion of it have escaped therefrom.
+
+The barometer is then inverted, and if it be desired to make an
+observation, suspend it vertically from its stand by the ring at top. The
+cistern must then be unscrewed, until the surface of the mercury is
+brought just level with the extreme end of the ivory or zero point fixed
+to the iron plug on which the glass cistern moves up and down.
+
+Should the elevation of the place where the barometer is to be used be
+considerably above the sea level, it will be well--after suspending it
+from the stand--to unscrew the cistern several turns, _holding the
+barometer in an oblique position_, as at great heights the mercury will
+fall considerably quicker than the cistern can be unscrewed, thereby
+filling it to overflowing; but by partly unscrewing the cistern first,
+room is given for the reception of a fall of mercury to the extent of
+several inches.
+
+The cistern must not be unscrewed when the _Instrument is_ INVERTED _more
+than_ two turns of the screw, otherwise the mercury will flow out through
+the groove.
+
+It is found safer when travelling to carry the barometer in a horizontal
+position, or with its cistern end uppermost.
+
+_To clean the Barometer._--Should at any time the mercury in the cistern
+become oxidised, and reading from its surface be difficult, it can be
+readily cleaned by removing the cistern and its contained mercury from the
+barometer frame by unscrewing it _when in a horizontal position_; this
+precaution is necessary that the mercury in the tube may not escape, and
+thereby allow air to enter. The cistern must then be emptied, and with a
+dry clean leather, or silk handkerchief, well cleaned.
+
+The operation of cleaning being performed, return the cistern to the
+frame, and screw it until the face is brought up against the under side of
+the shoulder, still keeping the instrument _horizontal_. The cistern is
+now ready for re-filling, to do which stand the barometer on end _head
+downwards_, and remove the small screw at bottom; through the aperture
+thus opened, pour in mercury, passing it through a paper funnel with a
+very small aperture. It is well to pass the mercury through a very small
+funnel two or three times before returning it to the barometer cistern, as
+by this process all particles of dust or oxide adhere to the paper, and
+are effectually removed.
+
+Should any small quantity of the mercury be lost during the operation of
+cleaning, it is of no importance so long as sufficient remains to allow of
+adjustment to the zero point. This latter constitutes one of the great
+advantages of this new instrument over the ordinary barometer; for, in the
+majority of cases, after an instrument has been compared carefully with a
+standard, should mercury be lost, there is no means of correcting the
+error unless a standard barometer be at hand; the new barometer is, in
+this respect, independent, a little mercury more or less being
+unimportant.
+
+
+=41. Short Tube Barometer.=--This is simply a tube shorter, as may be
+required, than that necessary to show the atmospheric pressure at the sea
+level. It is convenient for balloon purposes, and for use at mountain
+stations, being of course a special construction.
+
+
+=42. Method of Calculating Heights by the Barometer.=--The pressure of the
+atmosphere being measured by the barometer, it is evident that as the
+instrument is carried up a high mountain or elevated in a balloon, the
+length of the column must decrease as the atmospheric pressure decreases,
+in consequence of a stratum of air being left below. The pressure of air
+arises from its weight, or the attraction of gravitation upon it, and
+therefore the quantity of air below the barometer cistern cannot influence
+the height of the column. Hence it follows that a certain relation must
+exist between the difference of the barometric pressure at the foot and at
+the top of a hill or other elevation, and the difference of the absolute
+heights above the sea. Theoretical investigation, abundantly confirmed by
+practical results, has determined that the strata of air decrease in
+density in a geometrical proportion, while the elevations increase in an
+arithmetical one. Hence we have a method of determining differences of
+level, by observations made on the density of the air by means of the
+barometer. It is beyond our purpose to explain in detail the principles
+upon which this method is founded, or to give its mathematical
+investigation. We append Tables, which will be useful to practical
+persons,--surveyors, engineers, travellers, tourists, &c.,--who may carry
+a barometer as a travelling companion.
+
+Table I. is calculated from the formula, height in feet = 60,200 (log.
+29.922 - log. B) + 925; where 29.922 is the mean atmospheric pressure at
+32 deg. F., and the mean sea-level in latitude 45 deg.; and B is any other
+barometric pressure; the 925 being added to avoid minus signs in the
+Table.
+
+Table II. contains the correction necessary for the mean temperature of
+the stratum of air between the stations of observation; and is computed
+from Regnault's co-efficient for the expansion of air, which is .002036 of
+its volume at 32 deg. for each degree above that temperature.
+
+Table III. is the correction due to the difference of gravitation in any
+other latitude, and is found from the formula, _x_ = 1 + .00265 cos. 2
+lat.
+
+Table IV. is to correct for the diminution of gravity in ascending from
+the sea-level.
+
+To use these Tables: The barometer readings at the upper and lower
+stations having been corrected and reduced to temperature 32 deg. F., take out
+from Table I. the numbers opposite the corrected readings, and subtract
+the lower from the upper. Multiply this difference successively by the
+factors found in Tables II. and III. The factor from Table III. may be
+neglected unless precision is desired. Finally, add the correction taken
+from Table IV.
+
+TABLE I.
+
+_Approximate Height due to Barometric Pressure._
+
+  +----------------------------------------------+
+  |Inches.| Feet.||Inches.| Feet.||Inches.| Feet.|
+  |-------+------++-------+------++-------+------|
+  |  31.0 |    0 ||  28.2 | 2475 ||  25.4 | 5209 |
+  |  30.9 |   84 ||    .1 | 2568 ||    .3 | 5312 |
+  |    .8 |  169 ||  28.0 | 2661 ||    .2 | 5415 |
+  |    .7 |  254 ||  27.9 | 2754 ||    .1 | 5519 |
+  |    .6 |  339 ||    .8 | 2848 ||  25.0 | 5623 |
+  |    .5 |  425 ||    .7 | 2942 ||  24.9 | 5728 |
+  |    .4 |  511 ||    .6 | 3037 ||    .8 | 5833 |
+  |    .3 |  597 ||    .5 | 3132 ||    .7 | 5939 |
+  |    .2 |  683 ||    .4 | 3227 ||    .6 | 6045 |
+  |    .1 |  770 ||    .3 | 3323 ||    .5 | 6152 |
+  |  30.0 |  857 ||    .2 | 3419 ||    .4 | 6259 |
+  |  29.9 |  944 ||    .1 | 3515 ||    .3 | 6366 |
+  |    .8 | 1032 ||  27.0 | 3612 ||    .2 | 6474 |
+  |    .7 | 1120 ||  26.9 | 3709 ||    .1 | 6582 |
+  |    .6 | 1208 ||    .8 | 3806 ||  24.0 | 6691 |
+  |    .5 | 1296 ||    .7 | 3904 ||  23.9 | 6800 |
+  |    .4 | 1385 ||    .6 | 4002 ||    .8 | 6910 |
+  |    .3 | 1474 ||    .5 | 4100 ||    .7 | 7020 |
+  |    .2 | 1563 ||    .4 | 4199 ||    .6 | 7131 |
+  |    .1 | 1653 ||    .3 | 4298 ||    .5 | 7242 |
+  |  29.0 | 1743 ||    .2 | 4398 ||    .4 | 7353 |
+  |  28.9 | 1833 ||    .1 | 4498 ||    .3 | 7465 |
+  |    .8 | 1924 ||  26.0 | 4598 ||    .2 | 7577 |
+  |    .7 | 2015 ||  25.9 | 4699 ||    .1 | 7690 |
+  |    .6 | 2106 ||    .8 | 4800 ||  23.0 | 7803 |
+  |    .5 | 2198 ||    .7 | 4902 ||  22.9 | 7917 |
+  |    .4 | 2290 ||    .6 | 5004 ||    .8 | 8032 |
+  |    .3 | 2382 ||    .5 | 5106 ||    .7 | 8147 |
+  +----------------------------------------------+
+
+TABLE I.--_continued_.
+
+_Approximate Height due to Barometric Pressure._
+
+  +-------------------------------------------------+
+  |Inches.| Feet. ||Inches.| Feet. ||Inches.| Feet. |
+  |-------+-------++-------+-------++-------+-------|
+  | 22.6  |  8262 ||  18.9 | 12937 ||  15.2 | 18632 |
+  |   .5  |  8378 ||    .8 | 13076 ||    .1 | 18805 |
+  |   .4  |  8495 ||    .7 | 13215 ||  15.0 | 18979 |
+  |   .3  |  8612 ||    .6 | 13355 ||  14.9 | 19154 |
+  |   .2  |  8729 ||    .5 | 13496 ||    .8 | 19330 |
+  |   .1  |  8847 ||    .4 | 13638 ||    .7 | 19507 |
+  | 22.0  |  8966 ||    .3 | 13780 ||    .6 | 19685 |
+  | 21.9  |  9085 ||    .2 | 13923 ||    .5 | 19865 |
+  |   .8  |  9205 ||    .1 | 14067 ||    .4 | 20046 |
+  |   .7  |  9325 ||  18.0 | 14212 ||    .3 | 20228 |
+  |   .6  |  9446 ||  17.9 | 14358 ||    .2 | 20412 |
+  |   .5  |  9567 ||    .8 | 14505 ||    .1 | 20597 |
+  |   .4  |  9689 ||    .7 | 14652 ||  14.0 | 20783 |
+  |   .3  |  9811 ||    .6 | 14800 ||  13.9 | 20970 |
+  |   .2  |  9934 ||    .5 | 14949 ||    .8 | 21159 |
+  |   .1  | 10058 ||    .4 | 15099 ||    .7 | 21349 |
+  | 21.0  | 10182 ||    .3 | 15250 ||    .6 | 21541 |
+  | 20.9  | 10307 ||    .2 | 15402 ||    .5 | 21734 |
+  |   .8  | 10432 ||    .1 | 15554 ||    .4 | 21928 |
+  |   .7  | 10558 ||  17.0 | 15707 ||    .3 | 22124 |
+  |   .6  | 10684 ||  16.9 | 15861 ||    .2 | 22321 |
+  |   .5  | 10812 ||    .8 | 16016 ||    .1 | 22520 |
+  |   .4  | 10940 ||    .7 | 16172 ||  13.0 | 22720 |
+  |   .3  | 11069 ||    .6 | 16329 ||  12.9 | 22922 |
+  |   .2  | 11198 ||    .5 | 16487 ||    .8 | 23126 |
+  |   .1  | 11328 ||    .4 | 16646 ||    .7 | 23331 |
+  | 20.0  | 11458 ||    .3 | 16806 ||    .6 | 23538 |
+  | 19.9  | 11589 ||    .2 | 16967 ||    .5 | 23746 |
+  |   .8  | 11721 ||    .1 | 17129 ||    .4 | 23956 |
+  |   .7  | 11853 ||  16.0 | 17292 ||    .3 | 24168 |
+  |   .6  | 11986 ||  15.9 | 17456 ||    .2 | 24381 |
+  |   .5  | 12120 ||    .8 | 17621 ||    .1 | 24596 |
+  |   .4  | 12254 ||    .7 | 17787 ||  12.0 | 24813 |
+  |   .3  | 12389 ||    .6 | 17954 ||  11.9 | 25032 |
+  |   .2  | 12525 ||    .5 | 18122 ||    .8 | 25253 |
+  |   .1  | 12662 ||    .4 | 18291 ||    .7 | 25476 |
+  | 19.0  | 12799 ||    .3 | 18461 ||    .6 | 25700 |
+  +-------------------------------------------------+
+
+TABLE II.
+
+_Correction due to Mean Temperature of the Air._
+
+  +-------------------------------------------+
+  |Mean |Factor.||Mean |Factor.||Mean |Factor.|
+  |Temp.|       ||Temp.|       ||Temp.|       |
+  |-----+-------++-----+-------++-----+-------|
+  | 10 deg. | 0.955 || 35 deg. | 1.006 || 60 deg. | 1.057 |
+  | 11  |  .957 || 36  | 1.008 || 61  | 1.059 |
+  | 12  |  .959 || 37  | 1.010 || 62  | 1.061 |
+  | 13  |  .961 || 38  | 1.012 || 63  | 1.063 |
+  | 14  |  .963 || 39  | 1.014 || 64  | 1.065 |
+  | 15  |  .965 || 40  | 1.016 || 65  | 1.067 |
+  | 16  |  .967 || 41  | 1.018 || 66  | 1.069 |
+  | 17  |  .969 || 42  | 1.020 || 67  | 1.071 |
+  | 18  |  .971 || 43  | 1.022 || 68  | 1.073 |
+  | 19  |  .974 || 44  | 1.024 || 69  | 1.075 |
+  | 20  |  .976 || 45  | 1.026 || 70  | 1.077 |
+  | 21  |  .978 || 46  | 1.029 || 71  | 1.079 |
+  | 22  |  .980 || 47  | 1.031 || 72  | 1.081 |
+  | 23  |  .982 || 48  | 1.033 || 73  | 1.083 |
+  | 24  |  .984 || 49  | 1.035 || 74  | 1.086 |
+  | 25  |  .986 || 50  | 1.037 || 75  | 1.088 |
+  | 26  |  .988 || 51  | 1.039 || 76  | 1.090 |
+  | 27  |  .990 || 52  | 1.041 || 77  | 1.092 |
+  | 28  |  .992 || 53  | 1.043 || 78  | 1.094 |
+  | 29  |  .994 || 54  | 1.045 || 79  | 1.096 |
+  | 30  |  .996 || 55  | 1.047 || 80  | 1.098 |
+  | 31  | 0.998 || 56  | 1.049 || 81  | 1.100 |
+  | 32  | 1.000 || 57  | 1.051 || 82  | 1.102 |
+  | 33  | 1.002 || 58  | 1.053 || 83  | 1.104 |
+  | 34  | 1.004 || 59  | 1.055 || 84  | 1.106 |
+  +-------------------------------------------+
+
+TABLE III.
+
+  +-------------------------------------------------------+
+  |Latitude.|Factor.||Latitude.|Factor.||Latitude.|Factor.|
+  |---------+-------++---------+-------++---------+-------|
+  |   80 deg.   |0.99751||   50    |0.99954||   20    |1.00203|
+  |   75    |0.99770||   45    |1.00000||   15    |1.00230|
+  |   70    |0.99797||   40    |1.00046||   10    |1.00249|
+  |   65    |0.99830||   35    |1.00090||    5    |1.00261|
+  |   60    |0.99868||   30    |1.00132||    0    |1.00265|
+  |   55    |0.99910||   25    |1.00170||         |       |
+  +-------------------------------------------------------+
+
+TABLE IV.
+
+  +----------------------------------------------------+
+  |  Height in   |Correction||  Height in   |Correction|
+  |Thousand Feet.| Additive.||Thousand Feet.| Additive.|
+  |--------------+----------++--------------+----------|
+  |       1      |     3    ||      14      |    44    |
+  |       2      |     5    ||      15      |    48    |
+  |       3      |     8    ||      16      |    52    |
+  |       4      |    11    ||      17      |    56    |
+  |       5      |    14    ||      18      |    60    |
+  |       6      |    17    ||      19      |    65    |
+  |       7      |    20    ||      20      |    69    |
+  |       8      |    23    ||      21      |    74    |
+  |       9      |    26    ||      22      |    78    |
+  |      10      |    30    ||      23      |    83    |
+  |      11      |    33    ||      24      |    88    |
+  |      12      |    37    ||      25      |    93    |
+  |      13      |    41    ||      26      |    98    |
+  +----------------------------------------------------+
+
+EXAMPLE 1. On October 21st, 1852, when Mr. Welsh ascended in a balloon, at
+3h. 30m. p.m., the barometer, corrected and reduced, was 18.85, the air
+temperature 27 deg., while at Greenwich, 159 feet above the sea, the barometer
+at the same time was 29.97 inches, air temperature 49 deg., the balloon not
+being more than 5 miles S.W. from over Greenwich; required its elevation.
+
+                                            Feet.
+
+  Barometer in Balloon    18.85, Table I. = 13007
+      "     at Greenwich  29.97     "         883
+                                            -----
+                                            12124
+  Mean Temperature, 38 deg., Table II. Factor   1.012
+                                            -----
+                                            12269.
+                                            -----
+  Latitude 51-1/2 deg., Factor from Table III. .99941
+                                            -----
+                                            12262
+  Correction from Table IV.                    38
+                                            -----
+                                            12300
+  Elevation of Greenwich                      159
+                                            -----
+      "        Balloon                      12459 feet.
+                                            =====
+
+The following examples, from the balloon ascents of J. Glashier, Esq.,
+F.R.S., will serve for practice.[4]
+
+2. Ascended from Wolverhampton, 18th August, 1862, at 2h. 38m. p.m.;
+barometer (in all cases corrected and reduced to 32 deg. F) was 14.868, the
+temperature of the air 26 deg.; at the same time, at Wrottesley Hall, 531 feet
+above the sea, in latitude 52-1/2 deg. N, the barometer was 29.46, and the
+temperature of the air 65 deg.4; find the elevation of the balloon above the
+sea.
+
+  Height, 18,959 feet.
+
+3. From the same place an ascent was made 5th September, 1862, when at 1h.
+48m. p.m. barometer was 11.954, air O deg.; at Wrottesley Hall 29.38, air 56 deg.
+
+  Height, 23,923 feet.
+
+4. From the Crystal Palace a balloon ascent was made 20th August, 1862. At
+6h. 47m. p.m. barometer was 25.55, air 50 deg.5; and at the same time at
+Greenwich Observatory, at 159 feet above the sea, the barometer was 29.81,
+air 63 deg.
+
+  Height, 4,406 feet.
+
+5. From the same place an ascent was made 8th September, 1862. At 5 p.m.,
+the balloon being over Blackheath, barometer was 25.60, and the air 49 deg.5,
+while at Greenwich, barometer was 29.92, air 66 deg.4.
+
+  Height, 4,461 feet.
+
+
+
+
+CHAPTER V.
+
+SECONDARY BAROMETERS.
+
+
+=43. Desirability of Magnifying the Barometer Range.=--The limits within
+which the ordinary barometric column oscillates, do not exceed four inches
+for extreme range, while the ordinary range is confined to about two
+inches; hence it has often been felt that the public utility of the
+instrument would be greatly enhanced if by any means the scale indications
+could be increased in length. This object was sought to be obtained by
+bending the upper part of the tube from the vertical, so that the inches
+on the scale could be increased in length in proportion to the secant of
+the angle it made with the vertical. This was called "the diagonal
+barometer." The upper part of the tube has also been formed into a spiral,
+and the scale, placed along it, is thus greatly enlarged.
+
+But these methods of enlarging the indications cannot be so successfully
+accomplished, nor so cheaply nor so elegantly, as is done by the principle
+employed in the dial barometer. Hence they are not in use.
+
+
+[Illustration: Fig. 31.]
+
+=44. Howson's Long Range Barometer.=--Very recently quite a novel design
+has been patented by Mr. Howson, for a long range barometer. The
+construction requires neither distortion of the tube, nor mechanism for
+converting a short scale into a long one; but the mercury itself rises and
+falls, through an extended range, naturally, and in simple obedience to
+the varying pressure of the atmosphere. The tube is fixed, but its cistern
+is sustained by the mere pressure of the atmosphere. Looking at the
+instrument, it seems a perfect marvel. It appears as though the cistern
+with the mercury in it must fall to the ground. The bore of the tube is
+wide, about an inch across. A long glass rod is fixed to the bottom of the
+glass cistern, where a piece of cork or some elastic substance is also
+placed. The tube is filled with mercury; the glass rod is plunged into the
+tube as it is held top downwards, until the cork gets close up to the tube
+and fits tightly against it. The pressure against the cork simply prevents
+the mercury from coming out while the instrument is being inverted. When
+it is inverted, the mercury partly falls, and forms an ordinary barometric
+column. When the top is held, the cistern and glass rod, instead of
+falling away, remain perfectly suspended. There is no material support to
+the cistern; the tube only is fixed, the cistern hangs to it. Glass is
+many times lighter than mercury. When the glass rod is introduced, it
+displaces an equal volume of mercury. The glass rod, being so much lighter
+than mercury, floats and sustains the additional weight of the cistern by
+its buoyancy. In the mean time, the atmosphere is acting upon the mercury,
+keeping up the ordinary barometric column. Supposing there is a rise in
+the ordinary barometer, the atmosphere presses some more mercury up the
+tube. This mercury is taken out of the cistern, which of course becomes
+lighter, and therefore the rod and cistern float up a little higher, which
+thus causes the column of mercury to rise still more. The increased
+pressure and buoyancy thus acting together, increase the ascent in the
+barometric column, as shown by the fixed scale. One inch in the barometer
+might be represented by two or more inches in this instrument, according
+to construction. Supposing there was a decrease of pressure, the mercury
+would fall, come into the cistern, make it heavier, and increase the fall
+somewhat. Friction guides, at the top of the rod, prevent it coming into
+contact with the side of the tube when vertically suspended. The
+illustration, Fig. 31, shows the appearance of the instrument as framed in
+wood by the makers, Messrs. Negretti and Zambra.
+
+
+=45. McNeild's Long Range Barometer.=--A barometer designed by a gentleman
+named McNeild is on a directly opposite principle to the one just
+described. The tube is made to float on the mercury in the cistern. It is
+filled with mercury, inverted in the usual manner, then allowed to float,
+being held vertically by glass friction points or guides. By this
+contrivance, the ordinary range of the barometer is greatly increased. One
+inch rise or fall in the standard barometer may be represented by four or
+five inches in this instrument, so that it shows small variations in
+atmospheric pressure very distinctly. As the mercury falls in the tube
+with a decrease of pressure, the surface of the mercury in the cistern
+rises, and the floating tube rises also, which causes an additional
+descent in the column, as shown by fixed graduations on the tube. With an
+increase of pressure, some mercury will leave the cistern and rise in the
+tube, while the tube itself will fall, and so cause an additional ascent
+of mercury. This barometer is identical in principle with King's Barograph
+(see p. 34).
+
+The construction of Howson's and McNeild's Barometers has been assigned to
+Messrs. Negretti and Zambra. These instruments are usually made for
+domestic purposes with a scale of from three to five, and for public use
+from five to eight times the scale of the ordinary standard. Their
+sensitiveness is consequently increased in an equal proportion, and they
+have the additional advantage of not being affected by differences of
+level in the cistern. However, these novelties have not been sufficiently
+tried to determine their practical value for strictly scientific
+purposes; but as weather-glasses, for showing minute changes, they are
+superior to the common barometer.
+
+
+=46. The Water-glass Barometer.=--If a Florence flask, having a long neck,
+have a small quantity of water poured into it, and then be inverted and so
+supported that the open end dips into a vessel containing water, a small
+column of water will be confined in the neck of the bottle, the pressure
+of which, upon the surface of the exposed water, will be equal to the
+difference between the atmospheric pressure and the elasticity of the
+confined air in the body of the bottle. As the pressure of the atmosphere
+varies, this column will alter in height. But the elasticity of the
+confined air is also subject to variations, owing to changes of
+temperature. It follows, then, that the oscillations of the column are
+dependent on alterations of temperature and atmospheric pressure. Such an
+arrangement has been called "the Water-glass Barometer," and bears about
+the same relative value to the mercurial barometer, as an exponent of
+weather changes, that a cat-gut hygrometer bears to a thermometric
+hygrometer, as an indicator of relative moisture.
+
+
+47. SYMPIESOMETER.
+
+Nevertheless the instrument now about to be described, depending upon
+similar principles, but scientifically constructed and graduated, is a
+very useful and valuable substitute for the mercurial barometer. It
+consists of a glass tube, varying, according to the purposes for which the
+instrument is required, from six to twenty-four inches in length. The
+upper end is closed, and formed into a bulb; the lower is turned up,
+formed into a cistern, and open at top, through a pipette, or cone. A
+plug, moveable by a catch from below, can be made to close this opening,
+so as to render the instrument portable.
+
+[Illustration: Fig. 32.]
+
+The upper portion of the tube is filled with air; the lower portion, and
+part of the cistern, with sulphuric acid, coloured so as to render it
+plainly visible. Formerly, hydrogen and oil were used. It was found,
+however, that, by the process known to chemists as _osmosis_, this light
+gas in time partially escaped, and the remainder became mixed with air,
+the consequence being that the graduations were no longer correct. They
+are more durable as at present constructed. The liquid rises and falls in
+the tube with the variations of atmospheric pressure and temperature
+acting together. If the pressure were constant, the confined air would
+expand and contract for temperature only, and the instrument would act as
+a thermometer. In fact, the instrument is regarded as such in the
+manufacture; and the thermometric scales are ascertained and engraved on
+the scale. A good mercurial thermometer is also mounted on the same frame.
+If, therefore, at any time the mercurial and the air thermometers do not
+read alike, it must evidently be due to the atmospheric pressure acting
+upon the air in the tube; and it is further evident that, under these
+circumstances, the position of the top of the liquid may be marked to
+represent the barometric pressure at the time. In this manner a scale of
+pressure is ascertained by comparison with a standard barometer, extending
+generally from 27 to 31 inches.
+
+When made correctly, these instruments agree well with the mercurial
+barometer for a number of years, and their subsequent adjustment is not a
+matter of much expense.
+
+For use at sea, the liquid column is contracted at the bend. The
+sympiesometer is very sensitive, and feels the alterations in the
+atmospheric pressure sooner than the ordinary marine barometer.
+
+The scale is usually on silvered brass, mounted on a mahogany or rosewood
+frame, protected in front by plate glass. It is generally furnished with a
+revolving register, to record the observation, in order that it may be
+known whether the pressure has increased or decreased in the interval of
+observation.
+
+Small pocket sympiesometers are sometimes fitted with ivory scales, and
+protected by a neat velvet-lined pasteboard or morocco case.
+
+_How to take an Observation._--In practice, the indications of the
+atmospheric pressure are obtained from the sympiesometer by noting, first,
+the temperature of the mercurial thermometer; secondly, adjusting the
+pointer of the pressure scale to the same degree of temperature on the
+scale of the air column; thirdly, reading the height of the liquid on the
+sliding scale.
+
+_Directions for Use._--The sympiesometer should be carried and handled so
+as to keep the top always upwards, to prevent the air mechanically mixing
+with the liquid. Care should also be taken to screen it from casual rays
+of the sun or cabin fire.
+
+
+48. ANEROIDS.
+
+The beautiful and highly ingenious instrument called by the name
+_Aneroid_, is no less remarkable for the scientific principles of its
+construction and action, than for the nicety of its mechanism. It is a
+substitute, and perhaps the best of all substitutes, for the mercurial
+barometer. As its name implies, it is constructed "without fluid." It was
+invented by M. Vidi of Paris. In the general form in which it is made it
+consists of a brass cylindrical case about four inches in diameter and one
+and a half inch deep, faced with a dial graduated and marked similarly to
+the dial-plate of a "wheel-barometer," upon which the index or pointer
+shows the atmospheric pressure in inches and decimals of an inch in
+accordance with the mercurial barometer. Within the case, for ordinary
+sizes, is placed a flat metal box, generally not more than half an inch
+thick and about two inches or a little more in diameter, from which nearly
+all the air is exhausted. The top and bottom of this box is corrugated in
+concentric circles, so as to yield inwardly to external pressure, and
+return when the pressure is removed. The pressure of the atmosphere,
+acting externally, continually changes, while the elastic pressure of the
+small quantity of air within can only vary by its volume being increased
+or decreased, or by change of temperature. Leaving out of consideration,
+for the moment, the effect of temperature, we can readily perceive that as
+the pressure is lessened upon the outside of the box, the elastic force of
+the air within will force out the top and bottom of the box; and when the
+outer pressure is increased they will be forced in. Thus with the varying
+pressure of the atmosphere, the top and bottom of the box approach to and
+recede from each other by a small quantity; but the bottom being fixed,
+nearly all this motion takes place on the top. Thus the top of the box is
+like an elastic cushion, which rises and falls according as the
+compressing force lessens or increases. To the eye these expansions and
+contractions would not be perceptible, so small is the motion. But they
+are rendered very evident by a nice mechanical arrangement. To the box is
+attached a strong piece of iron, kept pressed upon it by a spring at one
+extremity; so that as the top of the box rises, the motion is made
+sensible at the point held by the spring, and when the top descends the
+spring draws the piece of iron into close contact with it. This piece of
+iron acts as a lever, having its fulcrum at one extremity, the power at
+the centre of the box-top, and the other extremity controlled by the
+spring. Thus it is evident that the small motion of the centre of the
+box-top is much increased at the spring extremity. The motion thus
+obtained is communicated to a system of levers; and, by the intervention
+of a piece of watch-chain and a fine spring passing round the arbour,
+turns the index to the right or left, according as the external pressure
+increases or decreases. Thus, when by increase of pressure the vacuum box
+is compressed, the mechanism transfers the movement to the index, and it
+moves to the right; when the vacuum box bulges out under diminished
+pressure, the mechanical motion is reversed, and the index moves to the
+left. As the index traverses the dial, it shows upon the scale the
+pressure corresponding with that which a good mercurial barometer would at
+the same time and place indicate; that is, supposing it correctly
+adjusted.
+
+A different and more elegant arrangement has since been adopted. A broad
+curved spring is connected to the top of the vacuum box, so as to be
+compressed by the top of the box yielding inward to increased pressure,
+and to relax itself and the box as the pressure is lessened. The system of
+levers is connected to this spring, which augments and transfers the
+motion to the index, in the manner already described. Increase of pressure
+causes the levers to slacken the piece of watch-chain connected with them
+and the arbour of the index. The spring now uncoils, winds the chain upon
+the arbour, and turns the index to the right. Decrease of pressure winds
+the chain off the barrel, tightens the spiral spring, which thus turns the
+index to the left. The graduations of the aneroid scale are obtained by
+comparisons with the correct standard reading of a mercurial barometer,
+under the normal and reduced atmospheric pressure. Reduced pressure is
+obtained by placing both instruments under the receiver of an air pump.
+
+[Illustration: Fig. 33.]
+
+Fig. 33 represents the latest improved mechanism of an aneroid. The outer
+case and the face of the instrument are removed, but the hand is attached
+by its collet to the arbour. _A_ is the corrugated box, which has been
+exhausted of air through the tube, _J_, and hermetically sealed by
+soldering. _B_ is a powerful curved spring, resting in gudgeons fixed on
+the frame-plate, and attached to a socket behind, _F_, in the top of the
+box. A lever, _C_, joined to the stout edge of the spring, is connected,
+by the bent lever at _D_, with the chain, _E_, the other end of which is
+coiled round, and fastened to the arbour, _F_. As the box, _A_, is
+compressed by the weight of the atmosphere increasing, the spring, _B_, is
+tightened, the lever, _C_, depressed, and the chain, _E_, uncoiled from
+_F_, which is thereby turned so that the hand, _H_, moves to the right. In
+the mean while the spiral spring, _G_, coiled round _F_, and fixed at one
+extremity to the frame-work and by the other to _F_, is compressed. When,
+therefore, the pressure decreases, _A_ and _B_ relax, by virtue of their
+elasticity; _E_ slackens, _G_ unwinds, turning _F_, which carries _H_ to
+the left. Near _J_ is shown an iron pillar, cast as part of the stock of
+the spring, _B_. A screw works in this pillar through the bottom of the
+plate, by means of which the spring, _B_, may be so adjusted to the box,
+_A_, as to set the hand, _H_, to read on the scale according to the
+indications of a mercurial barometer. The lever, _C_, is composed of brass
+and steel, soldered together, and adjusted by repeated trials to correct
+for the effects of temperature.
+
+A thermometer is sometimes attached to the aneroid, as it is convenient
+for indicating the temperature of the air. As regards the instrument
+itself, no correction for temperature can be applied with certainty. It
+should be set to read with the mercurial barometer at 32 deg. F. Then the
+readings from it are supposed to require no correction.
+
+In considering the effects of temperature upon the aneroid, they are found
+to be somewhat complex. There is the effect of expansion and contraction
+of the various metals of which the mechanism is composed; and there is the
+effect on the elasticity of the small portion of air in the box. An
+increase of temperature produces greater, a diminution less elasticity in
+this air. The compensation for effects of temperature is adjusted by the
+process of "trial and error," and only a few makers do it well. It is very
+often a mere sham. Admiral FitzRoy writes, in his _Barometer Manual_, "The
+known expansion and contraction of metals under varying temperatures,
+caused doubts as to the accuracy of the aneroid under such changes; but
+they were partly removed by introducing into the vacuum box a small
+portion of gas, as a compensation for the effects of heat or cold. The gas
+in the box, changing its bulk on a change of temperature, was intended to
+compensate for the effect on the metals of which the aneroid is made.
+Besides which, a further and more reliable compensation has lately been
+effected by a combination of brass and steel bars."
+
+"Aneroid barometers, if often compared with good mercurial columns, are
+similar in their indications, and valuable; but it must be remembered that
+they are not independent instruments, that they are set originally by a
+barometer, require adjustment occasionally, and may deteriorate in time,
+though slowly."
+
+"The aneroid is quick in showing the variation of atmospheric pressure;
+and to the navigator who knows the difficulty, at times, of using
+barometers, this instrument is a great boon, for it can be placed
+anywhere, quite out of harm's way, and is not affected by the ship's
+motion, although faithfully giving indication of increased or diminished
+pressure of air. In ascending or descending elevations, the hand of the
+aneroid may be seen to move (like the hand of a watch), showing the height
+above the level of the sea, or the difference of level between places of
+comparison."
+
+In the admiral's _Notes on Meteorology_, he says, "The aneroid is an
+excellent _weather glass_, if well made. Compensation for heat or cold has
+lately been introduced by efficient mechanism. In its _improved_
+condition, when the cost may be about L5, it is fit for measuring heights
+as far as 5,000 feet with approximate accuracy; but even at the price of
+L3, as a _weather-glass_ only, it is exceedingly valuable, because it can
+be carried anywhere; and if now and then compared with a good barometer,
+it may be relied on sufficiently. I have had one in constant use for ten
+years, and it appears to be as good now as at first. For a ship of war
+(considering concussion by the fire of guns), for boats, or to put in a
+drawer, or on a table, I believe there is nothing better than it for use
+as a common weather-glass."
+
+Colonel Sir H. James, R.E., in his _Instructions for taking Meteorological
+Observations_, says of the aneroid, "This is a most valuable instrument;
+it is extremely portable. I have had one in use for upwards of ten years,
+and find it to be the best form of barometer, as a "weather-glass," that
+has been made."
+
+One of the objects of Mr. Glaisher's experiments in balloons was "to
+compare the readings of an aneroid barometer with those of a mercurial
+barometer up to five miles." In the comparisons the readings of the
+mercurial barometer were corrected for index-error and temperature. The
+aneroid readings, says Mr. Glaisher, "prove all the observations made in
+the several ascents may be safely depended upon, and also that an aneroid
+barometer can be made to read correctly to pressures below twelve inches."
+As one of the general conclusions derived from his experiments he states,
+"that an aneroid barometer read correctly to the first place, and probably
+to the second place of decimals, to a pressure as low as seven inches."
+The two aneroids used by Mr. Glaisher were by Messrs. Negretti and Zambra.
+
+Aneroids are now manufactured almost perfectly compensated for
+temperature. Such an instrument therefore ought to show the same pressure
+in the external air at a temperature say of 40 deg., as it would in a room
+where the temperature at the same time may be 60 deg.; provided there is no
+difference of elevation. To test it thoroughly would require an
+examination and a comparison with barometer readings reduced to 32 deg. F.,
+conducted through a long range of temperature and under artificially
+reduced pressure. A practical method appears to be to compare the aneroid
+daily, or more often, for a few weeks with the readings of a mercurial
+barometer reduced to 32 deg.; and if the error so found be constant, the
+object of the compensation may be assumed to be attained, particularly if
+the temperature during the period has varied greatly.
+
+_Directions for using the Aneroid._--Aneroids are generally suspended with
+the dial vertical; but if they be placed with the dial horizontal, the
+indications differ a few hundredths of an inch in the two positions.
+Hence, if their indications are registered, they should be kept in the
+same position.
+
+The aneroid will not answer for exact scientific purposes, as it cannot be
+relied upon for a length of time. Its error of indication changes slowly,
+and hence the necessity of its being set from time to time with the
+reading of a good barometer. To allow of this being done, at the back of
+the outer case is the head of a screw in connection with the spring
+attached to the vacuum box. By applying a small turnscrew to this screw,
+the spring of the vacuum box may be tightened or relaxed, and the index
+made to move correspondingly to the right or left on the dial. By this
+means, besides being enabled to correct the aneroid at any time, "if the
+measure of a height rather greater than the aneroid will commonly show be
+required, it may be _re-set_ thus: When at the upper station (_within its
+range_), and having noted the reading carefully, touch the screw behind so
+as to bring back the hand a few inches (if the instrument will admit),
+then read off and start again. _Reverse the operation when descending._
+This may add some inches of measure _approximately_."--_FitzRoy._
+
+
+[Illustration: Fig. 34.]
+
+=49. Small Size Aneroids.=--The patent for the Aneroid having expired,
+Admiral FitzRoy urged upon Messrs. Negretti & Zambra the desirability of
+reducing the size at which it had hitherto been made, as well as of
+improving its mechanical arrangement, and compensation for temperature.
+They accordingly engaged skilful workmen, who, under their directions, and
+at their expense, by a great amount of labour and experiment, succeeded
+in reducing its dimensions to two inches in diameter, and an inch and a
+quarter thick. The exact size and appearance of this aneroid are shown in
+fig. 34. The compensation is carefully adjusted, and the graduations of
+the dial ascertained under reduced pressure, so that they are not quite
+equal, but more accurate.
+
+
+=50. Watch Aneroid.=--Subsequently the aneroid has been further reduced in
+size and it can now be had from an inch and a quarter to six inches in
+diameter. The smallest size can be enclosed in watch cases, fig. 35, or
+otherwise, so as to be adapted to the pocket. By a beautifully simple
+contrivance, a milled rim is adjusted to move round with hand pressure,
+and carry a fine index or pointer, outside and around the scale engraved
+on the dial, or face, for the purpose of marking the reading, so that the
+subsequent increase or decrease of pressure may be readily seen. These
+very small instruments are found to act quite as correctly as the largest,
+and are much more serviceable. Besides serving the purpose of a
+weather-glass in the house or away from home, if carried in the pocket,
+they are admirably suited to the exigencies of tourists and travellers.
+They may be had with scale sufficient to measure heights not exceeding
+8,000 feet; with a scale of elevation in feet, as well as of pressure in
+inches, engraved on the dial. The scale of elevation, which is for the
+temperature of 50 deg., was computed by Professor Airy, the Astronomer Royal,
+who kindly presented it to Messrs. Negretti and Zambra, at the same time
+suggesting its application. Moderate-sized aneroids, fitted in leathern
+sling cases, are also good travelling instruments, and will be found
+serviceable to pilots, fishermen, and for use in coasting and small
+vessels, where a mercurial barometer cannot be employed, because requiring
+too much space.
+
+[Illustration: Fig. 35.]
+
+Admiral FitzRoy, in a communication to the _Mercantile Marine Magazine_,
+December, 1860, says:--"Aneroids are now made more portable, so that a
+pilot or chief boatman may carry one in his pocket, as a railway guard
+carries his timekeeper; and, thus provided, pilots cruising for expected
+ships would be able to caution strangers arriving, if bad weather were
+impending, or give warning to coasters or fishing boats. Harbours of
+Refuge, however excellent and important, are not always accessible, even
+when most wanted, as in snow, rain, or darkness, when neither land, nor
+buoy, nor even a lighthouse-light can be seen."
+
+
+=51. Measurement of Heights by the Aneroid.=--For measuring heights not
+exceeding many hundred feet above the sea-level by means of the aneroid,
+the following simple method will suffice:--
+
+Divide the difference between the aneroid readings at the lower and upper
+stations by .0011; the quotient will give the approximate height in feet.
+
+Thus, supposing the aneroid to read at the
+
+  Lower Station                30.385 inches.
+  Upper Station                30.025
+                               ------
+                Difference       .360
+                               ======
+
+  Divided gives .360/.0011 = 327 feet.
+
+As an illustration of the mode in which the aneroid should be used in
+measuring heights, the following example is given:--
+
+A gentleman who ascended Helvellyn, August 12th, 1862, recorded the
+following observations with a pocket aneroid by Negretti and Zambra:--
+
+Near 10 a.m., at the first milestone from Ambleside, found by survey to be
+188 feet above the sea, the aneroid read 29.89 inches; about 1 p.m., at
+the summit of Helvellyn, 26.81; and at 5 p.m., at the milestone again,
+29.76. The temperature of the lower air was 57 deg., of the upper, 54 deg. Hence
+the height of the mountain is deduced as follows:--
+
+                                          Inches.
+
+  Reading at 10 a.m.                       29.89
+        "     5 p.m.                       29.76
+                                           ------
+              Mean                         29.825 Table I.[5]   1010
+  Upper Reading                            26.81     "          3796
+                                                               -----
+                   Difference                                   2786
+  Mean Temperature 55 deg.5, gives in Table II.                   1.048
+                                                               -----
+                                                                2920
+  Lat. 55 deg. N., gives in Table III.                             .9991
+                                                               -----
+                                                                2917
+                         Table IV.                                 5
+                                                               -----
+                   Difference of height                         2922
+  Height of lower station                                        188
+                                                               -----
+        "   Helvellyn                                           3110
+
+  In Sir J. Herschell's _Physical Geography_ it is given as     3115 ft.
+
+So near an agreement is attributable to the excellence of the aneroid, and
+the careful accuracy of the observer.
+
+
+52. METALLIC BAROMETER.
+
+This instrument, the invention of M. Bourdon, has a great resemblance to
+the aneroid, but is much simpler in arrangement. The inventor has applied
+the same principle to the construction of metallic steam-pressure gauges.
+We are here, however, only concerned with it as constructed to indicate
+atmospheric pressure. It consists of a long slender flattened metallic
+tube, partially exhausted of air, and hermetically closed at each end,
+then fixed upon its centre, and bent round so as to make the ends face
+each other. The transverse section of this tube is an elongated ellipse.
+The principle of action is this: interior pressure tends to straighten the
+tube, external pressure causes it to coil more. Hence as the atmospheric
+pressure decreases, the ends of the tube become more apart.
+
+This movement is augmented and transferred by a mechanical arrangement of
+small metallic levers to a radius bar, which carries a rack formed on the
+arc of its circle. This moves a pinion, upon the arbour of which a light
+pointer, or "hand," is poised, which indicates the pressure upon a dial.
+When the pressure increases, the ends of the tube approach each other, and
+the pointer moves from left to right over the dial. The whole mechanism is
+fixed in a brass case, having a hole at the back for adjusting the
+instrument to the mercurial barometer by means of a key, which sets the
+pointer without affecting the levers. The dial is generally open to show
+the mechanism, and is protected by a glass, to which is fitted a moveable
+index.
+
+This barometer is very sensitive, and has the advantage of occupying
+little space, although it has not yet been made so small as the aneroid.
+Both these instruments admit of a great variety of mounts to render them
+ornamental. The metallic barometer can be constructed with a small clock
+in its centre, so as to form a novel and beautiful drawing-room ornament.
+
+Admiral FitzRoy writes, "Metallic barometers, by Bourdon, have not yet
+been tested in very moist, hot, or cold air for a sufficient time. They
+are dependent, or secondary instruments, and liable to deterioration. For
+limited employment, when sufficiently compared, they may be very useful,
+especially in a few cases of electrical changes, _not foretold or shown by
+mercury_, which these seem to indicate remarkably."
+
+They are not so well adapted for travellers, nor for measurements of
+considerable elevations, as aneroids.
+
+
+
+
+CHAPTER VI.
+
+INSTRUMENTS FOR ASCERTAINING TEMPERATURE.
+
+
+=53. Temperature= is the energy with which heat affects our sensation of
+feeling.
+
+Bodies are said to possess the same temperature, when the amounts of heat
+which they respectively contain act outwardly with the same intensity of
+transfer or absorption, producing in the one case the sensation of warmth,
+in the other that of coldness. Instruments used for the determination and
+estimation of temperatures are called _Thermometers_.
+
+Experience proves that the same body always occupies the same space at the
+same temperature; and that for every increase or decrease of its
+temperature, it undergoes a definite dilatation or contraction of its
+volume. Provided, then, a body suffers no loss of substance or peculiar
+change of its constituent elements or atoms, while manifesting changes of
+temperature it will likewise exhibit alterations in volume; the latter
+may, therefore, be taken as exponents of the former. The expansion and
+contraction of bodies are adopted as arbitrary measures of changes of
+temperature; and any substance will serve for a thermometer in which these
+changes of volume are sensible, and can be rendered measureable.
+
+
+=54. Thermometric Substances.=--Thermometers for meteorological and
+domestic purposes are constructed with liquids, and generally either
+mercury or alcohol, because their alterations of volume for the same
+change of temperature are greater than those of solids; while being more
+manageable, they are preferred to gases. Mercury is of all substances the
+best adapted for thermometric purposes, as it maintains the liquid state
+through a great alteration of heat, has a more equable co-efficient of
+expansion than any other fluid, and is peculiarly sensitive to changes of
+temperature. The temperature of solidification of mercury, according to
+Fahrenheit's scale of temperature, is -40 deg.; and its temperature of
+ebullition is about 600 deg. Sulphuric ether, nitric acid, oil of sassafras,
+and other limpid fluids, have been employed for thermometers.
+
+
+=55. Description of the Thermometer.=--The ordinary thermometer consists
+of a glass tube of very fine bore, having a bulb of thin glass at one
+extremity, and closed at the other. The bulb and part of the tube contains
+mercury; the rest of the tube is a vacuum, and affords space for the
+expansion of the liquid. This arrangement renders very perceptible the
+alterations in volume of the mercury due to changes of temperature. It is
+true, the glass expands and contracts also; but only by about
+one-twentieth of the extent of the mercury. Regarding the bulb, then, as
+unalterable in size, all the changes in the bulk of the fluid must take
+place in the tube, and be exhibited by the expansion and contraction of
+the column, which variations are made to measure changes of temperature.
+
+
+56. STANDARD THERMOMETER.
+
+The peculiarities in the construction of thermometers will be best
+understood by describing the manufacture of a _Standard Thermometer_,
+which is one of the most accurate make, and the scale of which is divided
+independently of any comparison with another thermometer. Fig. 36 is an
+illustration of such an instrument, on a silvered brass scale.
+
+[Illustration: Fig. 36]
+
+_Selection of Tube._--In selecting the glass tube, much care is requisite
+to ascertain that its bore is perfectly uniform throughout. As received
+from the glass-house, the tubes are generally, in their interior, portions
+of very elongated cones, so that the bore is wider at one end than at the
+other. With due care, however, a proper length of tube can be selected, in
+which there is no appreciable difference of bore. This is ascertained by
+introducing into the tube a length of mercury of about a half or a third
+of an inch, and accurately measuring it in various positions in the tube.
+To accomplish this, the workman blows a bulb at one end of the tube, and
+heats the bulb a little to drive out some of the air. Then, placing the
+open end in mercury, upon cooling the elasticity of the enclosed air
+diminishes, and the superior pressure of the atmosphere drives in some
+mercury. The workman stops the process so soon as he judges sufficient
+mercury has entered. By cooling or heating the bulb, as necessary, the
+mercury is made to pass from one end of the tube to the other. Should the
+length of this portion of mercury alter in various parts of the bore, the
+tube must be rejected. If it is, as nearly as possible, one uniform
+length, the tube is set aside for filling.
+
+The _bulb_ is never blown by the breath, but by an elastic caoutchouc ball
+containing air, so that the introduction of moisture is avoided. The
+spherical form is to be preferred; for it is best adapted to resist the
+varying pressure of the atmosphere. The bulbs should not be too large, or
+the mercury will take some time to indicate sudden changes of temperature.
+Cylindrical bulbs are sometimes desirable, as they offer larger surfaces
+to the mercury, and enable thermometers to be made more sensitive.
+
+The _mercury_, with which the bulb is to be filled, should be quite pure,
+and freed from moisture and air by recent boiling.
+
+_Filling the Tube._--The filling is effected by heating the bulb with the
+flame of a spirit-lamp, while the open end is embedded in mercury. Upon
+allowing the bulb to cool, the atmospheric pressure drives some mercury
+into it; and the process of heating and cooling is thus continued until
+sufficient mercury is introduced. The mercury is next boiled in the tube,
+to expel any air or moisture that may be present. In order to close the
+tube and exclude all air, the artist ascertains that the tube contains
+the requisite quantity of mercury; then, by holding the bulb over the
+spirit flame, he causes the mercury to fill the whole of the tube, and
+dexterously removing it from the source of heat, he, at the same instant,
+closes it with the flame of a blow-pipe. If any air remain in the tube, it
+is easily detected; for if the instrument be inverted, the mercury will
+fall to the extremity of the tube, if there is a perfect vacuum, unless
+the tube be so finely capillary that its attraction for the mercury is
+sufficient to overcome the force of gravity, in which case the mercury
+will retain its position in every situation of the instrument. If,
+however, the mercury fall and does not reach quite to the extremity of the
+bore, some air is present, which must be removed.
+
+_The Graduation._--The thermometer is now prepared for graduation, the
+first part of which process is the determination of two fixed points.
+These are given by the temperatures of melting ice and of the vapour of
+boiling water. Melting ice has always the same temperature in every place
+and under all circumstances; provided only that the water from which the
+ice is congealed is free from salts. The temperature of the vapour of
+boiling water depends upon the pressure of the atmosphere, but is always
+constant for the same pressure.
+
+The fixed point corresponding to the temperature of melting ice is called
+the _freezing point_. It is obtained by keeping the bulb and the part of
+the tube occupied by mercury immersed in melting ice, until the mercury
+contracts to a certain point, where it remains stationary. This position
+of the end of the mercury is then marked upon the tube.
+
+The _boiling point_ is not so easily determined, for the barometer must be
+consulted about the same time. The boiling apparatus is generally
+constructed of copper. It consists of a cylindrical boiler, heated from
+the base by a spirit lamp or charcoal fire. An open tube two or three
+inches in diameter and of suitable length enters the top of the boiler.
+This tube is enveloped by another fixed to the top of the boiler but not
+opening into it, and so that the two tubes are about an inch apart. The
+object of the outer tube is to protect the inner tube from the cold
+temperature of the air. The outer tube has an opening at the top for the
+admission of the thermometer, and a hole near the bottom for the escape of
+steam through a spout. When the water is made to boil, the steam rises in
+the inner tube, fills the space between the tubes, and escapes at the
+spout. The thermometer is then passed down into the inner cylinder, and
+held securely from the top by means of a piece of caoutchouc. The tubes or
+cylinders should be of sufficient length to prevent the thermometer
+entering the water. This is necessary because the temperature of boiling
+water is influenced by any substance which it holds in chemical solution;
+and, moreover, its temperature increases with the depth, owing to the
+pressure of the upper stratum. The thermometer being thus surrounded with
+steam, the mercury rises in the tube. As it does so, the tube should be
+depressed so as always to keep the top of the mercury just perceptible.
+When the temperature of the vapour is attained, the mercury ceases to
+rise, and remains stationary. The position of the end of the mercury is
+now marked upon the tube, and the "_boiling-point_" is obtained.
+
+
+=57. Methods of ascertaining the exact Boiling Temperature.=--The normal
+boiling temperature of water all nations have tacitly agreed to fix under
+a normal barometric pressure of 29.922 inches of mercury, having the
+temperature of melting ice, in the latitude of 45 deg., and at the sea-level.
+If the atmospheric pressure at the time or place of graduating a
+thermometer does not equal this, the boiling temperature will be higher or
+lower according as the pressure is greater or less. Hence a reading must
+be taken from a reliable barometer, which must also be corrected for
+errors and temperature, and reduced for latitude, in order to compare the
+actual atmospheric pressure at the time with the assumed normal pressure.
+Tables of vapour tension, as they are termed, have been computed from
+accurate experimental investigations and theory,--giving the temperatures
+of the vapour of water for all probable pressures; Regnault's, the most
+recent, is considered the most accurate; and his investigations are based
+upon the standard pressure given above, and are for the same latitude. His
+Table, therefore, will give the temperature on the thermometric scale
+corresponding to the pressure.
+
+The Commissioners appointed by the British Government to construct
+standard weights and measures, decided that the normal boiling-point,
+212 deg., on the thermometer should represent the temperature of steam
+generated under an atmospheric pressure equal in inches of mercury, at the
+temperature of freezing water, to 29.922 + (cos. 2 latitude x .0766) +
+(.00000179 x height in feet above the sea-level). Hence, at London, lat.
+51 deg.30' N., we deduce 29.905 as the barometric pressure representing the
+normal boiling point of water,--the trifling correction due to height
+being neglected. If then, in the latitude of London, the barometric
+pressure, at the time of fixing the boiling point, be not 29.905 inches,
+that point will be higher or lower, according to the difference of the
+pressure from the normal. Near the sea-level about 0.59 inch of such
+difference is equivalent to 1 deg. Fahrenheit in the boiling point.
+
+Suppose, then, the atmospheric pressure at London to be 30.785 inches, the
+following calculation gives the corresponding boiling temperature for
+Fahrenheit's scale:--
+
+  Observed pressure    30.785
+  Normal       "       29.905
+                       ------
+  Difference             .880
+                      =======
+
+As 0.59 is to 0.88, so is 1 deg. to 1 deg.5.
+
+That is, the water boils at 1 deg.5 above its normal temperature; so that, in
+this case, the normal temperature to be placed on the scale, viz. 212 deg.,
+must be 1 deg.5 lower than the mark made on the tube at the height at which
+the mercury stood under the influence of the boiling water.
+
+The temperature of the vapour of boiling water may be found, at any time
+and place, as follows:--Multiply the atmospheric pressure by the factor
+due to the latitude, given in the annexed Table V., and with the result
+seek the temperature in Table VI.
+
+        TABLE V.                         TABLE VI.
+
+  +----------------------------------------------------------------+
+  |Latitude.| Factor. |||Temperature|Tension.||Temperature|Tension.|
+  |         |         ||| of Vapour.|        || of Vapour.|        |
+  |---------+---------+++-----------+--------++-----------+--------|
+  |Degrees. |         ||| Degrees.  |Inches. || Degrees.  |Inches. |
+  |    0    | 0.99735 |||    179    | 14.934 ||    197    | 22.036 |
+  |    5    | 0.99739 |||    180    | 15.271 ||    198    | 22.501 |
+  |   10    | 0.99751 |||    181    | 15.614 ||    199    | 22.974 |
+  |   15    | 0.99770 |||    182    | 15.963 ||    200    | 23.456 |
+  |   20    | 0.99797 |||    183    | 16.318 ||    201    | 23.946 |
+  |   25    | 0.99830 |||    184    | 16.680 ||    202    | 24.445 |
+  |   30    | 0.99868 |||    185    | 17.049 ||    203    | 24.952 |
+  |   35    | 0.99910 |||    186    | 17.425 ||    204    | 25.468 |
+  |   40    | 0.99954 |||    187    | 17.808 ||    205    | 25.993 |
+  |   45    | 1.00000 |||    188    | 18.197 ||    206    | 26.527 |
+  |   50    | 1.00046 |||    189    | 18.594 ||    207    | 27.070 |
+  |   55    | 1.00090 |||    190    | 18.998 ||    208    | 27.623 |
+  |   60    | 1.00132 |||    191    | 19.409 ||    209    | 28.185 |
+  |   65    | 1.00170 |||    192    | 19.828 ||    210    | 28.756 |
+  |   70    | 1.00203 |||    193    | 20.254 ||    211    | 29.335 |
+  |   75    | 1.00230 |||    194    | 20.688 ||    212    | 29.922 |
+  |   80    | 1.00249 |||    195    | 21.129 ||    213    | 30.515 |
+  |         |         |||    196    | 21.578 ||    214    | 31.115 |
+  +----------------------------------------------------------------+
+
+_How to use the Tables._--When the _temperature_ is known to decimals of a
+degree, take out the tension for the degree, and multiply the difference
+between it and the next tension by the decimals of the temperature, and
+add the product to the tension, for the degree.
+
+Required the tension corresponding to 197 deg.84.
+
+    deg.
+  197           = 22.036        .465 x .84 =   .391
+  198           = 22.501         197 deg.      = 22.036
+                  ------                     ------
+    Difference      .465         197.84    = 22.427
+                  ======                     ======
+
+When the _tension_ is given, take the difference between it and the next
+less tension in the Table, and divide this difference by the difference
+between the next less and next greater tensions. The quotient will be the
+decimals to add to the degree opposite the next less tension.
+
+Thus, for 23.214 inches, required the temperature.
+
+  Given 23.214             Next greater         23.456
+        22.974             Next less            22.974
+        ------                                  ------
+          .240                   Difference       .482
+                     .240
+                 And ----                    =      .5
+                     .482
+      Temperature opposite next less             199.0
+                                                 -----
+      Temperature required                       199.5
+                                                 =====
+
+A similar method of interpolation in taking out numerical quantities is
+applicable to almost all tables; and should be practised with all those
+given in this work.
+
+_Example._--Thus, in Liverpool, lat. 53 deg. 30' N., the barometer reading
+29.876 inches, its attached thermometer 55 deg., and the correction of the
+instrument being + .015 (including index error, capillarity and capacity),
+what temperature should be assigned for the boiling point marked on the
+thermometer?
+
+  Observed barometer               29.876
+  Correction                      +  .015
+                                   ------
+                                   29.891
+  Correction for temperature      -  .074
+                                   ------
+  Reduced reading                  29.817
+  Factor from Table V.            1.00077
+                                  -------
+                                   208719
+                                  208719
+                               29817
+                               -----------
+  Equivalent for lat. 45 deg.      29.83995909
+                               ===========
+
+In Table VI., 29.84 gives temperature 211 deg.86.
+
+
+=58. Displacement of the Freezing Point.=--Either the prolonged effect of
+the atmospheric pressure upon the thin glass of the bulbs of thermometers,
+or the gradual restoration of the equilibrium of the particles of the
+glass after having been greatly disturbed by the operation of boiling the
+mercury, seems to be the cause of the freezing points of standard
+thermometers reading from a few tenths to a degree higher in the course of
+some years, as has been repeatedly observed. To obviate this small error,
+it is our practice to place the tubes aside for about six months before
+fixing the freezing point, in order to give time for the glass to regain
+its former state of aggregation. The making of accurate thermometers is a
+task attended with many difficulties, the principal one being the
+liability of the zero or freezing point varying constantly, so much so,
+that a thermometer that is perfectly correct to-day, if immersed in
+boiling water, will be no longer accurate; at least, it will take some
+time before it again settles into its normal state. Then, again, if a
+thermometer is recently blown, filled, and graduated immediately, or, at
+least, before some months have elapsed, though every care may have been
+taken with the production of the instrument, it will require some
+correction; so that the instrument, however carefully made, should from
+time to time be plunged into finely-pounded ice, in order to verify the
+freezing point.
+
+
+=59. The Scale.=--The two fixed points having been determined, it is
+necessary to apply the scale. The thermometers in general use in the
+United Kingdom, the British Colonies, and North America are constructed
+with Fahrenheit's scale. Fahrenheit was a philosophical instrument maker
+of Amsterdam, who, about the year 1724, invented the scale which has given
+his name to the thermometer. The freezing point is marked 32 deg., the boiling
+point 212 deg., so that the intermediate space is divided into 180 equal
+parts, called degrees. "The principle which dictated this _peculiar
+division_ of the scale is as follows:--When the instrument stood at the
+greatest cold of Iceland, or 0 degree, it was computed to contain 11124
+equal parts of quicksilver, which, when plunged in melting snow, expanded
+to 11156 parts; hence the intermediate space was divided into 32 equal
+portions, and 32 was taken as the freezing point of water: when the
+thermometer was plunged in boiling water, the quicksilver was expanded to
+11336; and therefore 212 deg. was marked as the boiling point of that fluid.
+In _practice_, Fahrenheit determined the divisions of his scale from two
+fixed points, the freezing and boiling of water. _The theory_ of the
+division, if we may so speak, was derived from the lowest cold observed in
+Iceland, and the expansions of a given portion of mercury" (_Professor
+Trail_).
+
+The divisions of the scale can be carried beyond the fixed points, if
+requisite, by equal graduations. Fahrenheit's scale is very convenient in
+some respects. The meteorological observer is seldom troubled with
+negative signs, as the zero of the scale is much below freezing. Again,
+the divisions are more numerous, and consequently smaller, than on other
+scales in use; and the further subdivision into tenths of degrees, seems
+to give all the minuteness usually required.
+
+_Celcius_, a Swede, in 1742, proposed zero for the freezing point, and 100
+for the boiling point, all temperatures below zero being distinguishable
+by the sign (--) minus. This scale is known as the _centigrade_, and is in
+use in France, Sweden, and the southern part of Europe. It has the
+advantage of the decimal notation, with the embarrassment of the negative
+sign.
+
+_Reaumur_, a Frenchman, proposed zero for the freezing point, and 80 deg. for
+the boiling point, an arrangement inferior to the centigrade. It is,
+however, in use in Spain, Switzerland, and Germany.
+
+It is merely a simple arithmetical operation to change the indications of
+any one of these scales into the equivalents on the others. To facilitate
+such conversions, tables are convenient, when a large number of
+observations are under discussion; and they can be easily formed or
+obtained.
+
+In the absence of such tables, the following formulae will insure accuracy
+of method, and save thinking, when occasional conversions are wanted to be
+made:--F. stands for Fahrenheit, C. for Centigrade, and R. for Reaumur.
+
+  Given.   Required.     Solution.
+    F.        C.     =  (F.-32) 5/9
+    F.        R.     =  (F.-32) 4/9
+    C.        F.     =  9/5 C. + 32
+    C.        R.     =  4/5 C.
+    R.        F.     =  9/5 R. + 32
+    R.        C.     =  5/4 R.
+
+_Example._--Convert 25 deg. of Fahrenheit's scale into the corresponding
+temperature on the Centigrade scale.
+
+  Here C. = (25 - 32) 5/9
+       C. = -35/9 = -3.9
+
+or nearly 4 deg. _below_ zero of the Centigrade scale. The algebraical sign
+must be carefully attended-to in the calculations.
+
+
+=60. The method of testing Thermometers= for meteorological purposes is
+very simple. Such thermometers are seldom required to read above 120 deg. In
+these the freezing point having been determined, the divisions of the
+scale are ascertained by careful comparisons, with a standard thermometer,
+in water of the requisite temperature. "For the freezing point, the bulbs,
+and a considerable portion of the tubes of the thermometers, are immersed
+in pounded ice. For the higher temperatures, the thermometers are placed
+in a cylindrical glass vessel containing water of the required heat: the
+scales of the thermometers intended to be tested, together with the
+Standard with which they are to be compared, are read through the glass.
+In this way the scale readings may be tested at any required degree of
+temperature, and the usual practice is to test them at every ten degrees
+from 32 deg. to 92 deg. of Fahrenheit."--_FitzRoy._
+
+
+=61. Porcelain Scale Plates.=--Thermometer scales of brass, wood, or
+ivory, either by atmospheric influence or dipping in sea-water, are very
+liable to become soiled and discoloured, so much so that after a very
+little time the divisions are rendered nearly invisible. To obviate this
+inconvenience, Messrs. Negretti and Zambra were the first to introduce
+into extensive use thermometer and barometer scale-plates made of
+porcelain, having the divisions and figures engraved thereon by means of
+fluoric acid, and permanently burnt-in and blackened, so as always to
+present a clear legible scale. That these scales have been found superior
+to all others, may be inferred from the fact that all the thermometers now
+supplied to the various government departments are provided with such
+scales.
+
+They can be adapted to replace any of the old forms of brass or zinc
+scales, the divisions and figures of which have become obliterated or
+indistinct.
+
+
+=62. Enamelled Tubes.=--Nearly all thermometer tubes are now made with
+enamelled backs. This contrivance of enamelling the backs of the tubes
+enables the makers to use finer threads of mercury than had before been
+found practicable; for were it not for the great contrast between the dark
+thread of mercury and the white enamel on the glass, many of the
+thermometers now in use would be positively illegible. The enamelling of
+thermometers is an invention of Messrs. Negretti and Zambra. It is
+necessary to state this, as many persons, from interested motives, are
+anxious to ignore to whom the credit of the invention is due.
+
+
+=63. Thermometers of extreme Sensitiveness.=--Thermometers for delicate
+experiments are no novelty. Thermometers have been made with very delicate
+bulbs to contain a very small quantity of mercury. Such instruments have
+also been made with spiral or coiled tubular bulbs, but the thickness of
+glass required to keep these coils or spirals in shape, and in fact to
+prevent their falling to pieces, served to nullify the effect sought to be
+produced, viz. instantaneous action; and where a small thin bulb was
+employed, the indicating column was generally so fine that it was
+positively invisible except by the aid of a powerful lens. Messrs.
+Negretti and Zambra have now introduced a new form of thermometer, which
+combines sensitiveness and quickness of action, together with a good
+visible column. The bulb of this thermometer is of the gridiron form. Care
+has been taken in constructing the bulb, so that the objections attending
+spirals and other forms have been overcome; for whilst the reservoir or
+bulb is made of glass so thin that it is only by a spirit lamp and not a
+glass blower's blowpipe that it can be formed, yet it is still so rigid
+(owing to its peculiar configuration) that no variations in its
+indications can be detected, whether it be held in a horizontal, vertical,
+or oblique position, nor will any error be detected if it be stood on its
+own bulb. They have made thermometers with bulbs or reservoirs formed of
+about nine inches of excessively thin cylindrical glass, whose outer
+diameter is not more than a twentieth of an inch; so that, owing to the
+large surface presented, the indications are positively instantaneous.
+This form of thermometer was constructed expressly to meet the
+requirements of scientific balloon ascents, to enable thermometrical
+readings to be taken at the precise elevation. It was contemplated to
+procure a metallic thermometer, but on the production of this perfect
+instrument the idea was abandoned.
+
+
+64. VARIETIES OF THERMOMETERS.
+
+Fig. 37 is an illustration of boxwood scale thermometers for general use
+and common purposes.
+
+Fig. 38, Negretti and Zambra's Travelling Thermometer; it is fixed in a
+plated metal (silver or otherwise) case, similar to a pencil-case, and has
+the scale divided upon its stem.
+
+Fig. 39, Thermometer mounted on a slab of glass, upon which the scale is
+etched, the back being either oak, mahogany, or ebony.
+
+Fig. 40, Portable Thermometer, in a bronzed brass or German silver
+revolving case.
+
+Fig. 41, Pocket Thermometer, on ivory or metallic scale, in morocco or
+papier-mache case.
+
+[Illustration: Fig. 37.]
+
+[Illustration: Fig. 38.]
+
+[Illustration: Fig. 39.]
+
+[Illustration: Fig. 40.]
+
+[Illustration: Fig. 41.]
+
+Fig. 42, an Ornamental Drawing-room Thermometer, on ebony or ivory stand,
+with glass shade.
+
+Fig. 43, representation of highly carved or engine-turned design for
+thermometer mounts, in ivory or wood, for the drawing-room. Some have the
+addition of a sundial or compass at the top; they may also be formed for a
+watch-stand.
+
+Fig. 44, =Bath Thermometer=, having a float to admit of its being kept in
+the water.
+
+[Illustration: Fig. 42.]
+
+[Illustration: Fig. 43.]
+
+[Illustration: Fig. 44.]
+
+Fig. 45, Thermometer with ivory scale in glass cylinder, mounted on oak
+bracket with metal top, for out-door use; as at a window.
+
+Fig. 46, Thermometer for the window, on patent porcelain or glass scale,
+with oak bracket and convenient brass supports, for placing the instrument
+at any angle.
+
+Fig. 47, =Chemical Thermometer=, on boxwood scale, jointed near the bulb
+on a brass hinge, ranging from 300 deg. to 600 deg.
+
+Fig. 48, =Chemical Thermometer=, for acids, graduated on its own stem,
+suitable for insertion in the tubulure of retorts; they are also made
+insulated in glass cylinder to protect the graduated stem; ranging from 0 deg.
+to 600 deg.
+
+[Illustration: Fig. 45.]
+
+[Illustration: Fig. 46.]
+
+[Illustration: Fig. 47.]
+
+[Illustration: Fig. 48.]
+
+[Illustration: Fig. 49.]
+
+[Illustration: Fig. 50.]
+
+
+=65. Superheated Steam Thermometer.=--The great advantage gained by the
+use of superheated steam in marine and other steam-engines being now
+generally admitted by engineers, reliable thermometers, reading to 600 deg. at
+least, are of the utmost importance. To meet this want, Messrs. Negretti
+and Zambra have constructed for the purpose a substantial form of
+thermometer, on their patent porcelain scales, in strong and convenient
+metal mountings, with perforated protection to the bulb. The scales cannot
+be deteriorated by steam, heat, oil, or dirt; and an occasional wiping
+will be all that is necessary to keep the divisions and figures clean and
+visible for any length of time; while careful calibration of the
+thermometer tubes ensures the most accurate indications attainable. These
+thermometers are illustrated by figs. 49 & 50. A similar, but cheaper,
+construction is given to thermometers to be used with hot air, or hot
+water, apparatus.
+
+
+=66. Thermometer for Sugar Boiling= is protected by a metallic frame; and
+is usually from three to four feet long, the graduations being confined to
+a space of about twelve inches at the upper part of the instrument,
+allowing the bulb and greater part of the tube to be immersed in the
+boiling sugar. The graduations extend to 270 deg. or further. An index is
+sometimes attached to the scale, which may be set to any degree of heat
+required to be maintained.
+
+
+67. EARTH THERMOMETER.
+
+The Earth Thermometer is for ascertaining the temperature of the soil at
+various depths. It is protected by a brass frame, pointed and strengthened
+at the end to facilitate insertion into the ground, as in fig. 51.
+
+[Illustration: Fig. 51.]
+
+_Utility of a Knowledge of the Temperature of the Soil._--The temperature
+of the soil is an important element in the consideration of climate, as it
+concerns the vegetable kingdom.
+
+Dr. Daubeny, in his _Lectures on Climate_, gives the following statement
+with respect to some temperatures which have been observed just beneath
+the earth's surface, in different parts of the globe:--
+
+  --------------------------------------------------------------------
+     Country.      |          Temperature.          |   Authority.
+  -----------------+--------------------------------------------------
+  Tropics, often   |162-184 deg.                        | Humboldt.
+                   |                                |
+  Egypt            |133-144                         | Edwards & Colin.
+                   |                                |
+  Orinoco          |In white sand, 140              | Humboldt.
+                   |                                |
+  Chili            |113-118, among dry grass        | Boussingault.
+                   |                                |
+  Cape of Good Hope|150, under the soil of a bulb   | Herschell.
+                      garden                        |
+                   |                                |
+  Bermuda          |142, thermometer barely covered | Emmet.
+                   |  in earth                      |
+                   |                                |
+  China            |Water of the fields, 113;       | Meyer.
+                   |  adjacent sand, much higher;   |
+                   |  blackened sides of the boat   |
+                   |  at midday, 142-150            |
+                   |                                |
+  France           |118-122, and in one instance 127| Arago.
+  --------------------------------------------------------------------
+
+"The importance of this to vegetation may be estimated by the following
+considerations:--
+
+"It is known that every plant requires a certain amount of heat, varying
+in the case of each species, for the renewal of its growth, at the
+commencement of the season.
+
+"Now when this degree of heat has spurred into activity those parts that
+are above ground, and caused them to elaborate the sap, it is necessary
+that the subterranean portions should at the same time be excited by the
+heat of the ground to absorb the materials which are to supply the plant
+with nourishment. Unless the latter function is provided for, the aerial
+portions of the plant will languish from want of food to assimilate.
+Indeed, it is even advisable that the roots should take the start of the
+leaves, in order to have in readiness a store of food for the latter to
+draw upon." In another place the professor remarks:--"It has been
+calculated by Mr. Raikes, from experiments made at Chat Moss, that the
+temperature of the soil when drained averages 10 deg. more than it does when
+undrained; and this is not surprising, when we find that 1 lb. of water
+evaporated from 1,000 lbs. of soil will depress the whole by 10 deg., owing to
+the latent heat which it absorbs in its conversion into vapour."
+
+
+68. MARINE THERMOMETER.
+
+This instrument is a special construction to meet the requirements of
+navigation. It consists of a carefully constructed thermometer divided on
+its stem to degrees, which are sufficiently large to admit of subdivision
+into tenths of degrees by estimation, and ranging from 0 deg. to 130 deg. The
+scale is porcelain, having the degrees etched upon it, and burnt-in a
+permanent black. The instrument is made to slide into a japanned metallic
+case, for handy use and protection. It is therefore adapted for almost any
+ordinary purpose; and cannot be injuriously affected by any chemical
+action arising from air or sea-water. A set of these thermometers consists
+of six, carefully packed in a neat box; two having japanned metallic cases
+(fig. 52), the others being designed for use without the case, or to
+replace a breakage.
+
+[Illustration: Fig. 52.]
+
+This thermometer is employed in the Royal Navy, and for the observations
+made at sea for the Board of Trade.
+
+The thermometer is now considered a necessary instrument on board ship.
+Not only is it of invaluable utility in connection with the barometer as a
+guide to the weather, but its indications are of service in showing the
+presence of a warm or cold current in the sea; many of the great oceanic
+currents being characterised by the warmth or coldness of their waters. In
+seas visited by icebergs, the habitual use of the thermometer would
+indicate their proximity, as the water is rendered colder for some
+distance around by the thawing of huge masses of ice. The water over a
+shoal in the sea is generally colder than the surface-water of the
+surrounding ocean; which may result from the cold water being brought to
+the surface by the current of water encountering the shoal. With this fact
+navigators are well acquainted; and therefore a fall in the sea-water
+thermometer may forebode that shallow water is at hand. It has been
+ascertained that fish inhabit regions of the oceans and seas having the
+peculiar temperature suitable to their habits. The better and firmer sort
+of fish are found where cold waters exist. Those taken in warmer belts or
+streams of water, even in the same latitude, are far inferior in
+condition, and less approved by the palate. The fish of the Mediterranean,
+a warm sea, are generally poor and scarce. Fish taken in the cold waters
+between the American shore and the Gulf Stream are much esteemed; while in
+and on the other side of the stream they are said to be tasteless, and of
+no flavour. Between the coasts of China and the warm waters of the
+Japanese current, the seas abound with excellent fish; but in the warm
+waters of the current and beyond, they are never seen in such shoals.
+
+In fact, it is clearly ascertained that fishes are adapted to climates,
+like birds and beasts. It has been even affirmed, after careful
+investigation, that herrings, which abound in the British Seas, and form a
+most important branch of our fisheries, can only be found in a temperature
+varying from 54 deg. to 58 deg. Hence the thermometer, if brought into use by the
+fishermen, would guide them to the spots where they may with the best
+chance cast their nets on dark nights, when other indications are not
+perceptible.
+
+This thermometer in its metallic case is perfectly suited for dipping
+overboard, or placing in a bucket of water just taken from the sea, to
+ascertain its temperature.
+
+
+
+
+CHAPTER VII.
+
+SELF-REGISTERING THERMOMETERS.
+
+
+=69. Importance of Self-Registering Thermometers.=--Heat being apparently
+the most effective agent in producing meteorological phenomena, the
+determination of the highest temperature of the day, and the lowest during
+the night, is a prime essential to enable an estimate of the climate of
+any place to be formed. To observe these extremes by means of the ordinary
+thermometer would be impracticable, from the constant watchfulness which
+would be necessary. Hence, the utility and importance of self-recording
+thermometers are evident. A thermometer constructed to _register_ the
+highest temperature is usually called a _maximum thermometer_; one to show
+the lowest temperature is termed a _minimum thermometer_; and if made to
+record both extremes of temperature, it is designated a
+_maximum-and-minimum thermometer_. We will, for the sake of method,
+describe the instruments in use in this order.
+
+It would carry us beyond our scope to explain in detail the methods of
+dealing with temperature observations; but we may remark that half the sum
+of the maximum and minimum temperature of each day of twenty-four hours,
+is not what meteorologists designate the _mean daily temperature_,
+although it very frequently approximates to it. The mean temperature of
+the day is understood to be the average of twenty-four consecutive hourly
+readings of a thermometer; and meteorology now supplies formulae whereby
+this result can be deduced from two or three observations only in a day.
+But we would observe that the actual mean temperature of any place has not
+such an important influence upon life, either animal or vegetable, as the
+abruptness and magnitude of the variations of temperature. Climate,
+therefore, should be estimated more by the range of the thermometer than
+by the average of its indications. The Registrar General's returns prove
+that with a wide range of the thermometer, the mortality greatly
+increases; and it is now becoming apparent to meteorologists that the
+daily range of the thermometer marks the effects of temperature on the
+health of men, and the success of crops, better than any other
+meteorological fact of which we take cognizance. Now that self-registering
+thermometers are constructed with mercury, the most appropriate of all
+thermometric substances, not only for maxima, but likewise for minima
+temperatures, the determination of the diurnal range of temperature is
+rendered more certain, and observations at different places are more
+strictly comparable.
+
+
+MAXIMA THERMOMETERS.
+
+=70. Rutherford's Maximum Thermometer.=--The maximum thermometer, invented
+by Dr. John Rutherford, differs from an ordinary thermometer in having a
+small cylinder of steel, porcelain, or aluminium, moving freely in the
+tube beyond the mercury, so as to form an index. The stem of the
+thermometer is fixed horizontally on the frame, which must be suspended in
+the same position, as represented in fig. 53. The instrument is set by
+holding it bulb downward, so as to allow the index to fall by its own
+gravity into contact with the mercury. Increase of heat produces expansion
+of the mercury, which consequently pushes forward the index. When the
+temperature decreases, the mercury recedes from the index, leaving it so
+that the extremity which was in contact with the mercury indicates upon
+the scale the highest temperature since the instrument was last set.
+
+[Illustration: Fig. 53.]
+
+As it is easily constructed and is comparatively cheap, it is still
+employed for ordinary purposes. Its disadvantages are, firstly, its
+liability of soon getting out of order by the index becoming embedded in
+the mercury, or fixed by oxidation, thus rendering it altogether useless;
+secondly, the ease with which the index can be displaced by the wind
+moving the instrument, or other accidental disturbance, so as to cause it
+to give erroneous indications occasionally; and thirdly, its consequent
+total unfitness for use at sea.
+
+In the part of the tube beyond the mercury, a small quantity of air is
+enclosed for the purpose of preventing the metal flowing freely in the
+tube. This necessitates the construction of a larger bulb, which renders
+the thermometer less sensitive. Moreover, as it frequently happens that
+some mercury passes the index, particles of air insinuate themselves in
+the metal, and cause separations in the column, which very often can be
+removed only by a maker. To facilitate this re-adjustment, a small chamber
+is left at the end of the tube, and the mercury being expanded into it by
+heat until the index and air bubbles are forced into it, if possible, upon
+the cooling down again, by a little management, the mercury will contract,
+leaving the air and index behind. Yet sometimes the index cannot be moved
+in the least from its place of fixture, so that the instrument must be
+virtually reconstructed.
+
+
+=71. Phillip's Maximum Thermometer.=--A maximum thermometer, better
+perhaps in its action than Rutherford's, has been suggested by Professor
+John Phillips, of Oxford. A small portion of air is introduced into an
+ordinary thermometer, so as to cut off about half an inch of the mercurial
+thread near its end in the tube. This forms a maximum thermometer, when
+the stem is arranged horizontally. The isolated portion is pushed forward
+by expansion, and is left in this position when the mercury contracts.
+The end remote from the bulb shows on the scale the maximum temperature.
+
+When made with a capillary tube so fine that the attraction arising from
+capillarity overcomes the force of gravity, and prevents the mercury
+falling to the end of the tube when the instrument is inverted, it forms a
+very serviceable thermometer, quite portable and suitable for use on board
+ship. In such a tube a smart shake from a swing of the hand is required to
+bring the detached portion back to the column, so as to set the instrument
+for future observation; no ordinary motion will move it. When the
+thermometer has not this peculiarity, the mercury will flow to the end, if
+held bulb downward; and in this state it is not at all a satisfactory
+instrument, as the air is likely to be displaced, and a great deal of tact
+is requisite to again get it to divide the column suitably. It has been
+found in practice that the air bubble at different temperatures assumes
+different lengths, and if very small it disappears in a few years by
+oxidation and by diffusion with the mercury, so that the instrument
+becomes defective and uncertain in action,--results which led to the
+construction of the self-registering mercurial maximum thermometer,
+invented and patented by Messrs. Negretti and Zambra. It has been before
+the public about twelve years; we may therefore, now, safely speak of its
+merits.
+
+
+=72. Negretti and Zambra's Patent Maximum Thermometer= consists of a glass
+tube containing mercury fitted on an engraved scale, as shown in fig. 54.
+The part of the thermometer tube above the mercury is entirely free from
+air; and at the point A in the bend above the bulb, is inserted and fixed
+with the blow-pipe a small piece of solid glass, or enamel, which acts as
+a valve, allowing mercury to pass on one side of it when heat is applied,
+but not allowing it to return when the thermometer cools. When mercury has
+been once made to pass the contraction, which nothing but the expansive
+force of heat can effect, and has risen in the tube, the upper end of the
+column registers the maximum temperature. To return the mercury to the
+bulb, we must apply a force equal to that which raised it in the tube; the
+force employed is gravity, assisted when necessary by a little agitation
+of the instrument.
+
+[Illustration: Fig. 54.]
+
+The degrees are generally divided on the stems of these thermometers, but
+their frames of course bear a scale as well. The makers have various
+styles of framing in wood, metal, porcelain, and even glass. Each material
+is eligible according to requirements. Porcelain scales, having the marks
+_etched_ upon them by acid and permanently blackened and baked in,--by a
+process for which the inventors have a separate patent,--will be found
+very serviceable, as they do not corrode or tarnish by exposure to any
+kind of weather; while any amount of dust and dirt can readily be cleaned
+off.
+
+The chief recommendation of this thermometer is its simplicity of
+construction, enabling it to be used with confidence and safety. Of no
+other maximum thermometer can it be said that it is impossible to derange
+or put it out of order; hence, as regards durability, it surpasses all
+others. Nothing short of actual breakage can cause it to fail. Hence it is
+the most easily portable of all self-registering thermometers, an
+advantage which renders it suitable for travellers, and for transmission
+abroad. In the year 1852, the British Meteorological Society reported this
+thermometer to be "the best which has yet been constructed for maximum
+temperature, and particularly for sun observations." Since then eleven
+years have elapsed, and it is still without a rival.
+
+_Directions for use._ In using this thermometer for meteorological
+observations, it should be suspended by means of two brass plates B, C,
+attached for that purpose, in such manner that it hangs raised up a little
+at C, and so placed that it is in the shade, with the air passing freely
+to it from all sides; then, on an increase of heat, the mercury will pass
+up the tube as in an ordinary thermometer, and continue doing so as long
+as the heat increases. On a decrease of heat, the contraction of mercury
+will take place _below_ the _bend_ in the tube, leaving the whole column
+of mercury in the tube, thus registering the highest temperature, and
+showing such till the instrument is disturbed.
+
+To prepare the instrument for future observations, remove and hold it
+perpendicularly, with the bulb downward, and then shake it. The mercury
+will then descend in the tube, and indicate the temperature of the air at
+that time; and, when again suspended, is prepared for future observation.
+
+After the temperature has attained a maximum, there will be, with a
+decrease of heat, a slight contraction of mercury in the tube--as well as
+of that in the bulb--and hence doubts have arisen as to the accuracy of
+the registration; but calculation shows, and critical trial has proved,
+that the greatest daily range of temperature will not produce an error
+large enough to be appreciable on the scale.
+
+A very great advantage of this thermometer is that the mercury may be
+allowed to flow to the end of the tube without the maximum temperature
+attained during an experiment being lost. It can be employed with the bulb
+uppermost. All that is necessary for reading the maximum temperature is to
+slope the instrument so that the mercury flows gently towards the bulb. It
+will then stop at the contraction so as to show the maximum temperature on
+the scale. Afterwards the mercury is driven into the bulb by agitating the
+instrument while held in the hand. Hence the instrument is invaluable as a
+registering thermometer on board ship, as its indications are in no way
+affected by the motions and tremors of the vessel.
+
+For physiological experiments, such as taking the temperature of the
+mouth in fever, this thermometer is the only one that can be used with
+certainty, as it can be held in any position, without losing the maximum
+temperature attained.
+
+
+MINIMA THERMOMETERS.
+
+=73. Rutherford's Alcohol Minimum Thermometer=, fig. 55, consists of a
+glass tube, the bulb and part of the bore of which is filled with
+perfectly pure spirits of wine, in which moves freely a black glass index.
+A slight elevation of the thermometer, bulb uppermost, will cause the
+glass index to flow to the surface of the liquid, where it will remain,
+unless violently shaken. On a _decrease_ of temperature the alcohol
+recedes, taking with it the glass index; on an _increase_ of temperature
+the alcohol alone ascends in the tube, leaving the end of the index
+_farthest_ from the bulb indicating the minimum temperature.
+
+[Illustration: Fig. 55.]
+
+_Directions for using, &c._--Having caused the glass index to flow to the
+end of the column of spirit, by slightly tilting the thermometer, bulb
+uppermost, suspend the instrument (in the shade with the air passing
+freely to it on all sides) by the two brass plates attached for that
+purpose,--in such manner that the bulb is about half an inch lower than
+the upper, or the end of the thermometer farthest from the bulb; then, on
+a decrease of temperature, the spirits of wine will descend, carrying with
+it the glass index; on an increase of temperature, however, the spirits of
+wine will ascend in the tube, leaving that end of the small glass index
+farthest from the bulb indicating the minimum temperature. To reset the
+instrument, simply raise the bulb end of the thermometer a little, as
+before observed, and the index will again descend to the end of the
+column, ready for future observation.
+
+_Precautions._--1. By no means jerk or shake an alcohol minimum
+thermometer _when resetting_ it, for by so doing it is liable to
+disarrange the instrument, either by causing the index to leave the
+spirit, or by separating a portion of the spirit from the main column.
+
+2. As alcohol thermometers have a tendency to read lower by age, owing to
+the volatile nature of the fluid allowing particles in the form of vapour
+to rise and lodge in the tube, it becomes necessary to compare them
+occasionally with a mercurial thermometer whose index error is known; and
+if the difference be more than a few tenths of a degree, examine well the
+upper part of the tube to see if any alcohol is hanging in the bore
+thereof; if so, the detached portion of it can be joined to the main
+column by swinging the thermometer with a pendulous motion, _bulb
+downwards_.
+
+3. The spirit column is sometimes much separated by jolting in travelling.
+If the instrument is in such a condition when received, it should be held
+by the right hand, bulb downward, and the frame tapped smartly, but
+cautiously, against the palm of the left hand. The broken thread of spirit
+will soon begin to join, and by continuing the operation a sufficient time
+all the bubbles will disappear, and the thermometer become as good as
+ever.
+
+
+=74. Horticultural Minimum Thermometer.=--This instrument, represented in
+fig. 56, is a special construction of Rutherford's minimum thermometer to
+meet the requirements of horticulturists. It is desirable, if not
+essential, that gardeners should have the means of ascertaining to what
+temperature stoves and greenhouses descend on cold nights, especially in
+winter. This thermometer is mounted on a strong cast zinc frame, with the
+divisions and figures of the scale raised.
+
+[Illustration: Fig. 56.]
+
+The sunk surface of the frame is painted dark; the figures and division a
+bright colour, so that observations can be made without a close inspection
+of the instrument.
+
+The directions for using are the same as those given in the preceding
+section. It may be used as an ordinary thermometer, by simply hanging it
+from the top loop, in which position, the coloured liquid will always
+indicate the present temperature.
+
+It was a source of annoyance with the ordinary boxwood and flat metal
+scales, that after a time, exposure to a damp warm atmosphere favoured the
+growth of confervae upon them, and obliterated the divisions; the plan of
+raising the figures and divisions of the scale has been found to prevent
+the destruction of the instrument in this way.
+
+
+=75. Baudin's Alcohol Minimum Thermometer.=--This instrument resembles
+Rutherford's thermometer in appearance; its indications are given by the
+expansion and contraction of alcohol, and its minimum temperature is
+likewise registered by a glass index being pulled back and left behind by
+the alcohol, as in Rutherford's instrument. There is, however, a great
+improvement in Baudin's instrument; for whilst Rutherford's thermometer
+can only register in a horizontal position, Baudin's can be used either
+horizontally or vertically, as necessity may require. This important
+change is effected in the following manner:--Instead of the index in the
+thermometer being loose and free to run up and down according to the
+position in which the instrument is held, as in Rutherford's, the index in
+the new instrument is made to fit the bore of the tube as nearly tight as
+possible, so much so that in holding the thermometer even upside down, or
+shaking it, the index will not shift from its position; but, inasmuch as a
+minimum thermometer with an immoveable index could not be set when
+required for observation, and would consequently be useless, the inventor
+has introduced behind the index a piece of solid glass, about
+one-and-a-half inch in length, which moves freely in the alcohol. The
+addition of the weight of this piece of glass on the top of the index,
+when turned upside down, forces the index down to the edge of the alcohol;
+and it is there left, as in the case of the ordinary Rutherford's
+thermometer. It is, therefore, by turning the thermometer upside down, and
+letting the moveable piece of glass fall on the index, that the index is
+driven to the end of the alcohol; after this operation the thermometer is
+hung up either horizontally or vertically, and will then be ready for use.
+
+The index, although immoveable _per se_, is by the alcohol drawn back, as
+in the ordinary minimum, and its indications are read off on the scale
+from the top of the index.
+
+
+=76. Mercurial Minima Thermometers desirable.=--Alcohol does not expand
+equally for equal increments of heat, consequently errors are likely to
+exist in the scale indications unless the graduations are very
+accurately--not necessarily equally--made. On this account, as well as
+from the volatility of alcohol, and the intervention of gaseous partitions
+in the tube, a good and thoroughly reliable minimum thermometer was for a
+long time a desideratum. It was desirable to obtain a thermometer which
+should register the lowest temperature by mercury, the fluid in general
+use for meteorological thermometers. Several instruments have recently
+been invented to meet this requirement, which are suitable and
+satisfactory for land purposes, but one well adapted for use on board ship
+is still very much wanted.
+
+For very low temperatures, alcohol thermometers will always be required;
+as mercury freezes at -40 deg. F, and contracts very irregularly much before
+this point, while alcohol has never yet been frozen.
+
+
+=77. Negretti and Zambra's Patent Mercurial Minimum Thermometer=,
+represented by fig. 57, has a cylindrical bulb of large size, which, at
+first sight, might induce the idea that the instrument would not be
+sufficiently sensitive; but as length is given to the cylinder instead of
+increasing its diameter, it will be found as sensitive as a globular bulb
+of the same diameter, and much more so than an ordinary alcohol
+thermometer.
+
+[Illustration: Fig. 57.]
+
+The reason for having the bulb large is to allow the internal diameter of
+the thermometer tube to be larger than that generally used for
+thermometrical purposes, so that a steel index, pointed at both ends, may
+move freely within when required.
+
+The tube is blown, filled and regulated in the usual way, 60 deg. of
+temperature being about half-way up the tube. A small cylindrical bulb is
+then formed at the upper end of the tube, and then is introduced a steel
+needle pointed at both ends, that in contact with the mercury being
+abrupt, the other more prolonged. The open extremity of the tube is now
+drawn out into a fine capillary tube, and the bulb of the instrument
+warmed so as to cause the mercury to fill the tube completely. When the
+mercury reaches the capillary tube, the flame of a blow-pipe is applied;
+the glass is dexterously melted, the superfluous part taken away, and the
+tube left hermetically closed. During this operation, the steel index has
+been embedded in the heated mercury. As the instrument cools, if held
+upright, the mercury will recede and expose the needle, which will then
+follow the descending column simply by its own gravity. In this condition
+the thermometer resembles Rutherford's maximum, being a tube of mercury
+with a steel index floating on its surface; but it possesses these
+important advantages: it is quite free from air, so that the mercury can
+move with perfect freedom; and the index is pointed at both ends, to allow
+the mercury to pass, instead of being ground flat to prevent it.
+
+_To use the Thermometer_, it is suspended perpendicularly (figure 57) with
+the steel index resting on the surface of the mercurial column. As the
+mercury in the cylinder contracts, that in the tube descends, and the
+index, of its own gravity, follows it; on the contrary, as the mercury
+expands and rises in the tube, it passes the index on one side, and in
+rising, exerts a lateral pressure on the needle, and jams it to one side
+of the tube, where it remains firmly fixed, leaving the upper point of the
+needle indicating the minimum temperature. In this thermometer, the
+reading is always from the upper point of the needle, and not from the
+mercury itself.
+
+_To extricate the Needle_ from the mercury, a magnet is used, when, if the
+needle is embedded only a few degrees, it can readily be withdrawn without
+altering the position of the instrument. Should the magnet not be
+sufficient for the purpose, we simply turn the thermometer on its support
+from the upright position, slightly elevating the bulb (fig. 58 (=2=)).
+The mercury and index will then flow into the small reservoir. Should the
+index not freely leave the tube with the mercury, assist it with a magnet,
+and when the mercury and index are in the upper bulb (figure =2=), apply a
+magnet outside, which will attract and hold fast the index; and whilst
+thus holding it, again bring the thermometer to the upright position, when
+the mercury will immediately fall back into the tube, leaving the index
+attached to the magnet (figure =4=), with which it is guided down to the
+surface of the mercury, ready for another observation.
+
+[Illustration: Fig. 58.]
+
+Care must be taken not to withdraw the magnet until the index is in
+contact with mercury; for, if released before touching, it might plunge
+too deeply, and give a false indication. The rule for re-setting it will
+be to bring the needle-point in contact with the mercury, and then
+withdraw the magnet, having previously ascertained that no particles of
+mercury are attached to the index.
+
+It may sometimes, though rarely, happen, that from the time a minimum
+temperature is registered by the index, and by the time an observation is
+made, the mercury may have risen so high in the tube as to completely pass
+the index, as shown (figure =3=). Should it so happen, the space which the
+index occupies will readily be observed, as it will be pressed to one side
+of the tube, causing a different appearance in that part, although the
+point of the needle may not be seen. If such be the case, apply a magnet
+to the spot where you see the index is fixed: this will hold the needle
+firmly. Then, by slightly tilting the thermometer bulb uppermost, the
+mercury will flow into the top bulb, leaving the index attached to the
+magnet, and quite uncovered. Having taken the reading, draw the needle
+into the top bulb, and hold it there whilst you adjust the thermometer by
+again bringing it to the upright position.
+
+By contracting the bore of this thermometer, at the bend of the tube,
+sufficiently to keep the mercury from flowing out of its bulb with too
+much freedom by motion, the instrument becomes perfectly safe for
+transmission abroad.
+
+
+=78. Negretti & Zambra's Second Patent Mercurial Minimum Thermometer.=--In
+this thermometer a principle is used that has been long known to
+scientific men, viz. the affinity of mercury for platinum. If mercury be
+placed in contact with platinum under ordinary circumstances, no effect
+will take place; but if the mercury is once made to attack the platinum,
+the amalgamation is permanent and the contact perfect, so much so, that
+the principle was made use of in constructing standard barometers. A ring
+of platinum was fused round the end of the tube, dipping into the mercury;
+and the contact between the platinum and mercury became so perfect that
+air could not creep down the tube and up the bore, as in ordinary
+barometer tubes. This principle of adhesion or affinity of mercury for
+platinum has been brought into play for the purpose of arresting the
+mercury after it has reached the minimum temperature in a thermometer.
+This thermometer is made as follows:--behind the bulb is placed a
+supplementary chamber; in the space or neck between the bulb of the
+thermometer and the chamber, is placed a small piece of platinum; this may
+be of any shape or size, but the smaller the better. This is not to fit in
+the neck; it must, on the contrary, be rather loose; it may be fastened in
+position or not. The instrument is represented by fig. 59.
+
+[Illustration: Fig. 59.]
+
+_Directions for using._--Having suspended the thermometer in a horizontal
+position, the mercury is made to stand in exact contact with the platinum
+plug by slightly elevating the bulb end of the instrument. The thermometer
+is now ready for observation. On a decrease of temperature, the mercury
+will endeavour to contract first from the easier passage, viz. behind the
+bulb; but in consequence of the adhesion of the mercury to the platinum,
+it cannot recede from here, it is therefore forced to contract from the
+indicating tube, and will continue to do so as long as the temperature
+decreases; and as no indices are employed in this thermometer, the extreme
+end of the mercurial column will show "how cold it has been." On an
+increase of temperature the mercury will glide over the platinum plug and
+expand by the easier passage into the supplementary chamber, and there
+remain until a decrease of temperature again takes place, when the mercury
+that had gone into the supplementary chamber will be the first to recede,
+until it reaches the platinum plug, its further progress being arrested;
+it will then fall in the indicating tube, and there remain until re-set.
+
+
+=79. Casella's Mercurial Minimum Thermometer.=--The general form and
+arrangement of this instrument is shown in fig. 60. A tube with large
+bore, _a_, has at the end a _flat glass diaphragm_ formed by the abrupt
+junction of a small chamber, _b c_, the inlet to which at _b_ is larger
+than the bore of the indicating tube. The result of this is that on
+setting the thermometer, as described below, the contracting force of the
+mercury in cooling withdraws the fluid in the indicating stem only; whilst
+on its expanding with heat, the long column does not move, the increased
+bulk of mercury finding an easier passage into the small pear-shaped
+chamber attached.
+
+[Illustration: Fig. 60.]
+
+We believe that a small speck of air must be confined in the chamber, _b
+c_, to act as a spring to start the mercury from the chamber in the act of
+setting the thermometer. Were this air not present, the mercury would so
+adhere to the glass that no amount of shaking could induce it to flow from
+the chamber.
+
+_To set the Instrument_, place it in a horizontal position, with the back
+plate, _d_, suspended on a nail, and the lower part supported on a hook,
+_e_. The bulb end may now be gently raised or lowered, causing the mercury
+to flow slowly until the bent part, _a_, _is full_ and the chamber, _b c_,
+_quite empty_. At this point the flow of mercury in the long stem of the
+tube is arrested, _and indicates the exact temperature_ of the bulb or air
+at the time. On an increase of temperature the mercury will expand into
+the small chamber, _b c_; and a return of cold will cause its recession
+from this chamber only, until it reaches the diaphragm, _b_. Any further
+diminution of heat withdraws the mercury down the bore to whatever degree
+the cold may attain, where it remains until farther withdrawn by increased
+cold, or till re-set for future observation.
+
+
+MAXIMA AND MINIMA THERMOMETERS.
+
+=80. Rutherford's= arrangement for obtaining a complete instrument for the
+registration of heat and cold was simply mounting a maximum thermometer
+and a minimum thermometer upon the same frame or slab. Thus constructed,
+they are often called "day and night" thermometers, though somewhat
+inappropriately; for in temperate climates the temperature of the night
+sometimes exceeds that of the day, notwithstanding the reverse is the
+general law of temperature. Fig. 61 will explain the arrangement of
+Rutherford's day and night thermometer.
+
+[Illustration: Fig. 61.]
+
+
+=81. Sixe's Self-Registering Thermometer.=--The very ingenious and
+certainly elegant instrument about to be described was invented by James
+Sixe, of Colchester. It consists of a long cylindrical bulb, united to a
+tube of more than twice its length, bent round each side of it in the form
+of a syphon, and terminated in a smaller, oval-shaped bulb. Figure 62
+gives a representation of this instrument. The lower portion of the syphon
+is filled with mercury; the long bulb, the other parts of the tube, and
+part of the small bulb, with highly rectified alcohol. A steel index moves
+in the spirit in each limb of the syphon. The two indices are terminated
+at top and bottom with a bead of glass, to enable them to move with the
+least possible friction, and without causing separation of the spirit, or
+allowing mercury to pass easily. They would, from their weight, always
+rest upon the mercury; but each has a fine hair tied to its upper
+extremity and bent against the interior of the tube, which acts as a
+spring with sufficient elasticity to keep the index supported in the
+spirit in opposition to gravity.
+
+[Illustration: Fig. 62.]
+
+The instrument acts as follows:--A rise of temperature causes the spirit
+in the long bulb to expand and press some of the mercury into the other
+limb of the syphon, into which it rises also from its own expansion, and
+carries the index with it, until the greatest temperature is attained. The
+lower end of this index then indicates upon the engraved scale the maximum
+temperature. As the temperature falls the spirit and the mercury contract,
+and in returning towards the bulb the second index is met and carried up
+by the mercury until the lowest temperature occurs, when it is left to
+indicate upon the scale the minimum temperature. The limb of the syphon
+adjoining the bulb requires, therefore, a descending scale of
+thermometric degrees; the other limb, an ascending scale. The graduations
+must be obtained by comparisons with a standard thermometer under
+artificial temperatures, which should be done in this way for every 5 deg., in
+order to correct for the inequality in the bore of the tube, and the
+irregular expansion of the spirit. The instrument is set for observation
+by bringing the indices into contact with the mercury, by means of a small
+magnet, which attracts the steel through the glass, so that it is readily
+drawn up or down. They should be drawn nearly to the top of the limbs when
+it is desired to remove the instrument, which should be carefully carried
+in the vertical position; for should it be inverted, or laid flat, the
+spirit may get among the mercury, and so break up the column as to require
+the skill of a maker to put it in order again. For transmission by
+ordinary conveyances, it requires that attention be given to keep it
+vertical. The entanglement of a small portion of mercury with the indices
+is sometimes a source of annoyance in this instrument, for the readings
+are thereby rendered somewhat incorrect. Small breakages in the mercury,
+either from intervening bubbles of spirit or adhesion to the indices, may
+generally be rectified by cautiously tapping the frame of the instrument,
+so as to cause the mercury to unite by the assistance thus given to its
+superior gravity.
+
+These thermometers, when carefully made and adjusted to a standard
+thermometer, are strongly recommended for ordinary purposes, where strict
+scientific accuracy is not required. This is also the only fluid
+thermometer applicable for determining the temperature of the sea at
+depths.
+
+
+
+
+CHAPTER VIII.
+
+RADIATION THERMOMETERS.
+
+
+=82. Solar and Terrestrial Radiation considered.=--The surface of the
+earth absorbs the heat of the sun during the day, and radiates heat into
+space during the night. The envelope of gases and vapour, which we call
+the atmosphere, exerts highly important functions upon these processes.
+Thanks to the researches of Professor Tyndall, we are now enabled to
+understand these functions much more clearly than heretofore. His
+elaborate, patient, and remarkably sagacious series of experiments upon
+radiant heat, have satisfactorily demonstrated that _dry_ air is as
+transparent to radiant heat as the vacuum itself; while air _perfectly
+saturated_ with aqueous vapour absorbs more than five per cent. of radiant
+heat, estimated by the thermal unit adopted for the galvanometer
+indications of the effect upon a thermo-electric pile.
+
+Aqueous vapour, in the form of fog or mist, as is well known, gives to our
+sensation a feeling of cold, and interferes with the healthy action of the
+skin and the lungs; the cause being its property of absorbing heat from
+our person.
+
+Air containing moisture in an invisible state likewise exerts a remarkable
+influence in radiating and absorbing heat. By reason of these properties,
+aqueous vapour acts as a kind of blanket upon the ground, and maintains
+upon it a higher temperature than it would otherwise have. "Regarding the
+earth as a source of heat, no doubt at least ten per cent. of its heat is
+intercepted within ten feet of the surface." Thus vapour--whether
+transparent and invisible, or visible, as cloud, fog, or mist--is
+intimately connected with the important operations of solar and
+terrestrial radiation. Cloudy, or humid days, diminish the effect upon the
+soil of solar radiation; similar nights retard the radiation from the
+earth. A dry atmosphere is the most favourable for the direct transmission
+of the sun's rays; and the withdrawal of the sun from any region over
+which the air is dry, must be followed by very rapid cooling of the soil.
+"The removal, for a single summer night, of the aqueous vapour from the
+atmosphere which covers England, would be attended by the destruction of
+every plant which a freezing temperature could kill. In Sahara, where 'the
+soil is fire and the wind is flame,' the refrigeration at night is often
+painful to bear. Ice has been formed in this region at night. In
+Australia, also, the _diurnal range_ of temperature is very great,
+amounting, commonly, to between 40 and 50 degrees. In short, it may be
+safely predicted, that wherever the air is _dry_, the daily thermometric
+range will be great. This, however, is quite different from saying that
+when the air is _clear_, the thermometric range will be great. Great
+clearness to light is perfectly compatible with great opacity to heat; the
+atmosphere may be charged with aqueous vapour while a deep blue sky is
+overhead; and on such occasions the terrestrial radiation would,
+notwithstanding the 'clearness,' be intercepted." The great range of the
+thermometer is attributable to the absence of that protection against gain
+or loss of heat which is afforded when aqueous vapour is present in the
+air; and during such weather the rapid abstraction of moisture from the
+surface of plants and animals is very deleterious to their healthy
+condition. "The nipping of tender plants by frost, even when the air of
+the garden is some degrees above the freezing temperature, is also to be
+referred to chilling by radiation." Hence the practice of gardeners of
+spreading thin mats, of bad radiating material, over tender plants, is
+often attended with great benefit.
+
+By means of the process of terrestrial radiation ice is artificially
+formed in Bengal, "where the substance is never formed naturally. Shallow
+pits are dug, which are partially filled with straw, and on the straw flat
+pans containing water which had been boiled is exposed to the clear
+firmament. The water is a very powerful radiant, and sends off its heat
+into space. The heat thus lost cannot be supplied from the earth--this
+source being cut off by the non-conducting straw. Before sunrise a cake of
+ice is formed in each vessel.... To produce the ice in abundance, the
+atmosphere must not only be clear, but it must be comparatively free from
+aqueous vapour."
+
+Considering, therefore, the important consequences attending both
+terrestrial and solar radiation, it appears to us that observations from
+radiation thermometers are of much more utility in judging of climate than
+is usually supposed. These observations are very scanty; and what few are
+upon record are not very reliable, principally from bad exposure of the
+instruments, while the want of uniformity in construction may be another
+cause. Herschell's actinometer and Pouillet's pyrheliometer, instruments
+for ascertaining the absolute heating effect of the sun's rays, should,
+however, be more generally employed by meteorologists. In comparing
+observations on radiation it should be kept in mind, that "the difference
+between a thermometer which, properly confined [or shaded], gives the true
+temperature of the night air, and one which is permitted to radiate freely
+towards space, must be greater at high elevations than at low ones;"[6]
+because the higher the place, the less the thickness of the vapour-screen
+to intercept the radiation.
+
+
+=83. Solar Radiation Thermometer.=--"As the interchange of heat between
+two bodies by radiation depends upon the relative temperature which they
+respectively possess, the earth, by the rays transmitted from the sun
+during the day, must be continually gaining an accession of heat, which
+would be far from being counterbalanced by the opposite effect of its own
+radiation into space. Hence, from sunrise till two or three hours after
+mid-day, the earth goes on gradually increasing in temperature, the
+augmentation being greatest where the surface consists of materials
+calculated, from their colour and texture, to absorb heat, and where it is
+deficient in moisture, which, by its evaporation, would have a tendency
+to diminish it."[7] It is, therefore, important to have instruments for
+measuring the efficacy of solar radiation, apart from those for exhibiting
+the temperature of the place in the shade.
+
+[Illustration: Fig. 63.]
+
+Fig. 63 shows the arrangement of Negretti & Zambra's maximum thermometer,
+for registering the greatest heat of the sun's direct rays, hence called a
+_solar radiation thermometer_. It has a blackened bulb, the scale divided
+on its own stem, and the divisions protected by a glass shield. In use it
+should be placed nearly horizontally, resting on Y supports of wood or
+metal, with its bulb in the full rays of the sun, resting on grass, and,
+if possible, so that lateral winds should not strike the bulb; and at a
+sufficient distance from any wall, so that it does not receive any
+_reflected_ heat from the sun. Some observers place the thermometer as
+much as two feet from the ground. It would be very desirable if one
+uniform plan could be recognized: that of placing the instrument as
+indicated in the figure appears to be most generally adopted, and the
+least objectionable.
+
+
+=84. Vacuum Solar Radiation Thermometer.=--In order that the heat absorbed
+by the blackened bulb of the solar radiation thermometer may not in part
+be carried off by the currents of air which would come into contact with
+it, the instrument has been improved by Messrs. Negretti and Zambra into
+the _vacuum solar radiation thermometer_, as illustrated by fig. 64.
+
+[Illustration: Fig. 64.]
+
+This consists of a blackened-bulb radiation thermometer, enclosed in a
+glass tube and globe, from which all air is exhausted. Thus protected from
+the loss of heat which would ensue if the bulb were exposed, its
+indications are from 20 deg. to 30 deg. higher than when placed side by side with
+a similar instrument with the bulb exposed to the passing air. At times
+when the air has been in rapid motion, the difference between the reading
+of a thermometer giving the true temperature of the air in the shade, and
+an ordinary solar radiation thermometer, has been 20 deg. only, whilst the
+difference between the air temperature and the reading of a radiation
+thermometer in vacuo has been as large as 50 deg. It is also found that the
+readings are almost identical at distances from the earth varying from six
+inches to eighteen inches. By the use of this improvement, it is hoped
+that the amounts of solar radiation at different places may be rendered
+comparable; hitherto they have not been so; the results found at different
+places cannot be compared, as the bulbs of the thermometers are under very
+different circumstances as to exposure and currents of air. Important
+results are anticipated from this arrangement. The observations at
+different places are expected to present more agreement. Observers would
+do well to note carefully the effect of any remarkable degree of intensity
+in the solar heat upon particular plants, crops, fruit or other trees.
+
+
+=85. Terrestrial Radiation Thermometer= is an alcohol minimum thermometer,
+with the graduations etched upon the stem, and protected by a glass
+shield, as shown in figure 65, instead of being mounted on a frame. The
+bulb is transparent; that is to say, the spirit is not coloured.
+
+[Illustration: Fig. 65.]
+
+In use, it should be placed with its bulb fully exposed to the sky,
+resting on grass, the stem being supported by little forks of wood. The
+precautions required with this thermometer are similar to those for
+ordinary spirit thermometers, explained at page 76.
+
+
+[Illustration: Fig. 66.]
+
+=86. AEthrioscope.=--The celebrated experimental philosopher, Sir John
+Leslie, was the inventor of this instrument, the purpose of which is to
+give a comparative idea of the radiation proceeding from the surface of
+the earth towards the sky. It consists, as represented in fig. 66, of two
+glass bulbs united by a vertical glass tube, of so fine a bore that a
+little coloured liquid is supported in it by its own adhesion, there being
+air confined in each of the bulbs. The bulb, _A_, is enclosed in a highly
+polished brass sphere, _D_, made in halves and screwed together. The bulb,
+_B_, is blackened and placed in the centre of a metallic cup, _C_, which
+is well gilt on the inside, and which may be covered by a top, _F_. The
+brass coverings defend both bulbs from solar radiation, or any
+adventitious source of heat. When the top is on, the liquid remains at
+zero of the scale. On removing the top and presenting the instrument to a
+clear sky, either by night or by day, the bulb, _B_, is cooled by
+terrestrial radiation, while the bulb, _A_, retains the temperature of the
+air. The air confined in _B_, therefore, contracts; and the elasticity of
+that within _A_ forces the liquid up the tube, to a height proportionate
+to the intensity of the radiation. Such is the sensitiveness of the
+instrument, that the smallest cloud passing over it checks the rise of the
+liquid. Sir John Leslie says:--"Under a clear blue sky, the _aethrioscope_
+will sometimes indicate a cold of fifty millesimal degrees; yet, on other
+days, _when the air seems equally bright_, the effect is hardly 30 deg." This
+anomaly, according to Dr. Tyndall, is simply due to the difference in the
+quantity of aqueous vapour present in the atmosphere. The presence of
+invisible vapour intercepts the radiation from the aethrioscope, while its
+absence opens a door for the escape of this radiation into space.
+
+
+=87. Pouillet's Pyrheliometer.=--"This instrument is composed of a shallow
+cylinder of steel, _A_, fig. 67, which is filled with mercury. Into the
+cylinder a thermometer, _D_, is introduced, the stem of which is protected
+by a piece of brass tubing. We thus obtain the temperature of the mercury.
+The flat end of the cylinder is to be turned towards the sun, and the
+surface, _B_, thus presented is coated with lamp black. There is a collar
+and screw, _C_, by means of which the instrument may be attached to a
+stake driven into the ground, or into the snow, if the observations are
+made at considerable heights. It is necessary that the surface which
+receives the sun's rays should be perpendicular to the rays; and this is
+secured by appending to the brass tube which shields the stem of the
+thermometer, a disk, _E_, of precisely the same diameter as the steel
+cylinder. When the shadow of the cylinder accurately covers the disk, we
+are sure that the rays fall, as perpendiculars, on the upturned surface of
+the cylinder.
+
+[Illustration: Fig. 67.]
+
+"The observations are made in the following manner:--First, the instrument
+is permitted, not to receive the sun's rays, but to radiate its own heat
+for five minutes against an unclouded part of the firmament; the decrease
+of the temperature of the mercury consequent on this radiation is then
+noted. Next, the instrument is turned towards the sun, so that the solar
+rays fall perpendicularly upon it for five minutes; the augmentation of
+heat is now noted. Finally, the instrument is turned again towards the
+firmament, away from the sun, and allowed to radiate for another five
+minutes, the sinking of the thermometer being noted as before. In order to
+obtain the whole heating power of the sun, we must add to his observed
+heating power the quantity lost during the time of exposure, and this
+quantity is the mean of the first and last observations. Supposing the
+letter _R_ to represent the augmentation of temperature by five minutes'
+exposure to the sun, and that _t_ and _t^1_ represent the reductions of
+temperature observed before and after, then the whole force of the sun,
+which we may call _T_, would be thus expressed:--_T = R + 1/2(t + t^1)_.
+
+"The surface on which the sun's rays here fall is known; the quantity of
+mercury within the cylinder is also known; hence we can express the effect
+of the sun's heat upon a given area, by stating that it is competent, in
+five minutes, to raise so much mercury so many degrees in
+temperature."--_Dr. Tyndall's "Heat considered as a Mode of Motion."_
+
+
+[Illustration: Fig. 68.]
+
+=88. Sir John Herschell's Actinometer=, for ascertaining the absolute
+heating effect of the solar rays, in which _time_ is considered one of the
+elements of observation, is illustrated by fig. 68. The actinometer
+consists of a large cylindrical thermometer bulb, with a scale
+considerably lengthened, so that minute changes may be easily seen. The
+bulb is of transparent glass filled with a deep blue liquid, which is
+expanded when the rays of the sun fall direct on the bulb. To take an
+observation, the actinometer is placed in the shade for one minute and
+read off; it is then exposed for one minute to sunshine, and its
+indication recorded; it is finally restored to the shade, and its reading
+noted. The mean of the two readings in the shade, subtracted from that in
+the sun, gives the actual amount of expansion of the liquid produced by
+the sun's rays in one minute of time. For further information, see _Report
+of the Royal Society on Physics and Meteorology_; or _Kaemtz's
+Meteorology_, translated by C. V. Walker; or the _Admiralty Manual of
+Scientific Instructions_.
+
+
+
+
+CHAPTER IX.
+
+DEEP-SEA THERMOMETERS.
+
+
+=89. On Sixe's Principle.=--Thermometers for ascertaining the temperature
+of the sea at various depths are constructed to register either the
+maximum or minimum temperature, or both. The principle of each instrument
+is that of Sixe. There are very few parts of the ocean in which the
+temperature below is greater than at the surface, except in the Polar
+Seas, where it is generally found to be a few degrees warmer at
+considerable depths than at the surface. When the instrument is required
+to register only one temperature, it can be made narrower and more
+compact--a great advantage in sounding; and with less length of bulb and
+glass tube, so that the liability of error is diminished. Hence, the
+minimum is the most generally useful for deep-sea soundings. These
+thermometers must be sufficiently strong to withstand the pressure of the
+ocean at two or three miles of depth, where there may be a force exerted
+to compress them exceeding three or four hundred atmospheres (of 15 lbs.
+to the square inch).
+
+Many have been the contrivances for obtaining correct deep-sea
+indications. Thermometers and machines of various sorts have been
+suggested, adopted, and eventually abandoned as only approximate
+instruments. The principal reason for such instruments failing to give
+correct or reliable indications, has been that the weight or pressure on
+the bulbs at great depths has interfered with the correct reading of the
+instruments. Thermometers have been enclosed in strong water-tight cases
+to resist the pressure; but this contrivance has only had the tendency to
+retard the action, so much so as to throw a doubt on the indications
+obtained by the instrument so constructed.
+
+The thermometers constructed by Messrs. Negretti and Zambra for this
+purpose do not differ materially from those usually made under the
+denomination of Sixe's thermometers, except in the following most
+important particular:--The usual Sixe's thermometers have a central
+reservoir or cylinder containing alcohol; this reservoir, which is the
+only portion of the instrument likely to be affected by pressure, has
+been, in Negretti and Zambra's new instrument, superseded by a strong
+outer cylinder of glass, containing mercury and rarefied air; by this
+means the portion of the instrument susceptible of compression, has been
+so strengthened that no amount of pressure can possibly make the
+instrument vary. This instrument has been tested in every possible manner,
+and the results have been highly satisfactory, so much so as to place
+their reliability beyond any possible doubt.
+
+The scales are made of porcelain, and are firmly secured to a back of oak,
+which holds in a recess the bulb with its protecting shield, and is
+rounded off so as to fit easily and firmly in a stout cylindrical copper
+case, in which the thermometer is sent down when sounding (see fig. 69).
+The lid of the case is made to fit down closely, and water-tight. At the
+bottom of the case is a valve opening upward; and the lid has a similar
+valve. These allow the water to pass through the case as the instrument
+sinks, so that the least amount of obstruction is offered to the descent.
+At the lower end of the case is a stout brass spring, to protect the
+instrument from a sudden jar if it should touch the bottom while
+descending rapidly. As the instrument is drawn up, the valves close with
+the weight of water upon them, and it arrives at the surface filled with
+water brought up from its lowest position. The deep-sea thermometers used
+in the Royal Navy are of this pattern.
+
+[Illustration: Fig. 69.]
+
+
+=90. Johnson's Metallic Deep-Sea Thermometer.=--The objection to the
+employment of mercurial thermometers for ascertaining the temperature of
+the ocean at depths, arising from the compression of the bulbs, which was
+of such serious consequence previous to the modification made in the
+construction of the instrument by Messrs. Negretti and Zambra, led to the
+construction of a metallic thermometer altogether free from liability of
+disturbance from compression by the surrounding water; which, however, is
+certainly not so sensitive to changes of temperature as mercury. This
+instrument is the invention of Henry Johnson, Esq., F.R.A.S., and is thus
+described by him:--
+
+"During the year 1844 some experiments were made by James Glaisher, Esq.,
+F.R.S., on the temperature of the water of the Thames near Greenwich at
+the different seasons of the year; when that gentleman found that the
+indications of temperature were greatly affected by the pressure on the
+bulbs of the thermometers. At a depth of 25 feet this pressure would be
+nearly equal to the presence of three-fourths of an atmosphere. These
+observations demonstrate the importance of using in deep-sea soundings an
+instrument free from liability of disturbance from compression by the
+surrounding water, and have ultimately led to the construction of the
+thermometer now to be described.
+
+"The instrument is composed of solid metals of considerable specific
+gravity, viz. of brass and steel, the specific gravity of these metals
+being 8.39 and 7.81 respectively. They are therefore not liable to
+compression by the water, which under a pressure of 1,120 atmospheres, or
+at a depth of 5,000 fathoms in round numbers, acquires a density or
+specific gravity of 1.06. In the construction of this instrument,
+advantage has been taken of the well-known difference in the ratios of
+expansion and contraction by heat and cold of brass and steel, to form
+compound bars of thin bars of these metals riveted together; and which
+will be found to assume a slight curve in one direction when heat has
+expanded the brass more than the steel, and a slight one in the contrary
+direction when cold has contracted the brass more than the steel.
+
+"The indications of the instrument record the motions under changes of
+temperature of such compound bars; in which the proportion of brass, the
+more dilatable metal, is two-thirds, and of steel one-third.
+
+[Illustration: Fig. 70.]
+
+"Upon one end of a narrow plate of metal about a foot long, _a_, are fixed
+three scales of temperature, _h_, which ascend from 25 deg. to 100 deg. F., and
+which are shown more clearly in the drawing detached from the instrument.
+Upon one of these scales the present temperature is shown by the pointer,
+_e_, which turns upon a pivot in its centre. The register index, _g_, to
+the maximum temperature, and the index, _f_, to the minimum temperature,
+are moved along the other scales by the pin upon the moving pointer, at
+_e_, where they are retained by stiff friction. At equal distances from
+the centre of the pointer are two connecting pieces, _d d_, by which it is
+attached to the free ends of two compound bars, _b b_, and its movements
+correspond with the movements of the compound bars under variations of
+temperature. The other ends of the bars are fastened by the plate, _c_, to
+the plate, _a_, on which the scales of temperature are fixed. The
+connection of the bars with both sides of the centre of the pointer
+prevents disturbance of indication by lateral concussion. The case of the
+instrument has been improved at the suggestion of Admiral FitzRoy, and now
+presents to the water a smooth cylindrical surface, with rounded ends, and
+without projection of fastenings.
+
+"In surveying expeditions, this instrument would be found useful in giving
+notice of variation of depth of water, and of the necessity for taking
+soundings. A diminution of the temperature of water has been observed by
+scientific voyagers to accompany diminution of depth, as on nearing land,
+or approaching hidden rocks or shoals. Attention would also thus be
+attracted to the vicinity of icebergs."
+
+This thermometer might easily be modified to serve for several other
+important purposes, such as the determination of the temperature of
+intermittent hot springs, and mud volcanoes.
+
+[Illustration: Fig. 71.]
+
+The principle of this thermometer is not altogether new; but the duplicate
+arrangement of the bars, which effectually prevents the movement of the
+indices by any shaking, and the application are certainly novel. Professor
+Trail, in the _Library of Useful Knowledge_, writes:--"In 1803, Mr. James
+Crighton, of Glasgow, published a new 'metallic thermometer,' in which
+the unequal expansion of zinc and iron is the moving power. A bar is
+formed by uniting a plate of zinc (fig. 71), _c d_, 8 inches long, 1 inch
+broad, and 1/4 inch thick, to a plate of iron, _a b_, of the same length.
+The lower extremity of the compound bar is firmly attached to a mahogany
+board at _e e_; a pin, _f_, fixed to its upper end, plays in the forked
+opening in the short arm of the index, _g_. When the temperature is
+raised, the superior expansion of the zinc, _c d_, will bend the whole
+bar, as in the figure; and the index, _g_, will move along the graduated
+arc, from right to left, in proportion to the temperature. In order to
+convert it into a _register thermometer_, Crighton applied two slender
+hands, _h h_, on the axis of the index; these lie below the index, and are
+pushed in opposite directions by the stud, _i_,--a contrivance seemingly
+borrowed from the instrument of Fitzgerald," a complicated metallic
+thermometer, described by the Professor previously.
+
+
+
+
+CHAPTER X.
+
+BOILING-POINT THERMOMETERS.
+
+
+=91. Ebullition.=--The temperature at which a fluid _boils_ is called the
+_boiling-point_ of that particular fluid. It is different for different
+liquids; and, moreover, in the same liquid it varies with certain changes
+of circumstance. Thus the same liquid in various states of purity would
+have its boiling temperature altered in a slight degree. There is also an
+intimate connection with the pressure under which a fluid is boiled, and
+its temperature of ebullition. Liquids boiled in the open air are
+subjected to the atmospheric pressure, which is well known to vary at
+different times and places; and the boiling-point of the liquid exhibits
+corresponding changes. When the pressure is increased on the surface of
+any fluid, the temperature of ebullition rises; and with a decrease of
+pressure, the boiling goes on at a lower degree of heat.
+
+In the case of water, we commonly state the boiling-point to be 212 deg. F.;
+but it is only so at the level of the sea, under the mean pressure of the
+atmosphere, represented, in the latitude of London, by a column of 29.905
+inches of mercury, at a temperature of 32 deg. F., and when the water is fresh
+and does not contain any matter chemically dissolved in it. When steam is
+generated and confined in a boiler, the pressure upon the boiling water
+may be several times greater than that of the atmosphere. Experimentally
+it has been found, that if the pressure in the boiler be 25 lbs. on the
+square inch, the temperature of the boiling water, and of the steam
+likewise, is raised to 241 deg.; while under the exhausted receiver of an
+air-pump, water will boil at 185 deg., when the pressure is reduced to 17
+inches of mercury.
+
+
+=92. Relation between the Boiling-Point and Elevation.=--Now, as the
+atmospheric pressure is diminished by ascent, as shown by the fall of
+mercury in the barometer, it follows that in elevated localities water, or
+any other fluid, heated in the open air, will boil at a temperature lower
+than at the sea-level. Therefore, there must be some relation between the
+height of a hill, or mountain, and the temperature at which a fluid will
+boil at that height. Hence, the thermometer, as used to determine the
+boiling-point of fluids, is also an indicator of the atmospheric pressure;
+and may be used as a substitute for the barometer in measuring elevations.
+
+If the atmospheric pressure were constant at the sea-level, and always the
+same for definite heights, we might expect the boiling-points of fluids
+also to be in exact accordance with height; and the relation once
+ascertained, we could readily, by means of the thermometer and boiling
+water, determine an unknown height, or for a known elevation assert the
+boiling temperature of a liquid. However, as the atmospheric pressure is
+perpetually varying at the same place, within certain limits, so there
+are, as it were, sympathetic changes in the boiling temperatures of
+fluids. It follows from this, that heights can never be accurately
+measured, either by the barometer or the boiling-point thermometer, by
+simply observing at the places whose elevations are required. To determine
+a height with any approach to accuracy, it is necessary that a similar
+observation should be made at the same time at a lower station, not very
+remote laterally from the upper, and that they should be many times
+repeated. When such observations have been very carefully conducted, the
+height of the upper station above the lower may be ascertained with great
+precision, as has been repeatedly verified by subsequent trigonometrical
+measurement of elevations so determined. If the lower station be at the
+sea-level, of course the absolute height of the upper is at once obtained.
+
+
+=93. Mountain Thermometer; sometimes called Hypsometric Apparatus.=--We
+have now to examine the construction of the boiling-point thermometer, and
+its necessary appendages, as adapted for the determination of heights.
+
+Messrs. Negretti and Zambra's arrangement of the instrument is shown in
+figures 72 and 73.
+
+[Illustration: Fig. 72.]
+
+[Illustration: Fig. 73.]
+
+The thermometer is made with an elongated bulb, so as to be as sensitive
+as possible. The scale, about a foot long, is graduated on the stem, and
+ranges from 180 deg. to 214 deg., each degree being sufficiently large to show the
+divisions of tenths of a degree. A sliding metallic vernier might perhaps
+with advantage be attached to the stem, which would enable the observer to
+mark hundredths of a degree; which, however, he can pretty well do by
+estimation. The boiler is so contrived as to allow, not only the bulb, but
+the stem also of the thermometer, to be surrounded by the steam. The
+arrangement is readily understood by reference to the accompanying
+diagram, fig. 73.
+
+_C_, is a copper boiler, supported by a tripod stand so as to allow a
+spirit-lamp, _A_, made of metal to be placed underneath. The flame from
+the lamp may be surrounded by a fine wire gauze, _B_, which will prevent
+it being extinguished when experimenting in the external air. _E E E_, is
+a three-drawn telescope tube, proceeding from the boiler, and open also at
+top. Another tube, similarly constructed, envelops this, as shown by _D D
+D_. This tube is screwed to the top of the boiler, and has two openings,
+one at the top to admit the thermometer, the other low down, _G_, to give
+vent to the steam. As the steam is generated, it rises in the inner tube,
+passes down between the tubes, and flows away at _G_. The thermometer is
+passed down, supported by an india-rubber washer, fitting steam tight, so
+as to leave the top of the mercury, when the boiling-point is attained,
+sufficiently visible to make the observation. The telescopic movement, and
+the mode of supporting the thermometer, enable the observer always to keep
+the bulb near the water, and the double tube gives all the protection
+required to obtain a steady boiling-point. Some boiling-point thermometers
+are constructed with their scales altogether exposed to the air, which may
+be very cold, and consequently may contract to some extent the thread of
+mercury outside the boiler. The steam, having the same temperature as the
+boiling water, keeps the tube, throughout nearly its whole length, at the
+same degree of heat, in the apparatus described. The whole can be packed
+in a tin case very compactly and securely for travelling, as in fig. 72.
+
+_Directions for Using._--When the apparatus is required for practical use,
+sufficient water must be poured into the boiler to fill it about one
+third, through an opening, _F_, which must be afterwards closed by the
+screw plug. Then apply the lighted lamp. In a short time steam will issue
+from _G_; and the mercury in the thermometer, kept carefully immersed,
+will rise rapidly until it attains a stationary point, which is the
+boiling temperature. The observation should now be taken and recorded with
+as much accuracy as possible, and the temperature of the external air must
+be noted at the same time by an ordinary thermometer.
+
+The water employed should be pure. Distilled water would therefore be the
+best. If a substance is held mechanically suspended in water, it will not
+affect the boiling-point. Thus, muddy water would serve equally as well as
+distilled water. However, as it cannot be readily ascertained that nothing
+is dissolved chemically when water is dirty, we are only correct when we
+employ pure water.
+
+
+=94. Precautions to ensure correct Graduation.=--Those who possess a
+boiling-point thermometer should satisfy themselves that it has been
+correctly graduated. To do this, it is advisable to verify it with the
+reading of a standard barometer reduced to 32 deg. F. The table of "Vapour
+Tension" (given at p. 62) will furnish the means of comparison. Thus, if
+the reduced reading of the barometer, corrected also for latitude, be
+29.922, the thermometer should show 212 deg. as the boiling-point of water at
+the same time and place; if 29.745, the thermometer should read 211.7; and
+so on as per table. In this way the error of the chief point of the scale
+can be obtained. Other parts of the scale may be checked with a standard
+thermometer, by subjecting both to the same temperature, and comparing
+their indications. The graduations as fixed by some makers are not always
+to be trusted; and this essential test should be conducted with the utmost
+nicety and care.
+
+Admiral FitzRoy writes, in his _Notes on Meteorology_:--"Each degree of
+the boiling-point thermometer is equivalent to about 550 _feet of ascent_,
+or one-tenth to 55 feet; therefore, the smallest error in the graduation
+of the thermometer itself will affect the height deduced materially.
+
+"In the thermometer which is graduated from 212 deg. (the boiling-point) to
+180 deg., similarly to those intended for the purpose of measuring heights,
+there must have been a starting point, or zero, from which to begin the
+graduation. I have asked an optician in London how he fixed that zero, the
+boiling-point. 'By boiling water at my house,' he replied. 'Where is your
+house?' In such a part of the town, he answered. I said: 'What height is
+it above the sea?' to which he replied, 'I do not know;' and when I asked
+the state of the barometer when he boiled the water, whether the mercury
+was high or low, he said that he had not looked at it! Now, as this
+instrument is intended to measure heights and to decide differences of
+some hundred, if not thousand feet upwards, at least one should endeavour
+to ascertain a reliable starting point. From inquiries made, I believe
+that the determination of the boiling-point of ordinary thermometers has
+been very vague, not only from the extreme difficulties of the process
+itself (which are well known to opticians), but from the radical errors of
+not allowing for the pressure of the atmosphere at the time of
+graduation--which may be much, even an inch higher or lower, than the
+mean, or any _given height_--while the elevation of the place above the
+level of the sea is also unnoticed. Then there is another source of error,
+a minor one, perhaps: the inner limit, the 180 deg. point, is fixed only by
+comparison with another thermometer; it may be right, or it may be very
+much out, as may be the intermediate divisions; for the difficulty of
+ascertaining degree by degree is great: and it must be remembered that the
+measurement of a very high mountain depends upon those inner degrees from
+200 deg. down to 180 deg., thereabouts. Hence, the difficulty of making a reliable
+observation by boiling water seems to be greater than has been generally
+admitted."
+
+
+=95. Method of Calculating Heights from Observations with the Mountain
+Thermometer.=--Having considered how to make observations with the proper
+care and accuracy, it becomes necessary to know how to deduce the height
+by calculation. That a constant intimate relation exists between the
+boiling temperature of water and the pressure of the air, we have already
+learned. This knowledge is the result of elaborate experiments made by
+several scientific experimentalists, who have likewise constructed formulae
+and tables for the conversion of the boiling temperatures into the
+corresponding pressures of vapour, or, which is equivalent, of the
+atmosphere, when the operation is performed in the open air. As might be
+expected, there is not a perfect accord in the results arrived at by
+different persons. Regnault is the most recent, and his experiments are
+considered the most reliable.
+
+From Regnault's table of vapour tension, we can obtain the pressure in
+inches of mercury at 32 deg., which corresponds to the observed boiling-point;
+or _vice versa_, if required. From the pressure, the height may be deduced
+by the method for finding heights by means of the barometer.
+
+The following table expresses very nearly the elevation in feet
+corresponding to a fall of 1 deg. in the temperature of boiling water:--
+
+  Boiling Temperatures      Elevation in Feet
+      between.               for each Degree.
+
+  214 deg. and 210--                  520
+  210  and 200--                  530
+  200  and 190                    550
+  190  and 180                    570
+
+These numbers agree very well with the results of theory and actual
+observation. The assumption is that the boiling-point will be diminished
+1 deg. for each 520 feet of ascent until the temperature becomes 210 deg., then
+530 feet of elevation will lower it one degree until the water boils at
+200 deg., and so on; the air being at 32 deg.
+
+Let _H_ represent the vertical height in feet between two stations; _B_
+and _b_, the boiling-points of water at the lower and upper stations
+respectively; _f_, the factor found in the above table. Then
+
+  _H_ = _f_(_B_ - _b_)
+
+Further, let _m_ be the mean temperature of the stratum of air between the
+stations. Now, if the mean temperature is less than 32 deg., the column of air
+will be shorter; and if greater, longer than at 32 deg. According to
+Regnault, air expands 1/491.13 or .002036 of its volume at 32 deg., for each
+degree increase of heat. Calling the correction due to the mean
+temperature of air _C_, its value will be found from the equation,
+
+  _C_ = _H_ (_m_ - 32) .002036
+
+Calling the corrected height _H'_, it will be found from the formula,
+
+            _H'_ = _H_ + _H_ (_m_ - 32) .002036
+  that is,  _H'_ = _H_ { 1 + (_m_ - 32) .002036 }
+
+and substituting the value of _H_,
+
+  _H'_ = _f_(_B_ - _b_) { 1 + (_m_ - 32) .002036 }
+
+Strictly, according to theoretical considerations, there is a correction
+due to latitude, as in the determination of heights by the barometer; but
+its value is so small that it is practically of no importance.
+
+If a barometer be observed at one of the stations, the table of vapour
+tensions (p. 62) will be useful in converting the pressure into the
+corresponding boiling-point, or _vice versa_; so that the difference of
+height may be found either by the methods employed for the boiling-point
+thermometer or the barometer.
+
+In conclusion, it may be remarked that observers who have good instruments
+at considerable elevations, as sites on mountains or plateaus, would
+confer a benefit to science, by registering for a length of time the
+barometer along with the boiling temperature of water, as accurately as
+possible. Such observations would serve to verify the accuracy of
+theoretical deductions, and fix with certainty the theoretical scale with
+the barometer indications.
+
+_Example, in calculating Heights from the Observations of the
+Boiling-point of Water._--1. At Geneva the observed boiling-point of water
+was 209 deg.335; on the Great St. Bernard it was 197 deg.64; the mean
+temperature of the intermediate air was 63 deg.5; required the height of the
+Great St. Bernard above Geneva.
+
+Method by formula:--
+
+  _H'_ = _f_ (_B_ - _b_) { 1 + (_m_ - 32 deg.) .002036 }
+
+In this case _f_ is between 530 and 550, or 540.
+
+  _B_ = 209.335          _m_ = 63.5
+  _b_ = 197.64                 32
+        -------                -----
+         11.695                31.5
+  _f_  =    540                  .002036
+        -------                ---------
+         6315.3                 0.0641340
+          1.064                 1
+        -------                 -----
+  _H'_ = 6719.5 feet.           1.064
+         ======
+
+Method by Tables supplied with boiling-point apparatus made by Messrs.
+Negretti and Zambra:--
+
+  209.335 gives 1464 in Table I.
+  197.64      "     7736      "
+                    ----
+                    6272
+   63.5       "        1.07 in Table II.
+                    ----
+  Height            6711
+                    ====
+
+
+=96. Thermometers for Engineers.=--_1st. Salinometer._--Under the
+circumstances at which fresh water boils at 212 deg., sea water boils at
+213 deg.2. The boiling temperature is raised by the chemical solution of any
+substance in the water, and the more with the increase of matter
+dissolved.
+
+From a knowledge of this principle, marine engineers make use of the
+thermometer to determine the amount of salts held in solution by the water
+in the boilers of sea-going steamers. Common sea-water contains 1/33 of
+its volume of salt and other earthy matters. As evaporation proceeds, the
+solution becomes proportionally stronger, and more heat is required to
+produce steam. The following table from the work of Messrs. Main and
+Brown, on the Marine Steam-Engine, shows the relation between the
+boiling-point under the mean pressure of the atmosphere, or 80 inches of
+mercury, and the proportion of matter dissolved in the water:--
+
+  Proportion of Salt in 100 parts of water   0    Boiling-point 212 deg.
+            "                     "         1/33       "        213.2
+            "                     "         2/33       "        214.4
+            "                     "         3/33       "        215.5
+            "                     "         4/33       "        216.6
+            "                     "         5/33       "        217.9
+            "                     "         6/33       "        219.0
+            "                     "         7/33       "        220.2
+            "                     "         8/33       "        221.4
+            "                     "         9/33       "        222.5
+            "                     "        10/33       "        223.7
+            "                     "        11/33       "        224.9
+            "                     "        12/33       "        226.0
+
+When the salts in solution amount to 12/33, the water is saturated. It has
+also been ascertained that, when a solution of 4/33 is attained,
+incrustation of the substances commences on the boiler. Hence, it is a
+rule with engineers to expel some of the boiling water, when the
+thermometer indicates a temperature of 216 deg., and introduce some more cold
+water, in order to prevent incrustation, which not only injures the
+boiler, but opposes the passage of heat to the water. The thermometer used
+for this purpose should be very accurately graduated, and the scale must
+be considerably higher than, though it need not read much below 212 deg.
+
+_2nd. Pressure Gauge._--The elasticity of gases augments by increase of
+temperature, and _vice versa_; it follows, therefore, that when steam is
+generated in a closed boiler, its temperature rises beyond the boiling
+temperature of 212 deg., owing to the increased pressure upon the water. The
+law connecting the pressure and the corresponding temperature of steam is
+the same as that upon which the boiling of fluids under diminished
+atmospheric pressure takes place. Hence, the indications of the
+thermometer become exponents of steam pressure. Engineers are furnished,
+in works on the steam-engine, with tables, from which the pressure
+corresponding to a given temperature, or the converse, can be obtained by
+mere inspection.
+
+[Illustration: Fig. 74.]
+
+Fig. 74 represents the thermometer employed as a steam-pressure gauge. It
+is fitted in a brass case, with screw-plug and washers for closing the
+boiler when the thermometer is not in use. The scale shows the pressure
+corresponding to the temperature, from 15 to 120 lbs., above the
+atmospheric pressure, which is usually taken as 15 lbs. on the square
+inch.
+
+
+
+
+CHAPTER XI.
+
+INSTRUMENTS FOR ASCERTAINING THE HUMIDITY OF THE AIR.
+
+
+=97. Hygrometric Substances.=--The instruments devised for the purpose of
+ascertaining the humidity of the atmosphere are termed _hygrometers_. The
+earliest invented hygrometers were constructed of substances readily acted
+upon by the vapour in the air, such as hair, grass, seaweed, catgut, &c.,
+which all absorb moisture, and thereby increase in length, and when
+deprived of it by drying they contract. Toy-like hygrometers, upon the
+principle of absorption, are still common as ornaments for mantel-pieces.
+A useful little instrument of this class, formed from the beard of the
+wild oat, is made to resemble a watch in external appearance, and is
+designed to prove the dampness or dryness of beds: a moveable hand points
+out on the dial the hygrometric condition of the clothes upon which the
+instrument is laid.
+
+
+=98. Saussure's Hygrometer=, formerly used as a meteorologic instrument,
+but now regarded as an ornamental curiosity, is represented in fig. 75.
+Its action depends upon a prepared hair, fixed at one end to the frame of
+the instrument, and wound round a pulley at the other. The pulley carries
+a pointer which has a counterpoise sufficient to keep the hair stretched.
+By this means the shrinking and lengthening of the hair cause the pointer
+to traverse a graduated arc indicating the relative humidity.
+
+[Illustration: Fig. 75.]
+
+Such instruments, however ingenious, are not of scientific value; because
+they do not admit of rigid comparison, are liable to alter in their
+contractile and expansive properties, and cannot be made to indicate
+precisely alike.
+
+
+=99. Dew-Point.=--The amount of water which the air can sustain in an
+invisible form increases with the temperature; but for every definite
+temperature there is a limit to the amount of vapour which can be thus
+diffused. When the air is cooled, the vapour present may be more than it
+can sustain; part will then be condensed as dew, rain, hail or snow,
+according to the meteorologic circumstances. The temperature which the air
+has when it is so fully saturated with vapour that any excess will be
+deposited as dew, is called the _dew-point_.
+
+
+=100. Drosometer.=--"To measure the quantity of dew deposited each night,
+an instrument is used called a _Drosometer_. The most simple process
+consists in exposing to the open air bodies whose exact weight is known,
+and then weighing them afresh after they are covered with dew. According
+to Wells, locks of wool, weighing about eight grains, are to be preferred,
+which are to be divided [formed] into spherical masses of the diameter of
+about two inches."--_Koemtz._
+
+
+=101. Humidity.=--The proportion existing between the amount of vapour
+actually present in the air at any time, and the quantity necessary to
+completely saturate it, is called _the degree of humidity_. It is usually
+expressed in a centesimal scale, 0 being perfect dryness, and 100 complete
+saturation.
+
+The pressure, or tension, of vapour at the dew-point temperature, divided
+by the tension of vapour at the air temperature and the quotient
+multiplied by 100, gives the degree of humidity. (Regnault's Tables should
+be used.)
+
+Hence the utility of instruments for determining the dew-point.
+
+
+=102. Leslie's Hygrometer.=--This instrument consists of a glass syphon
+tube, terminated with a bulb or ball at each end, turned outwards from
+each other, as in fig. 76. The tube is partly filled with concentrated
+sulphuric acid, tinged by carmine. One of the balls is covered smoothly
+with fine muslin, and is kept continually moistened with pure water, drawn
+from a vase placed near it by the capillary attraction of a few strands of
+clean cotton-wick. The descent of the coloured liquid in the other stem
+will mark the diminution of temperature caused by the evaporation of the
+water from the humid surface. The drier the ambient air is, the more
+rapidly will the evaporation go on; and the cold produced will be greater.
+When the air is nearly saturated with moisture, the evaporation goes on
+slowly; the cold produced is moderate, because the ball regains a large
+portion of its lost heat from surrounding bodies; and the degree of
+refrigeration of the ball is an index of the dryness of the air.
+
+[Illustration: Fig. 76.]
+
+"Should the water become frozen on the ball, this hygrometer will still
+act; for evaporation goes on from the surface of ice in proportion to the
+dryness of the air. Leslie estimates, that when the ball is moist, air, at
+the temperature of the ball, will take up moisture equal to the
+sixteen-thousandth part of its weight, for each degree of his hygrometer;
+and as ice in melting requires one-seventh of the caloric consumed in
+converting water into vapour, when the ball is frozen, the hygrometer will
+sink more than when wet by 1 deg. in 7 deg.; and hence, in the frozen state, we
+must increase the value of the degrees one-seventh: so that each of them
+will correspond to an absorption of moisture equal to
+one-fourteen-thousandth part of the weight of the air.
+
+"When this hygrometer stands at 15 deg., the air feels damp; from 30 deg. to 40 deg.,
+we reckon it dry; from 50 deg. to 60 deg., very dry; and from 70 deg. upwards, we
+should call it intensely dry. A room would feel uncomfortable, and would
+probably be unwholesome, if the instrument in it did not reach 30 deg.[8] In
+thick fogs it keeps almost at the beginning of the scale. In winter, in
+our climate, it ranges from 5 deg. to 15 deg.; in summer often from 15 deg. to 55 deg.;
+and sometimes attains 80 deg. or 90 deg. The greatest degree of dryness ever
+noticed by Leslie was at Paris, in the month of September, when the
+hygrometer indicated 120 deg."--_Professor Trail, in "Library of Useful
+Knowledge."_
+
+In estimating the value of the indications of this hygrometer, it should
+be borne in mind that the scale adopted by Leslie was _millesimal_, that
+is to say, from the freezing to the boiling-point of water was divided
+into a thousand parts; ten millesimal degrees are therefore equal to one
+of the scale of Celsius.
+
+
+103. DANIEL'S HYGROMETER.
+
+This instrument was invented about the year 1820, by Professor Daniel, the
+distinguished author of _Meteorological Essays_; and it entirely
+superseded all hygrometers depending upon the absorption of moisture. The
+form of the instrument is shown in fig. 77.
+
+[Illustration: Fig. 77.]
+
+It consists of a glass tube, about one-eighth of an inch in diameter of
+bore, bent twice at right angles, and terminated, at each end, in a bulb
+about one inch and a quarter in diameter. In one limb of the tube is
+enclosed a delicate thermometer, which descends to the centre of the
+adjoining bulb, which is about three-parts filled with sulphuric ether.
+All the other parts of the tube are carefully freed from air, so that they
+are occupied by the vapour of the ether. This bulb is generally made of
+black glass; the other is transparent, but covered with a piece of fine
+muslin. The support for the tube has a thermometer attached, which shows
+the temperature of the external air. The tube can be removed from the
+stand, and the parts are made to pack, with a necessary phial of ether, in
+a small box, which can easily be got into the pocket.
+
+_How to use the Hygrometer._--This instrument gives the dew-point by
+direct observation, which must be made in the following manner:--Having
+fixed the tube upon the stand, with the bulbs vertically downward, the
+ether is all caused to flow into the lower ball by inclining the tube. The
+temperature of the air is noted by the exposed thermometer. Then some
+ether is poured, from a dropping tube fitting into the neck of the phial,
+upon the muslin-covered bulb. The rapid evaporation of this ether cools
+the bulb and causes condensation of the ethereal vapour in its interior.
+This gives rise to rapid evaporation of the ether in the lower bulb,
+whereby its temperature is greatly reduced. The air in the vicinity is
+deprived of its warmth by the cold bulb, and is soon cooled to the
+temperature at which it is perfectly saturated with the vapour which it
+contains. Cooled ever so little below this temperature, some aqueous
+vapour will be condensed, and will form a dew upon the black-glass bulb.
+At the first indication of the deposit of dew the reading of the internal
+thermometer is taken: which is the dew-point.
+
+This hygrometer has undeniable disadvantages. The surface upon which the
+dew condenses is small, and requires a peculiar direction of light in
+which to see it well. The observer, having his attention on the bulb and
+the thermometer, cannot always fix with precision the dew-point; and hence
+he is recommended to note the temperature at the appearance and at the
+disappearance of the dew, in order that the chance of error may be
+diminished. Without doubt, the necessarily long continuance of the
+observer near the instrument influences, to some extent, the observed
+temperatures; and the difficulty of not being always able to procure pure
+ether for the experiments is not the least of the drawbacks to the use of
+the instrument. Some of these disadvantages are obviated in Regnault's
+hygrometer.
+
+
+104. REGNAULT'S CONDENSER HYGROMETER
+
+(Fig. 78) consists of a tube, _C_, made of silver, very thin, and
+perfectly polished; the tube is larger at one end than the other, the
+large part being 1.8 inches in depth, by 0.8 in diameter; this is fitted
+tightly to a brass stand, _B_, with a telescopic arrangement for adjusting
+when making an observation.
+
+[Illustration: Fig. 78.]
+
+The tube, _C_, has a small lateral tubulure, to which is attached an
+India-rubber tube, with ivory mouth-piece; this tubulure enters _C_ at
+right angles near the top, and traverses it to the bottom of the largest
+part.
+
+A delicate thermometer, _D_, is inserted through a cork, or India-rubber
+washer, at the open end of the tube, _C_, the bulb of which descends to
+the centre of its largest part.
+
+_G_ is an attached thermometer for taking the temperature of the air, and
+_F_ is a bottle containing ether.
+
+_To use the Condenser Hygrometer_, a sufficient quantity of ether is
+poured into the silver tube to cover the thermometer bulb: on allowing air
+to pass bubble by bubble through the ether, by breathing in the tube, _E_,
+an uniform temperature will be obtained; if the ether continues to be
+agitated, by breathing briskly through the tube a rapid reduction of
+temperature will be the result; at the moment the ether is cooled down to
+the dew-point temperature, the external surface of that portion of the
+silver tube containing ether will become covered with a coating of
+moisture, and the degree shown by the thermometer at that instant will be
+the temperature of the dew-point.
+
+This form of hygrometer, for ascertaining by direct observation the
+dew-point, is so superior to Daniell's, both from its being more certain
+in its indications and economical in use, that Messrs. Negretti and
+Zambra have been induced to modify it, and reduce its price to little more
+than that of a good Daniell's Hygrometer.
+
+
+=105. Temperature of Evaporation.=--When the air is not saturated with
+vapour, evaporation is going on with more or less activity, according as
+the temperature is high or low, rising or falling. Now vapour cannot be
+formed without an expenditure of heat; as we invariably find that the
+process of evaporation lowers the temperature of the liquid from which the
+vapour is produced, and, by communication, that of contiguous substances
+also. Thus the emigrant, crossing the line under the scorching influence
+of the vertical sun, wraps a wet towel round his can of water, swings it
+in the breeze, to evaporate the moisture of the towel, and obtains a glass
+of cool water. So also, European residents in India, during the hot
+season, spread out mats in their apartments, and keep them wet, in order
+that the evaporation may cool the air. This principle has been applied,
+for the purpose of ascertaining the hygrometric condition of the air, in
+the instrument known as Mason's hygrometer, or psychrometer, which is now
+in general use, from its simplicity, accuracy, and ease of observing.
+
+
+106. MASON'S HYGROMETER.
+
+=The Dry and Wet Bulb Hygrometer, or Psychrometer=, known also as Mason's
+hygrometer (fig. 79), consists of two parallel thermometers, as nearly
+identical as possible, mounted on a wooden bracket, one marked _dry_, the
+other _wet_. The bulb of the wet thermometer is covered with thin muslin,
+and round the neck is twisted a conducting thread of lamp-wick, which
+passes into a vessel of water, placed at such a distance as to allow a
+length of conducting thread, of about three inches; the cup or glass is
+placed on one side, and a little beneath, so that the water within may not
+affect the reading of the _dry bulb thermometer_. In observing, the eye
+should be placed on a level with the top of the mercury in the tube, and
+the observer should refrain from breathing whilst taking an observation.
+
+[Illustration: Fig. 79.]
+
+The _dry_ bulb thermometer indicates the temperature of the air itself;
+while the wet bulb, cooled by evaporation, shows a lower temperature
+according to the rapidity of evaporation.
+
+_To find the Dew-point._--From the readings of the two thermometers, the
+dew-point can be deduced by formulae (that known as Apjohn's is considered
+the most theoretically true), or from the valuable Hygrometric Tables by
+J. Glaisher, Esq., F.R.S.
+
+For practical purposes in estimating the comparative humidity, the annexed
+table, which is a reduction from Mr. Glaisher's elaborate work, will be
+sufficient; it will at least serve to assist in familiarising the
+inexperienced in the value of the psychrometer's indications:--
+
+  +------------------------------------------+
+  |            | Difference between Dry-bulb |
+  |            |    and Wet-bulb Readings.   |
+  |Temperature |-----------------------------|
+  |   by the   | 2 deg. | 4 deg. | 6 deg. | 8 deg. | 10 deg.| 12 deg.|
+  |  Dry Bulb  |-----------------------------|
+  |Thermometer.|     Degree of Humidity.     |
+  |------------------------------------------|
+  |     34 deg.    | 79 | 63 | 50 | .. | .. | .. |
+  |     36     | 82 | 66 | 53 | .. | .. | .. |
+  |     38     | 83 | 68 | 56 | 45 | .. | .. |
+  |     40     | 84 | 70 | 58 | 47 | .. | .. |
+  |     42     | 84 | 71 | 59 | 49 | .. | .. |
+  |     44     | 85 | 72 | 60 | 50 | .. | .. |
+  |     46     | 86 | 73 | 61 | 51 | .. | .. |
+  |     48     | 86 | 73 | 62 | 52 | 44 | .. |
+  |     50     | 86 | 74 | 63 | 53 | 45 | .. |
+  |     52     | 86 | 74 | 64 | 54 | 46 | .. |
+  |     54     | 86 | 74 | 64 | 55 | 47 | .. |
+  |     56     | 87 | 75 | 65 | 56 | 48 | .. |
+  |     58     | 87 | 76 | 66 | 57 | 49 | .. |
+  |     60     | 88 | 76 | 66 | 58 | 50 | 43 |
+  |     62     | 88 | 77 | 67 | 58 | 50 | 44 |
+  |     64     | 88 | 77 | 67 | 59 | 51 | 45 |
+  |     66     | 88 | 78 | 68 | 60 | 52 | 45 |
+  |     68     | 88 | 78 | 68 | 60 | 52 | 46 |
+  |     70     | 88 | 78 | 69 | 61 | 53 | 47 |
+  |     72     | 89 | 79 | 69 | 61 | 54 | 48 |
+  |     74     | 89 | 79 | 70 | 62 | 55 | 48 |
+  |     76     | 89 | 79 | 71 | 63 | 55 | 49 |
+  |     78     | 89 | 79 | 71 | 63 | 56 | 50 |
+  |     80     | 90 | 80 | 71 | 63 | 56 | 50 |
+  |     82     | 90 | 80 | 72 | 64 | 57 | 51 |
+  |     84     | 90 | 80 | 72 | 64 | 57 | 51 |
+  |     86     | 90 | 80 | 72 | 64 | 58 | 52 |
+  +------------------------------------------+
+
+The total quantity of aqueous vapour which at any temperature can be
+diffused in the air being represented by 100, the per-centage of vapour
+actually present will be found in the table opposite the temperature of
+the dry thermometer, and under the difference between the dry-bulb and
+wet-bulb temperatures. The degree of humidity for intermediate
+temperatures and differences to those given in the table can be easily
+estimated sufficiently accurately for most practical purposes.
+
+The difference between the two thermometer readings taken from the reading
+of the wet bulb, gives the dew-point very nearly, when the air is at any
+temperature between freezing and 80 deg. This simple rule will be found
+serviceable to horticulturists, since it will enable them to estimate the
+chilling effect of dew or hoar-frost on tender plants.
+
+_Use as an Indicator of Weather._--In our climate, the usual difference
+between the thermometer readings,--in the open air, shaded from the sun,
+reflected heat, and currents of air,--ranges from one to twelve degrees.
+In hot and dry climates, as India and Australia, the range out of doors
+has been found as much as 30 deg., occasionally.
+
+When the moisture is frozen, the bulb should be wetted afresh, and the
+reading taken just before it again freezes; but the observation then is of
+little value, and for general purposes need not be taken, as the air is
+known to be dry in frosty weather.
+
+The muslin or cotton rag should be washed once or twice a week by pouring
+water over the bulb; and it should be replaced by a fresh piece at least
+once a month. Accuracy depends very much upon keeping the wet bulb clean,
+and not _too_ wet.
+
+In connection with the barometer, this hygrometer is very useful, not only
+on land, but especially at sea, where other kinds of hygrometers cannot be
+practically used. A fall in the barometer is indicative of coming wind or
+rain: if the hygrometer shows increasing dampness by the difference of the
+readings becoming smaller,--rain may therefore be anticipated. On the
+contrary, if the hygrometer shows continuing or increasing dryness, a
+stronger wind is probable, without rain.
+
+_Domestic Uses._--Mason's hygrometer is useful in regulating the moisture
+of the air of apartments; a difference in the thermometer readings of from
+5 deg. to 8 deg. being considered healthy. Many complaints require that the
+temperature and humidity of the air which the invalid breathes should be
+carefully regulated. Hence it is a valuable household instrument. In a
+room, it should be placed away from the fire as much as possible, but not
+exposed to draughts of air.
+
+Figs. 80 and 81 show cheap arrangements of the instrument for domestic
+purposes. Other arrangements are given to the instrument to make it
+suitable for exhibiting the hygrometrical state of the air in hot-houses,
+conservatories, malting-houses, warehouses, manufactories, &c.
+
+[Illustration: Fig. 80.]
+
+[Illustration: Fig. 81.]
+
+[Illustration: Fig. 82.]
+
+Fig. 82 shows the instrument arranged on brass tripod stand, with folding
+legs and metal cover, to render it portable.
+
+
+=107. Self-Registering Hygrometer.=--A maximum thermometer and a minimum
+thermometer, each fitted up as a wet-bulb thermometer, record the highest
+and lowest temperature of evaporation during the interval of observation.
+Negretti's mercurial maximum, and an alcohol minimum, answer best.
+
+
+=108. Causes of Dew.=--"The aqueous vapour of our atmosphere is a powerful
+radiant; but it is diffused through air which usually exceeds its own mass
+more than one hundred times. Not only, then, its own heat, but the heat of
+the large quantity of air which surrounds it, must be discharged by the
+vapour, before it can sink to its point of condensation. The retardation
+of chilling due to this cause enables good solid radiators, at the earth's
+surface, to outstrip the vapour in their speed of refrigeration; and
+hence, upon these bodies, aqueous vapour may be condensed to liquid, or
+even congealed to hoar-frost, while at a few feet above the surface it
+still maintains its gaseous state."[9] The amount of moisture so deposited
+will vary with different atmospheric conditions. If the sky be decidedly
+cloudy or misty, the heat radiated from the earth will be partly restored
+by counter-radiation from the visible vapour; the cooling of the earth's
+surface will, therefore, take place slowly, and little dew will be
+deposited. On the other hand, if the air contain transparent vapour, and
+the sky appear clear, the counter-radiation will be less, the earth will
+cool rapidly, and the deposit of dew will be copious; provided the night
+be comparatively calm, for, when the wind blows, the circulating air
+supplies heat to the radiating substances, and prevents any considerable
+chilling.
+
+The dew which falls in tropical countries greatly exceeds in abundance
+what we experience in our climate; because the air is there, from the
+great heat, capable of sustaining a large amount of vapour in the
+transparent state, and the conditions most favourable for a maximum
+reduction of temperature by radiation are present. At those places, or
+upon those substances which cool the lowest and most readily, the dew
+falls most copiously.
+
+
+[Illustration: Fig. 83.]
+
+=109. Plan of Exposing Thermometers=, &c.--Figure 83 is an illustration of
+a convenient slab for supporting thermometers in an exposed position
+attached to a stand (such as Glaisher's, described in Chapter XVI.) for
+ordinary scientific observations. It has a projecting ledge, _B_, to carry
+off rain from the instruments, the slab, _A_, being erected vertically.
+The hygrometer is placed at _E_, with the vase of water at _F_. An alcohol
+minimum thermometer is represented at _C_, in the position most favourable
+to its certain action; and at _D_ is shown one of Negretti & Zambra's
+maximum thermometers, the position of which may be more nearly horizontal
+than there exhibited, although a slight depression of the bulb-end of the
+frame is desirable, but not necessary, as this thermometer can be used in
+any position.
+
+
+
+
+CHAPTER XII.
+
+INSTRUMENTS USED FOR MEASURING THE RAINFALL.
+
+
+The instruments in use for measuring the quantity of rain which falls on a
+given spot are of very simple construction. Perhaps the simplest is:--
+
+
+=110. Howard's Rain-Gauge.=--It consists of a copper funnel, a stout glass
+or stone bottle, and a measuring glass. The bottle is to be placed upon
+the ground, with the funnel resting on its neck. A brass band or cylinder
+fixed upon the outer surface of the funnel envelops the neck of the
+bottle, and the pipe of the funnel extends nearly to the bottom of the
+bottle; so that loss by evaporation is avoided as much as possible. The
+receiving space of the funnel is formed by a brass ring, five inches in
+diameter, very accurately turned. The measuring vessel enables the
+observer to note the rainfall in inches, tenths, and hundredths of an
+inch.
+
+
+[Illustration: Fig. 84.]
+
+=111. Glaisher's Rain-Gauge.=--The rain-gauge designed by Mr. Glaisher,
+the well-known meteorologist, and used by most observers of the present
+day, is arranged for the reception of the water which falls upon its
+receiving surface only, and for the prevention of loss by evaporation. The
+rain is first collected in a funnel, _B_, (fig. 84,) the receiving surface
+of which is turned in a lathe. The conical surface of the funnel slopes to
+the pipe, _E_, at an angle of 60 deg. from the horizontal receiving surface.
+The tube, _E_, is of small aperture, and is bent up, in order to retain
+the last few drops of rain, so that the only opening for the escape of
+vapour may be closed as long as possible. The funnel, _B_, fits upon the
+cylinder, _A_, tightly in the groove, _D_. A copper can is placed inside
+the cylinder, _A_, to receive the rain from the funnel. Once or twice a
+day, or after a shower, this can should be taken out, and the water
+measured in the glass measure, _C_, which is graduated to hundredths of an
+inch, according to the calculated quantity of water, determined by the
+area of the receiving space. In use, this gauge should be partly sunk in
+the ground, so that the top may be about five inches above it. Thus
+situated, there will be little or no evaporation from it during any month
+of the year; and the readings need not be taken daily, although desirable.
+
+
+=112. Rain-Gauge with Float.=--In this construction the graduated glass
+measure is dispensed with. The cylinder of the gauge is made less in
+diameter than the funnel, and a hollow, very flattened spheroid of copper
+forming a float, and carrying a vertical graduated boxwood scale which
+moves through the orifice of the funnel, is placed in it. As the rain
+accumulates the float rises, and the amount of rain in the gauge is read
+upon the scale from the top of the gauge, a bar, having a hole at the
+centre for the passage of the scale, being fixed diametrically across the
+receiving space of the funnel. The gauge is provided at the bottom with a
+brass cock, by which the water may be allowed to flow out of it whenever
+necessary.
+
+This form of gauge is not very suitable for the measurement of small
+quantities; but is admirably adapted for localities where the rainfall is
+excessive.
+
+
+[Illustration: Fig. 85.]
+
+=113. Rain-Gauge with Side-Tube.=--This instrument, as represented in fig.
+85, is a cylindrical vessel, mounted on a base shaped as a frustum of a
+cone. This base may be filled with sand or gravel to make the instrument
+stable, so that when placed upon a lawn or in a garden it may have an
+ornamental appearance. The funnel for collecting the rain is larger in
+diameter than the cylinder. Parallel to the cylinder, and communicating
+with the lowest part of the interior and extending to its top, is a
+graduated glass tube, open at both ends. The rain collected will rise as
+high in this tube as in the cylinder, and its amount can therefore be read
+off without any trouble. The gauge is emptied by the brass tap at the
+bottom of the cylinder.
+
+
+=114. Admiral FitzRoy's Rain-Gauge.=--A form of rain-gauge, very well
+adapted for expeditious observation at any time, has been designed by
+Admiral FitzRoy, and extensively employed by his observers. It is
+cylindrical in shape, with the funnel let into the top; and the rainfall
+is collected in an inner and much smaller cylinder, so that a small fall
+is represented by a considerable depth of water in the gauge. The amount
+of rain which has fallen is ascertained by a dipping tube, similar in
+principle to the dipping syphon used by gaugers for taking out specimens
+of wines or spirits from casks by simply removing the bung. A short,
+vertical, tubular opening provided with a cap, which is attached to the
+instrument by a chain that it may not be lost, is formed in the funnel.
+The measuring tube, which has a small hole at each end, should be placed
+upright in the gauge; then the thumb should be pressed over the upper
+aperture, while the tube is lifted gently out, holding in the lower part a
+quantity of water representing the depth of the rain in the gauge, the
+upper edge of which is at the mark to be read off. The glass tube is
+graduated to inches and tenths; hundredths of an inch can be readily
+estimated by the eye. The marks are fixed by actual trial with a standard
+gauge, and are artificial, not true, inches.
+
+
+=115. Self-Registering Rain-Gauge.=--The rain-gauge can be combined with
+clock-work and other mechanism so as to be self-recording of the amount of
+rain, the time, and duration of its fall. For the details of construction
+the reader is referred to the next chapter, where he will find the
+instrument described in connection with Osler's anemometer, as the
+"pluviometer." To observe and duly record the times of commencement and
+termination of rain is very desirable. Scarcely any observer can attempt
+to do this even approximately from personal observation. Hence the want of
+a cheap and simple self-recording rain-gauge is much felt, the present
+construction being too expensive for all but a few individuals.
+
+In 1862, Mr. R. Strachan estimated the duration and amount of rain in
+London (Gray's Inn Road) as follows:--
+
+  +-------------------------------------------------------------------+
+  |  MONTHS.  |INCHES.|DAYS.|HOURS.||  MONTHS.   |INCHES.|DAYS.|HOURS.|
+  |-----------+-------+-----+------++------------+-------+-----+------|
+  |January.   | 1.86  | 19  |  88  ||July.       | 2.27  | 17  |  68  |
+  |February.  | 0.37  |  9  |  25  ||August.     | 2.45  | 12  |  72  |
+  |March.     | 3.40  | 22  | 130  ||September.  | 1.70  | 12  |  55  |
+  |April.     | 2.34  | 14  |  80  ||October.    | 3.23  | 21  |  94  |
+  |May.       | 3.04  | 16  |  90  ||November.   | 1.12  | 10  |  53  |
+  |June.      | 2.45  | 20  |  83  ||December.   | 1.44  | 17  |  66  |
+  +-------------------------------------------------------------------+
+
+"During the year 1862, the rainfall amounted to 25.67 inches. Rain fell on
+179 days, that is, on nearly every other day. The hours of rain were
+estimated at 904; therefore, if the rain had fallen continuously, it would
+have lasted nearly 38 days and nights."[10] The value of similar estimates
+of the rainfall by numerous observers would be very great to meteorology.
+
+
+=116. The principle of measurement= in all these gauges is the relation
+existing between the areas of the collecting and receiving surfaces; that
+is, between the area of the funnel into which the rain falls, and the area
+of the cylinder which receives it. In Howard's and Glaisher's gauges, this
+cylinder is virtually the measuring glass itself; in the others, above
+described, the measuring scales show the same depth of water as in the
+cylinder of the gauge.
+
+The cylinder being of less diameter than the funnel, and receiving all the
+rain collected by the funnel, it follows that its contents will have an
+increased depth. Now equal cylindrical volumes, having different
+diameters, are to each other in length inversely as the squares of the
+diameters. Hence, if the funnel be 9 inches and the cylinder 3 inches in
+diameter, a fall of 1 inch of rain will be represented in the gauge by 9
+inches; for 3 squared : 9 squared :: 1 : _x_ = 9. In this case, therefore, a length of
+nine inches of the measuring glass, tube, or scale, would represent an
+inch of rainfall, and be divided into tenths and hundredths of the
+artificial inch.
+
+
+=117. Position for Rain-Gauge, &c.=--Rain gauges should be placed on the
+ground, in any position exposed to a free fall of rain, snow, or hail,
+where neither walls, buildings, nor trees shelter or cause eddies of wind.
+They should be supported by a frame, or other means, to prevent them being
+blown down by the wind, but so that they can be readily emptied.
+
+During snow or frost, the gauge must be watched, and its contents melted
+by placing it in a warm room, either when the amount is to be measured, or
+the funnel is filled up with snow. A tin vessel of equal area to the
+funnel may at such times be useful as a substitute.
+
+Rain gauges are constructed of metal, usually copper, which, besides being
+readily workable, is little affected by atmospheric influences. If made of
+iron or zinc, they should be well japanned; if of copper, this is not so
+essential. The capacity of a gauge should be sufficient to contain at
+least the probable maximum fall of rain in a day at the locality. Those
+required for rainy districts must be of large size.
+
+
+=118. Causes of Rain.=--When the invisible vapour which is diffused in the
+atmosphere becomes sufficiently cooled, it appears visible as mist or
+cloud, and a further reduction of temperature causes its precipitation as
+rain, hail, or snow. The cooling of the higher regions of the atmosphere
+is doubtless the chief cause of this condensation; but the property which
+aqueous vapour possesses of radiating heat may also contribute to the
+result. Moreover, the law which regulates the amount of vapour which air
+at any particular temperature can sustain in a transparent state,
+determines that when two bodies of air at different temperatures,
+saturated with vapour, intermix, some moisture must be rendered visible;
+and hence, it is not only possible, but highly probable, that rain may
+result from the conflict of different winds. Let us imagine two cubic
+yards of air, both saturated with moisture, but having the respective
+temperatures of 50 and 70 degrees, to come into contact. There will be a
+tendency to equalize the temperature to a mean, which is 60 deg.; and during
+this process, some of the vapour will be condensed.
+
+  For in the air at 50 deg. there is                     110.7 grains of vapour[11]
+  and       "       70      "                        216.0       "
+                                                    ------
+  Total amount of vapour                             326.7       "
+  But two cubic yards of air at 60 deg. can only sustain 313.2       "
+                                                    ------
+  Hence there will be deposited                       13.5       " of rain.
+                                                    ======
+
+It may be conceded, therefore, that when a warm and moist current of air
+encounters a body of cold air which may not be extremely dry, the mixture
+is unable to retain the whole of the vapour in an invisible state; so that
+the excess becomes visible as mist or fog, and, when the temperature has
+become sufficiently lowered, rain. The British Isles are more or less
+enveloped in fog, or mist, at the commencement of easterly winds, which,
+with a sudden change of wind, is exhibited even in summer; while the
+south-westerly winds, warm, and arriving from the ocean, deposit large
+quantities of rain by the cooling effect of the land, colder by reason of
+its latitude. When rain occurs with a northerly wind, it is probably due
+to the deposition from an upper south-westerly current, often apparently
+proved by the movements of the upper clouds.
+
+
+=119. Laws of Rain-fall.=--Tropical countries have a dry and a wet season
+during the year: _dry_, when the sun is at the opposite side of the
+equator; _wet_, when the sun is overhead. With reference to the British
+Isles, the statistics collected by Mr. G. J. Symons indicate that: 1st.
+The stations of least rain are inland, or on the east or south-east
+coasts; the stations of greatest rain are on the western coasts. 2nd. The
+rain-fall is very large in the vicinity of mountain chains or groups,
+unless the station happens to be some miles to the north-eastward.
+
+It may be well to illustrate these remarks by quoting[12] the average fall
+at a few places, grouping them as--
+
+      Westerly.
+                           Inches.
+  Bodmin                    43
+  Bolton (Lancashire)       44
+  Coniston (Windermere)     71
+  Seathwaite               127
+  Torosay (I. of Mull)      75
+  Killaloe (Limerick)       38
+
+     Central.
+                           Inches.
+  Enfield                   23
+  Epping                    23
+  Derby                     24
+  York                      22
+  Stirling                  39
+  Perth                     29
+
+     Easterly.
+                           Inches.
+  Witham (Essex)            21
+  Patrington (Hull)         21
+  Sunderland                17
+  Inveresk (Edinburgh)      25
+  Pittenweem (Fife)         24
+  Dublin                    22
+
+Mr. Green, the celebrated aeronaut, has asserted from his experience,
+"that whenever a fall of rain happens, and the sky is entirely overcast,
+there will invariably be found to exist another stratum of cloud at a
+certain elevation above the former;" and the recent scientific balloon
+ascents by Mr. Glaisher have tended to confirm this theory. Mr. Glaisher
+says, "It would seem to be an established fact, that whenever rain is
+falling from an overcast sky, there is a second stratum above." "It would
+also seem that when the sky is overcast without rain, that there is no
+stratum of cloud above, but that the sun is shining on the upper surface.
+In every instance in which I have been up under these circumstances, I
+have found such to be the case, agreeing in this respect also with Mr.
+Green's observations."
+
+The amount of rain collected in a gauge placed near the surface of the
+earth is larger than in any gauge placed above it; and the higher the
+gauge is placed, the less water is collected. Mr. Glaisher contends that
+his balloon experiments corroborate this law.
+
+
+=120. Utility of Statistics of Rain-fall.=--The utility of knowing the
+rain-fall of any locality is sufficiently obvious, and little need be said
+upon the subject. The rain-gauge should be in the hands of every gardener
+and farmer. In the management of out-door plants and crops, as well as in
+the construction of cisterns and tanks for the supply of water, a rain
+gauge is a valuable assistant. By its use, the gardener will be guided in
+judging how far the supply of moisture to the earth is needed; and he will
+also see how beneficial is even a hasty shower to growing plants, when he
+considers that a fall of rain measuring the tenth of an inch in depth,
+corresponds to the deposit of about forty hogsheads per acre. The study of
+the rain-fall of a country is of considerable interest to agriculturists.
+The health and increase of domestic animals, the development of the
+productions of the land, as well as the daily labours of the farmer, are
+dependent upon the excess or deficiency of rain. "It must be a subject of
+great satisfaction and confidence to the husbandman to know at the
+beginning of a summer, by the certain evidence of meteorological results
+on record, that the season, in the ordinary course of things, may be
+expected to be a dry and warm one; or to find, in a certain period of it,
+that the average quantity of rain to be expected for the month has fallen.
+On the other hand, when there is reason, from the same source of
+information, to expect much rain, the man who has courage to begin his
+operations under an unfavourable sky, but with good ground to conclude,
+from the state of his instruments and his collateral knowledge, that a
+fair interval is approaching, may often be profiting by his observations;
+while his cautious neighbour who waited 'for the weather to settle' may
+find that he has let the opportunity go by. This superiority, however, is
+attainable by a very moderate share of application to the subject; and by
+the keeping of a plain diary of the barometer and rain-gauge, with the
+hygrometer and vane, under his daily notice."[13] The statistics of
+rain-fall are not only valuable and interesting in a meteorological point
+of view, and for agricultural purposes, but are also highly important in
+connection with sanitary arrangements for towns, and engineering
+operations. This is especially evident to the hydraulic engineer. As rain
+is an important source of water-supply to rivers, canals, and reservoirs,
+it is evident that a knowledge of the probable fall for any season or
+month, at a given place, as furnished by averages of the observations of
+former years, will be the data upon which the engineer will base his plans
+for providing for floods or droughts; while the measurement of the actual
+quantity which has just fallen, as gathered from the indications of a
+series of gauges, will suggest to him the precautions to adopt either to
+economise or conduct away the in-pouring waters.
+
+"When a canal is conducted across an undulating country, its course is
+necessarily governed by the accidents of the ground, and it alternately
+rises and falls. In this case, rising by a succession of levels, it
+necessarily arrives at a certain highest level, which is called by
+engineers the _summit level_. From this it again descends by a
+corresponding series of levels. Now, it is evident that, supposing the
+locks to be all equal in magnitude, the ascent of a vessel will require
+the descent of as much water from the summit to the lowest level as would
+fill a single lock; for this quantity of water must be discharged from
+each lock of the series when the vessel passes through it.
+
+"The same may be said of the process by which the vessel descends along
+the series of locks on the other side of the summit. It appears,
+therefore, that a supply of water must always be maintained on the summit
+level sufficient to fill a single lock twice for each vessel which crosses
+the summit.
+
+"It happens, fortunately, that by the laws of natural evaporation, rain is
+precipitated in greater quantities on elevated summits than on the
+intermediate valleys, so that the moving power, in this case, accommodates
+itself to the exigencies of intercommunication."--_Dr. Lardner's "Handbook
+of Natural Philosophy."_
+
+
+=121. New Form of Rain-Gauge.=--Since the foregoing pages were in type, a
+modification of Howard's rain-gauge has been arranged by Mr. Symons, which
+is compact in design, convenient in use, and low in price. It combines the
+advantages of most gauges; having solidity, and facility of measurement.
+The bottle is placed in a tin case, to the bottom of which are attached
+stout spikes, which, when forced into the earth, prevent its being upset
+either by wind or accident. The bottle being transparent, and slits made
+in the case, the fall of rain is seen at a glance, or with a race-glass,
+from a window. The funnel being attached to the cover of the case is
+thereby kept strictly horizontal, and the depth of rain can be accurately
+measured by lifting the bottle from its case and emptying it into a
+graduated glass jar.
+
+The funnel of this gauge is a very deep cone, to prevent the rain drops
+outsplashing. When properly placed, the receiving surface will be twelve
+inches above the ground, which experience has shown to be the most
+advantageous height.
+
+
+
+
+CHAPTER XIII.
+
+APPARATUS EMPLOYED FOR REGISTERING THE DIRECTION, PRESSURE, AND VELOCITY
+OF THE WIND.
+
+
+=122. The Vane.=--The instrument by which the wind's direction is most
+generally noted, is the vane, or weather-cock, and all that need be said
+of it here is that the points north, east, south and west, usually
+attached to it, should indicate the _true_ and not the _magnetic_
+directions; and that care should be taken to prevent its setting fast.
+Very complicated instruments are required for ascertaining the pressure
+and velocity of the wind, and these are called _Anemometers_. The simplest
+is _Lind's_.
+
+
+[Illustration: Fig. 86.]
+
+=123. Lind's Anemometer, or Wind-Gauge= (fig. 86), invented so late as
+1775, for showing the pressure of the wind, consists of a glass syphon,
+the limbs parallel to each other, and each limb the same diameter. One end
+of the syphon is bent at right angles to the limb, so as to present a
+horizontal opening to the wind. A graduated scale, divided to inches and
+tenths, is attached to the syphon tube, reading either way from a zero
+point in the centre of the scale. The whole instrument is mounted on a
+spindle, surmounted by a vane, and is moved freely in any direction by the
+wind, always presenting the open end towards the quarter from which the
+wind blows. To use the instrument, it is simply filled up to the zero
+point with water, and then exposed to the wind; the difference in the
+level of the water gives the force of the wind in inches and tenths, by
+adding together the amount of depression in one limb, and elevation in the
+other, the _sum of the two_ being the height of a column of water which
+the wind is capable of sustaining at that time.
+
+TABLE,
+
+Showing the Force of Wind on a square foot, for different heights of the
+column of Water in Lind's Wind-Gauge.
+
+  +-----------------------------------+
+  |Inches.|Force in|Common designation|
+  |       |  lbs.  |   of such Wind.  |
+  |-------+--------+------------------|
+  |  6    |  31.75 | A Hurricane.     |
+  |  5    |  26.04 | A violent Storm. |
+  |  4    |  20.83 | A great Storm.   |
+  |  3    |  15.62 | A Storm.         |
+  |  2    |  10.42 | A strong Wind.   |
+  |  1    |   5.21 | A high Wind.     |
+  |   .5  |   2.60 | A brisk Wind.    |
+  |   .1  |    .52 | A fresh Breeze.  |
+  |   .05 |    .26 | A gentle Breeze. |
+  | 0.    |   0.   | A Calm.          |
+  +-----------------------------------+
+
+
+=124. Modification of Lind's Gauge.=--_Sir W. Snow Harris_ has effected a
+modification of Lind's anemometer, with a view of obtaining a hand
+instrument for use at sea more especially. At present the force of the
+wind is estimated at sea by an arbitrary scale, suggested by Sir F.
+Beaufort, the late hydrographer; 0 being calm, 12 the strongest hurricane,
+and the intermediate numerals giving the varying strength of the wind.
+There has been a long-felt want of instrumental means for obtaining this
+data at sea, if merely for the sake of checking occasionally personal
+estimations, which may vary considerably among different observers.
+Harris's wind gauge is intended to be held by hand, while facing the wind,
+and keeping it in proper position by attending to a spirit-level attached.
+When in position, and held firmly, the tube has to be opened to the wind
+by pressure of the thumb acting upon jointed levers, controlled by
+springs. The pressure of the wind moves the enclosed liquid; and by
+withdrawing the thumb, the tube is closed so as to keep the liquid in its
+position; the reading is then taken from its scale, either in pounds on
+the square foot, miles per hour, or the ordinary designations of wind, as
+light, fresh, strong, &c.
+
+
+=125. Robinson's Anemometer.=--_Dr. Robinson_, of Armagh, is the inventor
+of a very successful anemometer, which determines the horizontal velocity
+of the wind. It was first used in 1850, in the meteorological and tidal
+observations made on the coast of Ireland under the direction of the Rev.
+Dr. Lloyd. No meteorological observatory should be without this valuable
+instrument, which is essential in determining the average velocity of the
+wind of a locality as distinguished from the most frequent wind of the
+same place. It is represented in fig. 87. Four hollow hemispherical cups,
+_A A_, are extended upon conjugate diameters, or arms, with their
+diametrical planes placed vertically, and facing the same way upon a
+vertical axis, _B_, which has at its lower extremity an endless screw,
+_D_. The axis is supported at _C_ so as to turn with as little friction as
+possible. The endless screw is placed in gear with a train of wheels and
+pinions. Each wheel carries an index over a stationary dial in front; or
+the index is fixed, and the graduations are placed upon the wheels
+themselves.
+
+[Illustration: Fig. 87.]
+
+Dr. Robinson has proved, both by theory and experiment, that the centre of
+any one of the cups so mounted and set in motion by the wind, revolves
+with one-third of the wind's velocity. If, therefore, the diametrical
+distance between the centres of the cups be one foot, the circle described
+by the centres in one revolution is 3.1416 feet, and the velocity of the
+wind will be three times this, or 9.42 feet, which must be referred to
+time for the absolute rate. The instrument is sometimes made with the
+centres of the cups 1.12 feet apart, so that the circle described is
+1/1500 of a mile in circumference. Hence, to produce one revolution of the
+cups, the wind must travel three times as fast, or 1/500 of a mile.
+Therefore, 500 revolutions will be produced by one mile of wind; so that
+the dials may be graduated to register the velocity in miles and tenths of
+miles. The simplest arrangement is with five dials, recording respectively
+10, 100, 1,000, 10,000 and 100,000 revolutions.
+
+_Directions for using Robinson's Anemometer._--The dials read off in the
+same manner as the register of a gas meter, commencing with the dial
+farthest from the endless screw.
+
+"The figures on the first dial indicate so many hundreds of thousands of
+revolutions; those on the second dial so many tens of thousands; those on
+the third, thousands; those on the fourth, hundreds; and those on the
+fifth so many tens.
+
+"The instrument should be read every morning at 9 o'clock; and, usually,
+it will only be necessary to read the first three dials. The figures can
+be entered as they are read off. Should the index point _between_ two
+figures, the less of the two is to be taken.
+
+"For example, if the first dial points to 7, or between 7 and 8; while the
+second dial indicates 4; and the third, 5; the entry to be made is 745
+(indicative of 745 _thousand_ revolutions).
+
+"Every time the index of the first dial is found to have passed zero (0),
+a cross or star is to be prefixed to the next (a lower) reading.
+
+"To ascertain how many _thousands_ of revolutions have been made during
+the month, it will simply be necessary to subtract the first reading from
+the last, and prefix to the three figures thus obtained a figure
+corresponding to the number of stars in the column. For every _thousand_
+revolutions there are two miles of wind: we have therefore only to
+multiply by 2 to find how many miles of wind have passed during the month.
+
+"Two entries must be made for the last day of each month (the one being
+written under the other), so as to bring the readings down to 9 A.M. on
+the 1st of the following month. The same entry which ends one month, will
+therefore begin the next. This repetition of one entry is necessary, in
+order to prevent losing a day's wind.
+
+  "The accompanying example of the           687
+  readings of an Anemometer for 13 days      773
+  will illustrate the method of making       822
+  the entries, &c.                           855
+                                             900
+  "In this instance, the first reading       953
+  (687) is less than the last (793).         990
+  When the first reading is greater than    *066
+  the last, it will be necessary to borrow   197
+  1,000 in making the subtractions,          323
+  and then deduct one from the number        414
+  of stars. Thus, if the first reading       597
+  of the series on the margin had            712
+  been 887, the result would have been       793
+  906 instead of 1106.                      ----
+                                            1106 thousands of revolutions.
+                                               2
+                                          +-----
+                                       13 | 2212 miles of wind in period.
+                                          +-----
+                                             170 miles of wind per day, on
+                                                 an average.
+
+"The foregoing directions are all which require to be regularly attended
+to. But it may be interesting at times to find the velocity of the wind
+during a period of a few minutes. This may be ascertained by observing the
+difference of two readings of all the dials, with an interval of some
+minutes between them, when a very brief calculation will suffice; but
+perhaps the simplest method is the following:--
+
+"Take two readings, with an interval of 12 minutes between them. The
+difference of these readings, divided by 10, is the velocity of the wind
+in miles per hour. Thus--if the reading of the five dials (from left to
+right) at noon is 15206, and at 12 minutes past 12 is 15348, the velocity
+of the wind is 14.2 miles per hour."--_Admiral FitzRoy, F.R.S._
+
+A lever and clutch are sometimes fitted to this anemometer, as in fig. 88,
+for throwing the train out of gear when not required to register. It may
+also be connected with clock-work so as to be self-recording, by causing
+the mechanism to impress a mark upon prepared paper moved by the
+apparatus, at certain intervals of time.
+
+[Illustration: Fig. 88.]
+
+This anemometer should be fixed in an exposed situation, as high above
+ground as may be convenient for reading. It may be made very portable, by
+the arms which carry the cups being fitted to unscrew or to fold down.
+When fitted in gimbals, it can be used at sea with much advantage.
+
+The pressure of the wind has been experimentally proved to vary as the
+square of the velocity; the relation being _V squared_ = 200 x _P_. From this
+formula, therefore, the pressure can be calculated corresponding to the
+observed velocity.
+
+
+=126. Whewell's Anemometer.=--This apparatus, the invention of the
+celebrated Dr. W. Whewell, registers the horizontal motion of the air with
+the direction. Its mechanism may be described in general terms, as
+follows:--
+
+A horizontal brass plate is attached to a vertical spindle, which passes
+through the axis of a fixed cylinder, being supported by a bearing at the
+lower end, and working in a collar at the upper. A vane is attached, by
+which the plate is moved about according to the direction of the wind. A
+fly, having eight fans, each fixed at an angle of 45 deg. with the axle, is
+placed upon the plate so that the axle is in the line of direction of the
+vane. An endless screw on the axle turns a vertical wheel having one
+hundred teeth, the axle to which has also an endless screw working into a
+horizontal wheel, having a like number of teeth, and which communicates
+motion to a vertical screw fifteen inches long. On this screw is placed a
+moveable nut, which carries a pencil. Round the cylinder is wrapped daily
+a paper divided for the points of the compass. The wind acting upon the
+vane will cause the plate to turn; and the screw which carries the pencil
+will travel with it, so that the pencil will mark upon the paper the
+direction of the wind. The fly will also be set in motion, and thereby the
+nut upon the screw will descend, so that the attached pencil will trace a
+vertical line upon the paper. When the fans on the axle are 2.3 inches
+from axis to end, and 1.9 inches wide, and the thread of the screw such
+that forty-five revolutions will cause the nut to descend two inches,
+75.85 miles of wind will cause the pencil to descend through a vertical
+space of two inches; but the actual trace upon the paper will be longer in
+proportion to the magnitude of change of azimuth, or direction, of the
+wind.
+
+
+=127. Osler's Anemometer, and Pluviometer.=--Mr. Follet Osler is the
+inventor of a self-recording apparatus which registers the direction and
+pressure of the wind, and the amount and duration of rain, upon the same
+sheet of paper. His apparatus has met with very much approbation, and has
+been erected in many observatories. The mechanism may be modified in
+various ways, and the following is a description of the simplest and most
+recent arrangement.
+
+[Illustration: Fig. 89.]
+
+The instrument, of which fig. 89 is a diagram rather than a picture,
+consists, first, of a vane, _V_, of a wedge-shape form, which is found to
+answer better than a flat vane; for the latter is always in a neutral
+line, and therefore is not sufficiently sensitive. A wind-mill governor
+has been substituted for the vane to get the direction of the wind, with
+advantage. At the lower end of the tube, _T T_, is a small pinion, working
+in a rack, _r_, which moves backwards and forwards as the wind presses the
+vane. To this rack a pencil, _x_, is attached, which marks the direction
+of the wind on a properly ruled paper, placed horizontally beneath, and so
+adjusted as to progress at the rate of half an inch per hour, by means of
+a simple contrivance connecting it with a good clock. The paper is shown
+in the illustration upon the table of the instrument.
+
+The pressure plate, _F_, for ascertaining the force of the wind, is one
+foot square, placed immediately beneath, and at right angles with the
+vane; it is supported by light bars, running horizontally on friction
+rollers, and communicating with flattened springs, 1, 2, 3, so that the
+plate, when affected by the pressure of the wind, acts upon them, and they
+transfer such action to a copper chain passing down the interior of the
+direction tube, and over a pulley at the bottom. A light copper wire
+connects this chain with the spring lever, _y y_, carrying a pencil which
+records the pressure upon the paper below. Mr. Osler much prefers a spring
+to any other means for ascertaining the force of the wind, because it is
+of the highest importance to have as little matter in motion as possible,
+otherwise the momentum acquired will cause the pressure plate to give very
+erroneous indications. The pressure plate is as light as is consistent
+with strength. It is kept before the wind by the vane, and is urged out by
+three or more springs, so that with light winds one only is compressed,
+and two, or more, according to the strength of the wind.
+
+The _pluviometer_ is placed on the right in the figure, _P P_ being the
+plane of the roof of the building. The rain funnel, _R_, exposes an area
+of about 200 square inches. The water collected in it is conveyed by a
+tube through the roof of the building into a glass vessel, _G_, so
+adjusted and graduated as to indicate a quarter of an inch of rain for
+every 200 square inches of surface, _i. e._ 50 cubic inches. _G_ is
+supported by spiral springs, _b b_, which are compressed by the
+accumulating rain. A glass tube, open at both ends, is cemented into the
+bottom of _G_, and over it is placed a larger one closed at the top like a
+bell glass. The smaller tube thus forms the long leg of a syphon, and the
+larger tube acts as the short leg. The water, having risen to the level of
+the top of the inner tube, drops over into a little copper tilt, _t_, in
+the globe, _S_, beneath the reservoir. This tilt is divided into two equal
+partitions by a slip of copper, and placed upon an axis not exactly
+balanced, but so that one end or the other preponderates. The water then
+drops into the end of the tilt which happens to be uppermost, and when
+quite full it falls over, throwing the water into the globe, _S_, from
+which it flows away by the waste pipe. In this way an imperfect vacuum is
+produced in the globe, quite sufficient to produce a draught in the small
+tube of the syphon, or the long leg; and the whole contents of the
+reservoir, _G_, immediately run off, and the spiral springs, _b b_,
+elevate the reservoir to its original position. To produce this action, a
+quarter of an inch of rain must have fallen. The registration is easily
+understood. A spring lever, _z_, carrying a pencil, is attached by a
+cord, _c_, to _S_. This spring always keeps the cord tight, so that as the
+apparatus descends during the fall of rain, the spring advances the pencil
+more and more from the zero of the scale upon the paper beneath, until a
+quarter of an inch has fallen, when the pencil is drawn back to zero by
+the ascent of the reservoir.
+
+The clock movement carries the registering paper forward by one of the
+wheels working into a rack attached to the frame.
+
+The adjustment of the instrument should be carefully made at its first
+erection. The scale for pressure should be established experimentally, by
+applying weights of 2, 4, 6, &c., lbs., to move the pressure plate.
+
+The registration trace for twenty-four hours is readily understood. The
+direction is recorded on the centre part; the pressure on one side, and
+the rain on the other. Lines parallel to the length of the paper show no
+rain, steady wind, and constant pressure. On the rain trace, a line
+parallel to the width of the paper shows that the pencil had been drawn
+back to zero, a quarter of an inch of rain having fallen. The hour lines
+are in the direction of the width of the paper.
+
+At the International Exhibition 1862, Messrs. Negretti and Zambra
+exhibited an improved Osler's anemometer, having combined with it
+Robinson's cups, so that the pressure and velocity appear on the same
+sheet, on which a line an inch in length is recorded at every ten miles;
+thus the complete instrument shows continuously the direction, pressure,
+and velocity of the wind.
+
+
+=128. Beckley's Anemometer.=--Mr. R. Beckley, of the Kew Observatory, has
+devised a self-registering anemometer, which consists of three principal
+parts: Robinson's cups for the determination of velocity; a double fan, or
+wind-mill governor, for obtaining the direction; and a clock to move a
+cylinder, around which registration paper is wrapped. The paper records
+the time, velocity, and direction of the wind for twenty-four hours, when
+it must be replaced. It has a cast-iron tubular support, or pedestal to
+carry the external parts--the cups and the fans,--which must be erected
+upon the roof of the building upon which it is desired to mount the
+instrument.
+
+The fans keep their axis at right angles to the wind; and with any change
+of direction they move, carrying with them an outer brass tube, which
+rests upon friction balls on the top of the pedestal, and is attached to a
+tubular shaft passing through the interior of the pedestal, and
+terminating with a mitre wheel. The mitre wheel, working with other cogged
+wheels, communicates the motion of the direction shaft to a cylinder
+carrying a pencil, to record the direction.
+
+The shaft carrying the cups is supported upon friction balls, placed in a
+groove formed on the top of the direction shaft, and passing through the
+interior of that shaft, comes out below the mitre wheel, where it is
+terminated in an endless screw, or worm.
+
+Upon the wind moving the cups, motion is given to the innermost shaft,
+thence to the worm-wheel, whence motion is given to a pencil which
+registers the velocity.
+
+De la Rue's metallic paper is used in registration, it having the property
+of receiving a trace from a brass pencil. The pencils can, therefore, be
+made in the most convenient form. Mr. Beckley forms each pencil of a strip
+of brass wrapped round a cylinder, making a very thin threaded screw, so
+that the contact of the pencil cylinder and the clock cylinder is a mere
+point of the metallic thread. The pencil cylinders are placed side by side
+upon the cylinder turned by the clock, and require no spring or other
+appliance to keep them to their work, but always make contact with the
+registration paper by their own gravity. They therefore require no
+attention, and being as long as the trace which they make, they will last
+a long time.
+
+The velocity pencil has only one turn on the cylinder, and its pitch is
+equal to a scale of fifty miles upon the paper. The direction pencil has
+likewise one turn on its cylinder, its pitch being equal to a scale of the
+cardinal points of the compass upon the paper.
+
+The clock gives a uniform motion of half an inch per hour to the cylinder
+upon which the paper is fastened.
+
+The registering mechanism of the instrument is very compact, requiring
+only a space of about 18 inches by 8 inches.
+
+In the Report of the British Association for 1858, Mr. Beckley has given a
+detailed description of his anemometer, with drawings of all the parts.
+
+
+=129. Self-Registering Lind's Anemometer.=--A Lind's wind-gauge, designed
+to register the maximum pressure, was exhibited at the International
+Exhibition 1862, by Mr. E. G. Wood. The bend of the syphon is contracted
+to obtain steadiness. On the leeward limb a hole is drilled corresponding
+in size with the contracted portion of the tube. The edge of the hole
+corresponds with the zero of the scale. On the pressure of the wind
+increasing, as much of the water as would have risen above the aperture
+flows away, and therefore the quantity left indicates the greatest
+pressure of the wind since the last setting of the instrument, which is
+done by filling it with water up to the zero point.
+
+
+=130. Anemometric Observations.=--To illustrate the value of anemometric
+observations, we quote from a paper by Mr. Hartnup, on the results
+obtained from Osler's Anemometer, at the Liverpool Observatory. The six
+years' observations, ending 1857, gave for the yearly average of the
+winds: North-easterly, on 60 days, at 7.8 miles per hour; North-westerly,
+on 112 days, at 15.4 miles per hour; South-easterly, on 115 days, at 11.0
+miles per hour; South-westerly, on 77 days, at 13.8 miles per hour; and
+one day calm. From the same observations, the average variation in the
+strength of the wind during the 24 hours is:--11 miles per hour, the
+minimum force, occurring at 1-1/2 a.m.; until 6 a.m. it remains much the
+same, being then 11.3 miles per hour; at 10 a.m. it is 13.4 miles per
+hour; at 1-1/2 p.m. the wind is at its maximum strength, being 14.8 miles
+per hour; at 5 p.m. it is again 13.4 miles per hour, and at 9 p.m. 11.3
+miles per hour. Hence it appears that the wind falls to its minimum force
+much more gradually than it rises to its maximum; that the decrease and
+increase are equal and contrary, so that the curve is symmetrical; and
+that generally the force of wind is less at night than during the day.
+
+"There is evidence," says Admiral FitzRoy, "in Mr. Hartnup's very valuable
+anemometrical results, which seems to prove that to his observatory, in a
+valley, with buildings and hills to the north-eastward, the real polar
+current does not blow from N.E., but nearer S.E. By his reliable digest of
+winds experienced there, it appears that those most prevalent were from
+W.N.W. and S.S.E. But in England, generally, the _prevailing_ winds are
+_believed_ to be westerly, inclining to south-westerly, and
+north-easterly; while of all winds, the south-easterly is about the
+rarest.
+
+"At Lord Wrottesley's observatory, in Staffordshire, about 530 feet above
+the sea, there appears to be considerably less strength of wind at any
+given time, when a gale is blowing _generally_, than occurs simultaneously
+at places along the sea-coast: whence the inference is, that undulations
+of the land's surface and hills, diminish the strength of wind materially
+by frictional resistance.
+
+"All the synoptic charts hitherto advanced at the Board of Trade exhibit a
+marked diminution of force inland compared with that on the sea-coast.
+Indeed, the coast itself offers similar evidence, in its stunted, sloping
+trees, and comparative barrenness."[14]
+
+
+
+
+CHAPTER XIV.
+
+INSTRUMENTS FOR INVESTIGATING ATMOSPHERIC ELECTRICITY.
+
+
+=131. Atmospheric Electroscope.=--The simplest instrument for ascertaining
+at any time the electric condition of the atmosphere is an electroscope
+composed of two equal pieces of gold leaf, suspended from a brass support,
+and insulated, as well as protected from the movement of the air, by a
+glass covering. Fig. 90 represents such an instrument. The cap of the
+brass support is fitted for the reception, in the vertical direction, of a
+metallic rod, not less than two or three feet in length. The top of the
+rod carries a clip. The instrument acts according to the law, that bodies
+similarly electrified repel each other; but when dissimilarly electrified,
+they attract each other. To make an observation, the instrument is placed
+in the open air, and a lighted piece of cigar fusee, or touch-paper, is
+fixed in the clip. The electricity of the air is collected by the
+substance undergoing combustion, and conducted by the rod to the gold
+leaf; and the pieces, being similarly electrified, separate more or less
+according to the amount of electricity present. The kind is determined by
+the effect of either an excited stick of sealing-wax or rod of glass upon
+the electrified gold leaf. A rod of glass, when rubbed briskly with a silk
+handkerchief or piece of woollen cloth, becomes positively electrified, or
+excited, as it is termed. A stick of sealing-wax, similarly treated,
+acquires the negative state. If, therefore, an excited glass rod be
+presented to the cap of the instrument, and it cause the pieces of gold
+leaf to diverge still further, the electric state of the air must be
+analogous to that of the glass, that is, _positive_; if they approach, it
+is _negative_. On the contrary, if a stick of sealing-wax be used, the
+pieces will be repelled more apart if they have acquired negative
+electricity from the air; and they will converge if they have a positive
+charge.
+
+[Illustration: Fig. 90.]
+
+By means of this very simple instrument, meteorological observers can
+readily ascertain the electric condition of the lower air at any time.
+
+NOTE.--A book containing strips of gold leaf is sent with the Electrometer
+to replace the gold leaves when torn or broken in use.
+
+To mount fresh gold leaves, unscrew the brass plate to which is attached
+the rod supporting the leaves; then moisten with the breath the flat piece
+of brass, and press it gently down on one strip of gold, whilst the book
+is only partly opened; the second leaf is attached in the same manner.
+
+
+=132. Volta's Electrometer= is similar to the instrument just described,
+except that instead of gold leaf two light pieces of straw, or two pith
+balls, are freely suspended from the conductor; the amount of the electric
+charge being estimated from the degrees of divergence, shown by a
+graduated arc.
+
+
+=133. Peltier's Electrometer= is a much superior instrument in point of
+sensibility. A tall glass tube an inch or more in diameter, is connected
+to a glass receiver, mounted on a base fitted with levelling screws. At
+the top of the tube is formed a globe from four to five inches in
+diameter, which is thickly gilt on the exterior, so as to form a good
+conducting surface. A wire passes from the ball down the tube into the
+receiver, where it is bent up, and ends in a steel point over the centre
+of the base. A bent wire, carrying a small magnetic needle, is balanced on
+the steel point, so that the magnet, with the fine wire, arranges itself
+horizontally in the direction of the magnetic meridian. If any cloud or
+portion of air in the neighbourhood be in an electrical state, it will act
+by induction upon the gilt ball, and the needle will be deflected from its
+north and south direction.
+
+A graduated circle indicates the number of degrees of the deflection,
+which will be greater or less according to the tension of the electricity.
+To ascertain whether the electricity is positive or negative, a stick of
+shellac or glass must be employed, as already described.
+
+
+=134. Bohnenberger's Electroscope= may be fitted with a metallic
+conductor, and used with great advantage for observing atmospheric
+electricity. "The principal parts of the instrument, as improved by
+Becquerel, are the following:--_A B_, fig. 91, is a small dry galvanic
+pile of from 500 to 800 pairs, about a quarter of an inch in diameter;
+when the plates are pressed together, such a pile will be from 2 to 2-1/2
+inches in length. The wires, which are bent so as to stand above the pile,
+terminate in two plates, _P_ and _M_, which are the poles of the pile.
+These plates, which are 2 inches by 1/2 an inch, are parallel and opposite
+to each other. It is convenient for their opposite sides to be slightly
+convex, for them to be gilded or coated with platinum, and for them to run
+on the polar wires, by the latter being made to pass through a small hole
+in them. One of these plates will always be in a state of positive, and
+the other of negative, electricity; between them suspend the very fine
+gold leaf, _D G_, which is attached to the conductor, _C D_, of copper
+wire. If the leaf hang exactly between the two plates, it is equally
+attracted by each, and will therefore be in a state of repose. The
+apparatus should be protected by a bell-glass, fitting exactly, and having
+an opening at the top through which the copper wire, _C D_, passes; the
+wire, however, is insulated by its being contained in a glass tube, which
+is made to adhere to the bell-glass by means of a small portion of shellac
+or gum-lac. Screw on a metal ball or plate, to impart to it the
+electricity you wish to test, which will be conveyed by the copper wire to
+the gold leaf, and the latter will immediately move towards the plate
+which has the opposite polarity. This electroscope is, beyond doubt, one
+of the most delicate ever constructed, and is well adapted to show small
+quantities of positive and negative electricity.
+
+[Illustration: Fig. 91.]
+
+"To ensure the susceptibility of electroscopes and electrometers placed
+under bell-glasses, precautions should be taken to render the air they
+contain as dry as possible, which may be effected by enclosing in a
+suitable vessel a little melted chloride of calcium beneath the glass."
+
+The galvanic pile employed in this electroscope is that invented by
+Zamboni. "It differs from the common hydro-electric batteries principally
+in this, that the presence of the electromotive liquid is dispensed with,
+and that in its place is substituted some moist substance of low
+conducting power, generally paper. The electromotors in these piles are
+composed for the most part of Dutch gold (copper) and silver (zinc) paper
+pressed one on the other, with their paper sides together, out of which
+discs are cut with a diameter of from a quarter of an inch to an inch.
+More powerful pairs of plates may be obtained by using only the silver
+paper and smearing its paper side with a thin coat of honey, on which some
+finely pulverized peroxide of manganese has been sprinkled, and all the
+sides similarly coated are presented one way. Powerful pairs of plates may
+also be made by pasting pure gold leaf on the paper side of zinc-paper.
+These plates are then to be arranged, just as in the ordinary voltaic
+pile, one above the other, so that the similar metallic surfaces may all
+lie one way; press them tightly together; tie them with pretty stout silk
+threads, and press them into a glass tube of convenient size. The metal
+rims of the tubes, which must be well connected with the outermost pairs
+of plates, form the poles of the pile, the negative pole being in the
+extreme zinc surface, and the positive in the extreme copper or manganese
+surface.
+
+"The electromotive energy called into action in these dry piles is less
+than that excited in the moist or hydro-electric piles, principally on
+account of the imperfect conduction of the paper. The accumulation of
+electricity at their poles also goes on less rapidly, and consequently the
+electrical tension continues for a long while unaltered; whereas, in all
+moist piles, even in the most constant of them, the tension is maintained,
+comparatively speaking, for but a short time, on account of the chemical
+action and decomposition of the electromotive fluid--causes of disturbance
+which do not exist in the dry pile."[15]
+
+
+=135. Thomson's Electrometer.=--Professor W. Thomson, of Glasgow, has
+devised an atmospheric electrometer, which is likely to become eminently
+successful, in the hands of skilful observers. It is mainly a torsion
+balance combined with a Leyden-jar. The index is an aluminium needle
+strung on a fine platinum wire, passing through its centre of gravity, and
+stretched firmly between two points. The needle and wire are carefully
+insulated from the greater part of the instrument, but are in metallic
+communication with two small plates fixed beside the two ends of the
+needle, and termed the repelling plates. A second pair of larger plates
+face the repelling plates, on the opposite side of the needle, but
+considerably farther from it. These plates are in connection with the
+inner coating of a Leyden-jar, and are termed the attracting plates. The
+whole instrument is enclosed in a metal cage, to protect the glass
+Leyden-jar and the delicate needle.
+
+The Leyden-jar should be charged when the instrument is used. Its effect
+is two-fold: it increases greatly the sensibility of the instrument, and
+enables the observer to distinguish between positive and negative
+electrification.
+
+The air inside the jar is kept dry by pumice-stone, slightly moistened
+with sulphuric acid; by which means very perfect insulation is maintained.
+
+Electrodes, or terminals, are brought outside the instrument, by which the
+Leyden-jar can be charged, and the needle system connected with the body,
+the electric state of which is to be tested.
+
+For the purpose of testing the electric state of the atmosphere, the
+instrument is provided with a conductor and support for a burning match,
+or, preferably, with an arrangement termed a water-dropping collector; by
+either of which means the electricity of the air is conveyed to the needle
+system.
+
+The needle abuts upon the repelling plates when not influenced by
+electricity, in which position it is at zero. It can always be brought
+back to zero by a torsion-head, turning one end of the platinum wire, but
+insulated from it, and provided with a graduated circle, so that the
+magnitude of the arc, that the torsion-head is moved through to bring the
+needle to zero, measures the force tending to deflect it.
+
+The action of the instrument is as follows:--The Leyden-jar is to be
+highly charged, say negatively; and the repelling plates are to be
+connected with the earth. The needle will then be deflected against a
+stop, under the combined influence of attraction from the Leyden-jar, or
+attracting plates, and repulsion from the repelling plates due to the
+positive charge induced on the needle and its plates by the Leyden-jar
+plates. The platinum wire must then be turned round by the torsion-head so
+as to bring back the needle to zero; and the number of degrees of torsion
+required will measure the force with which the needle is attracted. Next,
+let the needle plates be disconnected from the earth, and connected with
+the insulated body, the electric state of which is to be tested. In
+testing the atmosphere, the conductor and lighted match, or water-dropping
+apparatus, must be applied.
+
+If the electricity of the body be positive, it will augment the positive
+charge in the needle plates, induced by the Leyden-jar plates; and
+consequently the needle will be more deflected than by the action of the
+jar alone. If the electricity of the body be negative, it will tend to
+neutralize the positive charge; and the needle will be less deflected.
+Hence the kind of electricity present in the air becomes at once apparent,
+without the necessity of an experimental test. The platinum wire must then
+be turned till the needle is brought to zero, and the number of degrees
+observed; which is a measure of the intensity of the electrification.
+
+Any loss of charge from the Leyden-jar which may from time to time occur,
+reducing the sensibility inconveniently, may be made good by additions
+from a small electrophorus which accompanies the instrument.[16]
+
+The instrument may be made self-recording by the aid of clockwork and
+photography. To effect this, a clock gives motion to a cylinder, upon
+which photographic paper is mounted. The needle of the electrometer is
+made to carry a small reflector; and rays from a properly adjusted source
+of light are thrown by the reflector, through a small opening, upon the
+photographic paper. It is evident, that as the cylinder revolves, a trace
+will be left upon the paper, showing the magnitude of, and variations in,
+the deflection of the needle.
+
+
+=136. Fundamental Facts regarding Atmospheric Electricity.=--The _general_
+electrical condition of the atmosphere is _positive_ in relation to the
+surface of the earth and ocean, becoming more and more positive as the
+altitude increases. When the sky is overcast, and the clouds are moving in
+different directions, it is subject to great and sudden variations,
+changing rapidly from positive to negative, and the reverse. During fog,
+rain, hail, sleet, snow, and thunderstorms, the electrical state of the
+air undergoes many variations. The intensity of the electricity increases
+with hot weather following a series of wet days, or of wet weather coming
+after a continuance of dry days. The atmospheric electricity, in fact,
+seems to depend for its intensity and kind upon the direction and
+character of the prevailing wind, under ordinary circumstances. It has an
+annual and a diurnal variation. There is a greater diurnal change of
+tension in winter than in summer. By comparing observations from month to
+month, a gradual increase of tension is perceived from July to February,
+and a decrease from February to July. The intensity seems to vary with the
+temperature. The diurnal variation exhibits two periods of greatest and
+two of least intensity. In summer, the _maxima_ occur about 10 a.m. and 10
+p.m.; the _minima_ about 2 a.m. and noon. In winter, the _maxima_ take
+place near 10 a.m. and 8 p.m.; the _minima_ near 4 a.m. and 4 p.m.
+
+The researches of Saussure, Beccaria, Crosse, Quetelet, Thompson, and
+FitzRoy have tended to show that during the prevalence of polar currents
+of air positive electricity is developed, and becomes more or less active
+according to the greater or less coldness and strength of wind; but with
+winds from the equatorial direction there is little evidence of sensitive
+electricity, and when observable, it is of the negative kind. Storms and
+gales of wind are generally attended, in places, with lightning and
+thunder; and as the former are very often attributed to the conflict of
+polar and equatorial winds, the difference of the electric tension of
+these winds may account for the latter phenomena. It is not our intention
+to enter upon the general consideration of thunderstorms; the facts which
+we have given may be of service to the young observer; and finally, as it
+is interesting to be able to judge of the locality of a thunderstorm, the
+following simple rule will be of service, and sufficiently accurate:--Note
+by a second's watch the number of seconds which elapse from the sight of
+the lightning to the commencement of the thunder; divide them by five, and
+the quotient will be the distance in miles. Thus, if thunder is heard ten
+seconds after the lightning was seen, the distance from the seat of the
+storm will be about two miles. The interval between the flash and the roll
+has seldom been observed greater than seventy-two seconds.
+
+
+=137. Lightning Conductors.=--"The line of danger, whether from the
+burning or lifting power of lightning, is the line of strong and
+obstructed currents of air, of the greatest aerial friction."[17] Trees,
+church spires, wind-mills and other tall structures, obstruct the aerial
+currents, and hence their exposure to danger. The highest objects of the
+landscape, especially those that are nearest the thunder cloud, will
+receive the lightning stroke. The more elevated the object, the more
+likely is it to be struck. Of two or more objects, equally tall and near,
+the lightning is invariably found to select the best conductor of
+electricity, and even to make a circuitous path to get to it. Hence the
+application and evident advantage of metallic rods, called _lightning
+conductors_, attached to buildings and ships. A lightning conductor should
+be pointed at top, and extend some feet above the highest part of the
+edifice, or mast. It should be made of copper, which is a better
+conducting medium than iron, and more durable, being less corrosive. It
+must be unbroken throughout its length, and extend to the bottom of the
+building, and even some distance into the ground, so as to conduct the
+electricity into a well or moist soil. If it be connected with the lead
+and iron work in the structure of the house, it will be all the better, as
+affording a larger surface, and a readier means of exit for the fluid. In
+a ship, the lower end of the conductor should be led into communication
+with the hull, if of iron, and with the copper sheathing, if a wooden
+vessel; so that, spread over a large surface, it may escape more readily
+to the water.
+
+
+=138. Precautions against Lightning.=--Experience seems to warrant the
+assumption that any building or ship, fitted with a substantial lightning
+conductor, is safe from danger during a thunderstorm. Should a house or
+vessel be undefended by a conductor, it may be advisable to adopt a few
+precautions against danger. In a house, the fire-place should be avoided,
+because the lightning may enter by the chimney, its sooty lining being a
+good conductor. "Through chimneys, lightning has a way into most houses;
+and therefore, it is wise, by opening doors or windows, to give it a way
+out. Wherever the aerial current is fiercest, there the danger is
+greatest; and if we kept out of the way of currents or draughts, we keep
+out of the way of the lightning."[18] Lightning evinces as it were a
+preference for metallic substances, and will fly from place to place, even
+out of the direct line of its passage to the earth, to enter such bodies.
+It is therefore well to avoid, as much as possible, gildings, silvered
+mirrors, and articles of metal. The best place is perhaps the middle of
+the room, unless a draught passes, or a metallic lamp or chandelier should
+be hanging from the ceiling. The neighbourhood of bad conductors, such as
+glass windows, not being open, and on a thick bed of mattrasses, are safe
+places. The quality of trees as lightning conductors is considered to
+depend upon their height and moisture, those which are taller and
+relatively more humid being struck in preference to their fellows;
+therefore, it is unwise to seek shelter under tall and wet trees during a
+thunderstorm. In the absence of any other shelter, it would be better to
+lie down on the ground.
+
+
+
+
+CHAPTER XV.
+
+OZONE AND ITS INDICATORS.
+
+
+=139. Nature of Ozone.=--During the action of a powerful electric machine,
+and in the decomposition of water by the voltaic battery, a peculiar odour
+is perceptible, which is considered to arise from the generation of a
+substance to which the term ozone has been given, on account of its having
+been first detected by smell, which, for a long time after its discovery,
+was its only known characteristic. A similar odour is evolved by the
+influence of phosphorus on moist air, and in other cases of slow
+combustion. It is also traceable, by the smell, in air,--where a flash of
+lightning has passed immediately before. Afterwards it was established
+that the same element possessed an oxidising property. It was found to be
+liberated at the oxygen electrode when water was decomposed by an electric
+current; and has been regarded by some chemists as what is termed an
+_allotropic_ form of oxygen, while others speak of it as oxygen in the
+_nascent_ state, and some even regard it as intimately related to
+chlorine. So various are the existing notions of the nature of this
+obscure agent.
+
+Its oxidising property affords a ready means for its detection, even when
+the sense of smell completely fails. The methods of noting the presence
+and measuring the amount of ozone present in the air, are very simple;
+being the free exposure to the air, defended from rain and the direct rays
+of the sun, of prepared test-papers. There are two kinds of test-papers.
+One kind was invented by Dr. Schonbein, the original discoverer of ozone;
+and the other, which is more generally approved, by Dr. Moffat.
+
+
+=140. Schonbein's Ozonometer= consists of strips of paper, previously
+saturated with a solution of starch and iodide of potassium, and dried.
+The papers are suspended in a box, or otherwise properly exposed to the
+air, for a given interval, as twenty-four hours. The presence of ozone is
+shown by the test-paper acquiring a purple tint when momentarily immersed
+in water. The amount is estimated by the depth of the tint, according to a
+scale of ten tints furnished for the purpose, which are distinguished by
+numbers from 1 to 10. The ozone decomposes the compound which iodine forms
+with hydrogen, and, it is presumed, combines as oxygen with hydrogen,
+while the iodine unites with the starch, giving the blue colour when
+moist.
+
+
+=141. Dr. Moffat's Ozonometer= consists of papers prepared in a somewhat
+similar manner to Schonbein's; but they do not require immersion in water.
+The presence of ozone is shown by a brown tint, and the amount by the
+depth of tint according to a scale of ten tints, which is furnished with
+each box of the papers.
+
+Moffat's have the advantage of preserving their tint for years, if kept in
+the dark, or between the leaves of a book; and are simpler to use.
+
+
+[Illustration: Fig. 92.]
+
+=142. Sir James Clark's Ozone Cage= (fig. 92), consists of two cylinders
+of very fine wire gauze, one fitting into the other; the wire gauze being
+of such a fineness as to permit the free ingress of air, at the same time
+that it shuts out all light that would act injuriously on the test-paper,
+which is suspended by a clip or hook attached to the upper part of the
+inner cylinder.
+
+
+=143. Distribution and Effects of Ozone.=--Mr. Glaisher has found that
+"the amount of ozone at stations of low elevation is small; at stations of
+high elevation, it is almost always present; and at other and intermediate
+stations, it is generally so. The presence and amount of ozone would seem
+to be graduated by the elevation, and to increase from the lowest to the
+highest ground. The amount of ozone is less in towns than in the open
+country at the same elevation; and less at inland than at sea-side
+stations." It seems to abound most with winds from the sea, and to be most
+prevalent where the air is considered the purest and most salubrious. This
+may seem, says Admiral FitzRoy, in _The Weather Book_, to point to a
+connection between ozone and chlorine gas, which is in and over sea-water,
+and which _must_ be brought by any wind that blows from the sea. It
+prevails more over the ocean and near it than over land, especially land
+remote from the sea; and, says the Admiral, it affects the gastric juice,
+improves digestion, and has a tanning effect. Dr. Daubeny, in his
+_Lectures on Climate_, writes: "Its presence must have a sensible
+influence upon the purity of the air, by removing from it foetid and
+injurious organic effluvia. It is also quite possible that ozone may play
+an important part in regulating the functions of the vegetable kingdom
+likewise; and although it would be premature at present to speculate upon
+its specific office, yet, for this reason alone, it may be well to note
+the fact of its frequency, in conjunction with the different phases which
+vegetation assumes, persuaded that no principle can be generally diffused
+throughout nature, as appears to be the case, with this, without having
+some important and appropriate use assigned for it to fulfil."
+
+
+=144. Registering Ozonometer.=--Dr. E. Lancaster has contrived an
+ozonometer, the object of which is to secure the constant registration of
+ozone, so that the varying quantities present in the atmosphere may be
+detected and registered. For this purpose, an inch of ozone paper passes
+in each hour, by clock-work, beneath an opening in the cover of the
+instrument.
+
+
+
+
+CHAPTER XVI.
+
+INSTRUMENTS NOT DESCRIBED IN THE PRECEDING CHAPTERS.
+
+
+=145. Chemical Weather Glass.=--This curious instrument appears to have
+been invented more than a hundred years ago, but the original maker is not
+known. It is simply a glass vial about ten inches long and three quarters
+of an inch in diameter, which is nearly filled, and hermetically sealed,
+with the following mixture:--Two drachms of camphor, half a drachm of
+nitrate of potassium, half a drachm of chlorate of ammonium, dissolved in
+about two fluid ounces of absolute alcohol mixed with two ounces of
+distilled water. All the ingredients should be as pure as possible, and
+each vial filled separately. When the instruments are made in numbers and
+filled from a common mixture, some get more than the due proportion of the
+solid ingredients, and consequently such glasses do not exhibit that
+uniformity of appearance and changes, that undoubtedly should accompany
+similar influencing circumstances. It is in consequence of a want of
+precision and fixed principle of manufacture, that these interesting
+instruments are not properly appreciated, and more generally used.
+
+The glass should be kept quite undisturbed, exposed to the north, and
+shaded from the sun. Camphor is soluble in alcohol, but not in water,
+while both water and alcohol have different solvent powers, according to
+the temperature; hence, the solid ingredients being in excess for certain
+conditions of solution, depending upon temperature chiefly, and perhaps
+electricity and the action of light also, appear as crystals and disappear
+with the various changes that occur in the weather.
+
+The various appearances thus presented in the menstruum have been inferred
+to prognosticate atmospheric changes. The following rules have been
+deduced from careful study of the glass and weather:--
+
+1. During cold weather, beautiful fern-like or feathery crystallization is
+developed at the top, and sometimes even throughout the liquid. This is
+the normal state of the glass during winter. The crystallization increases
+with the coldness; and if the structure grows downward, the cold will
+continue.
+
+2. During warm and serene weather, the crystals dissolve, the upper and
+greater part of the liquid becoming perfectly clear. This is the normal
+state of the glass during summer. The less amount of crystallization, that
+is, the greater the clear portion of the liquid (for there is always some
+of the composition visible at the bottom), the greater the probability of
+continued fine dry weather.
+
+3. When the upper portion is clear, and flakes of the composition rise to
+the top and aggregate, it is a sign of increasing wind and stormy
+weather.
+
+4. In cold weather, if the top of the liquid becomes thick and cloudy, it
+denotes approaching rain.
+
+5. In warm weather, if small crystals rise in the liquid, which still
+maintains its clearness, rain may be expected.
+
+6. Sharpness in the points and features of the fern-like structure of the
+crystals, is a sign of fine weather; but when they begin to break up, and
+are badly defined, unsettled weather may be expected.
+
+Admiral FitzRoy, in _The Weather Book_, writes of this instrument as
+follows:--"Since 1825, we have generally had some of these glasses, as
+curiosities rather than otherwise; for nothing certain could be made of
+their variations until lately, when it was fairly demonstrated that if
+fixed undisturbed in free air, not exposed to radiation, fire, or sun, but
+in the ordinary light of a well-ventilated room, or, _preferably_, in the
+outer air, the chemical mixture in a so-called storm-glass varies in
+character with the _direction_ of the wind--not its force, _specially_
+(though it _may_ so vary in _appearance_, only from another cause,
+_electrical tension_).
+
+"As the atmospheric current veers toward, comes from, or is only
+_approaching_ from the polar direction, this chemical mixture--if closely,
+even microscopically watched--is found to grow like _fir_, _yew_, fern
+leaves, or hoar-frost--or like crystallizations.
+
+"As the wind, or great body of air, tends more from the _opposite_
+quarter, the lines or spikes--all regular, hard, or crisp
+features--gradually diminish, till they vanish.
+
+"Before, and in a continued southerly wind, the mixture sinks slowly
+downward in the vial, till it becomes shapeless, like melting white sugar.
+
+"Before, or during the continuance of a northerly wind (polar current),
+the crystallizations are beautiful (if the mixture is correct, the glass a
+_fixture_, and duly _placed_); but the least motion of the liquid disturbs
+them.
+
+"When the main currents meet, and turn _toward the west_, making
+_easterly_ winds, stars are more or less numerous, and the liquid dull, or
+less clear. When, and while they _combine by the west_, making westerly
+winds, the liquid is clear, and the crystallization well-defined, without
+loose stars.
+
+"While _any hard_ or _crisp_ features are visible below, above, or at the
+top of the liquid (where they form for polar winds), there is _plus_
+electricity in the air; a _mixture_ of polar current co-existing _in that
+locality_ with the opposite, or southerly.
+
+"When nothing but soft, melting, sugary substance is seen, the atmospheric
+current (feeble or strong as it may be) is southerly with _minus_
+electricity, unmixed with, and _uninfluenced_ by, the contrary wind.
+
+"Repeated trials with a delicate galvanometer, applied to measure electric
+tension in the air, have proved these facts, which are now found useful
+for aiding, with the barometer and thermometer, in forecasting weather.
+
+"Temperature affects the mixture much, but not solely; as many comparisons
+of winter with summer changes of temperature have fully proved.
+
+"A confused appearance of the mixture, with flaky spots, or stars, in
+motion, and less clearness of the liquid, indicates south-easterly wind,
+probably strong to a gale.
+
+"Clearness of the liquid, with more or less perfect crystallizations,
+accompanies a combination, or a contest, of the main currents, by the
+_west_, and very remarkable these differences are,--the results of these
+air currents acting on each other _from_ eastward, or from an entirely
+opposite direction, the _west_.
+
+"The glass should be wiped clean now and then,--and once or twice a year
+the mixture should be disturbed, by inverting and gently shaking the glass
+vial."
+
+
+[Illustration: Fig. 93.]
+
+=146. Leslie's Differential Thermometer.=--A glass tube having a large
+bulb at each extremity, and bent twice at right angles, as represented in
+figure 93, containing strong sulphuric acid tinged with carmine, and
+supported at the centre by a wooden stand, constitutes the differential
+thermometer as invented by Professor Leslie. The instrument is designed to
+exhibit and measure small differences of temperature. Each leg of the
+instrument is usually from three to six inches long, and the balls are
+about four inches apart. The calibre of the legs is about 1/50 inch, not
+more; the other part of the tube may be wider. The tube is filled with the
+liquid, the bulbs contain air. When both bulbs are heated alike, each
+scale indicates zero. The scale is divided so that the space between the
+freezing and the boiling-points of water is equal to 1,000 parts. When one
+bulb is heated more than the other, the difference of temperature is
+delicately shown by the descent of the coloured fluid from the heated
+ball. It is uninfluenced by changes in the temperature of the atmosphere;
+hence it is admirably adapted for experiments of radiant heat. The theory
+of the instrument is that gases expand equally for uniform increments of
+heat.
+
+
+=147. Rumford's Differential Thermometer= differs from that just described
+in simply containing only a small bubble of liquid, which lies in the
+centre of the tube, when both bulbs are similarly influenced. The bulbs
+and other parts of the tube contain air. When one bulb is more heated than
+the other, the bubble moves towards the one less heated; and the scale
+attached to the horizontal part of the tube affords a measurement of the
+difference of temperature.
+
+
+[Illustration: Fig. 94.]
+
+=148. Glaisher's Thermometer Stand.=--The thermometer stand consists of a
+horizontal board as a base, of a vertical board projecting upwards from
+one edge of the horizontal one, and of two parallel inclined boards,
+separated from each other by blocks of three inches in thickness,
+connected at the top with the vertical, and at the bottom with the
+horizontal board, and the air passes freely about and between them all. To
+the top of the inclined boards is connected a small projecting roof to
+prevent the rain falling on the bulbs of the instrument, which are carried
+on the face of the vertical board, with their bulbs projecting below it,
+so that the air plays freely on the bulbs from all sides. The whole frame
+revolves on an upright post firmly fixed to the ground, as shown in the
+engraving, fig. 94; and in use, the inclined side is always turned towards
+the sun.
+
+
+=149. Thermometer Screen, for use at Sea.=--This screen, or shade, was
+designed by Admiral FitzRoy, and has been in use for several years on
+board H.M. vessels and many merchant-ships. It is about twenty-four inches
+long by twelve wide and eight deep; having lattice-work sides, door, and
+bottom; with perforation also at top, so contrived that the air has free
+access to the interior, while the direct rays of the sun, rain, and sea
+spray are effectually excluded from the thermometers mounted inside. There
+is ample space for two thermometers placed side by side on brackets, at
+least three inches from each other or any part of the exterior of the
+screen. One thermometer should be fitted up as a "wet bulb" (see p. 105).
+A small vessel of water can easily be fixed inside the screen so as to
+retain its position and contents under the usual motions of the ship; and
+by means of a piece of cotton-wick, or muslin rag tied round the bulb of
+the thermometer and trailing into the cup of water, keep the bulb
+constantly moist.
+
+Self-registering thermometers should be protected by a similar screen. It
+has been found that thermometric observations made at sea are not valuable
+for scientific purposes unless the instruments have been duly protected by
+such a screen.
+
+
+=150. Anemoscope=, or Portable Wind Vane for travellers, with compass, bar
+needle, &c., shows the direct course of the wind to half a point of the
+compass.
+
+
+[Illustration: Fig. 95.]
+
+=151. Evaporating Dish, or Gauge= (fig. 95), for showing the amount of
+evaporation from the earth's surface. This gauge consists of a brass
+vessel, the area or evaporating surface of which is accurately determined;
+and also a glass cylindrical measure, graduated into inches, tenths, and
+hundredths of inches. In use, the evaporating gauge is nearly filled with
+water, the quantity having been previously measured by means of the glass
+cylinder; it is then placed out of doors, freely exposed to the action of
+the atmosphere; after exposure, the water is again measured, and the
+difference between the first and second measurement shows the amount of
+evaporation that has taken place. If rain has fallen during the exposure
+of the gauge, the quantity collected by it must be deducted from the
+measured quantity; the amount is shown by the quantity of rain collected
+in the rain gauge. The wire cage round the gauge is to prevent animals,
+birds, &c., from drinking the water.
+
+
+=152. Dr. Babington's Atmidometer=, or instrument for measuring the
+evaporation from water, _ice or snow_, consists of an oblong hollow bulb
+of glass or copper, beneath which and communicating with it by a
+contracted neck is a second globular bulb, duly weighted with mercury or
+shot. The upper bulb is surmounted by a small glass or metal stem, having
+a scale graduated to grains and half-grains; on the top of which is fixed
+horizontally a shallow metal pan. The bulbs are immersed in a vessel of
+water having a circular hole in the cover through which the stem rises.
+Distilled water is then gradually poured into the pan above, until the
+zero of the stem sinks to a level with the cover of the vessel. Thus
+adjusted, as the water in the pan evaporates, the stem ascends, and the
+amount of evaporation is indicated in grains. This instrument affords a
+means of measuring evaporation from _ice or snow_. An adjustment for
+temperature is necessary.
+
+
+=153. Cloud Reflector.=--At the International Exhibition 1862, Mr. J. T.
+Goddard exhibited a cloud mirror, for ascertaining the direction in which
+the clouds are moving.
+
+The mirror is laid on a horizontal support near a window, and fastened so
+that the point marked north may coincide with the south point of the
+horizon,--the several points will consequently be reversed. The edge of a
+conspicuous cloud is brought to the centre of the mirror, and the observer
+keeps perfectly still until it passes off at the margin, where the true
+point of the horizon _from which_ the clouds are coming can be read off.
+
+
+=154. Sunshine Recorder.=--Mr. Goddard also exhibited an instrument which
+he calls by this name. It works by letting the sun's rays pass through a
+narrow slit, and fall on photographic paper wound round a barrel moved by
+clock-work; the paper being changed daily, and the photographic impression
+developed and fixed in the usual manner.[19]
+
+
+155. SET OF PORTABLE INSTRUMENTS.
+
+In a small box, 8 in. by 8 in. by 4 in., a complete set of meteorological
+instruments have been packed. The lid of the box, by an ingenious
+arrangement, is made to take off and hang up; on it are permanently fixed
+for observation, a maximum and minimum, and a pair of dry and wet bulb
+thermometers. The interior of the box contains a maximum thermometer in
+vacuo for solar radiation, and a minimum for terrestrial purposes, one of
+Negretti and Zambra's small pocket aneroid barometers, pedometer for
+measuring distances, pocket compass, clinometer, and lastly a rain gauge.
+This latter instrument consists of an accurately turned brass ring having
+an india rubber body fastened to it to receive the rain, which is measured
+off by a small graduated glass, also contained in the box. Gentlemen
+travelling will find this compact observatory all that can be desired for
+meteorological observations.
+
+
+156. IMPLEMENTS.
+
+The practical meteorologist will find the following articles very useful,
+if not necessary. They scarcely require description; an enumeration will
+therefore suffice:--_Weather Diagrams_, or prepared printed and ruled
+forms, whereon to exhibit graphically the readings of the various
+instruments to render their indications useful in foretelling weather,
+&c.;--_Meteorological Registers_, or Record Books, for recording all
+observations, and the deductions;--_Cloud Pictures_, by which the clouds
+can be readily referred to their particular classification, very necessary
+to the inexperienced and learners;--Cyclone Glasses, or Horns, outline
+Maps with Wind-markers, are also useful, especially in forecasting
+weather.
+
+
+157. HYDROMETER.
+
+A simple kind of hydrometer is very much used at sea, as "a sea-water
+test;" and as the observations are usually recorded in a meteorological
+register or the ship's log-book, it may not be altogether out of place to
+give a description of it here.
+
+[Illustration: Fig. 96.]
+
+[Illustration: Fig. 97.]
+
+It is constructed of glass. If made of brass, the corrosive action of
+salt-water soon renders the instrument erroneous in its indications. The
+shapes usually given to the instruments are shown in figs. 96 and 97. A
+globular bulb is blown, and partly filled with mercury or small shot, to
+make the instrument float steadily in a vertical position. From the neck
+of the bulb the glass is expanded into an oval or a cylindrical shape, to
+give the instrument sufficient volume for flotation; finally, it is
+tapered off to a narrow upright stem which encloses an ivory scale, and is
+closed at the top. The divisions on the scale read downward, so as to
+measure the length of the stem which stands above the surface of any
+liquid in which the hydrometer is floated. The denser the fluid, the
+higher will the instrument rise; the rarer, the lower it will sink.
+
+The indications depend upon the hydrostatic principle, that floating
+bodies displace a quantity of the fluid which sustains them equal to their
+own weight. According, therefore, as the specific gravities of fluids
+differ from each other, so will vary the quantities of the fluids
+displaced by the same body when floated successively in each.
+
+The specific gravity of distilled water, at the temperature of 62 deg. _F_,
+being taken as unity, the depth to which the instrument sinks when gently
+immersed in such water is the zero of the scale. The graduations extend
+from 0 to 40; the latter being the mark which will be level with the
+surface when the instrument is placed in water, the specific gravity of
+which is 1.040. In recording observations, the last two figures
+only--being the figures on the scale--are written down. Sea-water usually
+ranges from 1.020 to 1.036.
+
+A small tin, copper, or glass cylinder is useful for containing the water
+to be tested. It should be wider than the hydrometer, and always filled to
+the brim. If fitted to a stand, which is supported by gimbals, it will be
+very convenient. Water in a bucket, basin, or other wide vessel, acquires
+motion at sea, and the eye cannot be brought low enough (on account of the
+edges) to read off the scale accurately.
+
+Errors of observation may occur with the hydrometer, if it be put into
+water without being clean, or without being carefully wiped. The
+instrument is extremely accurate if correctly used. It should be kept free
+from contact with the sides of the vessel; and all dust, smears, or
+greasiness, should be scrupulously avoided, by carefully wiping it with a
+clean cloth before and after use.
+
+Whenever the temperature of the water tested differs from 62 deg., a
+correction to the reading is necessary, for the expansion or contraction
+of the glass, as well as the water itself, in order to reduce all
+observations to one generally adopted standard.
+
+Negretti and Zambra's hydrometer, with thermometer in the stem, shows the
+density and temperature in one instrument.
+
+For the following Tables we are indebted to the kindness of Admiral
+FitzRoy:--
+
+TABLE for reducing observations made with a BRASS HYDROMETER, assuming the
+linear expansion of brass to be 0.000009555 for 1 deg. F. The correction is
+additive for all temperatures above 62 deg., and subtractive for temperatures
+below 62 deg.
+
+  +----------------------------------------------------------------------+
+  |_t deg._|Correction.||_t deg._|Correction.||_t deg._|Correction.||_t deg._|Correction.|
+  |----+-----------++----+-----------++----+-----------++----+-----------|
+  | 32 | -0.0014   || 48 | -0.0010   || 64 | +0.0002   || 80 | +0.0020   |
+  | 33 |   .0014   || 49 |   .0009   || 65 |   .0003   || 81 |   .0021   |
+  | 34 |   .0014   || 50 |   .0009   || 66 |   .0004   || 82 |   .0023   |
+  | 35 |   .0014   || 51 | -0.0008   || 67 |   .0005   || 83 |   .0024   |
+  | 36 |   .0014   || 52 |   .0008   || 68 | +0.0006   || 84 |   .0026   |
+  | 37 |   .0014   || 53 |   .0007   || 69 |   .0007   || 85 | +0.0027   |
+  | 38 | -0.0014   || 54 |   .0006   || 70 |   .0008   || 86 |   .0029   |
+  | 39 |   .0013   || 55 |   .0006   || 71 |   .0009   || 87 |   .0030   |
+  | 40 |   .0013   || 56 | -0.0005   || 72 |   .0010   || 88 |   .0032   |
+  | 41 |   .0013   || 57 |   .0004   || 73 |   .0011   || 89 |   .0033   |
+  | 42 |   .0013   || 58 |   .0003   || 74 | +0.0013   || 90 | +0.0035   |
+  | 43 |   .0012   || 59 |   .0003   || 75 |   .0014   || 91 |   .0036   |
+  | 44 | -0.0012   || 60 |   .0002   || 76 |   .0015   || 92 |   .0038   |
+  | 45 |   .0011   || 61 | -0.0001   || 77 |   .0016   || 93 |   .0040   |
+  | 46 |   .0011   || 62 |  0.0000   || 78 |   .0018   || 94 |   .0041   |
+  | 47 | -0.0010   || 63 | +0.0001   || 79 | +0.0019   || 95 | +0.0043   |
+  +----------------------------------------------------------------------+
+
+TABLE for reducing observations made with a GLASS HYDROMETER, assuming the
+linear expansion of glass to be 0.00000463 for 1 deg. F. The correction is
+additive for temperatures above 62 deg., and subtractive for temperatures
+below 62 deg.
+
+  +----------------------------------------------------------------------+
+  |_t deg._|Correction.||_t deg._|Correction.||_t deg._|Correction.||_t deg._|Correction.|
+  |----+-----------++----+-----------++----+-----------++----+-----------|
+  | 32 | -0.0019   || 48 | -0.0012   || 64 | +0.0002   || 80 | +0.0023   |
+  | 33 |   .0019   || 49 |   .0011   || 65 |   .0003   || 81 |   .0024   |
+  | 34 |   .0018   || 50 |   .0011   || 66 |   .0004   || 82 |   .0026   |
+  | 35 |   .0018   || 51 | -0.0010   || 67 |   .0005   || 83 |   .0027   |
+  | 36 |   .0018   || 52 |   .0009   || 68 | +0.0007   || 84 |   .0029   |
+  | 37 |   .0017   || 53 |   .0008   || 69 |   .0008   || 85 | +0.0031   |
+  | 38 | -0.0017   || 54 |   .0008   || 70 |   .0009   || 86 |   .0032   |
+  | 39 |   .0017   || 55 |   .0007   || 71 |   .0010   || 87 |   .0034   |
+  | 40 |   .0016   || 56 | -0.0006   || 72 |   .0012   || 88 |   .0036   |
+  | 41 |   .0016   || 57 |   .0005   || 73 |   .0013   || 89 |   .0037   |
+  | 42 |   .0015   || 58 |   .0004   || 74 | +0.0014   || 90 | +0.0039   |
+  | 43 |   .0015   || 59 |   .0003   || 75 |   .0016   || 91 |   .0041   |
+  | 44 | -0.0014   || 60 |   .0002   || 76 |   .0017   || 92 |   .0042   |
+  | 45 |   .0014   || 61 | -0.0001   || 77 |   .0018   || 93 |   .0044   |
+  | 46 |   .0013   || 62 |  0.0000   || 78 |   .0020   || 94 |   .0046   |
+  | 47 | -0.0013   || 63 | +0.0001   || 79 | +0.0021   || 95 | +0.0048   |
+  +----------------------------------------------------------------------+
+
+
+158. NEWMAN'S SELF-REGISTERING TIDE-GAUGE.
+
+At places where the phenomena of the tides are of much maritime
+importance, a continuous series of observations upon the rise and fall,
+and times of change, is essentially necessary as a basis for the
+construction of good tide tables; and as such observations should also be
+accompanied with the registration of atmospheric phenomena, we have no
+hesitation in inserting a description of an accurate self-registering
+tide-gauge.
+
+The tide-gauge, as shown in the illustration, consists of a cylinder, _A_,
+which is made to revolve on its axis once in twenty-four hours by the
+action of the clock, _B_. A chain, to which is attached the float, _D_,
+passes over the wheel, _C_, and on the axis of this wheel, _C_ (in about
+the middle of it) is a small toothed wheel, placed so as to be in contact
+with a larger toothed wheel carrying a cylinder, _E_, over which passes
+another smaller chain. This chain, passing along the upper surface of the
+cylinder, _A_, and round a second cylinder, _F_, at its further end, is
+acted on by a spring so as to be kept in a constant state of tension. In
+the middle of this chain a small tube is fixed for carrying a pencil,
+which, being gently pressed down by means of a small weight on the top of
+it, performs the duty of marking on paper placed round the cylinder the
+progress of the rise or fall of the tide as the cylinder revolves, and as
+it is drawn by the chain forward or backward by the rise or fall of the
+float. The paper is prepared with lines equidistant from each other, to
+correspond with the hours of the clock, crossed by others showing the
+number of feet of rise and fall.
+
+[Illustration]
+
+The cylinder while in action revolves from left to right to a spectator
+facing the clock, and the pencil is carried horizontally along the top of
+the cylinder; and the large wheel being made to revolve by the rise and
+fall of the float, turns the wheel with the small cylinder, _E_,
+attached to it. If the tide is _falling_, the small chain is wound round
+the cylinder, _E_, and the pencil is drawn towards the large wheel; but if
+the tide is _rising_, the small chain is wound on to the cylinder, _F_, by
+means of the spring contained in it, which constantly keeps it in a state
+of tension. Thus, by means of the rise and fall of the tide, a lateral
+progress is given to the pencil, while the cylinder is made to revolve on
+its axis by the clock, so that a line is traced on the paper showing the
+exact state of the tide continuously, without further attention than is
+necessary to change the paper once every day, and to keep the pencil
+carefully pointed; or a metallic pencil may be used, which will require
+little, if any, attention.
+
+A good self-registering tide-gauge is a valuable and important acquisition
+wherever tidal observations are required, and the only perfectly efficient
+instrument of this kind is that invented by the late Mr. John Newman, of
+Regent Street, London. It is now in action in several parts of the world,
+silently and _faithfully_ performing its duty, requiring no other kind of
+attention than that of a few minutes daily, and thus admitting the
+employment of the person on any other service whose duty it would
+otherwise have been to have registered the tide. It has done much by its
+faithful records in contributing to the construction of good tide tables
+for many places; for those unavoidable defects dependent on merely
+watching the surface on a divided scale are set aside by it, all erroneous
+conclusions excluded, and a true delineation of Nature's own making is
+preserved by it for the theorist.
+
+
+
+
+ADDENDA.
+
+
+1. French barometers are graduated to millimetres. An English inch is
+equal to 25.39954 millimetres. Hence, 30 inches on the English barometer
+scales correspond to 762 millimetres on the French barometer scales.
+Conversions from one scale to another can be effected by the following
+formulae:--
+
+  (1) Inches = millimetres divided by         25.39954
+  (2) Millimetres = inches multiplied by      25.39954
+
+Of course, a table of equivalent values should be drawn up and employed,
+when a large number of observations are to be converted from one scale to
+the other.
+
+
+2. In Germany, barometers are sometimes graduated with old French inches
+and lines,--the vernier generally indicating the tenth of a line.
+
+OLD FRENCH LINEAL MEASURE.
+
+                                              English Inches.
+  1 douzieme, or point                        =  0.0074
+  12 points              = 1 ligne            =  0.0888
+  12 lignes              = 1 pouce            =  1.065765
+  12 pouces              = 1 pied             = 12.7892
+  1 pied                 = 324.7 millimetres.
+
+"The Germans indicate inches by putting two accents after the number;
+lines, by putting three accents; 27" 3'''.85, means 27 inches 3 lines 85
+hundredths of a line; more frequently, they give the height in lines, and
+the preceding number becomes 327'''.85."--_Kaemtz._
+
+
+3. _Rule for finding Diameter of Bore of a Barometer Tube._
+
+"If the maker has not taken care to measure the interior diameter
+directly, it may be deduced from the exterior diameter. The exterior
+diameter is first measured by calipers, and, by deducting from this
+diameter 0.1 of an inch for tubes from .3 to .5 of an inch in external
+diameter, we have an approximation to the interior diameter of the
+tube."--_Kaemtz._
+
+
+4. WIND SCALES.
+
+  Sea Scale.       Wind.      Land Scale.
+  ----------       -----      -----------
+   0 to 3     =   Light     =   0 to 1
+   3 "  5     =   Moderate  =   1 "  2
+   5 "  7     =   Fresh     =   2 "  3
+   7 "  8     =   Strong    =   3 "  4
+   8 " 10     =   Heavy     =   4 "  5
+  10 " 12     =   Violent   =   5 "  6
+
+
+    Pressure in                  Velocity in
+      Pounds      (Land Scale).     Miles
+  (Avoirdupois)                   (Hourly).
+  -------------    ---------     -----------
+      1/2       =      1       =     10
+       5        =      2       =     32
+      10        =      3       =     45
+      21        =      4       =     65
+      26        =      5       =     72
+      32        =      6       =     80
+
+
+5. Letters to Denote the State of the Weather.
+
+  _b_ denotes blue sky, whether with clear or hazy atmosphere.
+  _c_    "    cloudy, that is detached opening clouds.
+  _d_    "    drizzling rain.
+  _f_    "    fog.
+  _h_    "    hail.
+  _l_    "    lightning.
+  _m_    "    misty, or hazy so as to interrupt the view.
+  _o_    "    overcast, gloomy, dull.
+  _p_    "    passing showers.
+  _q_    "    squally.
+  _r_    "    rain.
+  _s_    "    snow.
+  _t_    "    thunder.
+  _u_    "    ugly, threatening appearance of sky.
+  _v_    "    unusual visibility of distant objects.
+  _w_    "    wet, that is dew.
+
+A letter repeated denotes much, as _r r_, heavy rain; _f f_, dense fog;
+and a figure attached denotes duration in hours, as 14 _r_, 14 hours rain.
+
+By the combination of these letters, all the ordinary phenomena of the
+weather may be recorded with certainty and brevity.
+
+EXAMPLES.--_b c_, blue sky with less proportion of cloud. 2 _r r l l t_,
+heavy rain for two hours, with much lightning, and some thunder.
+
+The above methods of recording the force of wind and state of weather
+were originally proposed by Admiral Sir Francis Beaufort. They are now in
+general use at sea, and by many observers on land.
+
+
+6. Table of Expansion by Heat from 32 deg. to 212 deg. F.
+
+  Platinum              0.0008842  of the length.
+  Glass, Flint          0.0008117      "
+    "    with Lead      0.0008622      "
+  Brass                 0.0018708      "
+  Mercury               0.0180180      "
+  Water                 0.0433200, from 39 deg. to 212 deg.
+  Alcohol               0.1100       "  32 deg. to 174 deg.
+  Nitric Acid           0.1100
+  Sulphuric Acid        0.0600
+
+
+7. Table of Specific Gravity of Bodies at 32 deg. F. except water, which is
+taken at 39 deg.4.
+
+  Water                            1.000
+  Alcohol, pure                    0.791
+     "     proof                   0.916
+  Mercury                         13.596
+  Glass                      3 to  2.7
+  Brass                    7.8 to  8.54
+  Platinum                  21 to 22.00
+
+Weight of a cubic foot of water, at the temperature of comparison, 62.425
+lbs. avoirdupois.
+
+The pound avoirdupois contains 7,000 grains.
+
+Air is 813.67 times lighter than water.
+
+The linear expansions are the mean values of the results of various
+experimentalists. The specific gravities are as given in Professor
+Rankine's _Applied Mechanics_.
+
+
+8. Important Temperatures. Under the circumstances of--
+
+                                                         deg.
+  Water                          boiling at            212
+  Mercury                         boils at             660
+  Sulphuric Acid                     "                 590
+  Oil of Turpentine                  "                 560
+  Nitric Acid                        "                 242
+  Alcohol                            "                 174
+  A Saturated Solution of Salt       "                 218
+  Vital Heat                                            96
+  Olive Oil begins to solidify                          36
+  Fresh Water freezes                                   32
+  Sea Water freezes                                     28
+  Mercury freezes                                      -39
+
+
+9. TABLE OF METEOROLOGICAL ELEMENTS, FORMING EXPONENTS OF THE CLIMATE OF
+LONDON.
+
+  --------------------------------------------------------------------+
+  1841     |Mean Height of Barometer, reduced to 32 deg. F., at the mean  |
+   to      |    sea-level.                                            |
+  1861.    |       +--------------------------------------------------|
+           |       |Mean Monthly Range of Barometer.                  |
+  Months.  |       |    +---------------------------------------------|
+           |       |    |Mean of all the Highest Temperatures.        |
+           |       |    |    +----------------------------------------|
+           |       |    |    |Mean of all the Lowest Temperatures.    |
+           |       |    |    |    +-----------------------------------|
+           |       |    |    |    |Mean Temperature.                  |
+           |       |    |    |    |    +------------------------------|
+           |       |    |    |    |    |Mean Temperature of Dew-point.|
+           |       |    |    |    |    |    +-------------------------|
+           |       |    |    |    |    |    |Mean Degree of Humidity. |
+           |       |    |    |    |    |    |  +----------------------|
+           |       |    |    |    |    |    |  |Mean Number of  Rainy |
+           |       |    |    |    |    |    |  |  Days.               |
+           |       |    |    |    |    |    |  |   +------------------|
+           |       |    |    |    |    |    |  |   |Average Rainfall. |
+           |       |    |    |    |    |    |  |   |    +-------------|
+           |       |    |    |    |    |    |  |   |    |Average      |
+           |       |    |    |    |    |    |  |   |    |Amount of    |
+           |       |    |    |    |    |    |  |   |    |Cloud (10=   |
+           |       |    |    |    |    |    |  |   |    | overcast).  |
+           |       |    |    |    |    |    |  |   |    |   +---------|
+           |       |    |    |    |    |    |  |   |    |   |Prevalent|
+           |       |    |    |    |    |    |  |   |    |   |Winds.   |
+  ---------+-------+----+----+----+----+----+--+---+----+---+---------+
+           |Inches.| In.|  deg.  |  deg.  |  deg.  |  deg.  |  |   | In.|   |         |
+  ---------+-------+----+----+----+----+----+--+---+----+---+---------+
+  January  | 29.932|1.44|43.2|33.7|38.3|35.4|89| 11| 1.8|7.7|W. to N. |
+           |       |    |    |    |    |    |  |   |    |   |         |
+  February | 29.962|1.22|44.7|33.2|38.4|34.4|85| 10| 1.6|7.4|S. to W. |
+           |       |    |    |    |    |    |  |   |    |   |         |
+  March    | 29.967|1.23|50.0|35.3|41.7|36.4|82| 10| 1.5|6.6|N. to E. |
+           |       |    |    |    |    |    |  |   |    |   |         |
+           |       |    |    |    |    |    |  |   |    |   |         |
+  April    | 29.907|1.06|56.8|38.6|46.3|39.9|79| 11| 1.8|6.1|N. to E. |
+           |       |    |    |    |    |    |  |   |    |   |         |
+  May      | 29.931|1.02|64.4|44.2|52.8|45.5|76| 11| 2.1|6.1|S. to W. |
+           |       |    |    |    |    |    |  |   |    |   |         |
+  June     | 29.960|0.89|71.2|50.2|59.2|50.8|74| 11| 1.9|6.1|W. to N. |
+           |       |    |    |    |    |    |  |   |    |   |         |
+  July     | 29.970|0.79|73.8|53.2|61.9|53.9|76| 11| 2.7|6.9|W. to N. |
+           |       |    |    |    |    |    |  |   |    |   |         |
+  August   | 29.954|0.97|72.8|53.4|61.3|54.1|77| 11| 2.4|6.5|W. to N. |
+           |       |    |    |    |    |    |  |   |    |   |         |
+  September| 29.997|0.95|67.4|48.9|56.9|51.1|81| 12| 2.4|5.9|S. to W. |
+           |       |    |    |    |    |    |  |   |    |   |         |
+  October  | 29.860|1.33|58.3|43.7|50.2|46.0|87| 13| 2.8|6.9|S. to W. |
+           |       |    |    |    |    |    |  |   |    |   |         |
+  November | 29.929|1.53|49.3|37.7|43.4|40.1|89| 12| 2.4|7.2|  S.W.   |
+           |       |    |    |    |    |    |  |   |    |   |         |
+  December | 29.979|1.52|45.0|35.5|40.1|36.9|89| 12| 1.9|7.4|   W.    |
+           |       |    |    |    |    |    |  |   |    |   |         |
+  ---------+-------+----+----+----+----+----+--+---+----+---+---------+
+  Year     | 29.946|1.16|58.0|42.3|49.2|43.7|82|133|25.3|6.7|   --    |
+  ---------+-------+----+----+----+----+----+--+---+----+---+---------+
+               1     2     3    4    5    6   7  8    9   10    11
+  ---------------------------------------------------------------------
+
+             +-----------------------------------------------------+
+             |Sun above the Horizon on Middle Day.                 |
+             |       +---------------------------------------------|
+             |       |REMARKS.                                     |
+    ---------+-------+---------------------------------------------|
+             | Hours.|                                             |
+    ---------+-------+---------------------------------------------|
+    January  |  8-1/2|The majority of the nights are frosty.       |
+             |       |                                             |
+    February | 10    |10 frosty nights on the average.             |
+             |       |                                             |
+    March    | 12    |12 ditto   ditto     ditto.                  |
+             |       |   Strong winds.                             |
+             |       |                                             |
+    April    | 14    | 6 ditto   ditto     ditto.                  |
+             |       |                                             |
+    May      | 15-1/2|Very rarely frost.                           |
+             |       |                                             |
+    June     | 16-1/2|Sun attains greatest North Declination, 21st.|
+             |       |                                             |
+    July     | 16    |                                             |
+             |       |                                             |
+    August   | 14-1/2|                                             |
+             |       |                                             |
+    September| 12-1/2|                                             |
+             |       |                                             |
+    October  | 10-1/2|A few frosty nights. Heavy gales.            |
+             |       |                                             |
+    November |  9    | 11 nights frosty.                           |
+             |       |                                             |
+    December |  8    |Sun attains greatest South Declination, 21st.|
+             |       |                                             |
+    ---------+-------+---------------------------------------------|
+    Year     |   --  |                                             |
+    ---------+-------+---------------------------------------------|
+                 12               13                               |
+    ---------------------------------------------------------------+
+
+In the above Table, columns 1 to 10 are results obtained at the Royal
+Observatory, Greenwich, by J. Glaisher, Esq., F.R.S. The data contained in
+columns 2 and 10, are deduced from observations extending over the years
+1841 to 1855 inclusive, and are copied from Edward Hughes' _Third Reading
+Book_; the other columns are results of observations made during the
+twenty years ending 1861. The rest of the information is from Luke
+Howard's _Climate of London_.
+
+These valuable data indicate the characteristics of the weather in each
+month in the suburbs of London, and will be found tolerably accurate as
+indications of weather, and serviceable as standards for comparisons of
+observed results, at most places in England.
+
+
+
+
+STANDARD WORKS ON METEOROLOGY
+
+SUPPLIED BY NEGRETTI & ZAMBRA.
+
+  THE WEATHER BOOK: A MANUAL OF PRACTICAL METEOROLOGY.
+  By Vice-Admiral FITZROY, F.R.S., M.I.F., &c.
+  _Price_, L0 15 6
+
+  THE LAW OF STORMS,
+  By H. W. DOVE, F.R.S.
+  Translated by R. H. SCOTT, M.A.
+  _Price_, L0 10 6
+
+  L. F. KAEMTZ'S "COMPLETE COURSE OF METEOROLOGY,"
+  Translated by C. V. WALKER, Esq.
+  _Price_, L0 12 6
+
+  PRACTICAL METEOROLOGY,
+  By JOHN DREW, Ph.D., F.R.A.S.
+  _Price_, L0 5 0
+
+  HYGROMETRICAL TABLES,
+  Adapted to the use of the Wet and Dry Bulb Thermometer,
+  By JAMES GLAISHER, Esq., F.R.S.
+  _Price_, L0 2 6
+
+  TABLES OF THE CORRECTIONS FOR TEMPERATURES,
+  To reduce observations to the 32 deg. Fahrenheit, for Barometers with brass
+  scales extending from the cistern to the top of the mercurial column,
+  By JAMES GLAISHER, Esq., F.R.S.
+  _Price_, L0 1 0
+
+  TABLE OF THE DIURNAL RANGE OF THE BAROMETER,
+  By JAMES GLAISHER, Esq., F.R.S.
+  _Price_, L0 0 6
+
+  TABLES FOR CALCULATION OF HEIGHTS FROM OBSERVATIONS
+  ON THE BOILING-POINT OF WATER,
+  Adapted to the use of Negretti and Zambra's Boiling-point Apparatus.
+  _Price_, L0 1 0
+
+  A THERMOMETRICAL TABLE,
+  ON THE SCALES OF FAHRENHEIT, REAUMUR, AND CENTIGRADE,
+  By ALFRED S. TAYLOR, Esq., M.D., &c.
+  _Price_, in Sheet, with explanatory Pamphlet, L0 1 6
+
+  METEOROLOGICAL TABLES,
+  For the reduction of Barometrical and Hygrometrical Observations,
+  Determination of Heights by the Barometer and Boiling-point
+  Thermometer, &c.
+  By G. HARVEY SIMMONDS, M.B.M.S.
+  _Price_, L0 2 6
+
+  BAROMETER MANUAL,
+  Compiled by Vice-Admiral FITZROY, F.R.S.,
+  For the Board of Trade.
+  _Price_, L0 0 6
+
+  POCKET METEOROLOGICAL REGISTER AND NOTE-BOOK,
+  With Diagrams for exhibiting the Fluctuations of Barometer, &c.
+  Printed on metallic paper. _Price_, with Pencil, L0 3 0
+
+
+  LONDON:
+  PRINTED BY STRAHAN AND WILLIAMS,
+  7 LAWRENCE LAND, CHEAPSIDE, E.C.
+
+
+
+
+NEGRETTI & ZAMBRA'S
+
+PATENT RECORDING AND DEEP-SEA THERMOMETER.[20]
+
+
+This Thermometer differs from all other Registering or Recording
+Thermometers in the following important particulars:--
+
+I. The Thermometer contains only Mercury without any admixture of Alcohol
+or other fluid.
+
+II. It has no indices or springs, and its indications are by the column of
+Mercury only.
+
+III. It can be carried in any position, and cannot possibly be put out of
+order except by actual breakage of the instrument.
+
+And lastly, it will indicate and record the exact temperature at any hour
+of the day or night, or the exact temperature at any depth of the sea,
+irrespective of either warm or cold currents, or stratum through which the
+Thermometer may have to pass in its descent or ascent, this last very
+special quality renders this Thermometer superior for deep-sea
+temperatures to any others; for those now being used in the "Challenger"
+sounding expedition are liable to give erroneous indications owing to
+their indices slipping, and otherwise getting deranged--(This was proved
+by Messrs. Negretti and Zambra at a Meeting of the British Meteorological
+Society,) and _under certain conditions of temperature_ it is not possible
+by the old Thermometers to obtain true temperatures at certain depths
+which might be required. _Annexed is a copy of a report to the Admiralty
+from Captain G. S. Nares, of H.M.S. "Challenger," dated Melbourne, March
+25th, 1874, which we have taken from NATURE, July 30th, 1874, proving
+the assertion._
+
+"In the report to the Admiralty of Capt. G. S. Nares, of H.M.S.
+_Challenger_ dated Melbourne, March 25, 1874, Capt. Nares, speaking of the
+temperature of the ocean, especially near the pack edge of the ice,
+says:--'At a short distance from the pack, the surface water rose to 32 deg.,
+but at a depth of 40 fathoms we always found the temperature to be 29 deg.;
+this continued to 300 fathoms, the depth in which most of the icebergs
+float, after which there is a stratum of slightly warmer water of 33 deg. or
+34 deg. As the thermometers had to pass through these two belts of water
+before reaching the bottom, the indices registered those temperatures, and
+it was impossible to obtain the exact temperature of the bottom whilst
+near the ice, but the observations made in lower latitudes show that it is
+about 31 deg. More exact results could not have been obtained even had Mr.
+Siemens's apparatus been on board.' It seems to us that the difficulty
+mentioned is one which would certainly have been surmounted by Messrs.
+Negretti and Zambra's new Recording Thermometers, a description of which
+appeared in NATURE, vol. ix. p. 387; this being exactly one of the cases
+to which this instrument is peculiarly adapted. We believe the inventors
+and makers have greatly improved their Thermometer since our description
+appeared, and no doubt means will be taken by the Admiralty to transmit
+one to the _Challenger_."
+
+
+DESCRIPTION OF THE DEEP-SEA RECORDING THERMOMETER.
+
+In the first place, it must be observed that the bulb of the Thermometer
+is protected so as to resist the pressure of the ocean, which varies
+according to depth that of three thousand fathoms being something like
+three tons pressure on the square inch. The manner of protecting the bulb
+was invented by Messrs. Negretti and Zambra in 1857, and has been latterly
+copied by other persons and brought out as a new invention. The manner of
+protecting the bulb has been described by the late Admiral R. FitzRoy, in
+the first number of Meteorological Papers, page 55, published July 5th,
+1857, as follows:
+
+    "Referring to the erroneous readings of all thermometers, consequent
+    on their delicate bulbs being compressed by the great pressure of the
+    ocean, he says:--'With a view to obviate this failing, Messrs.
+    Negretti and Zambra undertook to make a case for the weak bulbs, which
+    should transmit temperature, but resist pressure. Accordingly a tube
+    of thick glass is sealed outside the delicate bulb, between which and
+    the casing is a space all round, which is nearly filled with mercury.
+    The small space not so filled is a vacuum, into which the mercury can
+    be expanded, or forced by heat or mechanical compression, without
+    doing injury to or even compressing the inner or much more delicate
+    bulb.'"
+
+[Illustration: Fig. 1.]
+
+The construction of this instrument for deep-sea temperatures is as
+follows:--
+
+    In shape it is like a syphon with parallel legs, all in one piece and
+    having a continuous communication, as in the annexed figure. The scale
+    of the Thermometer is pivoted on a centre and being attached in a
+    perpendicular position to a simple apparatus (which will be presently
+    described), is lowered to any depth that may be desired. In its
+    descent the Thermometer acts as an ordinary instrument, the mercury
+    rising or falling according to the temperature of the stratum through
+    which it passes; but so soon as the descent ceases, and a reverse
+    motion is given to the line, so as to pull the Thermometer towards the
+    surface, the instrument turns once on its centre, first bulb
+    uppermost, and afterwards bulb downwards. This causes the mercury,
+    which was in the left-hand column, first to pass into the dilated
+    syphon bend at the top, and thence into the right-hand tube, where it
+    remains, indicating on a graduated scale the exact temperature at the
+    time it was turned over. The woodcut, Fig. 1, shows the position of
+    the mercury _after_ the instrument has been thus turned on its centre.
+    A is the bulb; B the outer coating or protecting cylinder; C is the
+    space of rarefied air, which is reduced if the outer casing be
+    compressed; D is a small glass plug on the principle of Negretti and
+    Zambra's Patent Maximum Thermometer, which cuts off, in the moment of
+    turning, the mercury in the tube from that of the bulb, thereby
+    ensuring that none but the mercury in the tube can be transferred into
+    the indicating column; E is an enlargement made in the bend so as to
+    enable the mercury to pass quickly from one tube to another in
+    revolving; and F is the indicating tube or Thermometer proper. In its
+    action, as soon as the Thermometer is put in motion, and immediately
+    the tube has acquired a slightly oblique position, the mercury breaks
+    off at the point D, runs into the curved and enlarged portion E, and
+    eventually falls into the tube F when this tube resumes its original
+    perpendicular position.
+
+The contrivance for turning the Thermometer over may be described as a
+frame with a vertical propeller; to this frame the instrument is pivoted.
+On its descent through the water the propeller is lifted out of gear and
+revolves freely on its axis; but so soon as the instrument is pulled
+towards the surface the propeller falls into gear and revolves in the
+contrary direction, turning the Thermometer over once, and then becoming
+locked and immovable.
+
+
+_Directions for adjusting the Thermometer previous to its being lowered in
+the Sea._
+
+    I. The mercury must all be in the left-hand column.
+
+    II. The short peg at the back of the thermometer must be in front of
+    the stop plate S +; in order to effect this, pull the knob which stops
+    the Thermometer, and slightly turn the propeller, to make the
+    Thermometer advance sufficiently to escape the stop plate.
+
+Negretti & Zambra's Patent Atmospheric Recording Thermometer, Fig. 3,
+differs from the Deep-sea Thermometer by its not having the double or
+protected bulb, it not being required for resisting pressures. In this
+case the instrument is turned over by a simple clock movement, which can
+be set to any hour it may be desirable; the Thermometer is fixed on the
+clock, and when the hand arrives at the hour determined upon, and to which
+the clock is set as in setting an alarum clock, a spring is released and
+the Thermometer turns over as before described.
+
+[Illustration: Fig. 2.]
+
+[Illustration: Fig. 3.]
+
+Messrs. Negretti and Zambra have arranged a Wet and Dry Bulb Hygrometer
+upon the same plan.
+
+
+
+
+NEGRETTI & ZAMBRA'S PRICE LIST OF STANDARD METEOROLOGICAL AND OTHER
+PHILOSOPHICAL INSTRUMENTS.
+
+_The marginal figures in this List and the numbers of the wood engravings
+refer to paragraphs in "Negretti & Zambra's Treatise on Meteorological
+Instruments."_
+
+
+                                                                  L  s. d.
+
+  4 =Standard Barometers=, Fortin's arrangement, as Fig. 3
+  with mahogany board                                             8   8  0
+
+      Ditto    ditto with Millemetre and English scales           9   9  0
+
+      Ditto    ditto with tube, 0.45 internal diameter and
+                     millimetre scale                            10  10  0
+
+  =Observatory Standard Barometers=, extra large tubes and
+  cisterns                                          L25  0  0    35   0  0
+
+      Ditto    ditto arranged for observations being taken
+                     by the Cathetometer                         18  18  0
+
+  =Cathetometer=, for use with above                             21   0  0
+
+  9 =Self Compensating Standard Barometer=, Fig. 6               20   0  0
+
+  10 =Standard Barometer=, with electrical adjustment            15  15  0
+
+  11 =Pediment Barometers=, Fig 7                    L1  1  0     2   2  0
+
+     Ditto      ditto     Fig. 8                     L3  3  0     3  10  0
+
+     Ditto      ditto     Fig. 9                     L4 10  0     5  10  0
+
+     Ditto      ditto     Fig. 10                                 8  10  0
+
+     Ditto      ditto     ditto handsome carved mountings,
+     in mahogany, oak, or walnut wood    L8  8  0   L10 10  0    12  12  0
+
+  14 =FitzRoy's Storm or Sea Coast Fishery Barometer=, Fig. 12    5   5  0
+
+     Ditto    ditto with two verniers                             6   6  0
+
+     Ditto    ditto mounted in ornamental carved frames, oak,
+     walnut, or mahogany                             L6 10  0     8   8  0
+
+  19 =Marine Barometers=, ordinary forms, Figs. 13 and 14
+                                         L2  2s.     L2 10s.      3   3  0
+
+     Ditto    ditto   Best mounted                   L5  5s.      6   6  0
+
+  20 =The Board of Trade or Kew Marine Barometer=, Fig. 15,
+                                         L4  4s.     L5  5s.      6   6  0
+
+  22 =Negretti and Zambra's FitzRoy Marine or Gun Barometer=,
+       Fig. 16, with N. and Z.'s Patent Porcelain Scales, as
+       used in Her Majesty's Navy                                 5  10  0
+
+     Extra Tube for ditto                                         1  15  0
+
+  25 =Negretti and Zambra's Farmer's Barometer or Domestic
+  Weather Glass=, Fig. 17                                         2  10  0
+
+  28 =Negretti and Zambra's Miner's Barometers=
+                                         L1  1s.     L2  2s.      3   3  0
+
+  31 =Dial or Wheel Barometers=, Figs. 18, 19, 20, 21
+                                         L3  3s.     L4  4s.      5   5  0
+
+     Ditto  ditto  in carved ornamental mountings
+                             L5 10s.     L6  6s.     L8  8s.     10  10  0
+
+     Ditto  ditto  rosewood, inlaid with pearl or metal. Made
+     to order, Figs. 22 and 23. Price varying with size, &c.
+
+  37 =Gay Lussac's Syphon Tube Mountain Barometer=   L6  6  0     8   8  0
+
+  32 =Standard Syphon Barometer=, Gay Lussac's arrangement,
+     Fig. 24                                                      5   5  0
+
+  38 =Negretti and Zambra's Standard Mountain Barometer=, with
+     Fortin's cistern, with tripod stand and travelling case,
+     Fig. 30                                                     10  10  0
+
+  34 =Barograph, or Self-registering Barometer=, with syphon
+     mercurial tube. Negretti and Zambra's improved arrangement,
+     Fig. 26                                         18 18  0    25   0  0
+
+  =Negretti and Zambra's Self-recording Aneroid Barometer=,
+  with =Clock=                                                   22   0  0
+
+
+  48. =ANEROID BAROMETERS.=
+
+  =Aneroid Barometers,= with card dials 4-1/2 inches diameter,
+  best quality.                                                   2  10  0
+
+    Ditto    ditto      with silvered metal dial                  3   0  0
+
+    Ditto    ditto      with ditto and thermometer                3  10  0
+
+    Ditto    ditto         ditto   with corrected scale, as
+    supplied by Negretti and Zambra to the Royal Navy             5   5  0
+
+  =Aneroid Barometers=, with elegantly-chased dials               4   4  0
+
+    Ditto    ditto      with raised ring on dial                  5   5  0
+
+    Ditto    ditto         ditto    with thermometer              6   6  0
+
+  =Aneroid Barometer=, for altitude measurements with revolving
+  ring, carrying index, range of scale 20,000 feet 4-1/2 inches
+  diameter, with magnifier                                        8   8  0
+
+
+  =POCKET ANEROID BAROMETERS.= Fig. 34.
+
+  49 =Pocket Aneroid Barometer=, 2-3/4 inches diameter, with
+     silvered metal scale                                         3   3  0
+
+     Ditto   ditto  for measuring altitudes to 10,000 feet
+     compensated for temperature, in leather case                 5   5  0
+
+     Ditto   ditto  ditto to 20,000 feet, with magnifier          6   6  0
+
+
+  50 =WATCH-SIZE ANEROID BAROMETERS= in gilt metal cases (see figure 35.)
+
+  =Watch-Size Aneroid Barometer=, weather range      L3  3  0     4   4  0
+
+  Ditto      ditto  of best construction, extra thin, for
+  meteorological observations or altitude measurements to
+  10,000 feet                                                     5   5  0
+
+  Ditto      ditto  ditto  to 20,000 feet, compensated for
+  temperature                                                     6   6  0
+
+  Either of the above Watch-size Barometers may be had in Stout
+  Silver Cases at a cost of L2 2s. extra
+
+  _Watch-size Aneroid Barometers in Solid Gold, highly-finished cases.
+  L15 15s. to L21._
+
+  Table Stands for Aneroid Barometers of Carved Oak or other woods,
+  10s. 6d., 25s., 35s., to L5 5s.
+
+  =Ships' Aneroid Barometers=, in suitable mountings L2 10s. L3 3s.
+  L5 5s. L6 6s.
+
+
+  47 =Sympiesometer=, for Ship use                   L3  3  0     4  10  0
+
+     Ditto     ditto  Pocket form, Fig. 32                        4   4  0
+
+  The Sympiesometer is now rarely used, the Aneroid Barometer being
+  found equally sensitive and less liable to derangement.
+
+
+  56 =Independent Standard Thermometers=, Fig. 36                 5   5  0
+
+  57 =Standard Thermometers=, for Boiling Point Apparatus         1  10  0
+
+  =Chemists' or Brewers' Standard Reference Thermometers=
+                                                     L1  1s.      2   2  0
+
+  47 =Chemical Thermometers=, divided on the stem. Fig. 48.
+                                                    10s. 6d.      0  15  0
+
+  =Chemical Thermometers=, Boxwood Scale  7s.  6d.  10s. 6d.      0  12  6
+
+  64 =Thermometers on Boxwood Scales=, Fig. 37.
+                         1s.   1s.  6d.   2s.  6d.   3s. 6d.      0   4  6
+
+     Ditto   ditto   larger sizes                    7s. 6d.      0  12  6
+
+     Ditto   ditto   Engraved glass scales, Fig. 39
+                                    15s.   L1  1s.   L1  5s.      1  10  0
+
+
+  =POCKET THERMOMETERS, IN VARIOUS MOUNTINGS.=
+
+  Fig. 38   10s. 6d.    Fig. 40   10s. 6d. 15s.
+  Fig. 41    5s. 6d.   8s. 6d.   12s. 6d.
+
+  63 =Thermometers of extreme Sensitiveness=, 15s.   L1 10s.      2   2  0
+
+
+  =Drawing Room or Mantel Thermometers=, various mountings,
+
+  Figs. 42 and 43.  12s. 6d.  15s.  L1 1s.  L1 10s.  L2  2s.      2  10  0
+
+  =Bracket Window Thermometers=, Fig. 46
+                                  12s. 6d.    15s.   L1  1s.      1  10  0
+
+  =Bath Thermometers=, Figs. 44                     7s.  6d.      0  15  0
+
+  66 =Sugar Boiling Thermometers=           L1 12s.  L2  2s.      3   3  0
+
+  67 =Earth Thermometers=, Fig. 51                   L1 10s.      2   2  0
+
+     Hot Bed      Ditto                    12s. 6d.  L1  5s.      1  10  0
+
+  68 =Marine Thermometer=, Fig. 52          7s. 6d.  8s. 6d.      0  10  6
+
+  65 =Super Heated or Steam Pressure Thermometers=, Fig. 74,
+     Figs. 49 and 50               L1 5s.   L1 10s.  L2  2s.      2  10  0
+
+
+  =SELF-REGISTERING THERMOMETERS FOR HEAT.=
+
+  72 =Negretti and Zambra's Patent Standard Maximum
+     Self-Registering Thermometer=,[21] Fig. 54                   1   1  0
+
+  72 =Negretti and Zambra's Patent Maximum Thermometer=, on
+  boxwood scale                                                   0  10  6
+
+     Ditto,  ditto,  on Negretti and Zambra's Patent Solid
+     Porcelain or Metal Scales on oak mounting                    0  12  6
+
+  70 =Rutherford's Maximum Thermometer=, on boxwood or metal
+     scale, with steel index                5s. 6d.  7s. 6d.      0  10  6
+
+  71 =Phillip's Maximum Thermometer=, on boxwood or metal
+     scale, with air index                  7s. 6d.  10  6        0  12  6
+
+
+  =SELF-REGISTERING THERMOMETERS FOR COLD.=
+
+  73 =Negretti and Zambra's Standard Minimum Self-Registering
+     Thermometer=, Fig. 55                                        1   1  0
+
+  73 =Rutherford's Minimum Thermometer=, on boxwood or metal scale
+                                   3s. 6d.    5s. 6d.   7s. 6d.   0  10  6
+
+  73 =Rutherford's Minimum Thermometer=, on Negretti and Zambra's
+     Patent solid porcelain scale                      10s. 6d.   0  12  6
+
+     Ditto, on Negretti and Zambra's porcelain or metal scales
+     and oak mounting                                             0  12  6
+
+  74 =Negretti and Zambra's Horticultural Self-Registering
+     Thermometer.= The scale is made of stout zinc, enclosing
+     the tube; the figures and divisions are boldly marked for
+     quickly and easily reading the indications, Fig. 56          0   3  6
+
+
+  83 =Negretti and Zambra's Patent Solar Radiation Thermometer=,
+     Fig. 63                                                      1   5  0
+
+  84 Ditto,   ditto,   ditto, in vacuo, Fig. 64                   1  10  0
+
+     Ditto,   ditto,   ditto, improved form, with test gauge      2   2  0
+
+  85 =Negretti and Zambra's Terrestrial Radiation Thermometer=    1   5  0
+
+  Brass Stands for above, Fig. 65                                 0   5  0
+
+  76 and 77 =Negretti and Zambra's Patent Mercurial Minimum
+     Thermometers=                                   L2 10s.      2   2  0
+
+  81 =Maxima and Minima Thermometers=, on Sixe's arrangement,
+     Fig. 62, various forms of mounting
+                                  12s. 6d.    14s.    21s. 30s.   2   2  0
+
+  =Pocket Maxima and Minima Thermometers=, Negretti and Zambra's
+  Patent, in convenient cases                        L2  2s.      2  10  0
+
+  89 =Deep Sea Registering Thermometer=, with Negretti and
+     Zambra's improved protected bulb, in copper cylinder,
+     Fig. 69.                                                     2  10  0
+
+  89[21] =Negretti and Zambra's Improved Deep Sea Thermometer=,
+     with vulcanite mountings, in copper cylinder, with door,
+     small size                                                   2   5  0
+
+  90 =Negretti and Zambra's Patent Recording Deep Sea
+     Thermometer=                                                10  10  0
+
+  91 Ditto,   ditto,   ditto   =Recording Thermometer=            4   4  0
+
+  92 Ditto,   ditto,   ditto   =Hygrometer=                       6   6  0
+
+  93 =Improved Boiling Point Mountain Thermometer=, or
+     Hypsometric Apparatus, with Tables, Figs. 72 and 73, in
+     leather case with strap                                      5   5  0
+
+     Extra Thermometer for Ditto                                  1  10  0
+
+  106 =Negretti and Zambra's Standard Wet and Dry Bulb
+      Hygrometer=, Fig. 79                                        2   2  0
+
+      Wet and Dry Bulb Hygrometers, various mountings
+                                   30s.   25s.   21s.   14s.      0  10  6
+
+      Pocket Hygrometers, in box                     L2  2s.      2  10  0
+
+  103 =Daniell's Hygrometer=, Fig. 77                             3   3  0
+
+  104 =Regnault's Hygrometer=, Fig. 78               L3 10s.      5   5  0
+
+      Aspirator for Ditto                            L1 15s.      2  15  0
+
+
+  110 =Howard's Rain Gauge=, has a 5-inch copper Funnel, with
+      turned brass rim fitted to a stout stone-ware or glass
+      bottle, with a graduated glass measure, divided to 100ths
+      of an inch                                                  0  10  6
+
+      =Symons' Portable Rain Gauge=, (5-inch) with graduated
+      glass measure, japanned tin                                 0  10  6
+
+      Ditto   ditto   in stout copper                             0  15  0
+
+  111 =Glaisher's Rain Gauge=, the receiving surface is 8-inches
+      diameter, of stout japanned metal, with graduated glass
+      measure, Fig. 84                                            1   1  0
+
+      Ditto   ditto,  of stout copper                             1  10  0
+
+      Receiving Pots for ditto, extra 2s. and 3s. 6d.
+
+  113 =Rain Gauge=, having a receiving surface of 12 inches
+      diameter, and graduated glass gauge tube, divided to
+      hundredths of an inch, in japanned metal, with brass tap    2  10  0
+
+      Ditto   ditto, Fig. 85, in copper                           3  10  0
+
+      Ditto   ditto, with sliding rod instead of graduated tube,
+      japanned tin                                                2   2  0
+
+      =Rain Gauges=, of any form or area made to order, with suitable
+      measuring glasses.
+
+
+  123 =Lind's Anemometer=, Fig. 86                                2   2  0
+
+  125 =Robinson's Anemometer=, Fig. 87                            3   3  0
+
+      Ditto   ditto,   Improved arrangement          L4 10s.      5  15  0
+
+      Ditto   ditto,   with clutch movement, Fig. 88              6  15  0
+
+      =Negretti and Zambra's Improved Air Meter=, of extreme
+      sensitiveness, very portable                                4   4  0
+
+      Large Air Meters made to order.
+
+  127 =Osler's Self-Registering Anemometer and Rain Gauge=,
+      Fig. 89                                            L84 to 150   0  0
+
+  128 =Berkley's Anemometers= fitted up to order, _to suit the
+      Observatory_.
+
+  131 =Gold Leaf Electrometer=, Fig. 90                           1   1  0
+
+  133 =Peltier's Electrometer=                                    4   4  0
+
+  134 =Bohnenberger's Electroscope=, Fig. 91                      8   8  0
+
+  135 =Thompson's Electrometer=, to order
+
+      =Lightning Conductors= fitted up to order.
+
+  142 =Ozone Cage=, Fig. 92                                       0  18  0
+
+      Ditto  ditto, copper                                        1   5  0
+
+  146 =Leslie's Differential Thermometer=, Fig. 93    L1 10s.     2   2  0
+
+  148 =Thermometer Stand (Glaisher's)=                            3   3  0
+
+  149 =Thermometer Screen= for Sea use                            3   3  0
+
+  150 =Anemoscope=, or Portable Vane, Fig. 94                     2   5  0
+
+  151 =Evaporating Dish=, Fig. 95                                 1   2  6
+
+  157 =Sea Water Hydrometers=, Board of Trade Marine, Figs. 96
+      and 97                                                      0   5  6
+
+  158 =Newman's Self-Registering Tide Gauge=, Fig. 158, fitted
+      to the Building to order                              From 50   0  0
+
+
+  _Further Information as to Price, &c., will be found in_
+  NEGRETTI & ZAMBRA'S ENCYCLOPAEDIC CATALOGUE OF
+  MATHEMATICAL, PHILOSOPHICAL, OPTICAL, PHOTOGRAPHIC,
+  AND STANDARD METEOROLOGICAL INSTRUMENTS,
+
+  _Containing very numerous Comparative Tables of Reference,
+  and Illustrated by upwards of_
+
+  ELEVEN HUNDRED ENGRAVINGS.
+
+  Royal 8vo. Cloth, Gilt Lettered--Price 5s. 6d.
+
+
+
+
+FOOTNOTES:
+
+[1] Second Number of "Meteorological Papers," issued by the Board of
+Trade.
+
+[2] With reference to these barometers, we have received the subjoined
+testimonial, with permission to use it as we please.
+
+    "_Meteorologic Office, 12th June, 1863._
+
+    "MESSRS. NEGRETTI & ZAMBRA,
+
+    "The barometers which you have lately supplied to Her Majesty's ships
+    through this Office are much approved, being good for general service,
+    afloat or on land.
+
+    "(Signed)       R. FITZROY."
+
+[3] _Vide_ C. Daubeny, F.R.S., "On Climate."
+
+[4] _Vide_ Report of the British Association, 1862.
+
+[5] See page 42 for the Tables.
+
+[6] The quotations in this section are from Tyndall's _Heat considered as
+a Mode of Motion_.
+
+[7] Dr. Daubeny, F.R.S., _On Climate_.
+
+[8] Leslie _On the Relations of Air, Heat, and Moisture_.
+
+[9] Tyndall's _Heat considered as a Mode of Motion_.
+
+[10] Vide _Horological Journal_, Vol. V.
+
+[11] _Hygrometrical Tables_, by J. Glaisher, Esq., F.R.S.
+
+[12] Vide _Report of the British Association_, 1862. It may be added, for
+the information of those who are about to commence observing, that Mr.
+Symons, of Camden Road Villas, London, is desirous of securing returns of
+rain-fall from as many stations as possible, in order to render more
+complete his annual reports to the British Association.
+
+[13] Luke Howard's _Climate of London_.
+
+[14] Vide _Third Number of Meteorological Papers_, issued by the Board of
+Trade.
+
+[15] _Elements of Physics_, by C. F. Peschel.
+
+[16] This description is modified from that in Report of the Jurors for
+Class XIII. International Exhibition, 1862.
+
+[17] _All the Year Round_, No. 224.
+
+[18] _All the Year Round_, No. 224.
+
+[19] Vide _Jurors' Reports_.
+
+[20] See also page 90 of this Treatise.
+
+[21] These Instruments are the only Maximum Thermometers that can be
+recommended, as unless they be broken, they cannot be put out of
+adjustment. Fully described under the head of Standard Maximum
+Thermometers in our large Catalogue, and page 72 of our _Treatise on
+Meteorological Instruments_.
+
+
+
+
+
+
+End of the Project Gutenberg EBook of A Treatise on Meteorological
+Instruments, by Henry Negretti and Joseph Zambra
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