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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..d7b82bc --- /dev/null +++ b/.gitattributes @@ -0,0 +1,4 @@ +*.txt text eol=lf +*.htm text eol=lf +*.html text eol=lf +*.md text eol=lf diff --git a/LICENSE.txt b/LICENSE.txt new file mode 100644 index 0000000..6312041 --- /dev/null +++ b/LICENSE.txt @@ -0,0 +1,11 @@ +This eBook, including all associated images, markup, improvements, +metadata, and any other content or labor, has been confirmed to be +in the PUBLIC DOMAIN IN THE UNITED STATES. + +Procedures for determining public domain status are described in +the "Copyright How-To" at https://www.gutenberg.org. + +No investigation has been made concerning possible copyrights in +jurisdictions other than the United States. Anyone seeking to utilize +this eBook outside of the United States should confirm copyright +status under the laws that apply to them. diff --git a/README.md b/README.md new file mode 100644 index 0000000..c08bf99 --- /dev/null +++ b/README.md @@ -0,0 +1,2 @@ +Project Gutenberg (https://www.gutenberg.org) public repository for +eBook #68994 (https://www.gutenberg.org/ebooks/68994) diff --git a/old/68994-0.txt b/old/68994-0.txt deleted file mode 100644 index 7e4dc26..0000000 --- a/old/68994-0.txt +++ /dev/null @@ -1,3221 +0,0 @@ -The Project Gutenberg eBook of Lightning, Thunder and Lightning -Conductors, by Gerald Molloy - -This eBook is for the use of anyone anywhere in the United States and -most other parts of the world 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. If you are not located in the United States, you -will have to check the laws of the country where you are located before -using this eBook. - -Title: Lightning, Thunder and Lightning Conductors - -Author: Gerald Molloy - -Release Date: September 15, 2022 [eBook #68994] - -Language: English - -Produced by: deaurider and the Online Distributed Proofreading Team at - https://www.pgdp.net (This book was produced from images - made available by the HathiTrust Digital Library.) - -*** START OF THE PROJECT GUTENBERG EBOOK LIGHTNING, THUNDER AND -LIGHTNING CONDUCTORS *** - - - - - - LIGHTNING, THUNDER - - AND - - LIGHTNING CONDUCTORS. - - _WITH AN APPENDIX ON THE RECENT CONTROVERSY - ON LIGHTNING CONDUCTORS._ - - BY - - GERALD MOLLOY, D. D., D. Sc. - - _ILLUSTRATED._ - - [Illustration] - - NEW YORK: - - THE HUMBOLDT PUBLISHING CO., - - 28 LAFAYETTE PLACE. - - - - -LIGHTNING, THUNDER, AND LIGHTNING CONDUCTORS. - - - - -CONTENTS. - - - LECTURE I. Pages 5-26 - - LIGHTNING AND THUNDER. - - Identity of Lightning and Electricity--Franklin’s Experiment--Fatal - Experiment of Richman--Immediate Cause of Lightning--Illustration from - Electric Spark--What a Flash of Lightning Is--Duration of a Flash of - Lightning--Experiments of Professor Rood--Wheatstone’s - Experiments--Experiment with Rotating Disc--Brightness of a Flash of - Lightning--Various Forms of Lightning--Forked Lightning, Sheet - Lightning, Globe Lightning--St. Elmo’s Fire--Experimental - Illustration--Origin of Lightning--Length of a Flash of - Lightning--Physical Cause of Thunder--Rolling of Thunder--Succession - of Peals--Variation of Intensity--Distance of a Flash of Lightning - - - LECTURE II. Pages 26-53 - - LIGHTNING CONDUCTORS. - - Destructive Effects of Lightning--Destruction of - Buildings--Destruction of Ships at Sea--Destruction of Powder - Magazines--Experimental Illustrations--Destruction of Life by - Lightning--The Return Shock--Franklin’s Lightning Rods--Introduction - of Lightning Rods into England--The Battle of Balls and - Points--Functions of a Lightning Conductor--Conditions of a Lightning - Conductor--Mischief Done by Bad Conductors--Evil Effects of a Bad - Earth Contact--Danger from Rival Conductors--Insulation of Lightning - Conductors--Personal Safety in a Thunder Storm--Practical - Rules--Security Afforded by Lightning Rods - - - APPENDIX. Pages 55-62 - - RECENT CONTROVERSY ON LIGHTNING CONDUCTORS. - - Theory of Lightning Conductors Challenged--Lectures of Professor - Lodge--Short Account of his Views and Arguments--Effect of - Self-Induction on a Lightning Rod--Experiment on the Discharge of a - Leyden Jar--Outer Shell only of a Lightning Rod Acts as a - Conductor--Discussion at the Meeting of the British Association, - September, 1888--Statement by Mr. Preece--Lord Rayleigh and Sir - William Thomson--Professor Rowland and Professor Forbes--M. de - Fonvielle, Sir James Douglass, and Mr. Symons--Reply of Professor - Lodge--Concluding Remarks of Professor Fitzgerald, President - of the Section--Summary Showing the Present State of the Question - - - - -LIST OF ILLUSTRATIONS. - - - PAGE - - THE ELECTRIC SPARK: A TYPE OF A FLASH OF LIGHTNING, 8 - - CARDBOARD DISC WITH BLACK AND WHITE SECTORS; AS SEEN WHEN AT REST, 12 - - SAME DISC; AS SEEN WHEN IN RAPID ROTATION, 12 - - THE BRUSH DISCHARGE, ILLUSTRATING ST. ELMO’S FIRE, 17 - - ORIGIN OF SUCCESSIVE PEALS OF THUNDER, 22 - - VARIATIONS OF INTENSITY IN A PEAL OF THUNDER, 24 - - DISCHARGE OF LEYDEN JAR BATTERY THROUGH THIN WIRES, 27 - - GLASS VESSEL BROKEN BY DISCHARGE OF LEYDEN JAR BATTERY, 32 - - GUN COTTON SET ON FIRE BY ELECTRIC SPARK, 33 - - VOLTA’S PISTOL; EXPLOSION CAUSED BY ELECTRIC SPARK, 34 - - THE RETURN SHOCK ILLUSTRATED, 35 - - PROTECTION FROM LIGHTNING BY A CLOSED CONDUCTOR, 48 - - INDUCTION EFFECT OF LEYDEN JAR DISCHARGE, 56 - - - - -LECTURE I. - -LIGHTNING AND THUNDER. - - -The electricity produced by an ordinary electric machine exhibits, -under certain conditions, phenomena which bear a striking resemblance -to the phenomena attendant on lightning. In both cases there is a flash -of light; in both there is a report, which, in the case of lightning, -we call thunder; and, in both cases, intense heat is developed, which -is capable of setting fire to combustible bodies. Further, the spark -from an electric machine travels through space with extraordinary -rapidity, and so does a flash of lightning; the spark follows a zig-zag -course, and so does a flash of lightning; the spark moves silently and -harmlessly through metal rods and stout wires, while it forces its -way, with destructive effect, through bad conductors, and it is so, -too, with a flash of lightning. Lastly, the electricity of a machine -is capable of giving a severe shock to the human body; and we know -that lightning gives a shock so severe as usually to cause immediate -death. For these reasons it was long conjectured by scientific men -that lightning is, in its nature, identical with electricity; and that -it differs from the electricity of our machines only in this, that it -exists in a more powerful and destructive form. - - -=Identity of Lightning and Electricity.=--But it was reserved for -the celebrated Benjamin Franklin to demonstrate the truth of this -conjecture by direct experiment. He first conceived the idea of drawing -electricity from a thundercloud in the same way as it is drawn from -the conductor of an electric machine. For this purpose he proposed -to place a kind of sentry-box on the summit of a lofty tower, and to -erect, on the sentry-box, a metal rod, projecting twenty or thirty -feet upward into the air, pointed at the end, and having no electrical -communication with the earth. He predicted that when a thundercloud -would pass over the tower, the metal rod would become charged with -electricity, and that an observer, stationed in the sentry-box, might -draw from it, at pleasure, a succession of electric sparks. - -With the magnanimity of a really great man, Franklin published this -project to the world; being more solicitous to extend the domain of -science by new discoveries, than to secure for himself the glory -of having made them. The project was set forth in a letter to Mr. -Collinson, of London, which bears date July 29, 1750, and which, in the -course of a year or two, was translated into the principal languages -of Europe. Two years later the experiment suggested by Franklin was -made by Monsieur Dalibard, a wealthy man of science, at his villa near -Marly-la-Ville, a few miles from Paris. In the middle of an elevated -plain Monsieur Dalibard erected an iron rod, forty feet in length, one -inch in diameter, and ending above in a sharp steel point. The iron rod -rested on an insulating support, and was kept in position by means of -silk cords. - -In the absence of Monsieur Dalibard, who was called by business to -Paris, this apparatus was watched by an old dragoon, named Coiffier; -and on the afternoon of the tenth of May, 1752, he drew sparks from the -lower end of the rod at the time that a thundercloud was passing over -the neighborhood. Conscious of the importance that would be attached to -this phenomenon, the old dragoon summoned, in all haste, the prior of -Marly to come and witness it. The prior came without delay, and he was -followed by some of the principal inhabitants of the village. In the -presence of the little group, thus gathered together, the experiment -was repeated--electric sparks were again drawn, in rapid succession, -from the iron rod; the prediction of Franklin was fulfilled to the -letter; and the identity of lightning and electricity was, for the -first time, demonstrated to the world. - - -=Franklin’s Experiment.=--Meanwhile Franklin had been waiting, with -impatience, for the completion of the tower of Christchurch, in -Philadelphia, on which he intended to make the experiment himself. -He even collected money, it is said, to hasten on the building. -But, notwithstanding his exertions, the progress of the tower was -slow; and his active mind, which could ill brook delay, hit upon -another expedient, remarkable alike for its simplicity and for its -complete success. He constructed a boy’s kite, using, however, a silk -pockethandkerchief, instead of paper, that it might not be damaged by -rain. To the top of the kite he attached a pointed iron wire about a -foot long, and he provided a roll of hempen twine, which he knew to be -a conductor of electricity, for flying it. This was the apparatus with -which he proposed to explore the nature of a thundercloud. - -The thundercloud came late in the afternoon of the fourth of July, -1752, and Franklin sallied out with his kite, accompanied by his son, -and taking with him a common door-key and a Leyden jar. The kite -was soon high in air, and the philosopher awaited the result of his -experiment, standing, with his son, under the lee of a cowshed, partly -to protect himself from the rain that was coming, and partly, it is -said, to shield himself from the ridicule of passers-by, who, having -no sympathy with his philosophical speculations, might be inclined to -regard him as a lunatic. To guard against the danger of receiving a -flash of lightning through his body, he held the kite by means of a -silk ribbon, which was tied to the door-key, the door-key being itself -attached to the lower end of the hempen string. - -A flash of lightning soon came from the cloud, and a second, and a -third; but no sign of electricity could be observed in the kite, or -the hempen cord, or the key. Franklin was almost beginning to despair -of success, when suddenly he noticed that the little fibres of the -cord began to bristle up, just as they would if it were placed near an -electric machine in action. He presented the door-key to the knob of -the Leyden jar, and a spark passed between them. Presently a shower -began to fall; the cord, wetted by the rain, became a better conductor -than it had been before, and sparks came more freely. With these sparks -he now charged the Leyden jar, and found, to his intense delight, that -he could exhibit all the phenomena of electricity by means of the -lightning he had drawn from the clouds. - -In the following year a similar experiment, with even more striking -results, was carried out, in France, by de Romas. Though it is said he -had no knowledge of what Franklin had done in America, he, too, used -a kite; and, with a view of making the string a better conductor, he -interlaced with it a thin copper wire. Then, flying his kite in the -ordinary way, when it had risen to a height of about 550 feet, he drew -sparks from it which, we are told, were upwards of nine feet long, and -emitted a sound like the report of a pistol. - - -=Fatal Experiment of Richman.=--There can be no doubt that experiments -of this kind, made with the electricity of a thundercloud, were -extremely dangerous; and this was soon proved by a fatal accident. -Professor Richman, of St. Petersburgh, had erected on the roof of his -house a pointed iron rod, the lower end of which passed into a glass -vessel, intended, as we are informed, to measure the strength of the -charge which he expected to receive from the clouds. On the sixth of -August, 1753, observing the approach of a thunderstorm, he hastened -to his apparatus; and as he stood near it, with his head bent down, -to watch the effect, a flash of lightning passed through his body and -killed him on the spot. This catastrophe served to fix public attention -on the danger of such experiments, and gave occasion to the saying of -Voltaire: “There are some great lords whom we should always approach -with extreme precaution, and lightning is one of them.”[1] From this -time the practice of making experiments directly with the lightning of -the clouds seems to have been, by common consent, abandoned. - - -=Immediate Cause of Lightning.=--And now, having set before you some -of the most memorable experiments by which the identity of lightning -and electricity has been demonstrated, I will try to give you a clear -conception regarding the immediate cause of lightning, so far as the -subject is understood at the present day by scientific men. You know -that there are two kinds of electricity, which are called _positive_ -and _negative_; and that each of them repels electricity of the same -kind as itself, while it attracts electricity of the opposite kind. -Now, every thundercloud is charged with electricity of one kind or -the other, positive or negative; and, as it hovers over the earth, it -develops, by what is called _induction_, or influence, electricity -of the opposite kind in that part of the earth which is immediately -under it. Thus we have two bodies--the cloud and the earth--charged -with opposite kinds of electricity, and separated by a stratum of the -atmosphere. The two opposite electricities powerfully attract each -other; but for a time they are prevented from rushing together by the -intervening stratum of air, which is a non-conductor of electricity, -and acts as a barrier between them. As the electricity, however, -continues to accumulate, the attraction becomes stronger and stronger, -until at length it is able to overcome the resistance of this barrier; -a violent disruptive discharge then takes place between the cloud -and the earth, and the flash of lightning is the consequence of the -discharge. - -[Illustration: THE ELECTRIC SPARK; A TYPE OF A FLASH OF LIGHTNING.] - -The whole phenomenon may be illustrated, on a small scale, by means -of this electric machine of Carré’s which you see before you. When -my assistant turns the handle of the machine negative electricity is -developed in that large brass cylinder, which in our experiment will -represent the thundercloud. At a distance of five or six inches from -the cylinder I hold a brass ball, which is in electrical communication -with the earth through my body. The electrified brass cylinder acts -by induction, or influence on the brass ball, and develops in it, as -well as in my body, a charge of positive electricity. Now, the positive -electricity of the ball and the negative electricity of the cylinder -are mutually attracting each other, but the intervening stratum of air -offers a resistance which prevents a discharge from taking place. My -assistant, however, continues to work the machine; the two opposite -electricities rapidly accumulate on the cylinder and the ball; at -length their mutual attraction is strong enough to overcome the -resistance interposed between them; a disruptive discharge follows, -and at the same moment a spark is seen to pass, accompanied by a sharp -snapping report. - -This spark is a miniature flash of lightning; and the snapping report -is a diminutive peal of thunder. Furthermore, at the moment the spark -passes you may observe a slight convulsive movement in my hand and -wrist. This convulsive movement represents, on a small scale, the -violent shock, generally fatal to life, which is produced by a flash of -lightning when it passes through the body. - -I can continue to take sparks from the conductor as long as the machine -is worked; and it is interesting to observe that these sparks follow -an irregular zig-zag course, just as lightning does. The reason is the -same in both cases: a discharge between two electrified bodies takes -place along the line of least resistance; and, owing to the varying -condition of the atmosphere, as well as of the minute particles of -matter floating in it, the line of least resistance is almost always a -zig-zag line. - - -=What a Flash of Lightning is.=--Lightning, then, may be conceived as -an electrical discharge, sudden and violent in its character, which -takes place, through the atmosphere, between two bodies highly charged -with opposite kinds of electricity. Sometimes this electrical discharge -passes, as I have said, between a cloud and the earth; sometimes it -passes between one cloud and another; sometimes, on a smaller scale, -it takes place, between the great mass of a cloud and its outlying -fragments. - -But, if you ask me in what the discharge itself consists, I am utterly -unable to tell you. It is usual to speak and write on this subject as -if electricity were a material substance, a very subtle fluid, and as -if, at the moment the discharge takes place, this fluid passes like a -rapid stream, from the body that is positively electrified to the body -that is negatively electrified. But we must always remember that this -is only a conventional mode of expression, intended chiefly to assist -our conceptions, and to help us to talk about the phenomena. It does -not even profess to represent the objective truth. All that we know -for certain is this: that immediately before the discharge the two -bodies are highly electrified with opposite kinds of electricity; and, -that immediately after the discharge, they are found to have returned -to their ordinary condition, or, at least, to have become less highly -electrified than they were before. - -The flash of light that accompanies an electric discharge is often -supposed to be the electricity itself, passing from one body to -the other. But it is not; it is simply an effect produced by the -discharge. Heat is generated by the expenditure of electrical energy, -in overcoming the resistance offered by the atmosphere; and this heat -is so intense, that it produces a brilliant incandescence along the -path of the discharge. When a spark appears, for example, between the -conductor of the machine and this brass ball, it can be shown, by very -satisfactory evidence, that minute particles of these solid bodies are -first converted into vapor, and then made to glow with intense heat. -The gases, too, of which the air is composed, and the solid particles -floating in the air, are likewise raised to incandescence. So, too, -with lightning; the flash of light is due to the intense heat generated -by the electrical discharge, and owes its character to the composition -and the density of the atmosphere through which the discharge passes. - - -=Duration of a Flash of Lightning.=--How long does a flash of lightning -last? You are aware, I dare say, that when an impression of light is -made on the eye, the impression remains for a sensible interval of -time, not less than the tenth of a second, after the source of light -has been extinguished or removed. Hence we continue, in fact, to see -the light, for at least the tenth of a second, after the light has -ceased. Now, if you reflect how brief is the moment for which a flash -of lightning is visible, and if you deduct the tenth of a second from -that brief moment, you will see, at once, that the period of its actual -duration must be very short indeed. - -The exact duration of a flash of lightning is a question on which no -settled opinion has yet been accepted generally by scientific men. -Indeed, the most widely different statements have been made on the -subject, quite recently, by the highest authorities, each speaking -apparently with unhesitating confidence. Thus, for example, Professor -Mascart describes an experiment, which he says was made by Wheatstone, -and which showed that a flash of lightning lasts for less than -_one_-thousandth of a second;[2] Professor Everett describes the same -experiment, without saying by whom it was made, and gives, as the -result, that “the duration of the illumination produced by lightning -is certainly less than the _ten_-thousandth of a second;”[3] Professor -Tyndall, in his own picturesque way, tells us that “a flash of -lightning cleaves a cloud, appearing and disappearing in less than the -_hundred_-thousandth of a second;”[4] and according to Professor Tait, -of Edinburgh, “Wheatstone has shown that lightning certainly lasts less -than the _millionth_ of a second.”[5] - - -=Experiments of Professor Rood.=--I cannot say which of these -statements is best supported by actual observation; for none of the -writers I have quoted gives any reference to the original memoir from -which his statement is derived. As far as my own reading goes, I have -only come across one original record of experiments, made directly on -the flash of lightning itself, with a view to determine the period of -its duration. These experiments were carried out by Professor Ogden -Rood, of Columbia College, New York, between the years 1870 and 1873, -and are recorded in the _American Journal of Science and Arts_.[6] - -For the description of his apparatus, and for the details of his -observations, I must refer you to the memoir itself; but I may tell you -briefly that the results at which he arrived, if they be accepted, must -lead to a considerable modification of the views previously entertained -on the subject. In the first place, he satisfied himself that what -appears to the eye a single flash of lightning is usually, if not -always, multiple in its character; consisting, in fact, of a succession -of distinct flashes, which follow one another with such rapidity as -to make a continuous impression on the retina. Next, he proceeded to -measure approximately the duration of these several component flashes; -and he found that it varied over a wide range, amounting sometimes to -fully the twentieth of a second, and being sometimes less than the -sixteen-hundredth of a second. - - -=Wheatstone’s Experiments.=--These results are extremely interesting; -but we can hardly regard them as finally established, until they have -been confirmed by other observers. I may remark, however, that they -fit in very well with the experiments made by Professor Wheatstone, -many years ago, on the duration of the electric spark, which, as I told -you, is a miniature flash of lightning. In these classical experiments, -which leave nothing to be desired in point of accuracy, Professor -Wheatstone showed that a spark taken directly from a Leyden jar, or a -spark taken from the conductor of a powerful electric machine, that is, -just such a spark as you have seen here to-day, lasts for less than the -millionth of a second. - -But he also showed that the duration of the spark is greatly increased, -when a resisting wire is introduced into the path of the discharge. -Thus, for example, when the discharge from a Leyden jar was made to -pass through half a mile of copper wire, with breaks at intervals, the -sparks that appeared at these breaks were found to last for ¹⁄₂₄₀₀₀ -of a second.[7] Hence we should naturally expect that the period of -illumination would be still further increased, in the case of a flash -of lightning, where the resistance interposed is enormously greater -than in either of the experiments made by Wheatstone.[8] - - -=Experiment of the Rotating Disk.=--It would be tedious, on an occasion -like the present, to enter into an account of Wheatstone’s beautiful -and ingenious method of investigation, by which the above facts have -been established; but I will show you a much more simple experiment -which brings home very forcibly to the mind how exceedingly short -must be the duration of the electric spark. Here is a circular disk -of cardboard, the outer part of which, as you see, is divided into -sectors, black and white alternately, while the space about the centre -is entirely white. The disk is mounted on a stand, by means of which -I can make it rotate with great velocity. When it is put in rotation, -the effect on the eye is very striking--the central space remains white -as before, but in the outer rim the distinction of black and white -absolutely disappears and gives place to a uniform gray. This color is -due to the blending together of black and white in equal proportions; -the blending being effected, not on the cardboard disk, but on the -retina of the eye. - -[Illustration: CARDBOARD DISK AS SEEN WHEN AT REST.] - -[Illustration: SAME DISK AS SEEN WHEN IN RAPID ROTATION.] - -I mentioned just now that an impression made on the retina lasts for -the tenth of a second after the cause of it has been removed. Now, when -this disk is in rotation, the sectors follow one another so rapidly -that the particular part of space occupied at any moment by a white -sector will be occupied by a black sector within a time much less than -the tenth of a second. It follows that the impression made by each -white sector remains on the retina until the following black sector -comes into the same position; and, in like manner, the impression made -by each black sector remains until the following white sector takes up -the position of the black. Therefore, the impression made by the whole -outer rim is the impression of black and white combined--that is, the -impression of gray. - -So far, I dare say, the phenomenon is already familiar to you all. -But I propose now to show you the revolving disk illuminated by -the electric spark; and you will observe that, at the moment of -illumination, the black and white sectors come out as clearly and -distinctly as if the disk were standing still. - -For the success of this experiment it is desirable, not only to have -a brilliant spark in order to secure a good illumination of the disk, -but also to have a succession of such sparks, that you may see the -phenomenon frequently repeated, and thus be able to observe it at your -leisure. To attain these two objects, I have made the arrangement which -is here before you. - -In front of the disk is a large and very powerful Leyden jar. The -rod connected with the inner coating rises well above the mouth of -the jar, and ends in a brass ball nearly opposite the centre of the -disk. Connected with the outer coating of the jar is another rod -which likewise ends in a brass ball, and which is so adjusted that -the distance between the two balls is about an inch. The two rods are -connected respectively with the two conductors of a Holtz machine, so -that, when the machine is worked, the jar is first quickly charged, -and then it discharges itself, with a brilliant spark, between the -two brass balls. Thus, by continuing to work the machine, we can get, -as long as we choose, a succession of sparks following one another at -short and regular intervals right in front of the disk. - -Everything being now ready, and the room partially darkened, the -disk is put in rapid rotation; and you can see, by the twilight that -remains, the outer rim a uniform gray, and the central space white. -But when my assistant begins to turn the Holtz machine, and brilliant -sparks leap out at intervals, the revolving disk, illuminated for a -moment at each discharge, seems to be standing still, and shows the -black and white sectors distinctly visible. - -The reason of this is clear: So brief is the moment for which the spark -endures, that the disk, though in rapid motion, makes no sensible -advance during that small fraction of time; therefore, in the image on -the retina, the impression made by the white sectors remains distinct -from the impression made by the black, and the eye sees the disk as it -really is. - -I may notice, in passing, a very interesting consideration, suggested -by this experiment. A cannon ball is now commonly discharged with a -velocity of about 1,600 feet a second. Moving with this velocity it -is, as you know, under ordinary circumstances, altogether invisible to -the eye. But suppose it were illuminated, in the darkness of night, -by this electric spark, which lasts, we will say, for the millionth -of a second. During the moment of illumination, the cannon ball moves -through the millionth part of 1,600 feet, which is a little less than -the fiftieth of an inch. Practically, we may say that the cannon ball -does not sensibly change its place while the spark lasts. Further, the -impression it makes on the eye, from the position it occupies at the -moment of illumination, remains on the retina for at least the tenth -of a second. Therefore, if we are looking toward that particular part -of space where the cannon ball happens to be at the moment the spark -passes, we must see the cannon ball hanging motionless in the air, -though we know it is traveling at the rate of 1,600 feet a second, or -about 1,000 miles an hour. - - -=Brightness of a Flash of Lightning.=--I should like to say one word -about the brightness of a flash of lightning. Somewhat more than -thirty years ago, Professor Swan, of Edinburgh, showed that the eye -requires a sensible time--about the tenth of a second--to perceive -the full brightness of a luminous object. Further, he proved, by a -series of interesting experiments, that when a flash of light lasts -for less than the tenth of a second, its apparent brilliancy to the -eye is proportional to the time of its duration.[9] Now consider the -consequence of these facts in reference to the brightness of our -electric spark. If the spark lasted for the tenth of a second, we -should perceive its full brightness; if it lasted for the tenth part of -that time, we should see only the tenth part of its brightness; if it -lasted for the hundredth part, we should see only the hundredth part -of its brightness; and so on. But we know, in point of fact, that it -lasts for less than the millionth of a second, that is, less than the -hundred-thousandth part of the tenth of a second. Therefore we see only -the hundred-thousandth part of its real brightness. - -Here is a startling conclusion, and one, I may say, fully justified -by scientific evidence. That electric spark, brilliant as it appears -to us, is really a hundred thousand times as bright as it seems to -be. We cannot speak with the same precision of a flash of lightning; -because its duration has not yet been so exactly determined. But if we -suppose that a flash of lightning, in a particular case, lasts for the -thousandth of a second, it would follow, from the above experiments, -that the flash is a hundred times as bright, in fact, as it appears to -the eye. - - -=Various Forms of Lightning.=--The lightning of which I have spoken -hitherto is commonly called _forked_ lightning; a name which seems -to have been derived from the zig-zag line of light it presents to -the eye. But there are other forms under which the electricity of the -clouds often makes itself manifest; and to these I would now invite -your attention for a few moments. The most common of them all, at least -in this country, is that which is familiarly known by the name of -_sheet_ lightning. This is, probably, nothing else than the lighting up -of the atmosphere, or of the clouds, by forked lightning, which is not -itself directly visible. - -Generally speaking, after a flash of sheet lightning, we hear the -rolling of distant thunder. But it sometimes happens, especially in -summer time, that the atmosphere is again and again lit up by a sudden -glow of light, and yet no thunder is heard. This phenomenon is commonly -called _summer_ lightning, or _heat_ lightning. It is probably due, -in many cases, to electrical discharges in the higher regions of the -atmosphere, where the air is greatly rarified; and, in these cases, it -would seem to resemble the discharges obtained by means of an induction -coil in glass tubes containing rarified gases. But there is little -doubt that in many cases, too, summer lightning, like ordinary sheet -lightning, is due to forked lightning, which is so remote that we can -neither see the flash itself directly, nor hear the rolling of the -thunder. - -Perhaps the most distinct and satisfactory evidence on this subject, -derived from actual observation, is contained in the following letter -of Professor Tyndall, written in May, 1883: “Looking to the south -and south-east from the Bel Alp, the play of silent lightning among -the clouds and mountains is sometimes very wonderful. It may be seen -palpitating for hours, with a barely appreciable interval between -the thrills. Most of those who see it regard it as lightning without -thunder--Blitz ohne Donner, Wetterleuchten, I have heard it named by -German visitors. The Monte Generoso, overlooking the Lake of Lugano, is -about fifty miles from the Bel Alp, as the crow flies. The two points -are connected by telegraph; and frequently when the Wetterleuchten, -as seen from the Bel Alp, was in full play, I have telegraphed to the -proprietor of the Monte Generoso Hotel and learned, in every instance, -that our silent lightning co-existed in time with a thunderstorm more -or less terrific in upper Italy.”[10] - -Another form of lightning, described by many writers, is called _globe_ -lightning. It is said to appear as a ball of fire, about the size of -a child’s head, or even larger, which moves for a time slowly about, -and then, after the lapse of several seconds, explodes with a terrific -noise, sending forth flashes of fire in all directions, which burn -whatever they strike. Many accounts are on record of such phenomena; -but they are derived, for the most part, from the evidence of persons -who were not specially competent to observe, and to describe with -precision, the facts that fell under their observation. Hence these -accounts, while they are accepted by some, are rejected by others; -and it seems to me, in the present state of the question, that the -existence of globe lightning can hardly be regarded as a demonstrated -fact. At all events, if phenomena of this kind have really occurred, I -can only say that nothing we know about electricity, at present, will -enable us to account for them.[11] - - -=St. Elmo’s Fire.=--A much more authentic and, at the same time, very -interesting form, under which the electricity of the clouds sometimes -manifests its presence, is known by the name of St. Elmo’s fire. This -phenomenon at one time presents the appearance of a star, shining at -the points of the lances or bayonets of a company of soldiers; at -another, it takes the form of a tuft of bluish light, which seems to -stream away from the masts and spars of a ship at sea, or from the -pointed spire of a church. It was well known to the ancients. Cæsar, -in his Commentaries, tells us that, after a stormy night, the iron -points of the javelins of the fifth legion seemed to be on fire; and -Pliny says that he saw lights, like stars, shining on the lances of -the soldiers, keeping watch by night upon the ramparts. When two -such lights appeared at once, on the masts of a ship, they were -called Castor and Pollux, and were regarded by sailors as a sign of a -prosperous voyage. When only one appeared, it was called Helen, and was -taken as an unfavorable omen. - -In modern times St. Elmo’s fire has been witnessed by a host of -observers, and all its various phases have been repeatedly described. -In the memoirs of Forbin we read that, when he was sailing once, in -1696, among the Balearic Islands, a sudden storm came on during the -night, accompanied by lightning and thunder. “We saw on the vessel,” he -says, “more than thirty St. Elmo’s fires. Among the rest there was one -on the vane of the mainmast more than a foot and a half high. I sent a -man up to fetch it down. When he was aloft he cried out that it made -a noise like wetted gunpowder set on fire. I told him to take off the -vane and come down; but, scarcely had he removed it from its place, -when the fire left it and reappeared at the end of the mast, so that -it was impossible to take it away. It remained for a long time, and -gradually went out.” - -On the 14th of January, 1824, Monsieur Maxadorf happened to look at a -load of straw in the middle of a field just under a dense black cloud. -The straw seemed literally on fire--a streak of light went forth from -every blade; even the driver’s whip shone with a pale-blue flame. As -the black cloud passed away, the light gradually disappeared, after -having lasted about ten minutes. Again, it is related that on the 8th -of May, 1831, in Algiers, as the French artillery officers were walking -out after sunset without their caps, each one saw a tuft of blue light -on his neighbor’s head; and, when they stretched out their hands, a -tuft of light was seen at the end of every finger. Not infrequently a -traveler in the Alps sees the same luminous tuft on the point of his -alpenstock. And quite recently, during a thunderstorm, a whole forest -was observed to become luminous just before each flash of lightning, -and to become dark again at the moment of the discharge.[12] - -This phenomenon may be easily explained. It consists in a gradual and -comparatively silent electrical discharge between the earth and the -cloud; and generally, but not always, it has the effect of preventing -such an accumulation of electricity as would be necessary to produce -a flash of lightning. I can illustrate this kind of discharge with -the aid of our machine. If I hold a pointed metal rod toward the -large conductor, you can see, when the machine is worked and the room -darkened, how the point of the rod becomes luminous and shines like a -faint blue star. I substitute for the pointed rod the blunt handles of -a pair of pliers, and a tuft of blue light is at once developed at the -end of each handle, and seems to stream away with a hissing noise. I -now put aside the pliers, and open out my hand under the conductor--and -observe how I can set up, at pleasure, a luminous tuft at the tips -of my fingers. Now and then a spark passes, giving me a smart shock, -and showing how the electricity may sometimes accumulate so fast that -it cannot be sufficiently discharged by the luminous tuft. Lastly, I -present a small bushy branch of a tree to the conductor, and all its -leaves and twigs are aglow with bluish light, which ceases for a moment -when a spark escapes, to be again renewed when electricity is again -developed by the working of the machine. - -[Illustration: THE BRUSH DISCHARGE, ILLUSTRATING ST. ELMO’S FIRE.] - -Now, if you put a thundercloud in the place of that conductor, you can -easily realize how, through its influence, the lance and bayonet of -the soldier, the alpenstock of the traveler, the pointed spire of a -church, the masts of a ship at sea, the trees of a forest, can all be -made to glow with a silent electrical discharge which may or may not, -according to circumstances, culminate at intervals in a genuine flash -of lightning. - - -=Origin of Lightning.=--When we seek to account for the origin of -lightning, we are confronted at once with two questions of great -interest and importance--first, What are the sources from which the -electricity of the thundercloud is derived? and, secondly, How does -this electricity come to be developed in a form which so far transcends -in power the electricity of our machines? These questions have long -engaged the attention of scientific men, but I cannot say that they -have yet received a perfectly satisfactory solution. Nevertheless, -some facts of great scientific value have been established, and some -speculations have been put forward, which are well deserving of -consideration. - -In the first place, it is quite certain that the atmosphere which -surrounds our globe is almost always in a state of electrification. -Further, the electrical condition of the atmosphere would seem to be -as variable as the wind. It changes with the change of season; it -changes from day to day; it changes from hour to hour. The charge of -electricity is sometimes positive, sometimes negative; sometimes it -is strong, sometimes feeble; and the transition from one condition to -another is sometimes slow and gradual, sometimes sudden and violent. - -As a general rule, in fine, clear weather, the electricity of the -atmosphere is positive, and not very strongly developed. In wet weather -the charge may be either positive or negative, and is generally -strong, especially when there are sudden heavy showers. In fog it is -also strong, and almost always positive. In a snowstorm it is very -strong, and most frequently positive. Finally, in a thunderstorm it is -extremely strong, and generally negative; but it is subject to a sudden -change of sign, when a flash of lightning passes or when rain begins to -fall. - -So far I have simply stated facts, which have been ascertained -by careful observations, made at different stations by competent -observers, and extending over a period of many years. But as regards -the process by which the electricity of the atmosphere is developed, we -have, up to the present time, no certain knowledge. It has been said -that electricity may be generated in the atmosphere by the friction of -the air itself, and of the minute particles floating in it, against -the surface of the earth, against trees and buildings, against rocks, -cliffs, and mountains. But this opinion, however probable it may be, -has not yet been confirmed by any direct experimental investigation. - -The second theory is that the electricity of the atmosphere is due, in -great part at least, to the evaporation of salt water. Many years ago, -Pouillet, a French philosopher, made a series of experiments in the -laboratory, which seemed to show that evaporation is generally attended -with the development of electricity; and, in particular, he satisfied -himself that the vapor which passes off from the surface of salt water -is always positively electrified. Now, the atmosphere is everywhere -charged, more or less, with vapor which comes, almost entirely, from -the salt water of the ocean. Hence Pouillet inferred that the chief -source of atmospheric electricity is the evaporation of sea water. -This explanation would certainly go far to account for the presence -of electricity in the atmosphere, if the fact on which it rests were -established beyond dispute. But there is some reason to doubt whether -the development of electricity, in the experiments of Pouillet, was due -simply to the process of evaporation, and not rather to other causes, -the influence of which he did not sufficiently take into account. - -A conjecture has recently been started that electricity may be -generated by the mere impact of minute particles of water vapor against -minute particles of air.[13] If this conjecture could be established as -a fact, it would be amply sufficient to account for all the electricity -of the atmosphere. From the very nature of a gas, the molecules of -which it is composed are forever flying about with incredible velocity; -and therefore the particles of water vapor and the particles of air, -which exist together in the atmosphere, must be incessantly coming -into collision. Hence, however small may be the charge of electricity -developed at each individual impact, the total amount generated over -any considerable area, in a single day, must be very great indeed. -It is evident, however, that this method of explaining the origin of -atmospheric electricity can only be regarded as, at best, a probable -hypothesis, until the assumption on which it rests is supported by the -evidence of observation or experiment. - - -=Length of a Flash of Lightning.=--It would seem, then, that we are -not yet in a position to indicate with certainty the sources from -which the electricity of the atmosphere is derived. But whatever -these sources may be, there can be little doubt that the electricity -of the atmosphere is intimately associated with the minute particles -of water vapor of which the thundercloud is eventually built up. -This consideration is of great importance when we come to consider -the special properties of lightning, as compared with other forms of -electricity. The most striking characteristic of lightning is the -wonderful power it possesses of forcing its way through the resisting -medium of the air. In this respect it incomparably surpasses all forms -of electricity that have hitherto been produced by artificial means. -The spark of an ordinary electric machine can leap across a space of -three or four inches; the machine we have employed in our experiments -to-day can give, under favorable circumstances, a spark of nine or ten -inches; the longest electric spark ever yet produced artificially is -probably the spark of Mr. Spottiswoode’s gigantic induction coil; and -it does not exceed three feet six inches. But the length of a flash of -lightning is not to be measured in inches, or in feet or in yards; it -varies from one or two miles, for ordinary flashes, to eight or ten -miles in exceptional cases. - -This power of discharging itself violently through a resisting -medium, in which the thundercloud so far transcends the conductor of -an electric machine, is due to the property commonly known among -scientific men as electrical _potential_. The greater the distance to -which an electrified body can shoot its flashes through the air, the -higher must be its potential. Hence the potential of a thundercloud -must be exceedingly high, since its flashes can pierce the air to a -distance of several miles. And what I want to point out is, that we -are able to account for this exceedingly high potential, if we may -only assume that the minute particles of water vapor in the atmosphere -have, from any cause, received ever so small a charge of electricity. -The number of such particles that go to make up an ordinary drop of -rain are to be counted by millions of millions; and it is capable of -scientific proof that, as each new particle is added, in the building -up of the drop, a rise of potential is necessarily produced. It is -clear, therefore, that there is practically no limit to the potential -that may be developed by the simple agglomeration of very small cloud -particles, each carrying a very small charge of electricity.[14] - -This explanation, which traces the exceedingly high potential of -lightning to the building up of rain drops in the thundercloud, -suggests a reason why it so often happens that immediately after a -flash of lightning “the big rain comes dancing to the earth.” The -potential has been steadily rising as the drops have been getting -larger and larger, until at length the potential has become so high -that the thundercloud is able to discharge itself, and almost at the -same moment the drops have become so large that they can no longer be -held aloft against the attracting force of gravity. - - -=Physical Cause of Thunder.=--Let us now proceed to consider the -phenomenon of thunder, which is so intimately associated with -lightning, and which, though perfectly harmless in itself, and though -never heard until the real danger is past, often excites more terror -in the mind than the lightning flash itself. The sound of thunder, -like that of the electric spark, is due to a disturbance caused in -the air by the electric discharge. The air is first expanded by the -intense heat that is developed along the line of discharge, and then it -rushes back again to fill up the partial vacuum which its expansion has -produced. This sudden movement gives rise to a series of sound waves, -which reach the ear in the form of thunder. But there are certain -peculiar characteristics of thunder which are deserving of special -consideration. - - -=Rolling of Thunder.=--They may be classified, I think, under two -heads. First, the sound of thunder is not an instantaneous report -like the sound of the electric spark--it is a prolonged peal lasting, -sometimes, for several seconds. Secondly, each flash of lightning gives -rise, not to one peal only, but to a succession of peals following -one another at irregular intervals. These two phenomena, taken -together, produce that peculiar effect on the ear which is commonly -described as the _rolling_ of thunder; and both of them, I think, may -be sufficiently accounted for in accordance with the well-established -properties of sound. - -To understand why the sound of thunder reaches the ear as a prolonged -peal, we have only to remember that sound takes time to travel. Since a -flash of lightning is practically instantaneous, we may assume that the -sound is produced at the same moment all along the line of discharge. -But the sound waves, setting out at the same moment from all points -along the line of discharge, must reach the ear in successive instants -of time, arriving first from that point which is nearest to the -observer, and last from that point which is most distant. Suppose, for -example, that the nearest point of the flash is a mile distant from the -observer, and the farthest point two miles--the sound will take about -five seconds to come from the nearest point, and about ten seconds to -come from the farthest point; and moreover, in each successive instant -from the time the first sound reaches the ear, sound will continue -to arrive from the successive points between. Therefore the thunder, -though instantaneous in its origin, will reach the ear as a prolonged -peal extending over a period of five seconds. - - -=Succession of Peals.=--The succession of peals produced by a single -flash of lightning is due to several causes, each one of which may -contribute more or less, according to circumstances, toward the general -effect. First, if we accept the results arrived at by Professor Ogden -Rood, of Columbia College, what appears to the eye as a single flash -of lightning, consists, in fact, as a general rule, of a succession -of flashes, each one of which must naturally produce its own peal of -thunder; and although the several flashes, if they follow one another -at intervals of the tenth of a second, will make one continuous -impression on the eye, the several peals of thunder, under the same -conditions, will impress the ear as so many distinct peals. - -The next cause that I would mention is the zigzag path of the lightning -discharge. To make clear to you the influence of this circumstance, I -must ask your attention for a moment to the diagram on next page. Let -the broken line represent the path of a flash of lightning, and let O -represent the position of an observer. The sound will reach him first -from the point A, which is nearest to him, and then it will continue to -arrive in successive instants from the successive points along the line -A N and along the line A M, thus producing the effect of a continuous -peal. Meanwhile the sound waves have been traveling from the point B, -and in due time will reach the observer at O. Coming as they do in a -different direction from the former, they will strike the ear as the -beginning of a new peal which, in its turn, will be prolonged by the -sound waves arriving, in successive instants, from the successive -points along the line B M and B H. A little later, the sound will -arrive from the more distant point C, and a third peal will begin. And -so there will be several distinct peals proceeding, so to speak, from -several distinct points in the path of the lightning flash. - -[Illustration: ORIGIN OF SUCCESSIVE PEALS OF THUNDER.] - -A third cause to which the succession of peals may be referred is to -be found in the minor electrical discharges that must often take place -within the thundercloud itself. A thundercloud is not a continuous mass -like the metal cylinder of this electric machine--it has many outlying -fragments, more or less imperfectly connected with the principal body. -Moreover, the material of which the cloud is composed is only a very -imperfect conductor as compared with our brass cylinder. For these -two reasons it must often happen, about the time a flash of lightning -passes, that different parts of the cloud will be in such different -electrical conditions as to give rise to electrical discharges within -the cloud itself. Each of these discharges produces its own peal of -thunder; and thus we may have a number of minor peals, sometimes -preceding and sometimes following the great crash which is due to the -principal discharge. - -Lastly, the influence of echo has often a considerable share in -multiplying the number of peals of thunder. The waves of sound, going -forth in all directions, are reflected from the surfaces of mountains, -forests, clouds, and buildings, and coming back from different -quarters, and with varying intensity, reach the ear like the roar of -distant artillery. The striking effect of these reverberations in a -mountain district has been described by a great poet in words which, I -daresay, are familiar to most of you: - - “Far along, - From peak to peak, the rattling crags among, - Leaps the live thunder! Not from one lone cloud, - But every mountain now has found a tongue, - And Jura answers from her misty shroud - Back to the joyous Alps, that call to her aloud!” - - -=Variations of Intensity in Thunder.=--From what has been said, it -is easy to understand how the general roar of thunder is subject to -great changes of intensity, during the time it lasts, according to -the number of peals that may be arriving at the ear of an observer in -each particular moment. But every one must have observed that even an -individual peal of thunder often undergoes similar changes, swelling -out at one moment with great power, and the next moment rapidly -dying away. To account for this phenomenon, I would observe, first, -that there is no reason to suppose that the disturbance caused by -lightning is of exactly the same magnitude at every point of its path. -On the contrary, it would seem very probable that the amount of this -disturbance is, in some way, dependent on the resistance which the -discharge encounters. Hence the intensity of the sound waves sent forth -by a flash of lightning is probably very different at different parts -of its course; and each individual peal will swell out on the ear or -die away, according to the greater or less intensity of the sound waves -that reach the ear in each successive moment of time. - -But there is another influence at work which must produce variations -in the loudness of a peal of thunder, even though the sound waves, set -in motion by the lightning, were everywhere of equal intensity. This -influence depends on the position of the observer in relation to the -path of the lightning flash. At one part of its course the lightning -may follow a path which remains for a certain length at nearly the -same distance from the observer; then all the sound produced along -this length will reach the observer nearly at the same moment, and -will burst upon the ear with great intensity. At another part, the -lightning may for an equal length go right away from the observer; and -it is evident that the sound produced along this length will reach the -observer in successive instants, and consequently produce an effect -comparatively feeble. - -With a view to investigate this interesting question a little more -closely, let me suppose the position of the observer taken as a -centre, and a number of concentric circles drawn, cutting the path of -the lightning flash, and separated from one another by a distance of -110 feet, measured along the direction of the radius. It is evident -that all the sound produced between any two consecutive circles will -reach the ear within a period which must be measured by the time that -sound takes to travel 110 feet, that is, within the tenth of a second. -Hence, in order to determine the quantity of sound that reaches the -ear in successive periods of one-tenth of a second, we have only to -observe how much is produced between each two consecutive circles. -But on the supposition that the sound waves, set in motion by the -flash of lightning, are of equal intensity at every point of its path, -it is clear that the quantity of sound developed between each two -consecutive circles will be simply proportional to the length of the -path enclosed between them. - -With these principles established, let us now follow the course of a -peal of thunder, in the diagram before us. This broken line, drawn -almost at random, represents the path of a flash of lightning; the -observer is supposed to be placed at O, which is the centre of the -concentric circles; these circles are separated from one another by a -distance of 110 feet, measured in the direction of the radius; and we -want to consider how any one peal of thunder may vary in loudness in -the successive periods of one-tenth of a second. - -[Illustration: VARIATIONS OF INTENSITY IN A PEAL OF THUNDER.] - -Let us take, for example, the peal which begins when the sound waves -reach the ear from the point A. In the first unit of time the sound -that reaches the ear is the sound produced along the lines A B and A -C; in the second unit, the sound produced along the lines B D and C E; -in the third unit, the sound produced along D F and E G. So far the -peal has been fairly uniform in its intensity; though there has been a -slight falling off in the second and third units of time, as compared -with the first. But in the fourth unit there is a considerable falling -away of the sound; for the line F K is only about one-third as long -as D F and E G taken together; therefore the quantity of sound that -reaches the ear in the fourth unit of time is only one-third of that -which reaches it in each of the three preceding units; and consequently -the sound is only one-third as loud. In the fifth unit, however, the -peal must rise to a sudden crash; for the portion of the lightning path -inclosed between the fifth and sixth circles is about six times as -great as that between the fourth and fifth; therefore the intensity of -the sound will be suddenly increased about six-fold. After this sudden -crash, the sound as suddenly dies away in the sixth unit of time; it -continues feeble as the path of the lightning goes nearly straight -away from the observer; it swells again slightly in the ninth unit of -time; and then continues without much variation to the end. This is -only a single illustration, but it seems quite sufficient to show that -the changes of intensity in a peal of thunder must be largely due to -the position of the spectator in relation to the several parts of the -lightning flash. - - -=Distance of a Flash of Lightning.=--I need hardly remind you that, by -observing the interval that elapses between the flash of lightning and -the peal of thunder that follows it, we may estimate approximately the -distance of the nearest point of the discharge. Light travels with such -amazing velocity that we may assume, without any sensible error, that -we see the flash of lightning at the very moment in which the discharge -takes place. But sound, as we have seen, takes a sensible time to -travel even short distances; and therefore a measurable interval almost -always elapses between the moment in which the flash is seen and the -moment in which the peal of thunder first reaches the ear. And the -distance through which sound travels in this interval will be the -distance of the nearest point through which the discharge has passed. -Now, the velocity of sound in air varies slightly with the temperature; -but, at the ordinary temperature of our climate, we shall not be far -astray if we allow 1,100 feet for every second, or about one mile for -every five seconds. - -You will observe also that, by repeating this observation, we can -determine whether the thundercloud is coming toward us, or going away -from us. So long as the interval between each successive flash and the -corresponding peal of thunder, continues to get shorter and shorter, -the thundercloud is approaching; when the interval begins to increase, -the thundercloud is receding from us, and the danger is passed. - -The crash of thunder is terrific when the lightning is close at hand; -but it is a curious fact, that the sound does not seem to travel as -far as the report of an ordinary cannon. We have no authentic record -of thunder having been heard at a greater distance than from twelve to -fifteen miles, whereas the report of a single cannon has been heard -at five times that distance; and the roar of artillery, in battle, -at a greater distance still. On the occasion of the Queen’s visit to -Cherbourg, in August, 1858, the salute fired in honor of her arrival -was heard at Bonchurch, in the Isle of Wight, a distance of sixty -miles. It was also heard at Lyme Regis, in Dorsetshire, which is -eighty-five miles from Cherbourg, as the crow flies; and we are told -that, not only was it audible in its general effect, but the report of -individual guns was distinctly recognized. The artillery of Waterloo is -said to have been heard at the town of Creil, in France, 115 miles from -the field of battle; and the cannonading at the siege of Valenciennes, -in 1793, was heard, from day to day, at Deal, on the coast of England, -a distance of 120 miles.[15] - -So far, I have endeavored to set forth some general ideas on the nature -and origin of lightning, and of the thunder that accompanies it. In -my next Lecture I propose to give a short account of the destructive -effects of lightning, and to consider how these effects may best be -averted by means of lightning conductors. - - -NOTE TO PAGE 20. - -ON THE HIGH POTENTIAL OF A FLASH OF LIGHTNING. - -The potential of an electrified sphere is equal to the quantity of -electricity with which the sphere is charged, divided by the radius -of the sphere. Now the minute cloud particles, which go to make up -a drop of rain, may be taken to be very small spheres; and if _v_ -represent the potential of each one, _q_ the quantity of electricity -with which it is charged, and _r_ the radius of the sphere, we have _v_ -= _q_/_r_. Suppose 1,000 of these cloud particles to unite into one; -the quantity of electricity in the drop, thus formed, will be 1,000_q_; -and the radius, which increases in the ratio of the cube root of the -volume, will be 10_r_. Therefore the potential of the new sphere will -be 1000_q_/10_r_, or 100_q/r_; that is to say, it will be 100 times as -great as the potential of each of the cloud particles which compose -it. When a million of cloud particles are blended into a single drop, -the same process will show that the potential has been increased ten -thousandfold; and when a drop is produced by the agglomeration of a -million of millions of cloud particles, the potential of the drop -will be a hundred million times as great as that of the individual -particles.[16] - - -FOOTNOTES: - -[1] “Il y a des grands seigneurs dont il ne faut approcher qu’avec -d’extrêmes précautions. Le tonnerre est de ce nombre.”--Dict. Philos. -art. Foudre. - -[2] Electricité Statique, ii., 561. - -[3] Deschanel’s Natural Philosophy, Sixth Edition, p. 641. - -[4] Fragments of Science, Fifth Edition, p. 311. - -[5] Lecture on Thunderstorms, Nature, vol. xxii., p. 341. - -[6] Third Series, vol. v., p. 161. - -[7] Phil. Trans. Royal Society, 1834, vol. cxxv., pp. 583-591. - -[8] In experiments with a Leyden jar, Feddersen has shown that the -duration of the discharge is increased, not only by increasing the -striking distance, but also by increasing the size of the jar. Now, a -flash of lightning may be regarded as the discharge of a Leyden jar -of immense size, with an enormous striking distance; and therefore we -should expect that the duration of the discharge should be greatly -prolonged. See _American Journal of Science and Arts_, Third Series, -vol. i., p. 15. - -[9] See original paper by Swan, Trans. Royal Society, Edinburgh, 1849, -vol. xvi., pp. 581-603; also, a second paper, _ib._ 1861, vol. xxii., -pp. 33-39. - -[10] Nature, vol. xxviii., p. 54. - -[11] See, however, an attempt to account for this phenomenon in De -Larive’s Treatise on Electricity, London, 1853-8, vol. iii., pp. 199, -200; and another, quite recently, by Mr. Spottiswoode, in a Lecture on -the Electrical Discharge, delivered before the British Association at -York, in September, 1881, and published by Longmans, London, p. 42. -See also, for recent evidence regarding the phenomenon itself, Scott’s -Elementary Meteorology, pp. 175-8. - -[12] See Jamin, “Cours de Physique,” i., 480-1; Tomlinson, “The -Thunderstorm,” Third Edition, pp. 95-103; “Thunderstorms,” a Lecture by -Professor Tait, Nature, vol. xxii., p. 356. - -[13] Professor Tait, On Thunderstorms, Nature, vol. xxii., pp. 436-7. - -[14] See note at the end of this Lecture, p. 26. - -[15] See Tomlinson, The Thunderstorm, pp. 87-9. - -[16] See Tait on Thunderstorms, Nature, vol. xxii., p. 436. - - - - -LECTURE II. - -LIGHTNING CONDUCTORS. - - -The effects of lightning, on the bodies that it strikes, are analogous -to those which may be produced by the discharge of our electric -machines and Leyden jar batteries. When the discharge of a battery -traverses a metal conductor of sufficient dimensions to allow it an -easy passage, it makes its way along silently and harmlessly. But if -the conductor be so thin as to offer considerable resistance, then the -conductor itself is raised to intense heat, and may be melted, or even -converted into vapor, by the discharge. - -On opposite page is shown a board on which a number of very thin wires -have been stretched, over white paper, between brass balls. The wires -are so thin that the full charge of the battery before you, which -consists of nine large Leyden jars, is quite sufficient to convert them -in an instant into vapor. I have already, on former occasions, sent the -charge through two of these wires, and nothing remains of them now -but the traces of their vapor, which mark the path of the electric -discharge from ball to ball. At the present moment the battery stands -ready charged, and I am going to discharge it through a third wire, by -means of this insulated rod which I hold in my hand. The discharge has -passed; you saw a flash, and a little smoke; and now, if you look at -the paper, you will find that the wire is gone, but that it has left -behind the track of its incandescent vapor, marking the path of the -discharge. - -[Illustration: DISCHARGE OF LEYDEN JAR BATTERY THROUGH THIN WIRES.] - - -=Destruction of Buildings by Lightning.=--We learn from this experiment -that the electricity stored up in our battery passes, without visible -effect, through the stout wire of a discharging rod, but that it -instantly converts into vapor the thin wire stretched across the spark -board. And so it is with a flash of lightning. It passes harmlessly, as -every one knows, through a stout metal rod, but when it comes across -bell wires or telegraph wires, it melts them, or converts them into -vapor. On the sixteenth of July, 1759, a flash of lightning struck -a house in Southwark, on the south side of London, and followed the -line of the bell wire. After the lightning had passed, the wire was no -longer to be found; but the path of the lightning was clearly marked -by patches of vapor which were left, here and there, adhering to the -surface of the wall. In the year 1754, the lightning fell on a bell -tower at Newbury, in the United States of America, and having dashed -the roof to pieces, and scattered the fragments about, it reached the -bell. From this point it followed an iron wire, about as thick as a -knitting needle, melting it as it passed along, leaving behind a black -streak of vapor on the surface of the walls. - -Again, the electric discharge, passing through a bad conductor, -produces mechanical disturbance, and, if the substance be combustible, -often sets it on fire. So, too, as you know, the lightning flash, -falling on a church spire, dashes it to pieces, knocking the stones -about in all directions, while it sets fire to ships and wooden -buildings; and more than once it has caused great devastation by -exploding powder magazines. - -Let me give you one or two examples: In January, 1762, the -lightning fell on a church tower in Cornwall, and a stone--three -hundred-weight--was torn from its place and hurled to a distance of 180 -feet, while a smaller stone was projected as far as 1,200 feet from the -building. Again, in 1809, the lightning struck a house not far from -Manchester, and literally moved a massive wall twelve feet high and -three thick to a distance of several feet. You may form some conception -of the enormous force here brought into action, when I tell you that -the total weight of mason-work moved on this occasion was not less than -twenty-three tons. - -The church of St. George, at Leicester, was severely damaged by -lightning on the 1st of August, 1846. About 8 o’clock in the evening -the rector of the parish saw a vivid streak of light darting with -incredible velocity against the upper part of the spire. “For the -distance of forty feet on the eastern side, and nearly seventy on the -west, the massive stonework of the spire was instantly rent asunder and -laid in ruins. Large blocks of stone were hurled in all directions, -broken into small fragments, and in some cases, there is reason to -believe, reduced to powder. One fragment of considerable size was -hurled against the window of a house three hundred feet distant, -shattering to pieces the woodwork, and strewing the room within with -fine dust and fragments of glass. It has been computed that a hundred -tons of stone were, on this occasion, blown to a distance of thirty -feet in three seconds. In addition to the shivering of the spire, the -pinnacles at the angles of the tower were all more or less damaged, the -flying buttresses cracked through and violently shaken, many of the -open battlements at the base of the spire knocked away, the roof of the -church completely riddled, the roofs of the side entrances destroyed, -and the stone staircases of the gallery shattered.”[17] - -Lightning has been at all times the cause of great damage to property -by its power of setting fire to whatever is combustible. Fuller says, -in his Church History, that “scarcely a great abbey exists in England -which once, at least, has not been burned by lightning from heaven.” -He mentions, as examples, the Abbey of Croyland twice burned, the -Monastery of Canterbury twice, the Abbey of Peterborough twice; also -the Abbey of St. Mary’s, in Yorkshire, the Abbey of Norwich, and -several others. Sir William Snow Harris, writing about twenty years -ago, tells us that “the number of churches and church spires wholly or -partially destroyed by lightning is beyond all belief, and would be too -tedious a detail to enter upon. Within a comparatively few years, in -1822 for instance, we find the magnificent Cathedral of Rouen burned, -and, so lately as 1850, the beautiful Cathedral of Saragossa, in Spain, -struck by lightning during divine service and set on fire. In March of -last year a dispatch from our Minister at Brussels, Lord Howard de -Walden, dated the 24th of February, was forwarded by Lord Russell to -the Royal Society, stating that, on the preceding Sunday, a violent -thunderstorm had spread over Belgium; that twelve churches had been -struck by lightning; and that three of these fine old buildings had -been totally destroyed.”[18] - -Even in our own day the destruction caused by fires produced through -the agency of lightning is very great--far greater than is commonly -supposed. No general record of such fires is kept, and consequently -our information on the subject is very incomplete and inexact. I -may tell you, however, one small fact which, so far as it goes, -is precise enough and very significant. In the little province of -Schleswig-Holstein, which occupies an area less than one-fourth of the -area of Ireland, the Provincial Fire Assurance Association has paid in -sixteen years, for damage caused by lightning, somewhat over £100,000, -or at the rate of more than £6,000 a year. The total loss of property -every year in this province, due to fires caused by lightning, is -estimated at not less than £12,500.[19] - - -=Destruction of Ships at Sea.=--The destructive effects of lightning -on ships at sea, before the general adoption of lightning conductors, -seems almost incredible at the present day. From official records -it appears that the damage done to the Royal Navy of England alone -involved an expenditure of from £6,000 to £10,000 a year. We are -told by Sir William Snow Harris, who devoted himself for many years -to this subject with extraordinary zeal and complete success, that -between the year 1810 and the year 1815--that is, within a period of -five years--“no less than forty sail of the line, twenty frigates, and -twelve sloops and corvettes were placed _hors de combat_ by lightning. -In the merchant navy, within a comparatively small number of years, -no less than thirty-four ships, most of them large vessels with rich -cargoes, have been totally destroyed--been either burned or sunk--to -say nothing of a host of vessels partially destroyed or severely -damaged.”[20] - -And these statements, be it observed, take no account of ships that -were simply reported as missing, some of which, we can hardly doubt, -were struck by lightning in the open sea, and went down with all hands -on board. A famous ship of forty-four guns, the _Resistance_, was -struck by lightning in the Straits of Malacca, and the powder magazine -exploding, she went to the bottom. Of her whole crew only three were -saved, who happened to be picked up by a passing boat. It has been well -observed that, were it not for these three chance survivors, nothing -would have been known concerning the fate of the vessel, and she would -have been simply recorded as missing in the Admiralty lists. - -Nothing is more fearful to contemplate than the scene on board a ship -when she is struck by lightning in the open sea, with the winds howling -around, the waves rolling mountains high, the rain coming down in -torrents, and the vivid flashes lighting up the gloom at intervals, and -carrying death and destruction in their track. I will read you one or -two brief accounts of such a scene, given in the pithy but expressive -language of the sailor. In January, 1786, the _Thisbe_, of thirty-six -guns, was struck by lightning off the coast of Scilly, and reduced to -the condition of a wreck. Here is an extract from the ship’s log: “Four -A. M., strong gales; handed mainsail and main top-sail; hove to with -storm staysails; blowing very heavy, S. E. 4.15, a flash of lightning, -with tremendous thunder, disabled some of our people. A second flash -set the mainsail, main-top, and mizen staysails on fire. Obliged to cut -away the mainmast; this carried away mizen top-mast and fore top-sail -yard. Found foremast also shivered by the lightning. Fore top-mast went -over the side about 9 A. M. Set the foresail.”[21] - -A few years later, in March, 1796, the _Lowestoffe_ was struck in the -Mediterranean, and we read as follows in the log of the ship: “North -end of Minorca; heavy squalls; hail, rain, thunder, and lightning. -12.15, ship struck by lightning, which knocked three men from the -masthead, one killed. 12.30, ship again struck; main top-mast shivered -in pieces; many men struck senseless on the decks. Ship again struck, -and set on fire in the masts and rigging; mainmast shivered in pieces; -fore top-mast shivered; men benumbed on the decks, and knocked out -of the top; one man killed on the spot. 1.30, cut away the mainmast; -employed clearing wreck. 4, moderate; set the foresail.”[22] - -Again, in 1810, the _Repulse_, a ship of seventy-four guns, was struck, -off the coast of Spain. “The wind had been variable in the morning--and -at 12.35 there was a heavy squall, with rain, thunder, and lightning. -The ship was struck by two vivid flashes of lightning, which shivered -the maintop-gallant mast, and severely damaged the mainmast. Seven men -were killed on the spot; three others only survived a few days; and ten -others were maimed for life. After the second discharge the rain fell -in torrents. The ship was more completely crippled than if she had been -in action, and the squadron, then engaged on a critical service, lost -for a time one of its fastest and best ships.”[23] - - -=Destruction of Powder Magazines.=--Not less appalling is the -devastation caused by lightning when it falls on a powder magazine. -Here is a striking example: On the eighteenth of August, 1769, the -tower of St. Nazaire, at Brescia, was struck by lightning. Underneath -the tower about 200,000 pounds of gunpowder, belonging to the Republic -of Venice, were stored in vaults. The powder exploded, leveling to -the ground a great part of the beautiful city of Brescia, and burying -thousands of its inhabitants in the ruins. It is said that the tower -itself was blown up bodily to a great height in the air, and came down -in a shower of stones. This is, perhaps, the most fearful disaster of -the kind on record. But we are not without examples in our own times. -In the year 1856 the lightning fell on the Church of St. John, in the -Island of Rhodes. A large quantity of gunpowder had been deposited in -the vaults of the church. This was ignited by the flash; the building -was reduced to a mass of ruins, a large portion of the town was -destroyed, and a considerable number of the inhabitants were killed. -Again, in the following year, the magazine of Joudpore, in the Bombay -Presidency, was struck by lightning. Many thousand pounds of gunpowder -were blown up, five hundred houses were destroyed, and nearly a -thousand people are said to have been killed.[24] - - -=Experimental Illustrations.=--And now, before proceeding further, -I will make one or two experiments, with a view of showing that the -electricity of our machines is capable of producing effects similar -to those produced by lightning, though immeasurably inferior in point -of magnitude. Here is a common tumbler, about three-quarters full of -water. Into it I introduce two bent rods of brass, which are carefully -insulated below the surface of the water by a covering of india-rubber. -The points, however, are exposed, and come to within an inch of one -another, near the bottom of the tumbler. Outside the tumbler, the -brass rods are mounted on a stand, by means of which I can send the -full charge of this Leyden jar battery through the water, from point -to point. Since water is a bad conductor of electricity, as compared -with metals, the charge encounters great resistance in passing through -it, and in overcoming this resistance produces considerable mechanical -commotion, which is usually sufficient to shiver the glass to pieces. - -To charge the battery will take about twenty turns of this large Holtz -machine. Observe how the pith ball of the electroscope rises as the -machine is worked, showing that the charge is going in. And now it -remains stationary; which is a sign that the battery is fully charged, -and can receive no more. You will notice that the outside coating of -the battery has been already connected with one of the brass rods -dipping into the tumbler of water. By means of this discharger I will -now bring the inside coating into connection with the other rod. And -see, before contact is actually made, the spark has leaped across, and -our tumbler is violently burst asunder from top to bottom. - -[Illustration: GLASS VESSEL BROKEN BY DISCHARGE OF LEYDEN JAR BATTERY.] - -This will probably appear to you a very small affair, when compared -with the tearing asunder of solid masonry, and the hurling about of -stones by the ton weight. No doubt it is; and that is just one of -the lessons we have to learn from the experiment we have made. For, -not only does it show us that effects of this kind may be caused by -electricity artificially produced, but it brings home forcibly to the -mind how incomparably more powerful is the lightning of the clouds than -the electricity of our machines. - -The property which electricity has of setting fire to combustible -substances may be easily illustrated. This india-rubber tube is -connected with the gas pipe under the floor, and to the end of the -tube is fitted a brass stop-cock which I hold in my hand. I open -the cock, and allow the jet of gas to flow toward the conductor of -Carré’s machine, while my assistant turns the handle; a spark passes, -and the gas is lit. Again, my assistant stands on this insulating -stool, placing his hand on the large conductor of the machine, while -I turn the handle. His body becomes electrified, and when he presents -his knuckle to this vessel of spirits of wine, which is electrically -connected with the earth, a spark leaps across, and the spirits of wine -are at once in a blaze. Once more; I tie a little gun-cotton around -one knob of the discharging rod, and then use it to discharge a small -Leyden jar; at the moment of the discharge the gun-cotton is set on -fire. - -It would be easy to explode gunpowder with the electric spark, but the -smoke of the explosion would make the lecture-hall very unpleasant for -the remainder of the lecture. I propose, therefore, to substitute for -gunpowder an explosive mixture of oxygen and hydrogen, with which I -have filled this little metal flask, commonly known as Volta’s pistol. -By a very simple contrivance, the electric spark is discharged through -the mixture, when I hold the flask toward the conductor of the machine. -A cork is fitted tightly into the neck of the flask, and at the moment -the spark passes you hear a loud explosion, and you see the cork driven -violently up to the ceiling. - -[Illustration: GUN-COTTON SET ON FIRE BY ELECTRIC SPARK.] - - -=Destruction of Life.=--The last effect of lightning to which I shall -refer, and which, perhaps, more than any other, strikes us with terror, -is the sudden and utter extinction of life, when the lightning flash -descends on man or on beast. So swift is this effect, in most cases, -that death is, in all probability, absolutely painless, and the victim -is dead before he can feel that he is struck. I cannot give you, with -any degree of exactness, the number of people killed every year by -lightning, because the record of such deaths has been hitherto very -imperfectly kept, in almost all countries, and is, beyond doubt, very -incomplete. But perhaps you will be surprised to learn that the number -of deaths by lightning actually recorded is, on an average, in England -about 22 every year, in France 80, in Prussia 110, in Austria 212, in -European Russia 440.[25] - -So far as can be gathered from the existing sources of information, -it would seem that the number of persons killed by lightning is, on -the whole, about one in three of those who are struck. The rest are -sometimes only stunned, sometimes more or less burned, sometimes -made deaf for a time, sometimes partially paralyzed. On particular -occasions, however, especially when the lightning falls on a large -assembly of people, the number of persons struck down and slightly -injured, in proportion to the number killed, is very much increased. - -An interesting case of this kind is reported by Mr. Tomlinson. “On -the twenty-ninth of August, 1847, at the parish church of Welton, -Lincolnshire, while the congregation were engaged in singing the hymn -before the sermon, and the Rev. Mr. Williamson had just ascended the -pulpit, the lightning was seen to enter the church from the belfry, -and instantly an explosion occurred in the centre of the edifice. All -that could move made for the door, and Mr. Williamson descended from -the pulpit, endeavoring to allay the fears of the people. But attention -was now called to the fact that several of the congregation were lying -in different parts of the church, apparently dead, some of whom had -their clothing on fire. Five women were found injured, and having their -faces blackened and burned, and a boy had his clothes almost entirely -consumed. A respected old parishioner, Mr. Brownlow, aged sixty-eight, -was discovered lying at the bottom of his pew, immediately beneath one -of the chandeliers, quite dead. There were no marks on the body, but -the buttons of his waistcoat were melted, the right leg of his trousers -torn down, and his coat literally burnt off. His wife in the same pew -received no injury.”[26] - -[Illustration: VOLTA’S PISTOL; EXPLOSION CAUSED BY ELECTRIC SPARK.] - -Not less striking is the story told by Dr. Plummer, surgeon of the -Illinois Volunteers, in the _Medical and Surgical Reporter_ of June 19, -1865: “Our regiment was yesterday the scene of one of the most terrible -calamities which it has been my lot to witness. About two o’clock a -violent thunderstorm visited us. While the old guard was being turned -out to receive the new, a blinding flash of lightning was seen, -accompanied instantly by a terrific peal of thunder. The whole of the -old guard, together with part of the new, were thrown violently to the -earth. The shock was so severe and sudden that, in most cases, the rear -rank men were thrown across the front rank men. One man was instantly -killed, and thirty-two men were more or less severely burned by the -electric fluid. In some instances the men’s boots and shoes were rent -from their feet and torn to pieces, and, strange as it may appear, the -men were injured but little in the feet. In all cases the burns appear -as if they had been caused by scalding-hot water, in many instances the -skin being shriveled and torn off. The men all seem to be doing well, -and a part of them will be able to resume their duties in a few days.” - - -=The Return Shock.=--It sometimes happens that people are struck down -and even killed at the moment a discharge of lightning takes place -between a cloud and the earth, though they are very far from the -point where the flash is actually seen to pass; while others, who are -situated between them and the lightning, suffer very little, or perhaps -not at all. This curious phenomenon was first carefully investigated by -Lord Mahon in the year 1779, and was called by him the “return shock.” -His theory, which is now commonly accepted, may be easily understood -with the aid of the sketch before you. - -[Illustration: THE RETURN SHOCK ILLUSTRATED.] - -Let us suppose ABC to represent the outline of a thundercloud which -dips down toward the earth at A and at C. The electricity of the cloud -develops by inductive action a charge of the opposite kind in the earth -beneath it. But the inductive action is most powerful at E and F, where -the cloud comes nearest to the earth. Hence, bodies situated near these -points may be very highly electrified as compared with bodies at a -point between them, such as D. Now, when a flash of lightning passes at -E, the under part of the cloud is at once relieved of its electricity, -its inductive action ceases, and, therefore, a person situated at F -suddenly ceases to be electrified. This sudden change from a highly -electrified to a neutral state involves a shock to his system which -may be severe enough to stun or even to kill him. Meanwhile, people -at _D_, having been also electrified to some extent by the influence -of the thundercloud, must in like manner undergo a change in their -electrical condition when the flash of lightning passes, but this -change will be less violent because they were less highly electrified. - -Many experiments have been devised to illustrate this theory of Lord -Mahon. But the best illustration I know is furnished by this electric -machine of Carré’s. If you stand near one end of the large conductor -when the machine is in action and sparks are taken from the other end, -you will feel a distinct electric shock every time a spark passes. -The large conductor here takes the place of the cloud, the spark that -passes at one end represents the flash of lightning, and the observer -at the other end gets the return shock, though he is at a considerable -distance from the point where the flash is seen. - -An experiment of this kind, of course, cannot be made sensible to a -large audience like the present. But I can give you a good idea of the -effect by means of this tuft of colored papers. While the machine is in -action I hold the tuft of papers near that end of the conductor which -is farthest from the point where the discharge takes place. You see the -paper ribbons are electrified by induction, and, in virtue of mutual -repulsion, stand out from one another “like quills upon the fretful -porcupine.” But, when a spark passes, the inductive action ceases, the -paper ribbons cease to be electrified, and the whole tuft suddenly -collapses into its normal state. - -While fully accepting Lord Mahon’s theory of the return shock as -perfectly good so far as it goes, I would venture to point out another -influence which must often contribute largely to produce the effect in -question, and which is not dependent on the form of the cloud. It may -easily happen, from the nature of the surface in the district affected -by a thundercloud, that the point of most intense electrification--say -E in the figure--is in good electrical communication with a distant -point, such as F, while it is very imperfectly connected with a much -nearer point, D. In such a case it is evident that bodies at F will -share largely in the highly-electrified condition of E, and also share -largely in the sudden change of that condition the moment the flash of -lightning passes; whereas bodies at D will be less highly electrified -before the discharge, and less violently disturbed when the discharge -takes place. - -This principle may be illustrated by a very simple experiment. Here -is a brass chain about twenty feet long. One end of it I hand to any -one among the audience who will kindly take hold of it; the other end -I hold in my hand. I now stand near the conductor of the machine; and -will ask some one to stand about ten feet away from me, near the middle -of the chain, but without touching it. Now observe what happens when -the machine is worked and I take a spark from the conductor: My friend -at the far end of the chain, twenty feet away, gets a shock nearly -as severe as the one I get myself, because he is in good electrical -communication with the point where the discharge takes place. But -my more fortunate friend, who is ten feet nearer to the flash, is -hardly sensible of any effect, because he is connected with me only -through the floor of the hall, which is, comparatively speaking, a bad -conductor of electricity. - - -=Summary.=--Let me now briefly sum up the chief destructive effects -of lightning. First, with regard to good conductors: though it passes -harmlessly through them if they be large enough to afford it an easy -passage, it melts and converts them into vapor if they be of such -small dimensions as to offer considerable resistance. Secondly, -lightning acts with great mechanical force on bad conductors; it is -capable of tearing asunder large masses of masonry, and of projecting -the fragments to a considerable distance. Thirdly, it sets fire to -combustible materials. And lastly, it causes the instantaneous death of -men and animals. - - -=Franklin’s Lightning Rods.=--The object of lightning conductors is to -protect life and property from these destructive effects. Their use was -first suggested by Franklin, in 1749, even before his famous experiment -with the kite; and immediately after that experiment, in 1752, he set -up, on his own house, in Philadelphia, the first lightning conductor -ever made. He even devised an ingenious contrivance, by means of which -he received notice when a thundercloud was approaching. The contrivance -consisted of a peal of bells, which he hung on his lightning conductor, -and which were set ringing whenever the lightning conductor became -charged with electricity. - -Franklin’s lightning rods were soon adopted in America; and he himself -contributed very much to their popularity by the simple and lucid -instructions he issued every year, for the benefit of his countrymen, -in the annual publication known as “Poor Richard’s Almanac.” It is -very interesting at this distance of time to read the homely practical -rules laid down by this great philosopher and statesman; and, though -some modifications have been suggested by the experience of a hundred -and thirty years, especially as regards the dimensions of the lightning -conductor, it is surprising to find how accurately the general -principles of its construction, and of its action, are here set forth. - -“It has pleased God,” he says, “in His goodness to mankind, at length -to discover to them the means of securing their habitations and other -buildings from mischief by thunder and lightning. The method is this: -Provide a small iron rod, which may be made of the rod-iron used by -nailors, but of such a length that one end being three or four feet in -the moist ground, the other may be six or eight feet above the highest -part of the building. To the upper end of the rod fasten about a foot -of brass wire, the size of a common knitting needle, sharpened to a -fine point; the rod may be secured on the house by a few small staples. -If the house or barn be long, there may be a rod and point at each end, -and a middling wire along the ridge from one to the other. A house thus -furnished will not be damaged by lightning, it being attracted by the -points and passing through the metal into the ground, without hurting -anything. Vessels also having a sharp-pointed rod fixed on the top of -their masts, with a wire from the foot of the rod reaching down round -one of the shrouds to the water, will not be hurt by lightning.” - - -=Introduction of Lightning Rods into England.=--The progress of -lightning conductors was more slow in England and on the Continent of -Europe, owing to a fear, not unnatural, that they might, in some cases, -draw down the lightning where it would not otherwise have fallen. -People preferred to take their chance of escaping as they had escaped -before, rather than invite, as it were, the lightning to descend on -their houses, in the hope that an iron rod would convey it harmless -to the earth. But the immense amount of damage done every year by -lightning, soon led practical men to entertain a proposal which offered -complete immunity from all danger on such easy terms; and when it was -found that buildings protected by lightning conductors were, over and -over again, struck by lightning without suffering any harm, a general -conviction of their utility was gradually established in the public -mind. - -The first public building protected by a lightning rod in England was -St. Paul’s Cathedral, in London. On the eighteenth of June, 1764, the -beautiful steeple of Saint Bride’s Church, in the city, was struck by -lightning and reduced to ruin. This incident awakened the attention of -the dean and chapter of St. Paul’s to the danger of a similar calamity, -which seemed, as it were, impending over their own church. After long -deliberation, they referred the matter to the Royal Society, asking for -advice and instruction. A committee of scientific men was appointed by -the Royal Society to consider the question. Benjamin Franklin himself, -who happened to be in London at the time, as the representative of the -American States in their dispute with England, was nominated a member -of the committee. And the result of its deliberation was that, in the -year 1769, a number of lightning conductors were erected on St. Paul’s -Cathedral. - -It was on this occasion that arose the celebrated controversy about -the respective merits of points and balls. Franklin had recommended a -pointed conductor; but some members of the committee were of opinion -that the conductor should end in a ball and not in a point. The -decision of the committee was in favor of Franklin’s opinion, and -pointed conductors were accordingly adopted for St. Paul’s Cathedral. -But the controversy did not end here. The time was one of great -political excitement, and party spirit infused itself even into the -peaceful discussions of science. The weight of scientific opinion was -on the side of Franklin; but it was hinted, on the other side, that the -pointed conductors were tainted with republicanism, and pregnant with -danger to the empire. As a rule, the whigs were strongly in favor of -points; while the Tories were enthusiastic in their support of balls. - -For a time the Tories seemed to prevail. The king was on their side. -Experiments on a grand scale were conducted in his presence, at the -Pantheon, a large building in Oxford street; he was assured that these -experiments proved the great superiority of balls over points; and to -give practical effect to his convictions, his majesty directed that a -large cannon ball should be fixed on the end of the lightning conductor -attached to the royal palace at Kew. But the committee of the Royal -Society remained unconvinced. In course of time the heat of party -spirit abated; experience as well as reason was found to be in favor of -Franklin’s views; and the battle of the balls and points has long since -passed into the domain of history.[27] - - -=Functions of a Lightning Conductor.=--A lightning conductor fulfills -two functions. First, it favors a silent and gradual discharge of -electricity between the cloud and the earth, and thus tends to prevent -that accumulation which must of necessity take place before a flash -of lightning will pass. Secondly, if a flash of lightning come, the -lightning conductor offers it a safe channel through which it may pass -harmless to the earth. - -These two functions of a lightning conductor may be easily illustrated -by experiment. When our machine is in action, if I present my closed -hand to the large brass conductor, a spark passes between them, and I -feel, at the same moment, a slight electric shock. Here the conductor -of the machine, as usual, holds the place of the electrified cloud; -my closed hand represents, as it were, a lofty building that stands -out prominently on the surface of the earth; the spark is the flash of -lightning, and the electric shock just suggests the destructive power -of the sudden disruptive discharge. - -Now let me protect this building by a lightning conductor. For this -purpose, I take in my hand a brass rod, which I connect with the -earth by a brass chain. In the first instance, I will have a metal -ball on the end of my lightning conductor. You see the effect; sparks -pass rapidly, but I feel no shock. I can increase the strength of -the discharge by hanging this condensing jar on the conductor of the -machine. Sparks pass now, much more brilliant and powerful than before, -but still I get no shock. It is evident, therefore, that my lightning -rod does not prevent the flash from passing, but it conveys it harmless -to the ground. - -I next take a rod which is sharply pointed, and connecting it as before -with the earth by a brass chain, I present the sharp point to the -conductor of the machine. Observe how different is the result; there is -no disruptive discharge; no spark passes; no shock is felt. Electricity -still continues to be generated in the machine, and electricity is -generated, by induction, in the brass rod, and in my body. But these -two opposite electricities discharge themselves silently, by means of -this pointed rod, and no sensible effect of any kind is exhibited. - -These experiments are very simple, but they really put before us, in -the clearest possible way, the whole theory of lightning conductors. -In particular, they give us ocular demonstration that an efficient -lightning rod not only makes the lightning harmless when it comes, -but tends very much to prevent its coming. A remarkable example, on a -large scale, of this important property, is furnished by the town of -Pietermaritzburg, the capital of the colony of Natal, in South Africa. -This town is subject to the frequent visitation of thunderstorms, -at certain seasons of the year, and much damage was formerly done -by lightning, but since the erection of lightning conductors on the -principal buildings, the lightning has never fallen within the town. -Thunderclouds come as before, but they pass silently over the city, -and only begin to emit their lightning flashes when they reach the -open country, and have passed beyond the range of the lightning -conductors.[28] - -But it will often happen, even in the case of a pointed conductor, -that the accumulation of electricity goes on so fast that the silent -discharge is insufficient to keep it in check. A disruptive discharge -will then take place, from time to time, and a flash of lightning will -pass. Under these circumstances, the lightning conductor is called upon -to fulfill its second function, and to convey the lightning harmless to -the earth. - - -=Conditions of a Lightning Conductor.=--From the consideration of -the functions which it has to fulfill, we may now infer what are the -conditions necessary for an efficient lightning conductor. The first -condition is that the end of the conductor, projecting into the air, -should have, at least, one sharp point. Our experiments have shown us -that a pointed conductor tends, in a manner, to suppress the flash -of lightning altogether; whereas a blunt conductor, or one ending in -a ball, tends only to make it harmless when it comes. It is evident, -therefore, that the pointed conductor offers the greater security. - -But a fine point is very liable to be melted when the lightning falls -upon it, and thus to be rendered less efficient for future service. To -meet this danger, it has recently been suggested, by the Lightning Rod -Conference, that the extreme end of the conductor should be a blunt -point, destined to receive the full force of the lightning flash, when -it comes; and that, a little lower down, a number of very fine points -should be provided, with a view to favor the silent discharge. This -suggestion, which appears admirably fitted to provide for the twofold -function of a lightning conductor, deserves to be recorded in the exact -terms of the official report. - -“It seems best to separate the double functions of the point, -prolonging the upper terminal to the very summit, and merely beveling -it off, so that, if a disruptive discharge does take place, the full -conducting power of the rod may be ready to receive it. At the same -time, having regard to the importance of silent discharge from sharp -points, we suggest that, at one foot below the extreme top of the upper -terminal, there be firmly attached, by screws and solder, a copper -ring bearing three or four copper needles, each six inches long, and -tapering from a quarter of an inch diameter to as fine a point as can -be made; and with the object of rendering the sharpness as permanent -as possible, we advise that they be platinized, gilded, or nickel -plated.”[29] - -The second condition of a lightning conductor is, that it should be -made of such material, and of such dimensions, as to offer an easy -passage to the greatest flash of lightning likely to fall on it; -otherwise it might be melted by the discharge, and the lightning, -seeking for itself another path, might force its way through bad -conductors, which it would partly rend asunder, and partly consume -by fire. Copper is now generally regarded as the best material for -lightning conductors, and it is almost universally employed in these -countries. If it is used in the form of a rope, it should not be less -than half an inch in diameter; if a band of copper is preferred--and it -is often found more convenient by builders--it should be about an inch -and a half broad and an eighth of an inch thick. In France it has been -hitherto more usual to employ iron rods for lightning conductors, but -since iron is much inferior to copper in its conducting power, the iron -rod must be of much larger dimensions; it should be at least one inch -in diameter.[30] - -The third condition is that the lightning conductor should be -continuous throughout its whole length, and should be placed in good -electrical contact with the earth. This is a condition of the first -importance, and experience has shown that it is the one most likely -of all to be neglected. In a large town the best earth connection is -furnished by the system of water-mains and gas-mains, each of which -constitutes a great network of conductors everywhere in contact with -the earth. Two points, however, must be carefully attended to--first, -that the electrical contact between the lightning conductor and the -metal pipe should be absolutely perfect; and, secondly, that the pipe -selected should be of such large dimensions as to allow the lightning -an easy passage through it to the principal main. - -If no such system of water-pipes or gas-pipes is at hand, then the -lightning rod should be connected with moist earth by means of a bed of -charcoal or a metal plate not less than three feet square. This metal -plate should be always of the same material as the conductor, otherwise -a galvanic action would be set up between the two metals, which in -course of time might seriously damage the contact. Dry earth, sand, -rock, and shingle are bad conductors; and, if such materials exist near -the surface of the earth, the lightning rod must pass through them and -be carried down until it reaches water or permanently damp earth. - - -=Mischief Done by Bad Conductors.=--If the earth contact is bad, a -lightning conductor does more harm than good. It invites the lightning -down upon the building without providing for it, at the same time, a -free passage to earth. The consequence is that the lightning forces -a way for itself, violently bursting asunder whatever opposes its -progress, and setting fire to whatever is combustible. - -I will give you some recent and striking examples. In the month of May, -1879, the church of Laughton-en-le-Morthen, in England, though provided -with a conductor, was struck by lightning and sustained considerable -damage. On examination it was found that the lightning followed the -conductor down along the spire as far as the roof; then, changing its -course, it forced its way through a buttress of massive masonwork, -dislodging about two cartloads of stones, and leaped over to the leads -of the roof, about six feet distant. It now followed the leads until it -came to the cast-iron down-pipes intended to discharge the rain-water, -and through these it descended to the earth. When the earth contact -of the lightning conductor was examined, it was found exceedingly -deficient. The rod was simply bent underground, and buried in dry -loose rubbish at a depth not exceeding eighteen inches. This is a very -instructive example. The lightning had a choice of two paths--one by -the conductor prepared for it, the other by the leads of the roof -and the down-pipes--and, by a kind of instinct which, however we may -explain, we must always contemplate with wonder, it chose the path of -least resistance, though in doing so it had to burst its way at the -outset through a massive wall of solid masonry.[31] - -On the 5th of June, in the same year, a flash of lightning struck the -house of Mr. Osbaldiston, near Sheffield, and, notwithstanding the -supposed protection of a lightning conductor, it did damage to the -amount of about five hundred pounds. The lightning here followed the -conductor to a point about nine feet from the ground, then passed -through a thick wall to a gas-pipe at the back of the drawing-room -mirror. It melted the gas-pipe, set fire to the gas, smashed the -mirror to atoms, broke the Sevres vases on the chimney-piece, and -dashed the furniture about. In this case, as in the former, it was -found that the earth contact was bad; and, in addition, the conductor -itself was of too small dimensions. Hence, the electric discharge -found an easier path to earth through the gas-pipes, though to reach -them it had to force for itself a passage through a resisting mass of -non-conductors.[32] - -Again in the same year, on the 28th of May, the house of Mr. Tomes, of -Caterham, was struck by lightning, and some slight damage was done. -After a careful examination it was found that the greater part of -the discharge left the lightning conductor with which the house was -provided, and passed over the slope of the roof to an attic room, into -which it forced its way through a brick wall, and reached a small iron -cistern. This cistern was connected by an iron pipe of considerable -dimensions with two pumps in the basement story; and through them the -lightning found an easy passage to the earth, and did but little harm -on its way. When the earth contact of the lightning conductor was -examined, it was discovered that the end of the rod was simply stuck -into a dry chalky soil to a depth of about twelve inches. Thus in this -case, as in the two former, it was made quite clear that the lightning -conductor failed to fulfill its functions because the earth contact was -bad.[33] - -Cases are not uncommon in which builders provide underground a -carefully constructed reservoir of water, into which the lower end -of the lightning rod is introduced. The idea seems to prevail that a -reservoir of water constitutes a good earth contact; and this is quite -true of a natural reservoir, such as a lake, where the water is in -contact with moist earth over a considerable area. But an artificial -reservoir may have quite an opposite character, and practically -insulate the lightning conductor from the earth. One which came under -my notice lately, in the neighborhood of this city, consists of a large -earthenware pipe set on end in a bed of cement, and kept half full of -water. Now, the earthenware pipe is a good insulator, and so is the bed -of cement in which it rests; and the whole arrangement is identical, -in all essential features, with the apparatus of Professor Richman, in -which he introduced his lightning rod into a glass bottle, and by which -he lost his life a hundred and thirty years ago. - -A conductor mounted in this manner will, probably enough, draw down -lightning from the clouds; but it is more likely to discharge it, with -destructive effect, into the building it is intended to guard, than -to transmit it harmlessly to the earth. An example is at hand in the -case of Christ Church, in the town of Clevedon, in Somersetshire. -This church was provided with a very efficient system of lightning -conductors, five in number, corresponding to the four pinnacles and the -flagstaff, on the summit of the principal tower. The five conductors -consisted of good copper-wire rope; all were united together inside the -tower, through which they were carried down to earth, and there ended -in an earthenware drain. This kind of earth contact might be pretty -good as long as water was flowing in the drain; but whenever the drain -was dry the conductor was practically insulated from the earth. On the -fifteenth of March, 1876, the church was struck by lightning, which -for some distance followed the line of the conductor; then finding its -passage barred by the earthenware drain, which was dry at the time, it -burst through the walls of the church, displacing several hundredweight -of stone, and making its way to earth through the gas-pipe.[34] - -Another very instructive example is furnished by the lightning -conductor attached to the lighthouse of Berehaven, on the south-west -coast of Ireland. It consists of a half-inch copper-wire rope, which -is carried down the face of the tower “until it reaches the rock -at its base, where it terminates in _a small hole, three inches by -three inches, jumped out of the rock, about six inches under the -surface_.” Here, again, we have a good imitation of Professor Richman’s -experiment, with only this difference, that a small hole in the rock -is substituted for a glass bottle. A lightning conductor of this kind -fulfills two functions: it increases the chance of the lightning coming -down on the building, and it makes it positively certain that, having -come, it cannot get to earth without doing mischief. - -The lightning did come down on the Berehaven Lighthouse, about five -years ago. As might have been expected, it made no use of the lightning -conductor in finding a path to earth, but forced its way through the -building, dealing destruction around as it descended from stage to -stage. The Board of Irish Lights furnished a detailed report of this -accident to the Lightning Rod Conference, in March, 1880, from which -the above particulars have been derived.[35] - - -=Precaution Against Rival Conductors.=--But it is not enough to -provide a good lightning conductor, which is itself able to convey -the electric discharge harmless to the earth; we must take care that -there are no rival conductors near at hand in the building, to draw -off the lightning from the path prepared for it, and conduct it by -another route in which its course might be marked with destruction. -This precaution is of especial importance at the present day, owing to -the great extent to which metal, of various kinds, is employed in the -construction and fittings of modern buildings. I will take a typical -case which will bring home this point clearly to your minds. - -A great part of the roof of many large buildings is covered with lead. -The lead, at one or more points may come near the gutters intended -to collect the rain water; the gutters are in connection with the -cast-iron down-pipes into which the water flows, and these down-pipes -often pass into the earth, which, under the circumstances, is generally -moist, and, therefore, in good electrical contact with the metal pipes. -Here, then, is an irregular line of conductors, which, though it has -gaps here and there, may, under certain conditions, offer to the -lightning discharge a path not less free than the lightning conductor -itself. What is the consequence? The flash of lightning, or a part of -it, will quit the lightning rod, and make its way to earth through the -broken series of conductors, doing, perhaps, serious mischief, as it -leaps across, or bursts asunder, the non-conducting links in the chain. - -Another illustration may be taken from the gas and water-pipes, with -which almost all buildings in great cities are now provided, and which -constitute a network of conductors, spreading out over the walls and -ceilings, and stretching down into the earth, with which they have -the best possible electrical contact. Now, it often happens that a -lightning conductor, at some point in its course, comes within a short -distance of this network of pipes. In such a case, a portion of the -electrical discharge is apt to leave the lightning conductor, force -its way destructively through masses of masonry, enter the network of -pipes, melt the leaden gas-pipe, ignite the gas, and set the building -on fire. - -These are not merely the speculations of philosophers. All the various -incidents I have just described have occurred, over and over again, -during the last few years. You will remember, in some of the examples -I have already set before you, when the electric discharge failed to -find a sufficient path to earth through the lightning rod, it followed -some such broken series of chance conductors as we are now considering. -But this broken series of conductors seems to bring with it a special -danger of its own, even when the lightning conductor is otherwise in -efficient working order. I will give you just one case in point. - -On the fifth of June, 1879, the Church of Saint Marie, Rugby, was -struck by lightning and set on fire, and narrowly escaped being -burned to the ground. A number of workmen were engaged on that day in -repairing the spire of the church. About three o’clock they saw a dense -black cloud approaching, and they came down to take shelter within the -building. In a few minutes they heard a terrific crash just overhead; -at the same moment the gas was lighted under the organ loft and the -woodwork was set in a blaze. The men soon succeeded in putting out the -fire, and the church escaped with very little damage. - -Now, in this case there was no reason to suppose that the lightning -conductor was in any way defective. But about half-way up the spire -there was a peal of eight bells. Attached to these bells were iron -wires, about the eighth of an inch in diameter, leading from the -clappers down to the organ-loft, where they came within a short -distance of a gas-pipe fixed in the wall. It would seem that a great -part of the discharge was carried safely to earth by the lightning -conductor. But a part branched off at the bells in the spire, descended -by the iron wires, and forced its way into the organ loft, to reach -the network of gas-pipes, through which it passed down to the earth, -melting the soft leaden gas-pipe in its course and lighting the gas. - -The remedy for this danger is obvious. All large masses of metal used -in the structure of a building--the leads and gutters of the roof, -the cast-iron down-pipes, the iron gas and water mains--should be put -in good metallic connection with the lightning conductor, and, as -far as may be, with one another. Connected in this way they furnish -a continuous and effective line of conductors leading safely down to -earth; and, instead of being a dangerous rival, they become a useful -auxiliary to the lightning rod. - -I would observe, however, that the lightning conductor ought not to -be connected directly with the soft leaden pipes which are commonly -employed to convey gas and water to the several parts of a building. -Such pipes, as we have seen, are liable to be melted when any -considerable part of the lightning discharge passes through them; and -thus much harm might be done, and the building might even be set on -fire by the lighting of the gas. Every good end will be attained if the -conductor is put in metallic connection with the iron gas and water -_mains_ either inside or outside the building. - - -=Insulation of Lightning Conductors.=--It is a question often asked -whether a lightning rod should be insulated from the building it -is intended to protect. I believe that this practice was formerly -recommended by some writers, and I have observed that glass insulators -are still employed not infrequently by builders in the erection of -lightning conductors; but, from the principles I have set before -you to-day, it seems clear that any insulation of this kind is, to -say the least, altogether useless. The building to be protected is -itself in electrical communication with the earth, and the lightning -conductor, if efficient, is also in electrical communication with the -earth--therefore, the lightning conductor and the building are in -electrical communication with each other through the earth, and any -attempt at insulating them from one another above the earth is only -labor thrown away. - -Further, I have just shown you that the masses of metal employed in -the structure or decoration of a building ought to be electrically -connected with each other and with the lightning conductor. Now, if -this be done, the lightning conductor is, by the fact, in direct -communication with the building, and the glass insulators are utterly -futile. Again, the building itself, during a thunderstorm, becomes -highly electrified by the inductive action of the cloud, and needs to -be discharged through the conductor just as the surrounding earth needs -to be discharged; therefore, the more thoroughly it is connected with -the conductor, the more effectively will the conductor fulfill its -functions. - - -=Personal Safety in a Thunderstorm.=--I suppose there is hardly any one -to whom the question has not occurred, at some time or another, what -he had best do to secure his personal safety during a thunderstorm. -This question is of so much practical interest that I think I shall -be excused if I say a few words about it, though perhaps, strictly -speaking, it is somewhat beside the subject of lightning conductors. - -At the outset, perhaps, I shall surprise you when I say that you would -enjoy the most perfect security if you were in a chamber entirely -composed of metal plates, or in a cage constructed of metal bars, or -if you were incased, like the knights of old, in a complete suit of -metal armor. This kind of defense is looked upon as so perfect, among -scientific men, that Professor Tait does not hesitate to recommend -his adventurous young friends devoted to the cause of science to -provide themselves with a light suit of copper, and, thus protected, -take the first opportunity of plunging into a thundercloud, there -to investigate, at its source, the process by which lightning is -manufactured.[36] - -The reason why a metal covering affords complete protection is that, -when a conductor is electrified, the whole charge of electricity -exists on the outside surface of the conductor; and therefore, when a -discharge takes place, it is only the outside surface that is affected. -Thus, if you were completely incased in a metal covering, and then -charged with electricity by the inductive action of a thundercloud, it -is only the metal covering that would undergo any change of electrical -condition; and when the lightning flash would pass, it is only the -metal covering that would be discharged. - -Let me show you a very pretty and interesting experiment to illustrate -this principle: Here is a hollow brass cylinder, open at the ends, -mounted on an insulating stand. On the outside is erected a light brass -rod with two pith balls suspended from it by linen threads. Two pith -balls are also suspended by linen threads from the inner surface of -the cylinder. You know that these pith balls will indicate to us the -electrical condition of the surfaces to which they are attached. If the -surface be electrified, the pith balls attached to it will share in -its electrical condition, and will repel each other; if the surface be -neutral, the pith balls attached to it will be neutral, and will remain -at rest. - -I now put this apparatus under the influence of our thundercloud, that -is, the large brass conductor of our machine. The moment my assistant -turns the handle, the electricity begins to be developed on the -conductor, and you see, at once, the effect on the brass cylinder. The -pith balls attached to the outer surface fly asunder; those attached -to the inner surface remain at rest. And now a spark passes; our -thundercloud is discharged; the inductive action ceases; the pith balls -on the outside suddenly collapse, while those on the inside are in no -way affected. - -[Illustration: PROTECTION FROM LIGHTNING FURNISHED BY A CLOSED -CONDUCTOR.] - -It is not necessary that the brass cylinder should be insulated. To -vary the experiment, I will now connect it with the earth by a chain; -you will observe that the effect is precisely the same as before. -Flash after flash passes while the machine continues in action; the -outside pith balls fly about violently, being charged and discharged -alternately; the inside pith balls remain all the time at rest. Thus -you see clearly that, if you were sitting inside such a metal chamber -as this, or covered with a complete suit of metal armor, you would -be perfectly secure during a thunderstorm, whether the chamber were -electrically connected with the earth or insulated from it. - - -=Practical Rules.=--But it rarely happens, when a thunderstorm comes, -that an iron hut or a complete suit of armor is at hand, and you will -naturally ask me what you ought to do under ordinary circumstances. -First, let me tell you what you ought not to do. You ought not to take -shelter under a tree, or under a haystack, or under the lee of a house; -you ought not to stand on the bank of a river, or close to a large -sheet of water. If indoors, you ought not to stay near the fireplace, -or near any of the flues or chimneys; you ought not to stand under a -gasalier hanging from the ceiling; you ought not to remain close to the -gas pipes or water-pipes, or any large masses of metal, whether used in -the construction of the building, or lying loosely about. - -The necessity for these precautions is sufficiently evident from the -principles I have already put before you. You want to prevent your body -from becoming a link in that broken chain of conductors which, as we -have seen, the electric discharge between earth and cloud is likely to -follow. Now a tree is a better conductor than the air; and your body is -a better conductor than a tree. Hence, the lightning, in choosing the -path of least resistance, would leave the air to pass through the tree, -and would leave the tree to pass through you. A like danger would await -you if you stood under the lee of a haystack or of a house. - -The number of people who lose their lives by taking refuge under trees -in thunderstorms is very remarkable. As one instance out of many, I -may cite the following case which was reported in the _Times_, July -14, 1887: “Yesterday the funeral of a negress was being conducted in a -graveyard at Mount Pleasant, sixty miles north of Nashville, Tennessee, -when a storm came on, and the crowd ran for shelter under the trees. -Nine persons stood under a large oak, which the lightning struck, -killing everyone, including three clergymen, and the mother and two -sisters of the girl who had been buried.” - -Again, every large sheet of water constitutes practically a great -conductor, which offers a very perfect medium of discharge between the -earth round about and the cloud. Therefore, when a thundercloud is -overhead, the sheet of water is likely to become one end of the line of -the lightning discharge; and if you be standing near it, the line of -discharge may pass through your body. - -When lightning strikes a building, it is very apt to use the stack -of chimneys in making its way to earth, partly because the stack of -chimneys is generally the most prominent part of the building, and -partly because, on account of the heated air and the soot within the -chimney, it is usually a moderately good conductor. Therefore, if -you be indoors, you must keep well away from the chimneys; and for a -similar reason, you must keep as far as you can from large masses of -metal of every kind. - -Having pointed out the sources of danger which you must try to avoid -in a thunderstorm, I have nearly exhausted all the practical advice -that I have at my command. But there are some occasions on which it may -be possible, not only to avoid evident sources of danger, but to make -special provision for your own security. Thus, for example, in the open -country, if you stand a short distance from a wood, you may consider -yourself as practically protected by a lightning conductor. For a -wood, by its numerous branches and leaves, favors very much a quiet -discharge of electricity, thus tending to suppress altogether the flash -of lightning; and if the flash of lightning does come, it is much more -likely to strike the wood than to strike you, because the wood is a far -more prominent body, and offers, on the whole, an easier path to earth. -In like manner, if you place yourself near a tall solitary tree, some -twenty or thirty yards outside its longest branches, you will be in a -position of comparative safety. If the storm overtake you in the open -plain, far away from trees and buildings, you will be safer lying flat -on the ground than standing erect. - -In an ordinary dwelling house, the best situation is probably the -middle story, and the best position in the room is in the middle of -the floor; provided, of course, that there is no gasalier hanging from -the ceiling above or below you. Strictly speaking, the _middle of the -room_ would be a still safer position than the middle of the floor; -and nothing could be more perfect than the plan suggested by Franklin, -to get into “a hammock, or swinging bed, suspended by silk cords, and -equally distant from the walls on every side, as well as from the -ceiling and floor, above and below.” An interesting case has been -recently recorded, by a resident of Venezuela, which illustrates in a -remarkable way the excellence of this advice. “The lightning,” he says, -“struck a _rancho_--a small country house, built of wood and mud, and -thatched with straw or large leaves--where one man slept in a hammock, -another lay under the hammock on the ground, and three women were busy -about the floor; there were also several hens and a pig. The man in -the hammock did not receive any injury whatever, while the other four -persons and the animals were killed.”[37] - -But, as I can hardly hope that many of you when the thunderstorm -actually comes will find yourselves provided with a hammock, I would -recommend, as more generally useful, another plan of Franklin’s, which -is simply to sit on one chair in the middle of the floor and put your -feet up on another. This arrangement will approach very nearly to -absolute security if you take the further precaution, also mentioned by -Franklin, of putting a feather bed or a couple of hair mattresses under -the chairs.[38] - - -=Security Afforded by Lightning Rods.=--You might, perhaps, be inclined -to infer hastily, from the examples I have set before you, in the -course of this lecture, of buildings which were struck and severely -injured by lightning though provided with lightning conductors, that a -lightning rod affords a very imperfect protection to life and property. -But such an idea would be entirely at variance with the evidence at -hand on the subject. In all the cases to which I have referred, and in -many others which might easily have been cited, the damage was done -simply because the lightning rods were deficient in one or more of the -conditions on which I have so much insisted. Where these conditions are -fulfilled, the lightning flash will either not come down at all upon -the building, or, if it do come, it will be carried harmless to the -earth. - -Perhaps there is no one fact that so forcibly brings home to the -mind the complete protection afforded by lightning conductors as the -change which followed their introduction into the Royal Navy. I have -already told you that in former times the damage done by lightning to -ships of the Royal Navy was a regular source of expenditure, amounting -every year to several thousand pounds sterling. But, after the general -adoption of lightning conductors about forty years ago, through the -indefatigable exertions of Sir William Snow Harris, this source of -expenditure absolutely disappeared, and injury to life and property has -long been practically unknown in Her Majesty’s Fleet. - -I should say, however, that the trial of lightning conductors in the -Navy, though it lasted long enough to prove their perfect efficiency, -has almost come to an end in our own days. The great iron monsters -which in recent times have taken the place of the wooden ships of -Old England are quite independent of lightning rods in the common -sense of the word. Their ponderous masts are virtually lightning rods -of colossal dimensions, and their unsightly hulls are, so to speak, -earth-plates of enormous size in perfect electrical contact with the -ocean. To add to such structures lightning conductors of the common -kind would be nothing better than “wasteful and ridiculous excess.” - -As regards buildings on land, I may refer to the little province -of Schleswig-Holstein, of which I have already spoken to you. From -some cause or other this small peninsula is singularly exposed to -thunderstorms, and of late years it has been more abundantly provided -with lightning conductors than, perhaps, any other district of equal -extent in Europe. Now, as a simple illustration of the protection -afforded by these lightning conductors, I may mention that, on the -26th of May, 1878, a violent thunderstorm burst over the little town -of Utersen. Five several flashes of lightning fell in different parts -of the town, but not the slightest harm was done, each flash being -safely carried to earth by a lightning conductor. Further, it appears -from the records of the fire insurance company that, out of 552 -buildings injured by lightning during a period of eight years--from -1870 to 1878--only four had lightning conductors; and in these four -cases it was found, on examination, that the lightning conductors were -defective.[39] - -It would be easy to multiply evidence on this subject. But as I have -already trespassed, I fear, too far on your patience, I will content -myself with saying, in conclusion, that according to all the highest -authorities, both practical and theoretical, any structure provided -with a lightning conductor properly fitted up in conformity with the -principles I have set before you to-day is perfectly secure against -lightning. The lightning, indeed, may fall upon it, but it will pass -harmless to the earth; and the experience of more than a hundred years -has fully justified the simple and modest words of the great inventor -of lightning conductors: “It has pleased God, in His goodness to -mankind, at length to discover to them the means of securing their -habitations and other buildings from mischief by thunder and lightning.” - - -NOTE I. - -ON THE LIGHTNING CONDUCTOR AT BEREHAVEN.[40] - - It is satisfactory to know that the lightning conductor referred to - in my lecture as attached to the lighthouse at Berehaven has been - put in good order under the best scientific guidance. The following - interesting letter from Professor Tyndall, which appeared in the - _Times_, August 31, 1887, gives the history of the matter very - clearly, and fully bears out the views put forward in my lecture: - - “Your recent remarks on thunderstorms and their effects induce me to - submit to you the following facts and considerations. Some years ago - a rock lighthouse on the coast of Ireland was struck and damaged by - lightning. An engineer was sent down to report on the occurrence; - and, as I then held the honorable and responsible post of scientific - adviser to the Trinity House and Board of Trade, the report was - submitted to me. The lightning conductor had been carried down the - lighthouse tower, its lower extremity being carefully embedded in a - stone perforated to receive it. If the object had been to invite the - lightning to strike the tower, a better arrangement could hardly have - been adopted. - - “I gave directions to have the conductor immediately prolonged, and - to have added to it a large terminal plate of copper, which was to be - completely submerged in the sea. The obvious convenience of a chain as - a prolongation of the conductor caused the authorities in Ireland to - propose it; but I was obliged to veto the adoption of the chain. The - contact of link with link is never perfect. I had, moreover, beside - me a portion of a chain cable through which a lightning discharge had - passed, the electricity in passing from link to link encountering - a resistance sufficient to enable it to partially fuse the chain. - The abolition of resistance is absolutely necessary in connecting - a lightning conductor with the earth, and this is done by closely - embedding in the earth a plate of good conducting material and of - large area. The largeness of area makes atonement for the imperfect - conductivity of earth. The plate, in fact, constitutes a wide door - through which the electricity passes freely into the earth, its - disruptive and damaging effects being thereby avoided. - - “These truths are elementary, but they are often neglected. I watched - with interest some time ago the operation of setting up a lightning - conductor on the house of a neighbor of mine in the country. The - wire rope which formed part of the conductor was carried down the - wall and comfortably laid in the earth below without any terminal - plate whatever. I expostulated with the man who did the work, but - he obviously thought he knew more about the matter than I did. I am - credibly informed that this is a common way of dealing with lightning - conductors by ignorant practitioners, and the Bishop of Winchester’s - palace at Farnham has been mentioned to me as an edifice ‘protected’ - in this fashion. If my informant be correct, the ‘protection’ is a - mockery, a delusion, and a snare.” - - -NOTE II. - -BOOKS OF REFERENCE. - - As some of my readers may wish to pursue the study of lightning and - lightning conductors beyond the limits to which a popular lecture - must, of necessity, be confined, I subjoin a list of the books which - I think they would be likely to find most useful for the purpose. - Among ordinary text-books on physics--Jamin, Cours de Physique, - vol. i., pp. 470-494; Mascart, Traité d’Electricité Statique, vol. - ii., pp. 555-579; De Larive, A Treatise on Electricity, in three - volumes, London, 1853-8, vol. iii., pp. 90-201; Daguin, Traité - de Physique, vol. iii., pp. 209-280; Riess, Die Lehre von der - Reibungs-Elektricität, vol. ii., pp. 494-564; Müller-Pouillet, - Lehrbuch der Physik, Braunschweig, 1881, vol. iii., pp. 210-225; - Scott, Elementary Meteorology, chap. x. Of the numerous special - treatises and detached papers on the subject, I would recommend - Instruction sur les Paratonnerres adopté par l’Académie des Sciences, - Part i., 1823, Part ii., 1854, Part iii., 1867, Paris, 1874; Arago, - Sur le Tonnerre, Paris, 1837; also his Meteorological Essays, - translated by Sabine, London, 1855; Sir William Snow Harris, On the - Nature of Thunderstorms, London, 1843; also by the same writer, - A Treatise on Frictional Electricity, London, 1867; and various - papers on lightning conductors, from 1822 to 1859; Tomlinson, The - Thunderstorm, London, 1877; Anderson, Lightning Conductors, London, - 1880; Holtz, Ueber die Theorie, die Anlage, und die Prüfung der - Blitzableiter, Greifswald, 1878; Weber, Berichte über Blitzschläge - in der Provinz Schleswig-Holstein, Kiel, 1880-1; Tait, A Lecture - on Thunderstorms, delivered in the City Hall, Glasgow, in 1880, - Nature, vol. xxii.; Report of the Lightning Rod Conference, London, - 1882. This last-mentioned volume comes to us with very high - authority, representing, as it does, the joint labors of several - eminent scientific men selected from the following societies: The - Meteorological Society, the Royal Institute of British Architects, the - Society of Telegraph Engineers and Electricians, the Physical Society. - - Since the above was in print, two lectures given before the Society - of Arts by Professor Oliver Lodge, F. R. S., have appeared in the - _Electrician_, June and July, 1888, in which some new views are put - forward respecting lightning conductors, that seem deserving of - careful consideration. - - -FOOTNOTES: - -[17] The Thunderstorm, by Charles Tomlinson, F. R. S., Third Edition, -pp. 153-4. - -[18] Two Lectures on Atmospheric Electricity and Protection from -Lightning, published at the end of his Treatise on Frictional -Electricity, p. 273. - -[19] See Report of Lightning Rod Conference, p. 119. - -[20] _Loco citato._ - -[21] Sir William Snow Harris, _loco citato_, p. 274. - -[22] _Id._, p. 275. - -[23] The Thunderstorm, by Charles Tomlinson, F.R.S., Third Edition, p. -172. - -[24] See for these facts, Anderson, Lightning Conductors, p. 197; -Tomlinson, The Thunderstorm, pp. 167-9; Harris, _loco citato_, pp. -273-4. - -[25] See Anderson, Lightning Conductors, pp. 170-5. - -[26] The Thunderstorm, pp. 158-9. See also an account of four persons -who were struck on the Matterhorn, in July, 1869, all of whom were -hurt, and none killed: Whymper’s Scrambles Among the Alps, pp. 414, 415. - -[27] See Philosophical Transactions of the Royal Society, 1773, p. 42, -and 1778, part i., p. 232; Anderson’s Lightning Conductors, pp. 40-2; -Lighting Rod Conference, pp. 76-9. - -[28] See A Lecture on Thunderstorms, by Professor Tait of Edinburgh, -published in Nature, vol. xxii., p. 365. - -[29] Report of the Lightning Rod Conference, p. 4. - -[30] The dimensions here set forth are greater in some respects than -those “recommended as a minimum” in the report of the Lightning Rod -Conference, page 6. But it will be observed by those who consult the -report that the minimum recommended is just the size which, in the -preceding paragraph of the report, is said to have been actually melted -by a flash of lightning; and, therefore, it seems not to be a very -safe minimum. It will be also seen that there is some confusion in the -figures given, and that they contradict one another. For the dimensions -of iron rods, see the instructions adopted by the Academy of Science, -Paris, May 20, 1875; Lightning Rod Conference, pp. 67-8. - -[31] See letter of Mr. R. S. Newall, F. R. S., in the _Times_, May 30, -1879. - -[32] See Nature, June 12, 1879, vol. xx., p. 146. - -[33] See letter of Mr. Tomes in Nature, vol. xx., p. 145; also -Lightning Rod Conference, pp. 210-15. - -[34] See Anderson, Lightning Conductors, pp. 208-10. - -[35] See Lightning Rod Conference, pp. 208-10; see also the note at the -end of this Lecture, p. 52. - -[36] Lecture on Thunderstorms, Nature, vol. xxii., pp. 365, 437. See, -also, a very interesting paper by the late Professor J. Clerk Maxwell, -read before the British Association at Glasgow in 1876, and reprinted -in the report of the Lightning Rod Conference, pp. 109, 110. - -[37] Nature, vol. xxxi., p. 459. - -[38] See further information on this interesting subject in the Report -of the Lightning Rod Conference, pp. 233-5. - -[39] See “Die Theorie, die Anlage, und die Prüfung der Blitzableiter,” -von Doctor W. Holtz, Griefswald, 1878. - -[40] See page 44. - - - - -APPENDIX. - -RECENT CONTROVERSY ON LIGHTNING CONDUCTORS. - - -The lecture on lightning conductors contained in this volume fairly -represents, I think, the theory hitherto received on the subject. It -is, moreover, entirely in accord with the report of the Lightning Rod -Conference, brought out in 1883, by a committee of most eminent men, -representing several branches of science, who were specially chosen to -consider this question some ten years ago. - - -=Lectures of Professor Lodge.=--But, in the month of March, 1888, -two lectures were given before the Society of Arts, in London, by -Professor Oliver Lodge, in which this theory was directly challenged, -and attacked with cogent arguments, supported by striking and original -experiments. These lectures gave rise to an animated controversy, which -culminated in a formal discussion at the recent meeting of the British -Association in Bath. The discussion was carried on with great spirit, -and most of the leading representatives of physical and mechanical -science took an active part in it. The greater portion of this volume -was printed off before the meeting of the British Association took -place. But the discussion on the theory of lightning conductors seemed -to me so interesting and important that I thought it right, in the form -of an Appendix, to give some account of the questions at issue, and of -the opinions expressed upon them. - -Professor Lodge maintains[41] that the received theory of lightning -rods is open to two objections. First, it takes account only of the -conducting power of the lightning rod, and takes no account of the -phenomenon known as self-induction, or electrical inertia. Secondly, -it assumes that the whole substance of a lightning rod acts as a -conductor, in all cases of lightning discharge; whereas there is reason -to believe that, in many cases, it is only a thin outer shell that -really comes into action. I will deal with these two points separately. - - -=The Effect of Self-Induction.=--When an electric discharge begins -to pass through a conductor, a momentary back electro-motive force -is developed in the conductor, which obstructs its passage. This -phenomenon is called by some self-induction, by others electrical -inertia; but its existence is admitted by all. Now, when a flash -of lightning, so to say, falls on a lightning rod, the back -electro-motive force developed is very considerable; and it may -offer so great an obstruction that the discharge will find an easier -passage by some other route, such as the stone walls and woodwork, and -furniture of the building. - -According to this view, the obstruction which a flash of lightning -encounters in a conductor consists partly of the resistance of the -conductor, in the ordinary sense of the word resistance, and partly of -the back electro-motive force due to self-induction. The sum of these -two Professor Lodge calls the _impedance_ of the lightning rod; and he -considers that the impedance may be enormously great, even when the -resistance, in the ordinary sense, is comparatively small. - -In support of this view he has devised the following extremely -ingenious and remarkable experiment. A large Leyden jar, L, was -arranged in such a manner that, while it received a steady charge from -an electrical machine, it discharged itself, at intervals, across the -air space at A, between two brass balls. The discharge had then two -alternative paths before it; one through a conducting wire, C, the -other across a second air space, between two brass balls at B. During -the experiment, the two balls at A were kept at a fixed distance of one -inch apart; but the distance between the two balls at B was varied. -The conductor, C, used in the first instance, was a stout copper wire, -about forty feet long, and having a resistance of only one-fortieth of -an ohm. - -[Illustration: INDUCTION EFFECT OF LEYDEN JAR DISCHARGE. - - M Electrical Machine. - L Leyden Jar. - A B Air Spaces between Brass Knobs. - C Conducting Wire.] - -It was found that, so long as the distance between the B knobs was less -than 1.43 inches, all the discharges passed across between the knobs, -in the form of a spark. When the distance exceeded 1.43 inches, all -the discharges passed through the conductor, C, and no spark appeared -between the balls at B. And when the distance was exactly 1.43 inches, -the discharge sometimes took place between the knobs, and sometimes -followed the conductor, C. The interpretation given to these facts -is that the obstruction offered by the conductor C was about equal -to the resistance of 1.43 inches of air; and it is proposed to call -this distance, under the conditions of the experiment, the _critical -distance_. - -Coming now to the application of these results, Professor Lodge argues -that the conductor C, in his experiment, represents a lightning rod -of unimpeachable excellence; and yet, in certain cases, the discharge -refuses to follow the conductor, and prefers to leap across a -considerable space of air, notwithstanding the enormous resistance it -there encounters. In like manner, he says, a flash of lightning may, -in certain cases, leave a lightning rod fitted up in the most orthodox -manner, and force its way to earth through resisting masses of mason -work and such chance conductors as may come across its path. - -This conclusion, he admits, is altogether at variance with the received -views on the subject; but he contends that it is perfectly in accord -with the scientific theory of an electrical discharge. The moment -the discharge begins to pass in the conductor, it encounters the -obstruction due to self-induction; and this obstruction is so great -that the bad conductors offer, on the whole, an easier path to earth. - - -=Variation of the Experiment.=--When the experiment was varied by -substituting a thin iron wire for the stout copper wire at first -employed, a very curious result was obtained. The wire chosen was of -the same length as the copper, but had a resistance about 1,300 times -as great; its resistance being, in fact, 33.3 ohms. Nevertheless, in -this experiment, when the B knobs were at a distance of 1.43 inches, -no spark passed, which showed that the discharge always followed the -line of the conductor, and therefore that the conductor offered less -obstruction than 1.43 inches of air. The knobs were then brought -gradually nearer and nearer; and it was not until the distance was -considerably reduced that the sparks began to pass between them. When -the distance was exactly 1.03 inches, the discharge sometimes passed -between the knobs, and sometimes through the conductor; this was, -therefore, the _critical distance_, in the case of the iron wire. Thus -it appeared that the obstruction offered to the discharge by the iron -wire was much less than that offered by the copper, the one being equal -to a resistance of only 1.03 inches of air, the other to a resistance -of 1.43 inches. - -It does not appear that Professor Lodge undertakes to offer any -satisfactory explanation of this result. He has come to the conclusion, -from his various experiments, that, in the case of a sudden discharge, -difference of conducting power between fairly good conductors is a -matter of practically no account; and that difference of sectional area -is a matter of only trifling account. But he does not see why a thin -iron wire should have a _smaller_ impedance than a much thicker wire of -copper. He proposes to repeat the experiments so as to confirm or to -modify the result, which for the present seems to him anomalous.[42] - - -=The Outer Shell only of a Lightning Rod Acts as a Conductor.=--As a -consequence of self-induction or electrical inertia, Professor Lodge -contends that a lightning discharge in a conductor consists of a -series of oscillations. These oscillations follow one another with -extraordinary rapidity--there may be a hundred thousand in a second, -there may be a million. Now it has been shown that, when a current -starts in a conductor, it does not start at once all through its -section; it begins on the outside, and then gradually, but rapidly, -penetrates to the interior. From this he infers that the extremely -rapid oscillations of a lightning discharge have not time to penetrate -to the interior of a conductor. The electricity keeps surging to -and fro in the superficial layer or outer shell, while the interior -substance of the rod remains inert and takes no part in the action. A -conductor, therefore, will be most efficient for carrying off a flash -of lightning if it present the greatest possible amount of surface; a -thin, flat tape will be more efficient than a rod of the same mass; and -a number of detached wires more efficient than a solid cylinder. As for -existing lightning conductors, the greater part of their mass would, -in many cases, have no efficacy whatever in carrying off a flash of -lightning. - - -=The Discussion.=--The discussion at the meeting of the British -Association was opened by Mr. William H. Preece, F.R.S., Electrician -to the Post Office, who claimed to have 500,000 lightning conductors -under his control. He expressed his conviction that a lightning rod, -properly erected and duly maintained, was a perfect protection against -injury from lightning; and in support of this conviction he urged -very strongly the report of the Lightning Rod Conference. This report -represented the mature judgment of the most eminent scientific men, -who had devoted years to the study of the question; and he wished -particularly to bring before the meeting their clear and decisive -assertion--an assertion he was there to defend--that “there is no -authentic case on record where a properly constructed conductor failed -to do its duty.” - -The new views put forward by Professor Lodge were based, in great -measure, on his theory that a lightning discharge consisted of a series -of rapid oscillations. But this theory should be received with great -caution. It seemed to be nothing more than a deduction from certain -mathematical formulas, and was not supported by any solid basis of -observation or experiment. Besides, there were many facts against -it. They all knew that a flash of lightning magnetized steel bars, -deranged the compasses of ships at sea, and transmitted signals on -telegraph wires. But such effects could not be produced by a series of -oscillations, which, being equal and opposite, would neutralize each -other. It was alleged that these rapid oscillations occurred in the -discharge of a Leyden jar. That might be true, and probably was true; -but they were not dealing with Leyden jars, they were dealing with -flashes of lightning. If there was any analogy between the discharge of -a Leyden jar and a flash of lightning, it was to be found, not in the -external discharge employed by Professor Lodge in his experiments, but -in the bursting of the glass cylinder between the two coatings of the -jar. - -Lord Rayleigh thought the experiments of Professor Lodge were likely -to have important practical applications to lightning conductors. But -though these experiments were valuable as suggestions, they did not -furnish a sufficient ground for adopting any new system of protection. -It was only by experience with lightning conductors themselves that the -question could be finally settled. - -Sir William Thomson hoped for great fruit from the further -investigation of self-induction in the case of sudden electrical -discharges. He warmly encouraged Professor Lodge to continue his -researches; but he expressed no decided opinion on the question at -issue. Incidentally he observed that the best security for a gun-powder -magazine was an iron house; no lightning conductor at all, but an iron -roof, iron walls, and an iron floor. Wooden boards should, of course, -be placed over the floor to prevent the danger of sparks from people -walking on sheet-iron. This iron magazine might be placed on a dry -granite rock, or on wet ground; it might even be placed on a foundation -under water; it might be placed anywhere they pleased; no matter what -the surroundings were, the interior would be safe. He thought that was -an important practical conclusion which might safely be drawn from the -consideration of these electrical oscillations and the experiments -regarding them. - -Professor Rowland, of the Johns Hopkins University, America, said that -the question seemed to be whether the experiment of Professor Lodge -actually represented the case of lightning. He was very much disposed -to think it did not. In the experiment almost the whole circuit -consisted of good conductors; whereas, in the case of lightning, the -path of the discharge was, for the most part, through the air, and -therefore it might be an entirely different phenomenon. The air being a -very bad conductor, a flash of lightning might, perhaps, not consist of -oscillations, but rather of a single swing. Moreover, it was not at all -clear that the length of the spark, in the experiment, could be taken -as a measure of the obstruction offered by the conductor. Professor -George Forbes was greatly impressed with the beauty and significance of -Professor Lodge’s experiments, but he did not think the result so clear -that they should be warranted in abandoning the principles laid down by -the Lightning Rod Conference. - -M. de Fonvielle, of Paris, supported the views of Mr. Preece. He cited -the example of Paris, where they had erected a sufficient number -of lightning conductors, according to the received principles, and -calamities from lightning were practically unknown. He suggested that -the Eiffel Tower, which they were now building, and which would be -raised to the height of a thousand feet, would furnish an unrivalled -opportunity for experiments on lightning conductors. - -Sir James Douglass, Chief Engineer to the Corporation of Trinity House, -had a large experience with lighthouse towers. The lightning rods on -these towers had been erected and maintained during the last fifty -years entirely according to the advice of Faraday. They never had a -serious accident; and such minor accidents as did occur from time to -time were always traced to some defect in the conductor. They had now -established a more rigid system of inspection, and he, for one, should -feel perfectly safe in any tower where this system was carried out. - -Mr. Symons, F.R.S., Secretary to the Meteorological Society, had taken -part in a discussion on lightning conductors as long ago as 1859. It -had been a hobby with him all his life to investigate the circumstances -of every case he came across in which damage was done by lightning, -and the general impression left by his investigations entirely -coincided with the views just expressed by Sir James Douglass. He had -been a member of the Lightning Rod Conference, and was the editor of -their report; and he wished to enter his protest against the idea of -rejecting all that had hitherto been done in connection with lightning -conductors on the strength of mere laboratory experiments. - -Professor Lodge, in reply, said he could perfectly understand the -position of those who held that a lightning rod properly fitted up -never failed to do its duty, because, whenever it failed, they said -it was not properly fitted up. The great resource in such cases was -to ascribe the failure to bad earth contact. He thought a good earth -contact was a very good thing, but he could not understand why such -extraordinary importance should be attached to it. A lightning rod -had two ends--an earth end and a sky end--and he did not see why good -contact was more necessary at one end than at the other. If a few sharp -points sticking out from the conductor were sufficient for a good sky -contact, why were they not sufficient also for a good earth contact? - -Besides, though a bad earth contact might explain why a certain amount -of disruption should take place at the earth where the bad contact -existed, he did not see how it accounted for the flash shooting off -sideways half-way down the conductor. Again, what does a bad earth -contact mean? If an electrical engineer finds a resistance of a -hundred ohms, he will rightly pronounce the earth contact to be very -bad indeed. But why should the lightning flash leave a conductor -with a resistance of a hundred ohms in order to follow a line of -non-conductors where it encounters a resistance of many thousand ohms? - -He accepted the statement of Mr. Preece that his whole theory depended -on the existence of oscillations in the lightning discharge; but there -was good reason to believe they existed, because they were proved to -exist in the discharge of a Leyden jar. Mr. Preece objected that an -oscillating discharge could not produce magnetic effects, as a flash of -lightning was known to do. He confessed he was unable to explain how -an oscillating discharge produced such effects;[43] but that it could -produce them there was no doubt whatever, for the discharge of a Leyden -jar produces magnetic effects, and we have ocular demonstration that -the discharge of a Leyden jar is an oscillating discharge. - -As to the assurances we had received from electrical engineers that -a properly fitted lightning conductor never fails, he should like to -ask them how the Hotel de Ville, in Brussels, had been set on fire by -lightning on the 1st of last June. The system of lightning conductors -on this building had been erected in accordance with the received -theory, and had been held up by writers on the subject as the most -perfect in Europe. Unless some explanation were forthcoming to account -for its failure, we could no longer regard lightning conductors as a -perfect security against danger. - -The President of Section A, Professor Fitzgerald, in bringing the -discussion to a close, observed that one result of this meeting would -be to give a new interest to the phenomena of static electricity and -its practical applications. He was inclined himself to think that the -experiments of Professor Lodge were not quite analogous to the case of -a flash of lightning. In comparing the discharge of a Leyden jar with -a flash of lightning they should look for the analogy, not so much in -the external discharge through a series of conductors, but rather, -as Mr. Preece had observed, in the bursting of the glass between the -two coatings of the jar. As regarded the oscillations in a Leyden jar -discharge, he did not think such oscillations were at all necessary -to account for the phenomena observed in the experiments. Many of -the results which Professor Lodge seemed to think would require some -millions of oscillations per second would be produced by a single -discharge lasting for a millionth of a second. Improvements, perhaps, -were possible in our present system of lightning conductors, but -practical experience had shown, however we might reason on the matter, -that, on the whole, lightning conductors had been a great protection to -mankind from the dangers of lightning. - - -=Summary.=--I will now try to sum up the results of this interesting -discussion, and state briefly the conclusions which, as it seems to -me, may be deduced from it. First, I would remind my readers that a -lightning rod has two functions to fulfill. Its first function is to -promote a gradual, but rapid, discharge of electricity according as it -is developed, and thus to prevent such an accumulation as would lead -to a flash of lightning. Its second function is to convey the flash -of lightning, when it does come, harmless to the earth. Now, the new -views advanced by Professor Lodge in no way impugn the efficiency of -lightning rods as regards their first function; and it is evident that -the greater the number of lightning rods distributed over a given area, -the more perfectly will this function be fulfilled. This is a point of -great practical importance which seemed to me, in some degree, lost -sight of during the progress of the discussion. - -Secondly, it was practically admitted by the highest authorities that -the experiments and reasoning of Professor Lodge afford good grounds -for reconsidering the received theory of lightning conductors as -regards their second function--that of carrying the lightning flash -harmless to the earth. But there was undoubtedly a general feeling -that it would be rash to set aside, all at once, the received theory -on the strength of laboratory experiments made under conditions widely -different from those which actually exist in a lightning discharge. -Experiments are wanted on a larger scale; and, if possible, experiments -with lightning rods themselves. - -Thirdly, the testimony of electrical engineers who have had large -experience with lightning conductors seems almost unanimous that a -lightning conductor erected and maintained in accordance with the -conditions prescribed by the Lightning Rod Conference gives perfect -protection. It was certainly unfortunate that the Hotel de Ville, in -Brussels, which was reputed the best protected building in Europe, -should have been damaged by lightning just two months before the -discussion took place; but no certain conclusion can be drawn from -this catastrophe until we know exactly the conditions under which it -occurred. - -So the matter stands, awaiting further investigation. - - -FOOTNOTES: - -[41] See his Lectures, published in the _Electrician_, June 22, June -29, July 6, and July 13, 1888. - -[42] See paper read at the meeting of the British Association, in Bath, -1888, published in the _Electrician_, page 607. September 14. - -[43] See a very ingenious hypothesis, to account for this phenomenon, -suggested by Professor Ewing in the _Electrician_, p. 712. October 5, -1888. - - - - -Transcriber’s Notes - -Errors and omissions in punctuation have been corrected. - -Page 11: “continuous inpression” changed to “continuous impression” - -Page 41: “full conconducting power” changed to “full conducting power” - -Page 44: “it base” changed to “its base” - -Page 58: “follow one an-another” changed to “follow one another” - -*** END OF THE PROJECT GUTENBERG EBOOK LIGHTNING, THUNDER AND -LIGHTNING CONDUCTORS *** - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the -United States without permission and without paying copyright -royalties. 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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 <a href="https://www.gutenberg.org">www.gutenberg.org</a>. If you -are not located in the United States, you will have to check the laws of the -country where you are located before using this eBook. -</div> - -<p style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Title: Lightning, Thunder and Lightning Conductors</p> -<p style='display:block; margin-top:1em; margin-bottom:0; margin-left:2em; text-indent:-2em'>Author: Gerald Molloy</p> -<p style='display:block; text-indent:0; margin:1em 0'>Release Date: September 15, 2022 [eBook #68994]</p> -<p style='display:block; text-indent:0; margin:1em 0'>Language: English</p> - <p style='display:block; margin-top:1em; margin-bottom:0; margin-left:2em; text-indent:-2em; text-align:left'>Produced by: deaurider and the Online Distributed Proofreading Team at https://www.pgdp.net (This book was produced from images made available by the HathiTrust Digital Library.)</p> -<div style='margin-top:2em; margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK LIGHTNING, THUNDER AND LIGHTNING CONDUCTORS ***</div> - - - - -<h1> <span class="smcap big">Lightning, Thunder</span> -<br /><br /> -<span class="vsmall">AND</span><br /> -<br /><span class="smcap">Lightning Conductors</span>.</h1> -<p class="center p2"> -<i>WITH AN APPENDIX ON THE RECENT CONTROVERSY ON LIGHTNING CONDUCTORS.</i><br /> -</p> -<p class="center p2"> -<span class="small">BY</span><br /> -<br /><span class="big"> -GERALD MOLLOY, D. D., D. Sc.</span><br /> -</p> -<hr class="r5" /> -<p class="center"> -<i>ILLUSTRATED.</i><br /> -</p> -<hr class="r5" /> -<p class="center p2"><span class="figcenter" id="img001"> - <img src="images/001.jpg" class="w10" alt="publisher mark" /> -</span></p> -<p class="center p2"> -NEW YORK:<br /> -<span class="big">THE HUMBOLDT PUBLISHING CO.,</span><br /> -28 LAFAYETTE PLACE.<br /> -</p> -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_3">[Pg 3]</span></p> - -<p class="center xbig"><span class="smcap">Lightning, Thunder, and Lightning Conductors.</span></p> -</div> - -<hr class="r5" /> -<h2>CONTENTS.</h2> -<hr class="r5" /> - -<table class="autotable"> -<tr><td class="tdl"> -<a href="#LECTURE_I">LECTURE I.</a> -</td><td class="tdr page"><a href="#Page_5">Pages 5-26</a> -</td></tr> -<tr><td class="tdc" colspan="2"> -<a href="#LECTURE_I">LIGHTNING AND THUNDER.</a> -</td></tr> -<tr><td class="tdl" colspan="2"> - Identity of Lightning and Electricity—Franklin’s Experiment—Fatal Experiment of - Richman—Immediate Cause of Lightning—Illustration from Electric Spark—What - a Flash of Lightning Is—Duration of a Flash of Lightning—Experiments - of Professor Rood—Wheatstone’s Experiments—Experiment with - Rotating Disc—Brightness of a Flash of Lightning—Various Forms of - Lightning—Forked Lightning, Sheet Lightning, Globe Lightning—<abbr title="saint">St.</abbr> - Elmo’s Fire—Experimental Illustration—Origin of Lightning—Length of a - Flash of Lightning—Physical Cause of Thunder—Rolling of Thunder—Succession - of Peals—Variation of Intensity—Distance of a Flash of Lightning -</td></tr> -<tr><td class="tdl"> -<a href="#LECTURE_II">LECTURE II.</a> -</td><td class="tdr page"><a href="#Page_26">Pages 26-53</a> -</td></tr> -<tr><td class="tdc" colspan="2"> -<a href="#LECTURE_II">LIGHTNING CONDUCTORS.</a> -</td></tr> -<tr><td class="tdl" colspan="2"> - Destructive Effects of Lightning—Destruction of Buildings—Destruction of - Ships at Sea—Destruction of Powder Magazines—Experimental Illustrations—Destruction - of Life by Lightning—The Return Shock—Franklin’s - Lightning Rods—Introduction of Lightning Rods into England—The Battle - of Balls and Points—Functions of a Lightning Conductor—Conditions - of a Lightning Conductor—Mischief Done by Bad Conductors—Evil Effects - of a Bad Earth Contact—Danger from Rival Conductors—Insulation of - Lightning Conductors—Personal Safety in a Thunder Storm—Practical - Rules—Security Afforded by Lightning Rods -</td></tr> -<tr><td class="tdl"> -<a href="#APPENDIX">APPENDIX.</a> -</td><td class="tdr page"><a href="#Page_55">Pages 55-62</a> -</td></tr> -<tr><td class="tdc" colspan="2"> -<a href="#APPENDIX">RECENT CONTROVERSY ON LIGHTNING CONDUCTORS.</a> -</td></tr> -<tr><td class="tdl" colspan="2"> - Theory of Lightning Conductors Challenged—Lectures of Professor Lodge—Short - Account of his Views and Arguments—Effect of Self-Induction on a - Lightning Rod—Experiment on the Discharge of a Leyden Jar—Outer Shell - only of a Lightning Rod Acts as a Conductor—Discussion at the Meeting of<span class="pagenum" id="Page_4">[Pg 4]</span> - the British Association, September, 1888—Statement by <abbr title="mister">Mr.</abbr> Preece—Lord - Rayleigh and Sir William Thomson—Professor Rowland and Professor - Forbes—M. de Fonvielle, Sir James Douglass, and <abbr title="mister">Mr.</abbr> Symons—Reply of - Professor Lodge—Concluding Remarks of Professor Fitzgerald, President - of the Section—Summary Showing the Present State of the Question -</td></tr> -</table> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<h2 class="nobreak" id="LIST_OF_ILLUSTRATIONS">LIST OF ILLUSTRATIONS.</h2> -</div> - - -<table class="autotable"> -<tr><th></th><th class="tdr">PAGE</th></tr> -<tr><td class="tdl"> -<a href="#img002"><span class="smcap">The Electric Spark: A Type of a Flash of Lightning</span>,</a> -</td><td class="tdr page"> -<a href="#Page_8">8</a> -</td></tr> -<tr><td class="tdl"> -<a href="#img003"><span class="smcap">Cardboard Disc with Black and White Sectors; as Seen when at Rest</span>,</a> -</td><td class="tdr page"> -<a href="#Page_12">12</a> -</td></tr> -<tr><td class="tdl"> -<a href="#img004"><span class="smcap">Same Disc; as Seen when in Rapid Rotation</span>,</a> -</td><td class="tdr page"> -<a href="#Page_12">12</a> -</td></tr> -<tr><td class="tdl"> -<a href="#img005"><span class="smcap">The Brush Discharge, Illustrating <abbr title="saint">St.</abbr> Elmo’s Fire</span>,</a> -</td><td class="tdr page"> -<a href="#Page_17">17</a> -</td></tr> -<tr><td class="tdl"> -<a href="#img006"><span class="smcap">Origin of Successive Peals of Thunder</span>,</a> -</td><td class="tdr page"> -<a href="#Page_22">22</a> -</td></tr> -<tr><td class="tdl"> -<a href="#img007"><span class="smcap">Variations of Intensity in a Peal of Thunder</span>,</a> -</td><td class="tdr page"> -<a href="#Page_24">24</a> -</td></tr> -<tr><td class="tdl"> -<a href="#img008"><span class="smcap">Discharge of Leyden Jar Battery Through Thin Wires</span>,</a> -</td><td class="tdr page"> -<a href="#Page_27">27</a> -</td></tr> -<tr><td class="tdl"> -<a href="#img009"><span class="smcap">Glass Vessel Broken by Discharge of Leyden Jar Battery</span>,</a> -</td><td class="tdr page"> -<a href="#Page_32">32</a> -</td></tr> -<tr><td class="tdl"> -<a href="#img010"><span class="smcap">Gun Cotton Set on Fire by Electric Spark</span>,</a> -</td><td class="tdr page"> -<a href="#Page_33">33</a> -</td></tr> -<tr><td class="tdl"> -<a href="#img011"><span class="smcap">Volta’s Pistol; Explosion Caused by Electric Spark</span>,</a> -</td><td class="tdr page"> -<a href="#Page_34">34</a> -</td></tr> -<tr><td class="tdl"> -<a href="#img012"><span class="smcap">The Return Shock Illustrated</span>,</a> -</td><td class="tdr page"> -<a href="#Page_35">35</a> -</td></tr> -<tr><td class="tdl"> -<a href="#img013"><span class="smcap">Protection from Lightning by a Closed Conductor</span>,</a> -</td><td class="tdr page"> -<a href="#Page_48">48</a> -</td></tr> -<tr><td class="tdl"> -<a href="#img014"><span class="smcap">Induction Effect of Leyden Jar Discharge</span>,</a> -</td><td class="tdr page"> -<a href="#Page_56">56</a> -</td></tr> -</table> - -<p> -<span class="pagenum" id="Page_5">[Pg 5]</span></p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<h2 class="nobreak" id="LECTURE_I">LECTURE I.<br /><span class="small">LIGHTNING AND THUNDER.</span></h2> -</div> - - -<p>The electricity produced by an ordinary electric machine exhibits, -under certain conditions, phenomena which bear a striking resemblance -to the phenomena attendant on lightning. In both cases there is a flash -of light; in both there is a report, which, in the case of lightning, -we call thunder; and, in both cases, intense heat is developed, which -is capable of setting fire to combustible bodies. Further, the spark -from an electric machine travels through space with extraordinary -rapidity, and so does a flash of lightning; the spark follows a zig-zag -course, and so does a flash of lightning; the spark moves silently and -harmlessly through metal rods and stout wires, while it forces its -way, with destructive effect, through bad conductors, and it is so, -too, with a flash of lightning. Lastly, the electricity of a machine -is capable of giving a severe shock to the human body; and we know -that lightning gives a shock so severe as usually to cause immediate -death. For these reasons it was long conjectured by scientific men -that lightning is, in its nature, identical with electricity; and that -it differs from the electricity of our machines only in this, that it -exists in a more powerful and destructive form.</p> - - -<p><b>Identity of Lightning and Electricity.</b>—But it was reserved -for the celebrated Benjamin Franklin to demonstrate the truth of this -conjecture by direct experiment. He first conceived the idea of drawing -electricity from a thundercloud in the same way as it is drawn from -the conductor of an electric machine. For this purpose he proposed -to place a kind of sentry-box on the summit of a lofty tower, and to -erect, on the sentry-box, a metal rod, projecting twenty or thirty -feet upward into the air, pointed at the end, and having no electrical -communication with the earth. He predicted that when a thundercloud -would pass over the tower, the metal rod would become charged with -electricity, and that an observer, stationed in the sentry-box, might -draw from it, at pleasure, a succession of electric sparks.</p> - -<p>With the magnanimity of a really great man, Franklin published this -project to the world; being more solicitous to extend the domain of -science by new discoveries, than to secure for himself the glory -of<span class="pagenum" id="Page_6">[Pg 6]</span> having made them. The project was set forth in a letter to <abbr title="mister">Mr.</abbr> -Collinson, of London, which bears date July 29, 1750, and which, in the -course of a year or two, was translated into the principal languages -of Europe. Two years later the experiment suggested by Franklin was -made by Monsieur Dalibard, a wealthy man of science, at his villa near -Marly-la-Ville, a few miles from Paris. In the middle of an elevated -plain Monsieur Dalibard erected an iron rod, forty feet in length, one -inch in diameter, and ending above in a sharp steel point. The iron rod -rested on an insulating support, and was kept in position by means of -silk cords.</p> - -<p>In the absence of Monsieur Dalibard, who was called by business to -Paris, this apparatus was watched by an old dragoon, named Coiffier; -and on the afternoon of the tenth of May, 1752, he drew sparks from the -lower end of the rod at the time that a thundercloud was passing over -the neighborhood. Conscious of the importance that would be attached to -this phenomenon, the old dragoon summoned, in all haste, the prior of -Marly to come and witness it. The prior came without delay, and he was -followed by some of the principal inhabitants of the village. In the -presence of the little group, thus gathered together, the experiment -was repeated—electric sparks were again drawn, in rapid succession, -from the iron rod; the prediction of Franklin was fulfilled to the -letter; and the identity of lightning and electricity was, for the -first time, demonstrated to the world.</p> - - -<p><b>Franklin’s Experiment.</b>—Meanwhile Franklin had been waiting, -with impatience, for the completion of the tower of Christchurch, in -Philadelphia, on which he intended to make the experiment himself. -He even collected money, it is said, to hasten on the building. -But, notwithstanding his exertions, the progress of the tower was -slow; and his active mind, which could ill brook delay, hit upon -another expedient, remarkable alike for its simplicity and for its -complete success. He constructed a boy’s kite, using, however, a silk -pockethandkerchief, instead of paper, that it might not be damaged by -rain. To the top of the kite he attached a pointed iron wire about a -foot long, and he provided a roll of hempen twine, which he knew to be -a conductor of electricity, for flying it. This was the apparatus with -which he proposed to explore the nature of a thundercloud.</p> - -<p>The thundercloud came late in the afternoon of the fourth of July, -1752, and Franklin sallied out with his kite, accompanied by his son, -and taking with him a common door-key and a Leyden jar. The kite -was soon high in air, and the philosopher awaited the result of his -experiment, standing, with his son, under the lee of a cowshed, partly -to protect himself from the rain that was coming, and partly, it is -said, to shield himself from the ridicule of passers-by, who, having -no sympathy with his philosophical speculations, might be inclined to -regard him as a lunatic. To guard against the danger of receiving a<span class="pagenum" id="Page_7">[Pg 7]</span> -flash of lightning through his body, he held the kite by means of a -silk ribbon, which was tied to the door-key, the door-key being itself -attached to the lower end of the hempen string.</p> - -<p>A flash of lightning soon came from the cloud, and a second, and a -third; but no sign of electricity could be observed in the kite, or -the hempen cord, or the key. Franklin was almost beginning to despair -of success, when suddenly he noticed that the little fibres of the -cord began to bristle up, just as they would if it were placed near an -electric machine in action. He presented the door-key to the knob of -the Leyden jar, and a spark passed between them. Presently a shower -began to fall; the cord, wetted by the rain, became a better conductor -than it had been before, and sparks came more freely. With these sparks -he now charged the Leyden jar, and found, to his intense delight, that -he could exhibit all the phenomena of electricity by means of the -lightning he had drawn from the clouds.</p> - -<p>In the following year a similar experiment, with even more striking -results, was carried out, in France, by de Romas. Though it is said he -had no knowledge of what Franklin had done in America, he, too, used -a kite; and, with a view of making the string a better conductor, he -interlaced with it a thin copper wire. Then, flying his kite in the -ordinary way, when it had risen to a height of about 550 feet, he drew -sparks from it which, we are told, were upwards of nine feet long, and -emitted a sound like the report of a pistol.</p> - - -<p><b>Fatal Experiment of Richman.</b>—There can be no doubt that -experiments of this kind, made with the electricity of a thundercloud, -were extremely dangerous; and this was soon proved by a fatal accident. -Professor Richman, of <abbr title="saint">St.</abbr> Petersburgh, had erected on the roof of his -house a pointed iron rod, the lower end of which passed into a glass -vessel, intended, as we are informed, to measure the strength of the -charge which he expected to receive from the clouds. On the sixth of -August, 1753, observing the approach of a thunderstorm, he hastened -to his apparatus; and as he stood near it, with his head bent down, -to watch the effect, a flash of lightning passed through his body and -killed him on the spot. This catastrophe served to fix public attention -on the danger of such experiments, and gave occasion to the saying of -Voltaire: “There are some great lords whom we should always approach -with extreme precaution, and lightning is one of them.”<a id="FNanchor_1" href="#Footnote_1" class="fnanchor">[1]</a> From this -time the practice of making experiments directly with the lightning of -the clouds seems to have been, by common consent, abandoned.</p> - - -<p><b>Immediate Cause of Lightning.</b>—And now, having set before -you some of the most memorable experiments by which the identity of -lightning and electricity has been demonstrated, I will try to give<span class="pagenum" id="Page_8">[Pg 8]</span> -you a clear conception regarding the immediate cause of lightning, -so far as the subject is understood at the present day by scientific -men. You know that there are two kinds of electricity, which are -called <em>positive</em> and <em>negative</em>; and that each of them -repels electricity of the same kind as itself, while it attracts -electricity of the opposite kind. Now, every thundercloud is charged -with electricity of one kind or the other, positive or negative; -and, as it hovers over the earth, it develops, by what is called -<em>induction</em>, or influence, electricity of the opposite kind in -that part of the earth which is immediately under it. Thus we have -two bodies—the cloud and the earth—charged with opposite kinds of -electricity, and separated by a stratum of the atmosphere. The two -opposite electricities powerfully attract each other; but for a time -they are prevented from rushing together by the intervening stratum of -air, which is a non-conductor of electricity, and acts as a barrier -between them. As the electricity, however, continues to accumulate, the -attraction becomes stronger and stronger, until at length it is able to -overcome the resistance of this barrier; a violent disruptive discharge -then takes place between the cloud and the earth, and the flash of -lightning is the consequence of the discharge.</p> -<p class="center p2"><span class="figcenter" id="img002"> - <img src="images/002.jpg" class="w50" alt="THE ELECTRIC SPARK; A TYPE OF A FLASH OF LIGHTNING." /> -</span></p> -<p class="center caption">THE ELECTRIC SPARK; A TYPE OF A FLASH OF LIGHTNING.<br /></p> - - -<p>The whole phenomenon may be illustrated, on a small scale, by means -of this electric machine of Carré’s which you see before you. When -my assistant turns the handle of the machine negative electricity is -developed in that large brass cylinder, which in our experiment will -represent the thundercloud. At a distance of five or six inches from -the cylinder I hold a brass ball, which is in electrical communication -with the earth through my body. The electrified brass cylinder acts -by induction, or influence on the brass ball, and develops in it, as -well as in my body, a charge of positive electricity. Now, the positive -electricity of the ball and the negative electricity of the cylinder -are mutually attracting each other, but the intervening stratum of air -offers a resistance which prevents a discharge from taking place. My -assistant, however, continues to work the machine; the two opposite -electricities rapidly accumulate on the cylinder and the ball; at -length their mutual attraction is strong enough to overcome<span class="pagenum" id="Page_9">[Pg 9]</span> the -resistance interposed between them; a disruptive discharge follows, -and at the same moment a spark is seen to pass, accompanied by a sharp -snapping report.</p> - -<p>This spark is a miniature flash of lightning; and the snapping report -is a diminutive peal of thunder. Furthermore, at the moment the spark -passes you may observe a slight convulsive movement in my hand and -wrist. This convulsive movement represents, on a small scale, the -violent shock, generally fatal to life, which is produced by a flash of -lightning when it passes through the body.</p> - -<p>I can continue to take sparks from the conductor as long as the machine -is worked; and it is interesting to observe that these sparks follow -an irregular zig-zag course, just as lightning does. The reason is the -same in both cases: a discharge between two electrified bodies takes -place along the line of least resistance; and, owing to the varying -condition of the atmosphere, as well as of the minute particles of -matter floating in it, the line of least resistance is almost always a -zig-zag line.</p> - - -<p><b>What a Flash of Lightning is.</b>—Lightning, then, may be conceived -as an electrical discharge, sudden and violent in its character, which -takes place, through the atmosphere, between two bodies highly charged -with opposite kinds of electricity. Sometimes this electrical discharge -passes, as I have said, between a cloud and the earth; sometimes it -passes between one cloud and another; sometimes, on a smaller scale, -it takes place, between the great mass of a cloud and its outlying -fragments.</p> - -<p>But, if you ask me in what the discharge itself consists, I am utterly -unable to tell you. It is usual to speak and write on this subject as -if electricity were a material substance, a very subtle fluid, and as -if, at the moment the discharge takes place, this fluid passes like a -rapid stream, from the body that is positively electrified to the body -that is negatively electrified. But we must always remember that this -is only a conventional mode of expression, intended chiefly to assist -our conceptions, and to help us to talk about the phenomena. It does -not even profess to represent the objective truth. All that we know -for certain is this: that immediately before the discharge the two -bodies are highly electrified with opposite kinds of electricity; and, -that immediately after the discharge, they are found to have returned -to their ordinary condition, or, at least, to have become less highly -electrified than they were before.</p> - -<p>The flash of light that accompanies an electric discharge is often -supposed to be the electricity itself, passing from one body to -the other. But it is not; it is simply an effect produced by the -discharge. Heat is generated by the expenditure of electrical energy, -in overcoming the resistance offered by the atmosphere; and this heat -is so intense, that it produces a brilliant incandescence along the -path of<span class="pagenum" id="Page_10">[Pg 10]</span> the discharge. When a spark appears, for example, between the -conductor of the machine and this brass ball, it can be shown, by very -satisfactory evidence, that minute particles of these solid bodies are -first converted into vapor, and then made to glow with intense heat. -The gases, too, of which the air is composed, and the solid particles -floating in the air, are likewise raised to incandescence. So, too, -with lightning; the flash of light is due to the intense heat generated -by the electrical discharge, and owes its character to the composition -and the density of the atmosphere through which the discharge passes.</p> - - -<p><b>Duration of a Flash of Lightning.</b>—How long does a flash of -lightning last? You are aware, I dare say, that when an impression -of light is made on the eye, the impression remains for a sensible -interval of time, not less than the tenth of a second, after the source -of light has been extinguished or removed. Hence we continue, in fact, -to see the light, for at least the tenth of a second, after the light -has ceased. Now, if you reflect how brief is the moment for which a -flash of lightning is visible, and if you deduct the tenth of a second -from that brief moment, you will see, at once, that the period of its -actual duration must be very short indeed.</p> - -<p>The exact duration of a flash of lightning is a question on which no -settled opinion has yet been accepted generally by scientific men. -Indeed, the most widely different statements have been made on the -subject, quite recently, by the highest authorities, each speaking -apparently with unhesitating confidence. Thus, for example, Professor -Mascart describes an experiment, which he says was made by Wheatstone, -and which showed that a flash of lightning lasts for less than -<em>one</em>-thousandth of a second;<a id="FNanchor_2" href="#Footnote_2" class="fnanchor">[2]</a> Professor Everett describes the -same experiment, without saying by whom it was made, and gives, as the -result, that “the duration of the illumination produced by lightning -is certainly less than the <em>ten</em>-thousandth of a second;”<a id="FNanchor_3" href="#Footnote_3" class="fnanchor">[3]</a> -Professor Tyndall, in his own picturesque way, tells us that “a flash -of lightning cleaves a cloud, appearing and disappearing in less -than the <em>hundred</em>-thousandth of a second;”<a id="FNanchor_4" href="#Footnote_4" class="fnanchor">[4]</a> and according to -Professor Tait, of Edinburgh, “Wheatstone has shown that lightning -certainly lasts less than the <em>millionth</em> of a second.”<a id="FNanchor_5" href="#Footnote_5" class="fnanchor">[5]</a></p> - - -<p><b>Experiments of Professor Rood.</b>—I cannot say which of these -statements is best supported by actual observation; for none of the -writers I have quoted gives any reference to the original memoir from -which his statement is derived. As far as my own reading goes, I have -only come across one original record of experiments, made directly on -the flash of lightning itself, with a view to determine the period of -its duration. These experiments were carried out by<span class="pagenum" id="Page_11">[Pg 11]</span> Professor Ogden -Rood, of Columbia College, New York, between the years 1870 and 1873, -and are recorded in the <i>American Journal of Science and Arts</i>.<a id="FNanchor_6" href="#Footnote_6" class="fnanchor">[6]</a></p> - -<p>For the description of his apparatus, and for the details of his -observations, I must refer you to the memoir itself; but I may tell you -briefly that the results at which he arrived, if they be accepted, must -lead to a considerable modification of the views previously entertained -on the subject. In the first place, he satisfied himself that what -appears to the eye a single flash of lightning is usually, if not -always, multiple in its character; consisting, in fact, of a succession -of distinct flashes, which follow one another with such rapidity as -to make a continuous impression on the retina. Next, he proceeded to -measure approximately the duration of these several component flashes; -and he found that it varied over a wide range, amounting sometimes to -fully the twentieth of a second, and being sometimes less than the -sixteen-hundredth of a second.</p> - - -<p><b>Wheatstone’s Experiments.</b>—These results are extremely -interesting; but we can hardly regard them as finally established, -until they have been confirmed by other observers. I may remark, -however, that they fit in very well with the experiments made by -Professor Wheatstone, many years ago, on the duration of the electric -spark, which, as I told you, is a miniature flash of lightning. In -these classical experiments, which leave nothing to be desired in point -of accuracy, Professor Wheatstone showed that a spark taken directly -from a Leyden jar, or a spark taken from the conductor of a powerful -electric machine, that is, just such a spark as you have seen here -to-day, lasts for less than the millionth of a second.</p> - -<p>But he also showed that the duration of the spark is greatly increased, -when a resisting wire is introduced into the path of the discharge. -Thus, for example, when the discharge from a Leyden jar was made to -pass through half a mile of copper wire, with breaks at intervals, the -sparks that appeared at these breaks were found to last for ¹⁄₂₄₀₀₀ -of a second.<a id="FNanchor_7" href="#Footnote_7" class="fnanchor">[7]</a> Hence we should naturally expect that the period of -illumination would be still further increased, in the case of a flash -of lightning, where the resistance interposed is enormously greater -than in either of the experiments made by Wheatstone.<a id="FNanchor_8" href="#Footnote_8" class="fnanchor">[8]</a></p> - - -<p><b>Experiment of the Rotating Disk.</b>—It would be tedious, on an -occasion like the present, to enter into an account of Wheatstone’s -beautiful and ingenious method of investigation, by which the above -facts have been established; but I will show you a much more simple -experiment which brings home very forcibly to the mind how<span class="pagenum" id="Page_12">[Pg 12]</span> exceedingly -short must be the duration of the electric spark. Here is a circular -disk of cardboard, the outer part of which, as you see, is divided into -sectors, black and white alternately, while the space about the centre -is entirely white. The disk is mounted on a stand, by means of which -I can make it rotate with great velocity. When it is put in rotation, -the effect on the eye is very striking—the central space remains white -as before, but in the outer rim the distinction of black and white -absolutely disappears and gives place to a uniform gray. This color is -due to the blending together of black and white in equal proportions; -the blending being effected, not on the cardboard disk, but on the -retina of the eye.</p> - -<p class="center p2"><span class="figcenter" id="img003"> - <img src="images/003.jpg" class="w25" alt="CARDBOARD DISK AS SEEN WHEN AT REST." /> -</span></p> -<p class="center caption">CARDBOARD DISK AS SEEN WHEN AT REST.<br /></p> - - - -<p class="center p2"><span class="figcenter" id="img004"> - <img src="images/004.jpg" class="w25" alt="SAME DISK AS SEEN WHEN IN RAPID ROTATION." /> -</span></p> -<p class="center caption">SAME DISK AS SEEN WHEN IN RAPID ROTATION.<br /></p> - - - -<p>I mentioned just now that an impression made on the retina lasts for -the tenth of a second after the cause of it has been removed. Now, when -this disk is in rotation, the sectors follow one another so rapidly -that the particular part of space occupied at any moment by a white -sector will be occupied by a black sector within a time much less than -the tenth of a second. It follows that the impression made by each -white sector remains on the retina until the following black sector -comes into the same position; and, in like manner, the impression made -by each black sector remains until the following white sector takes up -the position of the black. Therefore, the impression made by the whole -outer rim is the impression of black and white combined—that is, the -impression of gray.</p> - -<p>So far, I dare say, the phenomenon is already familiar to you all. -But I propose now to show you the revolving disk illuminated by -the<span class="pagenum" id="Page_13">[Pg 13]</span> electric spark; and you will observe that, at the moment of -illumination, the black and white sectors come out as clearly and -distinctly as if the disk were standing still.</p> - -<p>For the success of this experiment it is desirable, not only to have -a brilliant spark in order to secure a good illumination of the disk, -but also to have a succession of such sparks, that you may see the -phenomenon frequently repeated, and thus be able to observe it at your -leisure. To attain these two objects, I have made the arrangement which -is here before you.</p> - -<p>In front of the disk is a large and very powerful Leyden jar. The -rod connected with the inner coating rises well above the mouth of -the jar, and ends in a brass ball nearly opposite the centre of the -disk. Connected with the outer coating of the jar is another rod -which likewise ends in a brass ball, and which is so adjusted that -the distance between the two balls is about an inch. The two rods are -connected respectively with the two conductors of a Holtz machine, so -that, when the machine is worked, the jar is first quickly charged, -and then it discharges itself, with a brilliant spark, between the -two brass balls. Thus, by continuing to work the machine, we can get, -as long as we choose, a succession of sparks following one another at -short and regular intervals right in front of the disk.</p> - -<p>Everything being now ready, and the room partially darkened, the -disk is put in rapid rotation; and you can see, by the twilight that -remains, the outer rim a uniform gray, and the central space white. -But when my assistant begins to turn the Holtz machine, and brilliant -sparks leap out at intervals, the revolving disk, illuminated for a -moment at each discharge, seems to be standing still, and shows the -black and white sectors distinctly visible.</p> - -<p>The reason of this is clear: So brief is the moment for which the spark -endures, that the disk, though in rapid motion, makes no sensible -advance during that small fraction of time; therefore, in the image on -the retina, the impression made by the white sectors remains distinct -from the impression made by the black, and the eye sees the disk as it -really is.</p> - -<p>I may notice, in passing, a very interesting consideration, suggested -by this experiment. A cannon ball is now commonly discharged with a -velocity of about 1,600 feet a second. Moving with this velocity it -is, as you know, under ordinary circumstances, altogether invisible to -the eye. But suppose it were illuminated, in the darkness of night, -by this electric spark, which lasts, we will say, for the millionth -of a second. During the moment of illumination, the cannon ball moves -through the millionth part of 1,600 feet, which is a little less than -the fiftieth of an inch. Practically, we may say that the cannon ball -does not sensibly change its place while the spark lasts. Further, the -impression it makes on the eye, from the position it occupies at the<span class="pagenum" id="Page_14">[Pg 14]</span> -moment of illumination, remains on the retina for at least the tenth -of a second. Therefore, if we are looking toward that particular part -of space where the cannon ball happens to be at the moment the spark -passes, we must see the cannon ball hanging motionless in the air, -though we know it is traveling at the rate of 1,600 feet a second, or -about 1,000 miles an hour.</p> - - -<p><b>Brightness of a Flash of Lightning.</b>—I should like to say one -word about the brightness of a flash of lightning. Somewhat more than -thirty years ago, Professor Swan, of Edinburgh, showed that the eye -requires a sensible time—about the tenth of a second—to perceive -the full brightness of a luminous object. Further, he proved, by a -series of interesting experiments, that when a flash of light lasts -for less than the tenth of a second, its apparent brilliancy to the -eye is proportional to the time of its duration.<a id="FNanchor_9" href="#Footnote_9" class="fnanchor">[9]</a> Now consider the -consequence of these facts in reference to the brightness of our -electric spark. If the spark lasted for the tenth of a second, we -should perceive its full brightness; if it lasted for the tenth part of -that time, we should see only the tenth part of its brightness; if it -lasted for the hundredth part, we should see only the hundredth part -of its brightness; and so on. But we know, in point of fact, that it -lasts for less than the millionth of a second, that is, less than the -hundred-thousandth part of the tenth of a second. Therefore we see only -the hundred-thousandth part of its real brightness.</p> - -<p>Here is a startling conclusion, and one, I may say, fully justified -by scientific evidence. That electric spark, brilliant as it appears -to us, is really a hundred thousand times as bright as it seems to -be. We cannot speak with the same precision of a flash of lightning; -because its duration has not yet been so exactly determined. But if we -suppose that a flash of lightning, in a particular case, lasts for the -thousandth of a second, it would follow, from the above experiments, -that the flash is a hundred times as bright, in fact, as it appears to -the eye.</p> - - -<p><b>Various Forms of Lightning.</b>—The lightning of which I have -spoken hitherto is commonly called <em>forked</em> lightning; a name -which seems to have been derived from the zig-zag line of light -it presents to the eye. But there are other forms under which the -electricity of the clouds often makes itself manifest; and to these I -would now invite your attention for a few moments. The most common of -them all, at least in this country, is that which is familiarly known -by the name of <em>sheet</em> lightning. This is, probably, nothing else -than the lighting up of the atmosphere, or of the clouds, by forked -lightning, which is not itself directly visible.</p> - -<p>Generally speaking, after a flash of sheet lightning, we hear the -rolling of distant thunder. But it sometimes happens, especially -in<span class="pagenum" id="Page_15">[Pg 15]</span> summer time, that the atmosphere is again and again lit up by a -sudden glow of light, and yet no thunder is heard. This phenomenon is -commonly called <em>summer</em> lightning, or <em>heat</em> lightning. It -is probably due, in many cases, to electrical discharges in the higher -regions of the atmosphere, where the air is greatly rarified; and, -in these cases, it would seem to resemble the discharges obtained by -means of an induction coil in glass tubes containing rarified gases. -But there is little doubt that in many cases, too, summer lightning, -like ordinary sheet lightning, is due to forked lightning, which is so -remote that we can neither see the flash itself directly, nor hear the -rolling of the thunder.</p> - -<p>Perhaps the most distinct and satisfactory evidence on this subject, -derived from actual observation, is contained in the following letter -of Professor Tyndall, written in May, 1883: “Looking to the south -and south-east from the Bel Alp, the play of silent lightning among -the clouds and mountains is sometimes very wonderful. It may be seen -palpitating for hours, with a barely appreciable interval between -the thrills. Most of those who see it regard it as lightning without -thunder—Blitz ohne Donner, Wetterleuchten, I have heard it named by -German visitors. The Monte Generoso, overlooking the Lake of Lugano, is -about fifty miles from the Bel Alp, as the crow flies. The two points -are connected by telegraph; and frequently when the Wetterleuchten, -as seen from the Bel Alp, was in full play, I have telegraphed to the -proprietor of the Monte Generoso Hotel and learned, in every instance, -that our silent lightning co-existed in time with a thunderstorm more -or less terrific in upper Italy.”<a id="FNanchor_10" href="#Footnote_10" class="fnanchor">[10]</a></p> - -<p>Another form of lightning, described by many writers, is called -<em>globe</em> lightning. It is said to appear as a ball of fire, about -the size of a child’s head, or even larger, which moves for a time -slowly about, and then, after the lapse of several seconds, explodes -with a terrific noise, sending forth flashes of fire in all directions, -which burn whatever they strike. Many accounts are on record of such -phenomena; but they are derived, for the most part, from the evidence -of persons who were not specially competent to observe, and to describe -with precision, the facts that fell under their observation. Hence -these accounts, while they are accepted by some, are rejected by -others; and it seems to me, in the present state of the question, -that the existence of globe lightning can hardly be regarded as a -demonstrated fact. At all events, if phenomena of this kind have really -occurred, I can only say that nothing we know about electricity, at -present, will enable us to account for them.<a id="FNanchor_11" href="#Footnote_11" class="fnanchor">[11]</a></p> - - -<p><b><abbr title="saint">St.</abbr> Elmo’s Fire.</b>—A much more authentic and, at the same -time,<span class="pagenum" id="Page_16">[Pg 16]</span> very interesting form, under which the electricity of the -clouds sometimes manifests its presence, is known by the name of <abbr title="saint">St.</abbr> -Elmo’s fire. This phenomenon at one time presents the appearance of -a star, shining at the points of the lances or bayonets of a company -of soldiers; at another, it takes the form of a tuft of bluish light, -which seems to stream away from the masts and spars of a ship at -sea, or from the pointed spire of a church. It was well known to the -ancients. Cæsar, in his Commentaries, tells us that, after a stormy -night, the iron points of the javelins of the fifth legion seemed to be -on fire; and Pliny says that he saw lights, like stars, shining on the -lances of the soldiers, keeping watch by night upon the ramparts. When -two such lights appeared at once, on the masts of a ship, they were -called Castor and Pollux, and were regarded by sailors as a sign of a -prosperous voyage. When only one appeared, it was called Helen, and was -taken as an unfavorable omen.</p> - -<p>In modern times <abbr title="saint">St.</abbr> Elmo’s fire has been witnessed by a host of -observers, and all its various phases have been repeatedly described. -In the memoirs of Forbin we read that, when he was sailing once, in -1696, among the Balearic Islands, a sudden storm came on during the -night, accompanied by lightning and thunder. “We saw on the vessel,” he -says, “more than thirty <abbr title="saint">St.</abbr> Elmo’s fires. Among the rest there was one -on the vane of the mainmast more than a foot and a half high. I sent a -man up to fetch it down. When he was aloft he cried out that it made -a noise like wetted gunpowder set on fire. I told him to take off the -vane and come down; but, scarcely had he removed it from its place, -when the fire left it and reappeared at the end of the mast, so that -it was impossible to take it away. It remained for a long time, and -gradually went out.”</p> - -<p>On the 14th of January, 1824, Monsieur Maxadorf happened to look at a -load of straw in the middle of a field just under a dense black cloud. -The straw seemed literally on fire—a streak of light went forth from -every blade; even the driver’s whip shone with a pale-blue flame. As -the black cloud passed away, the light gradually disappeared, after -having lasted about ten minutes. Again, it is related that on the 8th -of May, 1831, in Algiers, as the French artillery officers were walking -out after sunset without their caps, each one saw a tuft of blue light -on his neighbor’s head; and, when they stretched out their hands, a -tuft of light was seen at the end of every finger. Not infrequently a -traveler in the Alps sees the same luminous tuft on the point of his -alpenstock. And quite recently, during a thunderstorm, a whole forest -was observed to become luminous just before each flash of lightning, -and to become dark again at the moment of the discharge.<a id="FNanchor_12" href="#Footnote_12" class="fnanchor">[12]</a></p> - -<p><span class="pagenum" id="Page_17">[Pg 17]</span></p> - -<p>This phenomenon may be easily explained. It consists in a gradual and -comparatively silent electrical discharge between the earth and the -cloud; and generally, but not always, it has the effect of preventing -such an accumulation of electricity as would be necessary to produce -a flash of lightning. I can illustrate this kind of discharge with -the aid of our machine. If I hold a pointed metal rod toward the -large conductor, you can see, when the machine is worked and the room -darkened, how the point of the rod becomes luminous and shines like a -faint blue star. I substitute for the pointed rod the blunt handles of -a pair of pliers, and a tuft of blue light is at once developed at the -end of each handle, and seems to stream away with a hissing noise. I -now put aside the pliers, and open out my hand under the conductor—and -observe how I can set up, at pleasure, a luminous tuft at the tips -of my fingers. Now and then a spark passes, giving me a smart shock, -and showing how the electricity may sometimes accumulate so fast that -it cannot be sufficiently discharged by the luminous tuft. Lastly, I -present a small bushy branch of a tree to the conductor, and all its -leaves and twigs are aglow with bluish light, which ceases for a moment -when a spark escapes, to be again renewed when electricity is again -developed by the working of the machine.</p> - - -<p class="center p2"><span class="figcenter" id="img005"> - <img src="images/005.jpg" class="w50" alt="THE BRUSH DISCHARGE, ILLUSTRATING ST. ELMO’S FIRE." /> -</span></p> -<p class="center caption">THE BRUSH DISCHARGE, ILLUSTRATING ST. ELMO’S FIRE.<br /></p> - - - -<p>Now, if you put a thundercloud in the place of that conductor, you can -easily realize how, through its influence, the lance and bayonet of -the soldier, the alpenstock of the traveler, the pointed spire of a -church, the masts of a ship at sea, the trees of a forest, can all be -made to glow with a silent electrical discharge which may or may not, -according to circumstances, culminate at intervals in a genuine flash -of lightning.</p> - - -<p><b>Origin of Lightning.</b>—When we seek to account for the origin -of lightning, we are confronted at once with two questions of great -interest and importance—first, What are the sources from which the -electricity of the thundercloud is derived? and, secondly, How does -this electricity come to be developed in a form which so far transcends -in power the electricity of our machines? These questions have long<span class="pagenum" id="Page_18">[Pg 18]</span> -engaged the attention of scientific men, but I cannot say that they -have yet received a perfectly satisfactory solution. Nevertheless, -some facts of great scientific value have been established, and some -speculations have been put forward, which are well deserving of -consideration.</p> - -<p>In the first place, it is quite certain that the atmosphere which -surrounds our globe is almost always in a state of electrification. -Further, the electrical condition of the atmosphere would seem to be -as variable as the wind. It changes with the change of season; it -changes from day to day; it changes from hour to hour. The charge of -electricity is sometimes positive, sometimes negative; sometimes it -is strong, sometimes feeble; and the transition from one condition to -another is sometimes slow and gradual, sometimes sudden and violent.</p> - -<p>As a general rule, in fine, clear weather, the electricity of the -atmosphere is positive, and not very strongly developed. In wet weather -the charge may be either positive or negative, and is generally -strong, especially when there are sudden heavy showers. In fog it is -also strong, and almost always positive. In a snowstorm it is very -strong, and most frequently positive. Finally, in a thunderstorm it is -extremely strong, and generally negative; but it is subject to a sudden -change of sign, when a flash of lightning passes or when rain begins to -fall.</p> - -<p>So far I have simply stated facts, which have been ascertained -by careful observations, made at different stations by competent -observers, and extending over a period of many years. But as regards -the process by which the electricity of the atmosphere is developed, we -have, up to the present time, no certain knowledge. It has been said -that electricity may be generated in the atmosphere by the friction of -the air itself, and of the minute particles floating in it, against -the surface of the earth, against trees and buildings, against rocks, -cliffs, and mountains. But this opinion, however probable it may be, -has not yet been confirmed by any direct experimental investigation.</p> - -<p>The second theory is that the electricity of the atmosphere is due, in -great part at least, to the evaporation of salt water. Many years ago, -Pouillet, a French philosopher, made a series of experiments in the -laboratory, which seemed to show that evaporation is generally attended -with the development of electricity; and, in particular, he satisfied -himself that the vapor which passes off from the surface of salt water -is always positively electrified. Now, the atmosphere is everywhere -charged, more or less, with vapor which comes, almost entirely, from -the salt water of the ocean. Hence Pouillet inferred that the chief -source of atmospheric electricity is the evaporation of sea water. -This explanation would certainly go far to account for the presence -of electricity in the atmosphere, if the fact on which it<span class="pagenum" id="Page_19">[Pg 19]</span> rests were -established beyond dispute. But there is some reason to doubt whether -the development of electricity, in the experiments of Pouillet, was due -simply to the process of evaporation, and not rather to other causes, -the influence of which he did not sufficiently take into account.</p> - -<p>A conjecture has recently been started that electricity may be -generated by the mere impact of minute particles of water vapor against -minute particles of air.<a id="FNanchor_13" href="#Footnote_13" class="fnanchor">[13]</a> If this conjecture could be established as -a fact, it would be amply sufficient to account for all the electricity -of the atmosphere. From the very nature of a gas, the molecules of -which it is composed are forever flying about with incredible velocity; -and therefore the particles of water vapor and the particles of air, -which exist together in the atmosphere, must be incessantly coming -into collision. Hence, however small may be the charge of electricity -developed at each individual impact, the total amount generated over -any considerable area, in a single day, must be very great indeed. -It is evident, however, that this method of explaining the origin of -atmospheric electricity can only be regarded as, at best, a probable -hypothesis, until the assumption on which it rests is supported by the -evidence of observation or experiment.</p> - - -<p><b>Length of a Flash of Lightning.</b>—It would seem, then, that -we are not yet in a position to indicate with certainty the sources -from which the electricity of the atmosphere is derived. But whatever -these sources may be, there can be little doubt that the electricity -of the atmosphere is intimately associated with the minute particles -of water vapor of which the thundercloud is eventually built up. -This consideration is of great importance when we come to consider -the special properties of lightning, as compared with other forms of -electricity. The most striking characteristic of lightning is the -wonderful power it possesses of forcing its way through the resisting -medium of the air. In this respect it incomparably surpasses all forms -of electricity that have hitherto been produced by artificial means. -The spark of an ordinary electric machine can leap across a space of -three or four inches; the machine we have employed in our experiments -to-day can give, under favorable circumstances, a spark of nine or ten -inches; the longest electric spark ever yet produced artificially is -probably the spark of <abbr title="mister">Mr.</abbr> Spottiswoode’s gigantic induction coil; and -it does not exceed three feet six inches. But the length of a flash of -lightning is not to be measured in inches, or in feet or in yards; it -varies from one or two miles, for ordinary flashes, to eight or ten -miles in exceptional cases.</p> - -<p>This power of discharging itself violently through a resisting -medium, in which the thundercloud so far transcends the conductor of -an electric machine, is due to the property commonly known among<span class="pagenum" id="Page_20">[Pg 20]</span> -scientific men as electrical <em>potential</em>. The greater the distance -to which an electrified body can shoot its flashes through the air, the -higher must be its potential. Hence the potential of a thundercloud -must be exceedingly high, since its flashes can pierce the air to a -distance of several miles. And what I want to point out is, that we -are able to account for this exceedingly high potential, if we may -only assume that the minute particles of water vapor in the atmosphere -have, from any cause, received ever so small a charge of electricity. -The number of such particles that go to make up an ordinary drop of -rain are to be counted by millions of millions; and it is capable of -scientific proof that, as each new particle is added, in the building -up of the drop, a rise of potential is necessarily produced. It is -clear, therefore, that there is practically no limit to the potential -that may be developed by the simple agglomeration of very small cloud -particles, each carrying a very small charge of electricity.<a id="FNanchor_14" href="#Footnote_14" class="fnanchor">[14]</a></p> - -<p>This explanation, which traces the exceedingly high potential of -lightning to the building up of rain drops in the thundercloud, -suggests a reason why it so often happens that immediately after a -flash of lightning “the big rain comes dancing to the earth.” The -potential has been steadily rising as the drops have been getting -larger and larger, until at length the potential has become so high -that the thundercloud is able to discharge itself, and almost at the -same moment the drops have become so large that they can no longer be -held aloft against the attracting force of gravity.</p> - - -<p><b>Physical Cause of Thunder.</b>—Let us now proceed to consider -the phenomenon of thunder, which is so intimately associated with -lightning, and which, though perfectly harmless in itself, and though -never heard until the real danger is past, often excites more terror -in the mind than the lightning flash itself. The sound of thunder, -like that of the electric spark, is due to a disturbance caused in -the air by the electric discharge. The air is first expanded by the -intense heat that is developed along the line of discharge, and then it -rushes back again to fill up the partial vacuum which its expansion has -produced. This sudden movement gives rise to a series of sound waves, -which reach the ear in the form of thunder. But there are certain -peculiar characteristics of thunder which are deserving of special -consideration.</p> - - -<p><b>Rolling of Thunder.</b>—They may be classified, I think, under -two heads. First, the sound of thunder is not an instantaneous report -like the sound of the electric spark—it is a prolonged peal lasting, -sometimes, for several seconds. Secondly, each flash of lightning gives -rise, not to one peal only, but to a succession of peals following one -another at irregular intervals. These two phenomena, taken together, -produce that peculiar effect on the ear which is commonly<span class="pagenum" id="Page_21">[Pg 21]</span> described -as the <em>rolling</em> of thunder; and both of them, I think, may be -sufficiently accounted for in accordance with the well-established -properties of sound.</p> - -<p>To understand why the sound of thunder reaches the ear as a prolonged -peal, we have only to remember that sound takes time to travel. Since a -flash of lightning is practically instantaneous, we may assume that the -sound is produced at the same moment all along the line of discharge. -But the sound waves, setting out at the same moment from all points -along the line of discharge, must reach the ear in successive instants -of time, arriving first from that point which is nearest to the -observer, and last from that point which is most distant. Suppose, for -example, that the nearest point of the flash is a mile distant from the -observer, and the farthest point two miles—the sound will take about -five seconds to come from the nearest point, and about ten seconds to -come from the farthest point; and moreover, in each successive instant -from the time the first sound reaches the ear, sound will continue -to arrive from the successive points between. Therefore the thunder, -though instantaneous in its origin, will reach the ear as a prolonged -peal extending over a period of five seconds.</p> - - -<p><b>Succession of Peals.</b>—The succession of peals produced by -a single flash of lightning is due to several causes, each one of -which may contribute more or less, according to circumstances, toward -the general effect. First, if we accept the results arrived at by -Professor Ogden Rood, of Columbia College, what appears to the eye as a -single flash of lightning, consists, in fact, as a general rule, of a -succession of flashes, each one of which must naturally produce its own -peal of thunder; and although the several flashes, if they follow one -another at intervals of the tenth of a second, will make one continuous -impression on the eye, the several peals of thunder, under the same -conditions, will impress the ear as so many distinct peals.</p> - -<p>The next cause that I would mention is the zigzag path of the lightning -discharge. To make clear to you the influence of this circumstance, I -must ask your attention for a moment to the <a href="#img006">diagram</a> on next page. Let -the broken line represent the path of a flash of lightning, and let <span class="allsmcap">O</span> -represent the position of an observer. The sound will reach him first -from the point <span class="allsmcap">A</span>, which is nearest to him, and then it will -continue to arrive in successive instants from the successive points -along the line <span class="allsmcap">A N</span> and along the line <span class="allsmcap">A M</span>, thus -producing the effect of a continuous peal. Meanwhile the sound waves -have been traveling from the point <span class="allsmcap">B</span>, and in due time will -reach the observer at <span class="allsmcap">O</span>. Coming as they do in a different -direction from the former, they will strike the ear as the beginning -of a new peal which, in its turn, will be prolonged by the sound waves -arriving, in successive instants, from the successive points along the -line <span class="allsmcap">B M</span> and <span class="allsmcap">B H</span>. A little later,<span class="pagenum" id="Page_22">[Pg 22]</span> the sound will -arrive from the more distant point <span class="allsmcap">C</span>, and a third peal will -begin. And so there will be several distinct peals proceeding, so to -speak, from several distinct points in the path of the lightning flash.</p> - -<p class="center p2"><span class="figcenter" id="img006"> - <img src="images/006.jpg" class="w50" alt="ORIGIN OF SUCCESSIVE PEALS OF THUNDER." /> -</span></p> -<p class="center caption">ORIGIN OF SUCCESSIVE PEALS OF THUNDER.<br /></p> - -<p>A third cause to which the succession of peals may be referred is to -be found in the minor electrical discharges that must often take place -within the thundercloud itself. A thundercloud is not a continuous mass -like the metal cylinder of this electric machine—it has many outlying -fragments, more or less imperfectly connected with the principal body. -Moreover, the material of which the cloud is composed is only a very -imperfect conductor as compared with our brass cylinder. For these -two reasons it must often happen, about the time a flash of lightning -passes, that different parts of the cloud will be in such different -electrical conditions as to give rise to electrical discharges within -the cloud itself. Each of these discharges produces its own peal of -thunder; and thus we may have a number of minor peals, sometimes -preceding and sometimes following the great crash which is due to the -principal discharge.</p> - -<p>Lastly, the influence of echo has often a considerable share in -multiplying the number of peals of thunder. The waves of sound, going -forth in all directions, are reflected from the surfaces of mountains, -forests, clouds, and buildings, and coming back from different -quarters, and with varying intensity, reach the ear like the roar of -distant artillery. The striking effect of these reverberations in a -mountain district has been described by a great poet in words which, I -daresay, are familiar to most of you:</p> - -<p class="poetry"> -<span style="margin-left: 16em;">“Far along,</span><br /> -<span style="margin-left: 1em;">From peak to peak, the rattling crags among,</span><br /> -<span style="margin-left: 2em;">Leaps the live thunder! Not from one lone cloud,</span><br /> -<span style="margin-left: 1em;">But every mountain now has found a tongue,</span><br /> -<span style="margin-left: 2em;">And Jura answers from her misty shroud</span><br /> -<span style="margin-left: 2em;">Back to the joyous Alps, that call to her aloud!”</span><br /> -</p> - -<p><span class="pagenum" id="Page_23">[Pg 23]</span></p> - - -<p><b>Variations of Intensity in Thunder.</b>—From what has been said, -it is easy to understand how the general roar of thunder is subject -to great changes of intensity, during the time it lasts, according to -the number of peals that may be arriving at the ear of an observer in -each particular moment. But every one must have observed that even an -individual peal of thunder often undergoes similar changes, swelling -out at one moment with great power, and the next moment rapidly -dying away. To account for this phenomenon, I would observe, first, -that there is no reason to suppose that the disturbance caused by -lightning is of exactly the same magnitude at every point of its path. -On the contrary, it would seem very probable that the amount of this -disturbance is, in some way, dependent on the resistance which the -discharge encounters. Hence the intensity of the sound waves sent forth -by a flash of lightning is probably very different at different parts -of its course; and each individual peal will swell out on the ear or -die away, according to the greater or less intensity of the sound waves -that reach the ear in each successive moment of time.</p> - -<p>But there is another influence at work which must produce variations -in the loudness of a peal of thunder, even though the sound waves, set -in motion by the lightning, were everywhere of equal intensity. This -influence depends on the position of the observer in relation to the -path of the lightning flash. At one part of its course the lightning -may follow a path which remains for a certain length at nearly the -same distance from the observer; then all the sound produced along -this length will reach the observer nearly at the same moment, and -will burst upon the ear with great intensity. At another part, the -lightning may for an equal length go right away from the observer; and -it is evident that the sound produced along this length will reach the -observer in successive instants, and consequently produce an effect -comparatively feeble.</p> - -<p>With a view to investigate this interesting question a little more -closely, let me suppose the position of the observer taken as a -centre, and a number of concentric circles drawn, cutting the path of -the lightning flash, and separated from one another by a distance of -110 feet, measured along the direction of the radius. It is evident -that all the sound produced between any two consecutive circles will -reach the ear within a period which must be measured by the time that -sound takes to travel 110 feet, that is, within the tenth of a second. -Hence, in order to determine the quantity of sound that reaches the -ear in successive periods of one-tenth of a second, we have only to -observe how much is produced between each two consecutive circles. -But on the supposition that the sound waves, set in motion by the -flash of lightning, are of equal intensity at every point of its path, -it is clear that the quantity of sound developed between each<span class="pagenum" id="Page_24">[Pg 24]</span> two -consecutive circles will be simply proportional to the length of the -path enclosed between them.</p> - -<p>With these principles established, let us now follow the course of a -peal of thunder, in the diagram before us. This broken line, drawn -almost at random, represents the path of a flash of lightning; the -observer is supposed to be placed at <span class="allsmcap">O</span>, which is the centre of -the concentric circles; these circles are separated from one another by -a distance of 110 feet, measured in the direction of the radius; and we -want to consider how any one peal of thunder may vary in loudness in -the successive periods of one-tenth of a second.</p> - - - -<p class="center p2"><span class="figcenter" id="img007"> - <img src="images/007.jpg" class="w50" alt="VARIATIONS OF INTENSITY IN A PEAL OF THUNDER." /> -</span></p> -<p class="center caption">VARIATIONS OF INTENSITY IN A PEAL OF THUNDER.<br /></p> - -<p>Let us take, for example, the peal which begins when the sound waves -reach the ear from the point <span class="allsmcap">A</span>. In the first unit of time the -sound that reaches the ear is the sound produced along the lines <span class="allsmcap">A -B</span> and <span class="allsmcap">A C</span>; in the second unit, the sound produced along -the lines <span class="allsmcap">B D</span> and <span class="allsmcap">C E</span>; in the third unit, the sound -produced along <span class="allsmcap">D F</span> and <span class="allsmcap">E G</span>. So far the peal has been -fairly uniform in its intensity; though there has been a slight falling -off in the second and third units of time, as compared with the first. -But in the fourth unit there is a considerable falling away of the -sound; for the line <span class="allsmcap">F K</span> is only about one-third as long as -<span class="allsmcap">D F</span> and <span class="allsmcap">E G</span> taken together; therefore the quantity of -sound that reaches the ear in the fourth unit of time is only one-third -of that which reaches it in each of the three preceding units; and -consequently the sound is only one-third as loud. In the fifth unit, -however, the peal must rise to a sudden crash; for the portion of the -lightning path inclosed between the fifth and sixth circles is about -six times as great as that between the fourth and fifth; therefore -the intensity of the sound will be suddenly increased about six-fold. -After this sudden crash, the sound as suddenly dies away in the sixth -unit of time; it continues feeble as the path of the lightning goes -nearly straight away from the observer; it swells again slightly in the -ninth unit of time; and then continues without much variation to the -end. This is only a single illustration, but it seems quite sufficient -to show that the changes of intensity in a peal of<span class="pagenum" id="Page_25">[Pg 25]</span> thunder must be -largely due to the position of the spectator in relation to the several -parts of the lightning flash.</p> - - -<p><b>Distance of a Flash of Lightning.</b>—I need hardly remind you -that, by observing the interval that elapses between the flash of -lightning and the peal of thunder that follows it, we may estimate -approximately the distance of the nearest point of the discharge. -Light travels with such amazing velocity that we may assume, without -any sensible error, that we see the flash of lightning at the very -moment in which the discharge takes place. But sound, as we have seen, -takes a sensible time to travel even short distances; and therefore a -measurable interval almost always elapses between the moment in which -the flash is seen and the moment in which the peal of thunder first -reaches the ear. And the distance through which sound travels in this -interval will be the distance of the nearest point through which the -discharge has passed. Now, the velocity of sound in air varies slightly -with the temperature; but, at the ordinary temperature of our climate, -we shall not be far astray if we allow 1,100 feet for every second, or -about one mile for every five seconds.</p> - -<p>You will observe also that, by repeating this observation, we can -determine whether the thundercloud is coming toward us, or going away -from us. So long as the interval between each successive flash and the -corresponding peal of thunder, continues to get shorter and shorter, -the thundercloud is approaching; when the interval begins to increase, -the thundercloud is receding from us, and the danger is passed.</p> - -<p>The crash of thunder is terrific when the lightning is close at hand; -but it is a curious fact, that the sound does not seem to travel as -far as the report of an ordinary cannon. We have no authentic record -of thunder having been heard at a greater distance than from twelve to -fifteen miles, whereas the report of a single cannon has been heard -at five times that distance; and the roar of artillery, in battle, -at a greater distance still. On the occasion of the Queen’s visit to -Cherbourg, in August, 1858, the salute fired in honor of her arrival -was heard at Bonchurch, in the Isle of Wight, a distance of sixty -miles. It was also heard at Lyme Regis, in Dorsetshire, which is -eighty-five miles from Cherbourg, as the crow flies; and we are told -that, not only was it audible in its general effect, but the report of -individual guns was distinctly recognized. The artillery of Waterloo is -said to have been heard at the town of Creil, in France, 115 miles from -the field of battle; and the cannonading at the siege of Valenciennes, -in 1793, was heard, from day to day, at Deal, on the coast of England, -a distance of 120 miles.<a id="FNanchor_15" href="#Footnote_15" class="fnanchor">[15]</a></p> - -<p>So far, I have endeavored to set forth some general ideas on the nature -and origin of lightning, and of the thunder that accompanies<span class="pagenum" id="Page_26">[Pg 26]</span> it. In -my next Lecture I propose to give a short account of the destructive -effects of lightning, and to consider how these effects may best be -averted by means of lightning conductors.</p> - -<hr class="r5" /> -<h3><span class="smcap">Note to Page 20.</span></h3> - -<p class="center caption"><span class="smcap">On the High Potential of a Flash of Lightning.</span></p> -<div class="blockquot"> -<p>The potential of an electrified sphere is equal to the quantity of -electricity with which the sphere is charged, divided by the radius -of the sphere. Now the minute cloud particles, which go to make -up a drop of rain, may be taken to be very small spheres; and if -<i>v</i> represent the potential of each one, <i>q</i> the quantity -of electricity with which it is charged, and <i>r</i> the radius of -the sphere, we have <i>v</i> = <i>q</i>/<i>r</i>. Suppose 1,000 of -these cloud particles to unite into one; the quantity of electricity -in the drop, thus formed, will be 1,000<i>q</i>; and the radius, -which increases in the ratio of the cube root of the volume, will -be 10<i>r</i>. Therefore the potential of the new sphere will be -1000<i>q</i>/10<i>r</i>, or 100<i>q/r</i>; that is to say, it will be -100 times as great as the potential of each of the cloud particles -which compose it. When a million of cloud particles are blended -into a single drop, the same process will show that the potential -has been increased ten thousandfold; and when a drop is produced by -the agglomeration of a million of millions of cloud particles, the -potential of the drop will be a hundred million times as great as that -of the individual particles.<a id="FNanchor_16" href="#Footnote_16" class="fnanchor">[16]</a></p> - -</div> -<div class="footnotes"><h3>FOOTNOTES:</h3> - -<div class="footnote"> - -<p><a id="Footnote_1" href="#FNanchor_1" class="label">[1]</a> “Il y a des grands seigneurs dont il ne faut approcher -qu’avec d’extrêmes précautions. Le tonnerre est de ce nombre.”—Dict. -Philos. art. Foudre.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_2" href="#FNanchor_2" class="label">[2]</a> Electricité Statique, ii., 561.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_3" href="#FNanchor_3" class="label">[3]</a> Deschanel’s Natural Philosophy, Sixth Edition, <abbr title="page">p.</abbr> 641.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_4" href="#FNanchor_4" class="label">[4]</a> Fragments of Science, Fifth Edition, <abbr title="page">p.</abbr> 311.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_5" href="#FNanchor_5" class="label">[5]</a> Lecture on Thunderstorms, Nature, <abbr title="volume">vol.</abbr> xxii., <abbr title="page">p.</abbr> 341.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_6" href="#FNanchor_6" class="label">[6]</a> Third Series, <abbr title="volume">vol.</abbr> v., <abbr title="page">p.</abbr> 161.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_7" href="#FNanchor_7" class="label">[7]</a> Phil. Trans. Royal Society, 1834, <abbr title="volume">vol.</abbr> cxxv., <abbr title="pages">pp.</abbr> 583-591.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_8" href="#FNanchor_8" class="label">[8]</a> In experiments with a Leyden jar, Feddersen has shown that -the duration of the discharge is increased, not only by increasing the -striking distance, but also by increasing the size of the jar. Now, a -flash of lightning may be regarded as the discharge of a Leyden jar -of immense size, with an enormous striking distance; and therefore we -should expect that the duration of the discharge should be greatly -prolonged. See <i>American Journal of Science and Arts</i>, Third -Series, <abbr title="volume">vol.</abbr> i., <abbr title="page">p.</abbr> 15.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_9" href="#FNanchor_9" class="label">[9]</a> See original paper by Swan, Trans. Royal Society, -Edinburgh, 1849, <abbr title="volume">vol.</abbr> xvi., <abbr title="pages">pp.</abbr> 581-603; also, a second paper, -<i>ib.</i> 1861, <abbr title="volume">vol.</abbr> xxii., <abbr title="pages">pp.</abbr> 33-39.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_10" href="#FNanchor_10" class="label">[10]</a> Nature, <abbr title="volume">vol.</abbr> xxviii., <abbr title="page">p.</abbr> 54.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_11" href="#FNanchor_11" class="label">[11]</a> See, however, an attempt to account for this phenomenon -in De Larive’s Treatise on Electricity, London, 1853-8, <abbr title="volume">vol.</abbr> iii., -<abbr title="pages">pp.</abbr> 199, 200; and another, quite recently, by <abbr title="mister">Mr.</abbr> Spottiswoode, in -a Lecture on the Electrical Discharge, delivered before the British -Association at York, in September, 1881, and published by Longmans, -London, <abbr title="page">p.</abbr> 42. See also, for recent evidence regarding the phenomenon -itself, Scott’s Elementary Meteorology, <abbr title="pages">pp.</abbr> 175-8.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_12" href="#FNanchor_12" class="label">[12]</a> See Jamin, “Cours de Physique,” i., 480-1; Tomlinson, -“The Thunderstorm,” Third Edition, <abbr title="pages">pp.</abbr> 95-103; “Thunderstorms,” a -Lecture by Professor Tait, Nature, <abbr title="volume">vol.</abbr> xxii., <abbr title="page">p.</abbr> 356.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_13" href="#FNanchor_13" class="label">[13]</a> Professor Tait, On Thunderstorms, Nature, <abbr title="volume">vol.</abbr> xxii., <abbr title="pages">pp.</abbr> -436-7.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_14" href="#FNanchor_14" class="label">[14]</a> See note at the end of this Lecture, <a href="#Page_26"><abbr title="page">p.</abbr> 26</a>.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_15" href="#FNanchor_15" class="label">[15]</a> See Tomlinson, The Thunderstorm, <abbr title="pages">pp.</abbr> 87-9.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_16" href="#FNanchor_16" class="label">[16]</a> See Tait on Thunderstorms, Nature, <abbr title="volume">vol.</abbr> xxii., <abbr title="page">p.</abbr> 436.</p> - -</div> -</div> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<h2 class="nobreak" id="LECTURE_II">LECTURE II.<br /><span class="small">LIGHTNING CONDUCTORS.</span></h2> -</div> - - - -<p>The effects of lightning, on the bodies that it strikes, are analogous -to those which may be produced by the discharge of our electric -machines and Leyden jar batteries. When the discharge of a battery -traverses a metal conductor of sufficient dimensions to allow it an -easy passage, it makes its way along silently and harmlessly. But if -the conductor be so thin as to offer considerable resistance, then the -conductor itself is raised to intense heat, and may be melted, or even -converted into vapor, by the discharge.</p> - -<p>On opposite page is shown a board on which a number of very thin wires -have been stretched, over white paper, between brass balls. The wires -are so thin that the full charge of the battery before you, which -consists of nine large Leyden jars, is quite sufficient to convert them -in an instant into vapor. I have already, on former occasions, sent the -charge through two of these wires, and nothing remains of them now<span class="pagenum" id="Page_27">[Pg 27]</span> -but the traces of their vapor, which mark the path of the electric -discharge from ball to ball. At the present moment the battery stands -ready charged, and I am going to discharge it through a third wire, by -means of this insulated rod which I hold in my hand. The discharge has -passed; you saw a flash, and a little smoke; and now, if you look at -the paper, you will find that the wire is gone, but that it has left -behind the track of its incandescent vapor, marking the path of the -discharge.</p> - -<p class="center p2"><span class="figcenter" id="img008"> - <img src="images/008.jpg" class="w50" alt="DISCHARGE OF LEYDEN JAR BATTERY THROUGH THIN WIRES." /> -</span></p> -<p class="center caption">DISCHARGE OF LEYDEN JAR BATTERY THROUGH THIN WIRES.<br /></p> - -<p><b>Destruction of Buildings by Lightning.</b>—We learn from this -experiment that the electricity stored up in our battery passes, -without visible effect, through the stout wire of a discharging rod, -but that it instantly converts into vapor the thin wire stretched -across the spark board. And so it is with a flash of lightning. It -passes harmlessly, as every one knows, through a stout metal rod, but -when it comes across bell wires or telegraph wires, it melts them, or -converts them into vapor. On the sixteenth of July, 1759, a flash of -lightning struck a house in Southwark, on the south side of London, and -followed the line of the bell wire. After the lightning had passed, -the wire was no longer to be found; but the path of the lightning was -clearly marked by patches of vapor which were left, here and there, -adhering to the surface of the wall. In the year 1754, the lightning -fell on a bell tower at Newbury, in the United States of America, and -having dashed the roof to pieces, and scattered the fragments about, -it reached the bell. From this point it followed an iron wire, about -as thick as a knitting needle, melting it as it passed along, leaving -behind a black streak of vapor on the surface of the walls.</p> - -<p>Again, the electric discharge, passing through a bad conductor, -produces mechanical disturbance, and, if the substance be combustible, -often sets it on fire. So, too, as you know, the lightning flash, -falling on a church spire, dashes it to pieces, knocking the stones -about in all directions, while it sets fire to ships and wooden -buildings; and more than once it has caused great devastation by -exploding powder magazines.</p> - -<p><span class="pagenum" id="Page_28">[Pg 28]</span></p> - -<p>Let me give you one or two examples: In January, 1762, the -lightning fell on a church tower in Cornwall, and a stone—three -hundred-weight—was torn from its place and hurled to a distance of 180 -feet, while a smaller stone was projected as far as 1,200 feet from the -building. Again, in 1809, the lightning struck a house not far from -Manchester, and literally moved a massive wall twelve feet high and -three thick to a distance of several feet. You may form some conception -of the enormous force here brought into action, when I tell you that -the total weight of mason-work moved on this occasion was not less than -twenty-three tons.</p> - -<p>The church of <abbr title="saint">St.</abbr> George, at Leicester, was severely damaged by -lightning on the 1st of August, 1846. About 8 o’clock in the evening -the rector of the parish saw a vivid streak of light darting with -incredible velocity against the upper part of the spire. “For the -distance of forty feet on the eastern side, and nearly seventy on the -west, the massive stonework of the spire was instantly rent asunder and -laid in ruins. Large blocks of stone were hurled in all directions, -broken into small fragments, and in some cases, there is reason to -believe, reduced to powder. One fragment of considerable size was -hurled against the window of a house three hundred feet distant, -shattering to pieces the woodwork, and strewing the room within with -fine dust and fragments of glass. It has been computed that a hundred -tons of stone were, on this occasion, blown to a distance of thirty -feet in three seconds. In addition to the shivering of the spire, the -pinnacles at the angles of the tower were all more or less damaged, the -flying buttresses cracked through and violently shaken, many of the -open battlements at the base of the spire knocked away, the roof of the -church completely riddled, the roofs of the side entrances destroyed, -and the stone staircases of the gallery shattered.”<a id="FNanchor_17" href="#Footnote_17" class="fnanchor">[17]</a></p> - -<p>Lightning has been at all times the cause of great damage to property -by its power of setting fire to whatever is combustible. Fuller says, -in his Church History, that “scarcely a great abbey exists in England -which once, at least, has not been burned by lightning from heaven.” -He mentions, as examples, the Abbey of Croyland twice burned, the -Monastery of Canterbury twice, the Abbey of Peterborough twice; also -the Abbey of <abbr title="saint">St.</abbr> Mary’s, in Yorkshire, the Abbey of Norwich, and -several others. Sir William Snow Harris, writing about twenty years -ago, tells us that “the number of churches and church spires wholly or -partially destroyed by lightning is beyond all belief, and would be too -tedious a detail to enter upon. Within a comparatively few years, in -1822 for instance, we find the magnificent Cathedral of Rouen burned, -and, so lately as 1850, the beautiful Cathedral of Saragossa, in Spain, -struck by lightning during divine service and set on fire. In March of -last year a dispatch from our Minister at Brussels,<span class="pagenum" id="Page_29">[Pg 29]</span> Lord Howard de -Walden, dated the 24th of February, was forwarded by Lord Russell to -the Royal Society, stating that, on the preceding Sunday, a violent -thunderstorm had spread over Belgium; that twelve churches had been -struck by lightning; and that three of these fine old buildings had -been totally destroyed.”<a id="FNanchor_18" href="#Footnote_18" class="fnanchor">[18]</a></p> - -<p>Even in our own day the destruction caused by fires produced through -the agency of lightning is very great—far greater than is commonly -supposed. No general record of such fires is kept, and consequently -our information on the subject is very incomplete and inexact. I -may tell you, however, one small fact which, so far as it goes, -is precise enough and very significant. In the little province of -Schleswig-Holstein, which occupies an area less than one-fourth of the -area of Ireland, the Provincial Fire Assurance Association has paid in -sixteen years, for damage caused by lightning, somewhat over £100,000, -or at the rate of more than £6,000 a year. The total loss of property -every year in this province, due to fires caused by lightning, is -estimated at not less than £12,500.<a id="FNanchor_19" href="#Footnote_19" class="fnanchor">[19]</a></p> - - -<p><b>Destruction of Ships at Sea.</b>—The destructive effects of -lightning on ships at sea, before the general adoption of lightning -conductors, seems almost incredible at the present day. From official -records it appears that the damage done to the Royal Navy of England -alone involved an expenditure of from £6,000 to £10,000 a year. We are -told by Sir William Snow Harris, who devoted himself for many years -to this subject with extraordinary zeal and complete success, that -between the year 1810 and the year 1815—that is, within a period of -five years—“no less than forty sail of the line, twenty frigates, -and twelve sloops and corvettes were placed <i lang="fr" xml:lang="fr">hors de combat</i> by -lightning. In the merchant navy, within a comparatively small number -of years, no less than thirty-four ships, most of them large vessels -with rich cargoes, have been totally destroyed—been either burned -or sunk—to say nothing of a host of vessels partially destroyed or -severely damaged.”<a id="FNanchor_20" href="#Footnote_20" class="fnanchor">[20]</a></p> - -<p>And these statements, be it observed, take no account of ships that -were simply reported as missing, some of which, we can hardly doubt, -were struck by lightning in the open sea, and went down with all hands -on board. A famous ship of forty-four guns, the <i>Resistance</i>, was -struck by lightning in the Straits of Malacca, and the powder magazine -exploding, she went to the bottom. Of her whole crew only three were -saved, who happened to be picked up by a passing boat. It has been well -observed that, were it not for these three chance survivors, nothing -would have been known concerning the fate of the vessel, and she would -have been simply recorded as missing in the Admiralty lists.</p> - -<p><span class="pagenum" id="Page_30">[Pg 30]</span></p> - -<p>Nothing is more fearful to contemplate than the scene on board a -ship when she is struck by lightning in the open sea, with the winds -howling around, the waves rolling mountains high, the rain coming down -in torrents, and the vivid flashes lighting up the gloom at intervals, -and carrying death and destruction in their track. I will read you -one or two brief accounts of such a scene, given in the pithy but -expressive language of the sailor. In January, 1786, the <i>Thisbe</i>, -of thirty-six guns, was struck by lightning off the coast of Scilly, -and reduced to the condition of a wreck. Here is an extract from the -ship’s log: “Four <span class="allsmcap">A. M.</span>, strong gales; handed mainsail and -main top-sail; hove to with storm staysails; blowing very heavy, S. -E. 4.15, a flash of lightning, with tremendous thunder, disabled some -of our people. A second flash set the mainsail, main-top, and mizen -staysails on fire. Obliged to cut away the mainmast; this carried away -mizen top-mast and fore top-sail yard. Found foremast also shivered by -the lightning. Fore top-mast went over the side about 9 <span class="allsmcap">A. M.</span> -Set the foresail.”<a id="FNanchor_21" href="#Footnote_21" class="fnanchor">[21]</a></p> - -<p>A few years later, in March, 1796, the <i>Lowestoffe</i> was struck -in the Mediterranean, and we read as follows in the log of the ship: -“North end of Minorca; heavy squalls; hail, rain, thunder, and -lightning. 12.15, ship struck by lightning, which knocked three men -from the masthead, one killed. 12.30, ship again struck; main top-mast -shivered in pieces; many men struck senseless on the decks. Ship again -struck, and set on fire in the masts and rigging; mainmast shivered -in pieces; fore top-mast shivered; men benumbed on the decks, and -knocked out of the top; one man killed on the spot. 1.30, cut away the -mainmast; employed clearing wreck. 4, moderate; set the foresail.”<a id="FNanchor_22" href="#Footnote_22" class="fnanchor">[22]</a></p> - -<p>Again, in 1810, the <i>Repulse</i>, a ship of seventy-four guns, was -struck, off the coast of Spain. “The wind had been variable in the -morning—and at 12.35 there was a heavy squall, with rain, thunder, and -lightning. The ship was struck by two vivid flashes of lightning, which -shivered the maintop-gallant mast, and severely damaged the mainmast. -Seven men were killed on the spot; three others only survived a few -days; and ten others were maimed for life. After the second discharge -the rain fell in torrents. The ship was more completely crippled than -if she had been in action, and the squadron, then engaged on a critical -service, lost for a time one of its fastest and best ships.”<a id="FNanchor_23" href="#Footnote_23" class="fnanchor">[23]</a></p> - - -<p><b>Destruction of Powder Magazines.</b>—Not less appalling is the -devastation caused by lightning when it falls on a powder magazine. -Here is a striking example: On the eighteenth of August, 1769, the -tower of <abbr title="saint">St.</abbr> Nazaire, at Brescia, was struck by lightning. Underneath -the tower about 200,000 pounds of gunpowder, belonging<span class="pagenum" id="Page_31">[Pg 31]</span> to the Republic -of Venice, were stored in vaults. The powder exploded, leveling to -the ground a great part of the beautiful city of Brescia, and burying -thousands of its inhabitants in the ruins. It is said that the tower -itself was blown up bodily to a great height in the air, and came down -in a shower of stones. This is, perhaps, the most fearful disaster of -the kind on record. But we are not without examples in our own times. -In the year 1856 the lightning fell on the Church of <abbr title="saint">St.</abbr> John, in the -Island of Rhodes. A large quantity of gunpowder had been deposited in -the vaults of the church. This was ignited by the flash; the building -was reduced to a mass of ruins, a large portion of the town was -destroyed, and a considerable number of the inhabitants were killed. -Again, in the following year, the magazine of Joudpore, in the Bombay -Presidency, was struck by lightning. Many thousand pounds of gunpowder -were blown up, five hundred houses were destroyed, and nearly a -thousand people are said to have been killed.<a id="FNanchor_24" href="#Footnote_24" class="fnanchor">[24]</a></p> - - -<p><b>Experimental Illustrations.</b>—And now, before proceeding further, -I will make one or two experiments, with a view of showing that the -electricity of our machines is capable of producing effects similar -to those produced by lightning, though immeasurably inferior in point -of magnitude. Here is a common tumbler, about three-quarters full of -water. Into it I introduce two bent rods of brass, which are carefully -insulated below the surface of the water by a covering of india-rubber. -The points, however, are exposed, and come to within an inch of one -another, near the bottom of the tumbler. Outside the tumbler, the -brass rods are mounted on a stand, by means of which I can send the -full charge of this Leyden jar battery through the water, from point -to point. Since water is a bad conductor of electricity, as compared -with metals, the charge encounters great resistance in passing through -it, and in overcoming this resistance produces considerable mechanical -commotion, which is usually sufficient to shiver the glass to pieces.</p> - -<p>To charge the battery will take about twenty turns of this large Holtz -machine. Observe how the pith ball of the electroscope rises as the -machine is worked, showing that the charge is going in. And now it -remains stationary; which is a sign that the battery is fully charged, -and can receive no more. You will notice that the outside coating of -the battery has been already connected with one of the brass rods -dipping into the tumbler of water. By means of this discharger I will -now bring the inside coating into connection with the other rod. And -see, before contact is actually made, the spark has leaped across, and -our tumbler is violently burst asunder from top to bottom.</p> - -<p><span class="pagenum" id="Page_32">[Pg 32]</span></p> - - - -<p class="center p2"><span class="figcenter" id="img009"> - <img src="images/009.jpg" class="w50" alt="GLASS VESSEL BROKEN BY DISCHARGE OF LEYDEN JAR BATTERY." /> -</span></p> -<p class="center caption">GLASS VESSEL BROKEN BY DISCHARGE OF LEYDEN JAR BATTERY.<br /></p> - - -<p>This will probably appear to you a very small affair, when compared -with the tearing asunder of solid masonry, and the hurling about of -stones by the ton weight. No doubt it is; and that is just one of -the lessons we have to learn from the experiment we have made. For, -not only does it show us that effects of this kind may be caused by -electricity artificially produced, but it brings home forcibly to the -mind how incomparably more powerful is the lightning of the clouds than -the electricity of our machines.</p> - -<p>The property which electricity has of setting fire to combustible -substances may be easily illustrated. This india-rubber tube is -connected with the gas pipe under the floor, and to the end of the -tube is fitted a brass stop-cock which I hold in my hand. I open -the cock, and allow the jet of gas to flow toward the conductor of -Carré’s machine, while my assistant turns the handle; a spark passes, -and the gas is lit. Again, my assistant stands on this insulating -stool, placing his hand on the large conductor of the machine, while -I turn the handle. His body becomes electrified, and when he presents -his knuckle to this vessel of spirits of wine, which is electrically -connected with the earth, a spark leaps across, and the spirits of wine -are at once in a blaze. Once more; I tie a little gun-cotton around -one knob of the discharging rod, and then use it to discharge a small -Leyden jar; at the moment of the discharge the gun-cotton is set on -fire.</p> - -<p>It would be easy to explode gunpowder with the electric spark, but the -smoke of the explosion would make the lecture-hall very unpleasant for -the remainder of the lecture. I propose, therefore, to<span class="pagenum" id="Page_33">[Pg 33]</span> substitute for -gunpowder an explosive mixture of oxygen and hydrogen, with which I -have filled this little metal flask, commonly known as Volta’s pistol. -By a very simple contrivance, the electric spark is discharged through -the mixture, when I hold the flask toward the conductor of the machine. -A cork is fitted tightly into the neck of the flask, and at the moment -the spark passes you hear a loud explosion, and you see the cork driven -violently up to the ceiling.</p> - - - -<p class="center p2"><span class="figcenter" id="img010"> - <img src="images/010.jpg" class="w50" alt="GUN-COTTON SET ON FIRE BY ELECTRIC SPARK." /> -</span></p> -<p class="center caption">GUN-COTTON SET ON FIRE BY ELECTRIC SPARK.<br /></p> - - -<p><b>Destruction of Life.</b>—The last effect of lightning to which I -shall refer, and which, perhaps, more than any other, strikes us with -terror, is the sudden and utter extinction of life, when the lightning -flash descends on man or on beast. So swift is this effect, in most -cases, that death is, in all probability, absolutely painless, and the -victim is dead before he can feel that he is struck. I cannot give you, -with any degree of exactness, the number of people killed every year -by lightning, because the record of such deaths has been hitherto very -imperfectly kept, in almost all countries, and is, beyond doubt, very -incomplete. But perhaps you will be surprised to learn that the number -of deaths by lightning actually recorded is, on an average, in England -about 22 every year, in France 80, in Prussia 110, in Austria 212, in -European Russia 440.<a id="FNanchor_25" href="#Footnote_25" class="fnanchor">[25]</a></p> - -<p>So far as can be gathered from the existing sources of information, -it would seem that the number of persons killed by lightning is, on -the whole, about one in three of those who are struck. The rest are -sometimes only stunned, sometimes more or less burned, sometimes -made deaf for a time, sometimes partially paralyzed. On particular -occasions, however, especially when the lightning falls on a large -assembly of people, the number of persons struck down and slightly -injured, in proportion to the number killed, is very much increased.</p> - -<p>An interesting case of this kind is reported by <abbr title="mister">Mr.</abbr> Tomlinson. “On -the twenty-ninth of August, 1847, at the parish church of Welton,<span class="pagenum" id="Page_34">[Pg 34]</span> -Lincolnshire, while the congregation were engaged in singing the hymn -before the sermon, and the <abbr title="reverend">Rev.</abbr> <abbr title="mister">Mr.</abbr> Williamson had just ascended the -pulpit, the lightning was seen to enter the church from the belfry, -and instantly an explosion occurred in the centre of the edifice. All -that could move made for the door, and <abbr title="mister">Mr.</abbr> Williamson descended from -the pulpit, endeavoring to allay the fears of the people. But attention -was now called to the fact that several of the congregation were lying -in different parts of the church, apparently dead, some of whom had -their clothing on fire. Five women were found injured, and having their -faces blackened and burned, and a boy had his clothes almost entirely -consumed. A respected old parishioner, <abbr title="mister">Mr.</abbr> Brownlow, aged sixty-eight, -was discovered lying at the bottom of his pew, immediately beneath one -of the chandeliers, quite dead. There were no marks on the body, but -the buttons of his waistcoat were melted, the right leg of his trousers -torn down, and his coat literally burnt off. His wife in the same pew -received no injury.”<a id="FNanchor_26" href="#Footnote_26" class="fnanchor">[26]</a></p> - - -<p class="center p2"><span class="figcenter" id="img011"> - <img src="images/011.jpg" class="w50" alt="VOLTA’S PISTOL; EXPLOSION CAUSED BY ELECTRIC SPARK." /> -</span></p> -<p class="center caption">VOLTA’S PISTOL; EXPLOSION CAUSED BY ELECTRIC SPARK.<br /></p> - -<p>Not less striking is the story told by <abbr title="doctor">Dr.</abbr> Plummer, surgeon of the -Illinois Volunteers, in the <i>Medical and Surgical Reporter</i> of -June 19, 1865: “Our regiment was yesterday the scene of one of the -most terrible calamities which it has been my lot to witness. About -two o’clock a violent thunderstorm visited us. While the old guard was -being turned out to receive the new, a blinding flash of lightning was -seen, accompanied instantly by a terrific peal of thunder. The whole -of the old guard, together with part of the new, were thrown violently -to the earth. The shock was so severe and sudden that, in most cases, -the rear rank men were thrown across the front rank men. One<span class="pagenum" id="Page_35">[Pg 35]</span> man was -instantly killed, and thirty-two men were more or less severely burned -by the electric fluid. In some instances the men’s boots and shoes -were rent from their feet and torn to pieces, and, strange as it may -appear, the men were injured but little in the feet. In all cases the -burns appear as if they had been caused by scalding-hot water, in many -instances the skin being shriveled and torn off. The men all seem to be -doing well, and a part of them will be able to resume their duties in a -few days.”</p> - - -<p><b>The Return Shock.</b>—It sometimes happens that people are struck -down and even killed at the moment a discharge of lightning takes -place between a cloud and the earth, though they are very far from the -point where the flash is actually seen to pass; while others, who are -situated between them and the lightning, suffer very little, or perhaps -not at all. This curious phenomenon was first carefully investigated by -Lord Mahon in the year 1779, and was called by him the “return shock.” -His theory, which is now commonly accepted, may be easily understood -with the aid of the sketch before you.</p> - - -<p class="center p2"><span class="figcenter" id="img012"> - <img src="images/012.jpg" class="w50" alt="THE RETURN SHOCK ILLUSTRATED." /> -</span></p> -<p class="center caption">THE RETURN SHOCK ILLUSTRATED.<br /></p> - -<p>Let us suppose <span class="allsmcap">ABC</span> to represent the outline of a thundercloud -which dips down toward the earth at <span class="allsmcap">A</span> and at <span class="allsmcap">C</span>. The -electricity of the cloud develops by inductive action a charge of the -opposite kind in the earth beneath it. But the inductive action is most -powerful at <span class="allsmcap">E</span> and <span class="allsmcap">F</span>, where the cloud comes nearest to -the earth. Hence, bodies situated near these points may be very highly -electrified as compared with bodies at a point between them, such -as <span class="allsmcap">D</span>. Now, when a flash of lightning passes at <span class="allsmcap">E</span>, -the under part of the cloud is at once relieved of its electricity, -its inductive action ceases, and, therefore, a person situated at -<span class="allsmcap">F</span> suddenly ceases to be electrified. This sudden change from -a highly electrified to a neutral state involves a shock to his system -which may be severe enough to stun or even to kill him.<span class="pagenum" id="Page_36">[Pg 36]</span> Meanwhile, -people at <i>D</i>, having been also electrified to some extent by the -influence of the thundercloud, must in like manner undergo a change in -their electrical condition when the flash of lightning passes, but this -change will be less violent because they were less highly electrified.</p> - -<p>Many experiments have been devised to illustrate this theory of Lord -Mahon. But the best illustration I know is furnished by this electric -machine of Carré’s. If you stand near one end of the large conductor -when the machine is in action and sparks are taken from the other end, -you will feel a distinct electric shock every time a spark passes. -The large conductor here takes the place of the cloud, the spark that -passes at one end represents the flash of lightning, and the observer -at the other end gets the return shock, though he is at a considerable -distance from the point where the flash is seen.</p> - -<p>An experiment of this kind, of course, cannot be made sensible to a -large audience like the present. But I can give you a good idea of the -effect by means of this tuft of colored papers. While the machine is in -action I hold the tuft of papers near that end of the conductor which -is farthest from the point where the discharge takes place. You see the -paper ribbons are electrified by induction, and, in virtue of mutual -repulsion, stand out from one another “like quills upon the fretful -porcupine.” But, when a spark passes, the inductive action ceases, the -paper ribbons cease to be electrified, and the whole tuft suddenly -collapses into its normal state.</p> - -<p>While fully accepting Lord Mahon’s theory of the return shock as -perfectly good so far as it goes, I would venture to point out another -influence which must often contribute largely to produce the effect in -question, and which is not dependent on the form of the cloud. It may -easily happen, from the nature of the surface in the district affected -by a thundercloud, that the point of most intense electrification—say -<span class="allsmcap">E</span> in the figure—is in good electrical communication with -a distant point, such as <span class="allsmcap">F</span>, while it is very imperfectly -connected with a much nearer point, <span class="allsmcap">D</span>. In such a case it -is evident that bodies at <span class="allsmcap">F</span> will share largely in the -highly-electrified condition of <span class="allsmcap">E</span>, and also share largely in -the sudden change of that condition the moment the flash of lightning -passes; whereas bodies at <span class="allsmcap">D</span> will be less highly electrified -before the discharge, and less violently disturbed when the discharge -takes place.</p> - -<p>This principle may be illustrated by a very simple experiment. Here -is a brass chain about twenty feet long. One end of it I hand to any -one among the audience who will kindly take hold of it; the other end -I hold in my hand. I now stand near the conductor of the machine; and -will ask some one to stand about ten feet away from me, near the middle -of the chain, but without touching it. Now observe what happens when -the machine is worked and I take a spark<span class="pagenum" id="Page_37">[Pg 37]</span> from the conductor: My friend -at the far end of the chain, twenty feet away, gets a shock nearly -as severe as the one I get myself, because he is in good electrical -communication with the point where the discharge takes place. But -my more fortunate friend, who is ten feet nearer to the flash, is -hardly sensible of any effect, because he is connected with me only -through the floor of the hall, which is, comparatively speaking, a bad -conductor of electricity.</p> - - -<p><b>Summary.</b>—Let me now briefly sum up the chief destructive -effects of lightning. First, with regard to good conductors: though it -passes harmlessly through them if they be large enough to afford it -an easy passage, it melts and converts them into vapor if they be of -such small dimensions as to offer considerable resistance. Secondly, -lightning acts with great mechanical force on bad conductors; it is -capable of tearing asunder large masses of masonry, and of projecting -the fragments to a considerable distance. Thirdly, it sets fire to -combustible materials. And lastly, it causes the instantaneous death of -men and animals.</p> - - -<p><b>Franklin’s Lightning Rods.</b>—The object of lightning conductors -is to protect life and property from these destructive effects. Their -use was first suggested by Franklin, in 1749, even before his famous -experiment with the kite; and immediately after that experiment, in -1752, he set up, on his own house, in Philadelphia, the first lightning -conductor ever made. He even devised an ingenious contrivance, by -means of which he received notice when a thundercloud was approaching. -The contrivance consisted of a peal of bells, which he hung on his -lightning conductor, and which were set ringing whenever the lightning -conductor became charged with electricity.</p> - -<p>Franklin’s lightning rods were soon adopted in America; and he himself -contributed very much to their popularity by the simple and lucid -instructions he issued every year, for the benefit of his countrymen, -in the annual publication known as “Poor Richard’s Almanac.” It is -very interesting at this distance of time to read the homely practical -rules laid down by this great philosopher and statesman; and, though -some modifications have been suggested by the experience of a hundred -and thirty years, especially as regards the dimensions of the lightning -conductor, it is surprising to find how accurately the general -principles of its construction, and of its action, are here set forth.</p> - -<p>“It has pleased God,” he says, “in His goodness to mankind, at length -to discover to them the means of securing their habitations and other -buildings from mischief by thunder and lightning. The method is this: -Provide a small iron rod, which may be made of the rod-iron used by -nailors, but of such a length that one end being three or four feet in -the moist ground, the other may be six or eight feet above the highest -part of the building. To the upper end of the<span class="pagenum" id="Page_38">[Pg 38]</span> rod fasten about a foot -of brass wire, the size of a common knitting needle, sharpened to a -fine point; the rod may be secured on the house by a few small staples. -If the house or barn be long, there may be a rod and point at each end, -and a middling wire along the ridge from one to the other. A house thus -furnished will not be damaged by lightning, it being attracted by the -points and passing through the metal into the ground, without hurting -anything. Vessels also having a sharp-pointed rod fixed on the top of -their masts, with a wire from the foot of the rod reaching down round -one of the shrouds to the water, will not be hurt by lightning.”</p> - - -<p><b>Introduction of Lightning Rods into England.</b>—The progress of -lightning conductors was more slow in England and on the Continent of -Europe, owing to a fear, not unnatural, that they might, in some cases, -draw down the lightning where it would not otherwise have fallen. -People preferred to take their chance of escaping as they had escaped -before, rather than invite, as it were, the lightning to descend on -their houses, in the hope that an iron rod would convey it harmless -to the earth. But the immense amount of damage done every year by -lightning, soon led practical men to entertain a proposal which offered -complete immunity from all danger on such easy terms; and when it was -found that buildings protected by lightning conductors were, over and -over again, struck by lightning without suffering any harm, a general -conviction of their utility was gradually established in the public -mind.</p> - -<p>The first public building protected by a lightning rod in England was -<abbr title="saint">St.</abbr> Paul’s Cathedral, in London. On the eighteenth of June, 1764, the -beautiful steeple of Saint Bride’s Church, in the city, was struck by -lightning and reduced to ruin. This incident awakened the attention of -the dean and chapter of <abbr title="saint">St.</abbr> Paul’s to the danger of a similar calamity, -which seemed, as it were, impending over their own church. After long -deliberation, they referred the matter to the Royal Society, asking for -advice and instruction. A committee of scientific men was appointed by -the Royal Society to consider the question. Benjamin Franklin himself, -who happened to be in London at the time, as the representative of the -American States in their dispute with England, was nominated a member -of the committee. And the result of its deliberation was that, in the -year 1769, a number of lightning conductors were erected on <abbr title="saint">St.</abbr> Paul’s -Cathedral.</p> - -<p>It was on this occasion that arose the celebrated controversy about -the respective merits of points and balls. Franklin had recommended a -pointed conductor; but some members of the committee were of opinion -that the conductor should end in a ball and not in a point. The -decision of the committee was in favor of Franklin’s opinion, and -pointed conductors were accordingly adopted for <abbr title="saint">St.</abbr> Paul’s Cathedral. -But the controversy did not end here. The time was one of great<span class="pagenum" id="Page_39">[Pg 39]</span> -political excitement, and party spirit infused itself even into the -peaceful discussions of science. The weight of scientific opinion was -on the side of Franklin; but it was hinted, on the other side, that the -pointed conductors were tainted with republicanism, and pregnant with -danger to the empire. As a rule, the whigs were strongly in favor of -points; while the Tories were enthusiastic in their support of balls.</p> - -<p>For a time the Tories seemed to prevail. The king was on their side. -Experiments on a grand scale were conducted in his presence, at the -Pantheon, a large building in Oxford street; he was assured that these -experiments proved the great superiority of balls over points; and to -give practical effect to his convictions, his majesty directed that a -large cannon ball should be fixed on the end of the lightning conductor -attached to the royal palace at Kew. But the committee of the Royal -Society remained unconvinced. In course of time the heat of party -spirit abated; experience as well as reason was found to be in favor of -Franklin’s views; and the battle of the balls and points has long since -passed into the domain of history.<a id="FNanchor_27" href="#Footnote_27" class="fnanchor">[27]</a></p> - - -<p><b>Functions of a Lightning Conductor.</b>—A lightning conductor -fulfills two functions. First, it favors a silent and gradual discharge -of electricity between the cloud and the earth, and thus tends to -prevent that accumulation which must of necessity take place before a -flash of lightning will pass. Secondly, if a flash of lightning come, -the lightning conductor offers it a safe channel through which it may -pass harmless to the earth.</p> - -<p>These two functions of a lightning conductor may be easily illustrated -by experiment. When our machine is in action, if I present my closed -hand to the large brass conductor, a spark passes between them, and I -feel, at the same moment, a slight electric shock. Here the conductor -of the machine, as usual, holds the place of the electrified cloud; -my closed hand represents, as it were, a lofty building that stands -out prominently on the surface of the earth; the spark is the flash of -lightning, and the electric shock just suggests the destructive power -of the sudden disruptive discharge.</p> - -<p>Now let me protect this building by a lightning conductor. For this -purpose, I take in my hand a brass rod, which I connect with the -earth by a brass chain. In the first instance, I will have a metal -ball on the end of my lightning conductor. You see the effect; sparks -pass rapidly, but I feel no shock. I can increase the strength of -the discharge by hanging this condensing jar on the conductor of the -machine. Sparks pass now, much more brilliant and powerful than before, -but still I get no shock. It is evident, therefore, that my lightning -rod does not prevent the flash from passing, but it conveys it harmless -to the ground.</p> - -<p><span class="pagenum" id="Page_40">[Pg 40]</span></p> - -<p>I next take a rod which is sharply pointed, and connecting it as before -with the earth by a brass chain, I present the sharp point to the -conductor of the machine. Observe how different is the result; there is -no disruptive discharge; no spark passes; no shock is felt. Electricity -still continues to be generated in the machine, and electricity is -generated, by induction, in the brass rod, and in my body. But these -two opposite electricities discharge themselves silently, by means of -this pointed rod, and no sensible effect of any kind is exhibited.</p> - -<p>These experiments are very simple, but they really put before us, in -the clearest possible way, the whole theory of lightning conductors. -In particular, they give us ocular demonstration that an efficient -lightning rod not only makes the lightning harmless when it comes, -but tends very much to prevent its coming. A remarkable example, on a -large scale, of this important property, is furnished by the town of -Pietermaritzburg, the capital of the colony of Natal, in South Africa. -This town is subject to the frequent visitation of thunderstorms, -at certain seasons of the year, and much damage was formerly done -by lightning, but since the erection of lightning conductors on the -principal buildings, the lightning has never fallen within the town. -Thunderclouds come as before, but they pass silently over the city, -and only begin to emit their lightning flashes when they reach the -open country, and have passed beyond the range of the lightning -conductors.<a id="FNanchor_28" href="#Footnote_28" class="fnanchor">[28]</a></p> - -<p>But it will often happen, even in the case of a pointed conductor, -that the accumulation of electricity goes on so fast that the silent -discharge is insufficient to keep it in check. A disruptive discharge -will then take place, from time to time, and a flash of lightning will -pass. Under these circumstances, the lightning conductor is called upon -to fulfill its second function, and to convey the lightning harmless to -the earth.</p> - - -<p><b>Conditions of a Lightning Conductor.</b>—From the consideration of -the functions which it has to fulfill, we may now infer what are the -conditions necessary for an efficient lightning conductor. The first -condition is that the end of the conductor, projecting into the air, -should have, at least, one sharp point. Our experiments have shown us -that a pointed conductor tends, in a manner, to suppress the flash -of lightning altogether; whereas a blunt conductor, or one ending in -a ball, tends only to make it harmless when it comes. It is evident, -therefore, that the pointed conductor offers the greater security.</p> - -<p>But a fine point is very liable to be melted when the lightning falls -upon it, and thus to be rendered less efficient for future service. To -meet this danger, it has recently been suggested, by the Lightning Rod -Conference, that the extreme end of the conductor should be a blunt -point, destined to receive the full force of the lightning flash,<span class="pagenum" id="Page_41">[Pg 41]</span> when -it comes; and that, a little lower down, a number of very fine points -should be provided, with a view to favor the silent discharge. This -suggestion, which appears admirably fitted to provide for the twofold -function of a lightning conductor, deserves to be recorded in the exact -terms of the official report.</p> - -<p>“It seems best to separate the double functions of the point, -prolonging the upper terminal to the very summit, and merely beveling -it off, so that, if a disruptive discharge does take place, the full -conducting power of the rod may be ready to receive it. At the same -time, having regard to the importance of silent discharge from sharp -points, we suggest that, at one foot below the extreme top of the upper -terminal, there be firmly attached, by screws and solder, a copper -ring bearing three or four copper needles, each six inches long, and -tapering from a quarter of an inch diameter to as fine a point as can -be made; and with the object of rendering the sharpness as permanent -as possible, we advise that they be platinized, gilded, or nickel -plated.”<a id="FNanchor_29" href="#Footnote_29" class="fnanchor">[29]</a></p> - -<p>The second condition of a lightning conductor is, that it should be -made of such material, and of such dimensions, as to offer an easy -passage to the greatest flash of lightning likely to fall on it; -otherwise it might be melted by the discharge, and the lightning, -seeking for itself another path, might force its way through bad -conductors, which it would partly rend asunder, and partly consume -by fire. Copper is now generally regarded as the best material for -lightning conductors, and it is almost universally employed in these -countries. If it is used in the form of a rope, it should not be less -than half an inch in diameter; if a band of copper is preferred—and it -is often found more convenient by builders—it should be about an inch -and a half broad and an eighth of an inch thick. In France it has been -hitherto more usual to employ iron rods for lightning conductors, but -since iron is much inferior to copper in its conducting power, the iron -rod must be of much larger dimensions; it should be at least one inch -in diameter.<a id="FNanchor_30" href="#Footnote_30" class="fnanchor">[30]</a></p> - -<p>The third condition is that the lightning conductor should be -continuous throughout its whole length, and should be placed in good -electrical contact with the earth. This is a condition of the first -importance, and experience has shown that it is the one most likely -of all to be neglected. In a large town the best earth connection is -furnished by the system of water-mains and gas-mains, each of which -constitutes a great network of conductors everywhere in contact with<span class="pagenum" id="Page_42">[Pg 42]</span> -the earth. Two points, however, must be carefully attended to—first, -that the electrical contact between the lightning conductor and the -metal pipe should be absolutely perfect; and, secondly, that the pipe -selected should be of such large dimensions as to allow the lightning -an easy passage through it to the principal main.</p> - -<p>If no such system of water-pipes or gas-pipes is at hand, then the -lightning rod should be connected with moist earth by means of a bed of -charcoal or a metal plate not less than three feet square. This metal -plate should be always of the same material as the conductor, otherwise -a galvanic action would be set up between the two metals, which in -course of time might seriously damage the contact. Dry earth, sand, -rock, and shingle are bad conductors; and, if such materials exist near -the surface of the earth, the lightning rod must pass through them and -be carried down until it reaches water or permanently damp earth.</p> - - -<p><b>Mischief Done by Bad Conductors.</b>—If the earth contact is bad, a -lightning conductor does more harm than good. It invites the lightning -down upon the building without providing for it, at the same time, a -free passage to earth. The consequence is that the lightning forces -a way for itself, violently bursting asunder whatever opposes its -progress, and setting fire to whatever is combustible.</p> - -<p>I will give you some recent and striking examples. In the month of May, -1879, the church of Laughton-en-le-Morthen, in England, though provided -with a conductor, was struck by lightning and sustained considerable -damage. On examination it was found that the lightning followed the -conductor down along the spire as far as the roof; then, changing its -course, it forced its way through a buttress of massive masonwork, -dislodging about two cartloads of stones, and leaped over to the leads -of the roof, about six feet distant. It now followed the leads until it -came to the cast-iron down-pipes intended to discharge the rain-water, -and through these it descended to the earth. When the earth contact -of the lightning conductor was examined, it was found exceedingly -deficient. The rod was simply bent underground, and buried in dry -loose rubbish at a depth not exceeding eighteen inches. This is a very -instructive example. The lightning had a choice of two paths—one by -the conductor prepared for it, the other by the leads of the roof -and the down-pipes—and, by a kind of instinct which, however we may -explain, we must always contemplate with wonder, it chose the path of -least resistance, though in doing so it had to burst its way at the -outset through a massive wall of solid masonry.<a id="FNanchor_31" href="#Footnote_31" class="fnanchor">[31]</a></p> - -<p>On the 5th of June, in the same year, a flash of lightning struck the -house of <abbr title="mister">Mr.</abbr> Osbaldiston, near Sheffield, and, notwithstanding the -supposed protection of a lightning conductor, it did damage to the<span class="pagenum" id="Page_43">[Pg 43]</span> -amount of about five hundred pounds. The lightning here followed the -conductor to a point about nine feet from the ground, then passed -through a thick wall to a gas-pipe at the back of the drawing-room -mirror. It melted the gas-pipe, set fire to the gas, smashed the -mirror to atoms, broke the Sevres vases on the chimney-piece, and -dashed the furniture about. In this case, as in the former, it was -found that the earth contact was bad; and, in addition, the conductor -itself was of too small dimensions. Hence, the electric discharge -found an easier path to earth through the gas-pipes, though to reach -them it had to force for itself a passage through a resisting mass of -non-conductors.<a id="FNanchor_32" href="#Footnote_32" class="fnanchor">[32]</a></p> - -<p>Again in the same year, on the 28th of May, the house of <abbr title="mister">Mr.</abbr> Tomes, of -Caterham, was struck by lightning, and some slight damage was done. -After a careful examination it was found that the greater part of -the discharge left the lightning conductor with which the house was -provided, and passed over the slope of the roof to an attic room, into -which it forced its way through a brick wall, and reached a small iron -cistern. This cistern was connected by an iron pipe of considerable -dimensions with two pumps in the basement story; and through them the -lightning found an easy passage to the earth, and did but little harm -on its way. When the earth contact of the lightning conductor was -examined, it was discovered that the end of the rod was simply stuck -into a dry chalky soil to a depth of about twelve inches. Thus in this -case, as in the two former, it was made quite clear that the lightning -conductor failed to fulfill its functions because the earth contact was -bad.<a id="FNanchor_33" href="#Footnote_33" class="fnanchor">[33]</a></p> - -<p>Cases are not uncommon in which builders provide underground a -carefully constructed reservoir of water, into which the lower end -of the lightning rod is introduced. The idea seems to prevail that a -reservoir of water constitutes a good earth contact; and this is quite -true of a natural reservoir, such as a lake, where the water is in -contact with moist earth over a considerable area. But an artificial -reservoir may have quite an opposite character, and practically -insulate the lightning conductor from the earth. One which came under -my notice lately, in the neighborhood of this city, consists of a large -earthenware pipe set on end in a bed of cement, and kept half full of -water. Now, the earthenware pipe is a good insulator, and so is the bed -of cement in which it rests; and the whole arrangement is identical, -in all essential features, with the apparatus of Professor Richman, in -which he introduced his lightning rod into a glass bottle, and by which -he lost his life a hundred and thirty years ago.</p> - -<p>A conductor mounted in this manner will, probably enough, draw down -lightning from the clouds; but it is more likely to discharge it, with -destructive effect, into the building it is intended to guard, than -to transmit it harmlessly to the earth. An example is at hand in the<span class="pagenum" id="Page_44">[Pg 44]</span> -case of Christ Church, in the town of Clevedon, in Somersetshire. -This church was provided with a very efficient system of lightning -conductors, five in number, corresponding to the four pinnacles and the -flagstaff, on the summit of the principal tower. The five conductors -consisted of good copper-wire rope; all were united together inside the -tower, through which they were carried down to earth, and there ended -in an earthenware drain. This kind of earth contact might be pretty -good as long as water was flowing in the drain; but whenever the drain -was dry the conductor was practically insulated from the earth. On the -fifteenth of March, 1876, the church was struck by lightning, which -for some distance followed the line of the conductor; then finding its -passage barred by the earthenware drain, which was dry at the time, it -burst through the walls of the church, displacing several hundredweight -of stone, and making its way to earth through the gas-pipe.<a id="FNanchor_34" href="#Footnote_34" class="fnanchor">[34]</a></p> - -<p>Another very instructive example is furnished by the lightning -conductor attached to the lighthouse of Berehaven, on the south-west -coast of Ireland. It consists of a half-inch copper-wire rope, which -is carried down the face of the tower “until it reaches the rock at -its base, where it terminates in <i>a small hole, three inches by -three inches, jumped out of the rock, about six inches under the -surface</i>.” Here, again, we have a good imitation of Professor -Richman’s experiment, with only this difference, that a small hole -in the rock is substituted for a glass bottle. A lightning conductor -of this kind fulfills two functions: it increases the chance of the -lightning coming down on the building, and it makes it positively -certain that, having come, it cannot get to earth without doing -mischief.</p> - -<p>The lightning did come down on the Berehaven Lighthouse, about five -years ago. As might have been expected, it made no use of the lightning -conductor in finding a path to earth, but forced its way through the -building, dealing destruction around as it descended from stage to -stage. The Board of Irish Lights furnished a detailed report of this -accident to the Lightning Rod Conference, in March, 1880, from which -the above particulars have been derived.<a id="FNanchor_35" href="#Footnote_35" class="fnanchor">[35]</a></p> - - -<p><b>Precaution Against Rival Conductors.</b>—But it is not enough to -provide a good lightning conductor, which is itself able to convey -the electric discharge harmless to the earth; we must take care that -there are no rival conductors near at hand in the building, to draw -off the lightning from the path prepared for it, and conduct it by -another route in which its course might be marked with destruction. -This precaution is of especial importance at the present day, owing to -the great extent to which metal, of various kinds, is employed in the -construction and fittings of modern buildings. I will take a typical -case which will bring home this point clearly to your minds.</p> - -<p><span class="pagenum" id="Page_45">[Pg 45]</span></p> - -<p>A great part of the roof of many large buildings is covered with lead. -The lead, at one or more points may come near the gutters intended -to collect the rain water; the gutters are in connection with the -cast-iron down-pipes into which the water flows, and these down-pipes -often pass into the earth, which, under the circumstances, is generally -moist, and, therefore, in good electrical contact with the metal pipes. -Here, then, is an irregular line of conductors, which, though it has -gaps here and there, may, under certain conditions, offer to the -lightning discharge a path not less free than the lightning conductor -itself. What is the consequence? The flash of lightning, or a part of -it, will quit the lightning rod, and make its way to earth through the -broken series of conductors, doing, perhaps, serious mischief, as it -leaps across, or bursts asunder, the non-conducting links in the chain.</p> - -<p>Another illustration may be taken from the gas and water-pipes, with -which almost all buildings in great cities are now provided, and which -constitute a network of conductors, spreading out over the walls and -ceilings, and stretching down into the earth, with which they have -the best possible electrical contact. Now, it often happens that a -lightning conductor, at some point in its course, comes within a short -distance of this network of pipes. In such a case, a portion of the -electrical discharge is apt to leave the lightning conductor, force -its way destructively through masses of masonry, enter the network of -pipes, melt the leaden gas-pipe, ignite the gas, and set the building -on fire.</p> - -<p>These are not merely the speculations of philosophers. All the various -incidents I have just described have occurred, over and over again, -during the last few years. You will remember, in some of the examples -I have already set before you, when the electric discharge failed to -find a sufficient path to earth through the lightning rod, it followed -some such broken series of chance conductors as we are now considering. -But this broken series of conductors seems to bring with it a special -danger of its own, even when the lightning conductor is otherwise in -efficient working order. I will give you just one case in point.</p> - -<p>On the fifth of June, 1879, the Church of Saint Marie, Rugby, was -struck by lightning and set on fire, and narrowly escaped being -burned to the ground. A number of workmen were engaged on that day in -repairing the spire of the church. About three o’clock they saw a dense -black cloud approaching, and they came down to take shelter within the -building. In a few minutes they heard a terrific crash just overhead; -at the same moment the gas was lighted under the organ loft and the -woodwork was set in a blaze. The men soon succeeded in putting out the -fire, and the church escaped with very little damage.</p> - -<p>Now, in this case there was no reason to suppose that the lightning<span class="pagenum" id="Page_46">[Pg 46]</span> -conductor was in any way defective. But about half-way up the spire -there was a peal of eight bells. Attached to these bells were iron -wires, about the eighth of an inch in diameter, leading from the -clappers down to the organ-loft, where they came within a short -distance of a gas-pipe fixed in the wall. It would seem that a great -part of the discharge was carried safely to earth by the lightning -conductor. But a part branched off at the bells in the spire, descended -by the iron wires, and forced its way into the organ loft, to reach -the network of gas-pipes, through which it passed down to the earth, -melting the soft leaden gas-pipe in its course and lighting the gas.</p> - -<p>The remedy for this danger is obvious. All large masses of metal used -in the structure of a building—the leads and gutters of the roof, -the cast-iron down-pipes, the iron gas and water mains—should be put -in good metallic connection with the lightning conductor, and, as -far as may be, with one another. Connected in this way they furnish -a continuous and effective line of conductors leading safely down to -earth; and, instead of being a dangerous rival, they become a useful -auxiliary to the lightning rod.</p> - -<p>I would observe, however, that the lightning conductor ought not to -be connected directly with the soft leaden pipes which are commonly -employed to convey gas and water to the several parts of a building. -Such pipes, as we have seen, are liable to be melted when any -considerable part of the lightning discharge passes through them; and -thus much harm might be done, and the building might even be set on -fire by the lighting of the gas. Every good end will be attained if the -conductor is put in metallic connection with the iron gas and water -<em>mains</em> either inside or outside the building.</p> - - -<p><b>Insulation of Lightning Conductors.</b>—It is a question often -asked whether a lightning rod should be insulated from the building -it is intended to protect. I believe that this practice was formerly -recommended by some writers, and I have observed that glass insulators -are still employed not infrequently by builders in the erection of -lightning conductors; but, from the principles I have set before -you to-day, it seems clear that any insulation of this kind is, to -say the least, altogether useless. The building to be protected is -itself in electrical communication with the earth, and the lightning -conductor, if efficient, is also in electrical communication with the -earth—therefore, the lightning conductor and the building are in -electrical communication with each other through the earth, and any -attempt at insulating them from one another above the earth is only -labor thrown away.</p> - -<p>Further, I have just shown you that the masses of metal employed in -the structure or decoration of a building ought to be electrically -connected with each other and with the lightning conductor. Now, if -this be done, the lightning conductor is, by the fact, in direct -communication with the building, and the glass insulators are utterly<span class="pagenum" id="Page_47">[Pg 47]</span> -futile. Again, the building itself, during a thunderstorm, becomes -highly electrified by the inductive action of the cloud, and needs to -be discharged through the conductor just as the surrounding earth needs -to be discharged; therefore, the more thoroughly it is connected with -the conductor, the more effectively will the conductor fulfill its -functions.</p> - - -<p><b>Personal Safety in a Thunderstorm.</b>—I suppose there is hardly -any one to whom the question has not occurred, at some time or -another, what he had best do to secure his personal safety during a -thunderstorm. This question is of so much practical interest that I -think I shall be excused if I say a few words about it, though perhaps, -strictly speaking, it is somewhat beside the subject of lightning -conductors.</p> - -<p>At the outset, perhaps, I shall surprise you when I say that you would -enjoy the most perfect security if you were in a chamber entirely -composed of metal plates, or in a cage constructed of metal bars, or -if you were incased, like the knights of old, in a complete suit of -metal armor. This kind of defense is looked upon as so perfect, among -scientific men, that Professor Tait does not hesitate to recommend -his adventurous young friends devoted to the cause of science to -provide themselves with a light suit of copper, and, thus protected, -take the first opportunity of plunging into a thundercloud, there -to investigate, at its source, the process by which lightning is -manufactured.<a id="FNanchor_36" href="#Footnote_36" class="fnanchor">[36]</a></p> - -<p>The reason why a metal covering affords complete protection is that, -when a conductor is electrified, the whole charge of electricity -exists on the outside surface of the conductor; and therefore, when a -discharge takes place, it is only the outside surface that is affected. -Thus, if you were completely incased in a metal covering, and then -charged with electricity by the inductive action of a thundercloud, it -is only the metal covering that would undergo any change of electrical -condition; and when the lightning flash would pass, it is only the -metal covering that would be discharged.</p> - -<p>Let me show you a very pretty and interesting experiment to illustrate -this principle: Here is a hollow brass cylinder, open at the ends, -mounted on an insulating stand. On the outside is erected a light brass -rod with two pith balls suspended from it by linen threads. Two pith -balls are also suspended by linen threads from the inner surface of -the cylinder. You know that these pith balls will indicate to us the -electrical condition of the surfaces to which they are attached. If the -surface be electrified, the pith balls attached to it will share in -its electrical condition, and will repel each other; if the surface be -neutral, the pith balls attached to it will be neutral, and will remain -at rest.</p> - -<p><span class="pagenum" id="Page_48">[Pg 48]</span></p> - -<p>I now put this apparatus under the influence of our thundercloud, that -is, the large brass conductor of our machine. The moment my assistant -turns the handle, the electricity begins to be developed on the -conductor, and you see, at once, the effect on the brass cylinder. The -pith balls attached to the outer surface fly asunder; those attached -to the inner surface remain at rest. And now a spark passes; our -thundercloud is discharged; the inductive action ceases; the pith balls -on the outside suddenly collapse, while those on the inside are in no -way affected.</p> - - -<p class="center p2"><span class="figcenter" id="img013"> - <img src="images/013.jpg" class="w25" alt="PROTECTION FROM LIGHTNING FURNISHED BY A CLOSED -CONDUCTOR." /> -</span></p> -<p class="center caption">PROTECTION FROM LIGHTNING FURNISHED BY A CLOSED -CONDUCTOR.<br /></p> - -<p>It is not necessary that the brass cylinder should be insulated. To -vary the experiment, I will now connect it with the earth by a chain; -you will observe that the effect is precisely the same as before. -Flash after flash passes while the machine continues in action; the -outside pith balls fly about violently, being charged and discharged -alternately; the inside pith balls remain all the time at rest. Thus -you see clearly that, if you were sitting inside such a metal chamber -as this, or covered with a complete suit of metal armor, you would -be perfectly secure during a thunderstorm, whether the chamber were -electrically connected with the earth or insulated from it.</p> - -<p><span class="pagenum" id="Page_49">[Pg 49]</span></p> - - -<p><b>Practical Rules.</b>—But it rarely happens, when a thunderstorm -comes, that an iron hut or a complete suit of armor is at hand, -and you will naturally ask me what you ought to do under ordinary -circumstances. First, let me tell you what you ought not to do. You -ought not to take shelter under a tree, or under a haystack, or under -the lee of a house; you ought not to stand on the bank of a river, or -close to a large sheet of water. If indoors, you ought not to stay -near the fireplace, or near any of the flues or chimneys; you ought -not to stand under a gasalier hanging from the ceiling; you ought not -to remain close to the gas pipes or water-pipes, or any large masses -of metal, whether used in the construction of the building, or lying -loosely about.</p> - -<p>The necessity for these precautions is sufficiently evident from the -principles I have already put before you. You want to prevent your body -from becoming a link in that broken chain of conductors which, as we -have seen, the electric discharge between earth and cloud is likely to -follow. Now a tree is a better conductor than the air; and your body is -a better conductor than a tree. Hence, the lightning, in choosing the -path of least resistance, would leave the air to pass through the tree, -and would leave the tree to pass through you. A like danger would await -you if you stood under the lee of a haystack or of a house.</p> - -<p>The number of people who lose their lives by taking refuge under trees -in thunderstorms is very remarkable. As one instance out of many, I may -cite the following case which was reported in the <i>Times</i>, July -14, 1887: “Yesterday the funeral of a negress was being conducted in a -graveyard at Mount Pleasant, sixty miles north of Nashville, Tennessee, -when a storm came on, and the crowd ran for shelter under the trees. -Nine persons stood under a large oak, which the lightning struck, -killing everyone, including three clergymen, and the mother and two -sisters of the girl who had been buried.”</p> - -<p>Again, every large sheet of water constitutes practically a great -conductor, which offers a very perfect medium of discharge between the -earth round about and the cloud. Therefore, when a thundercloud is -overhead, the sheet of water is likely to become one end of the line of -the lightning discharge; and if you be standing near it, the line of -discharge may pass through your body.</p> - -<p>When lightning strikes a building, it is very apt to use the stack -of chimneys in making its way to earth, partly because the stack of -chimneys is generally the most prominent part of the building, and -partly because, on account of the heated air and the soot within the -chimney, it is usually a moderately good conductor. Therefore, if -you be indoors, you must keep well away from the chimneys; and for a -similar reason, you must keep as far as you can from large masses of -metal of every kind.</p> - -<p><span class="pagenum" id="Page_50">[Pg 50]</span></p> - -<p>Having pointed out the sources of danger which you must try to avoid -in a thunderstorm, I have nearly exhausted all the practical advice -that I have at my command. But there are some occasions on which it may -be possible, not only to avoid evident sources of danger, but to make -special provision for your own security. Thus, for example, in the open -country, if you stand a short distance from a wood, you may consider -yourself as practically protected by a lightning conductor. For a -wood, by its numerous branches and leaves, favors very much a quiet -discharge of electricity, thus tending to suppress altogether the flash -of lightning; and if the flash of lightning does come, it is much more -likely to strike the wood than to strike you, because the wood is a far -more prominent body, and offers, on the whole, an easier path to earth. -In like manner, if you place yourself near a tall solitary tree, some -twenty or thirty yards outside its longest branches, you will be in a -position of comparative safety. If the storm overtake you in the open -plain, far away from trees and buildings, you will be safer lying flat -on the ground than standing erect.</p> - -<p>In an ordinary dwelling house, the best situation is probably the -middle story, and the best position in the room is in the middle of -the floor; provided, of course, that there is no gasalier hanging from -the ceiling above or below you. Strictly speaking, the <em>middle of -the room</em> would be a still safer position than the middle of the -floor; and nothing could be more perfect than the plan suggested by -Franklin, to get into “a hammock, or swinging bed, suspended by silk -cords, and equally distant from the walls on every side, as well as -from the ceiling and floor, above and below.” An interesting case has -been recently recorded, by a resident of Venezuela, which illustrates -in a remarkable way the excellence of this advice. “The lightning,” he -says, “struck a <i>rancho</i>—a small country house, built of wood and -mud, and thatched with straw or large leaves—where one man slept in a -hammock, another lay under the hammock on the ground, and three women -were busy about the floor; there were also several hens and a pig. The -man in the hammock did not receive any injury whatever, while the other -four persons and the animals were killed.”<a id="FNanchor_37" href="#Footnote_37" class="fnanchor">[37]</a></p> - -<p>But, as I can hardly hope that many of you when the thunderstorm -actually comes will find yourselves provided with a hammock, I would -recommend, as more generally useful, another plan of Franklin’s, which -is simply to sit on one chair in the middle of the floor and put your -feet up on another. This arrangement will approach very nearly to -absolute security if you take the further precaution, also mentioned by -Franklin, of putting a feather bed or a couple of hair mattresses under -the chairs.<a id="FNanchor_38" href="#Footnote_38" class="fnanchor">[38]</a></p> - -<p><span class="pagenum" id="Page_51">[Pg 51]</span></p> -<p><b>Security Afforded by Lightning Rods.</b>—You might, perhaps, be -inclined to infer hastily, from the examples I have set before you, in -the course of this lecture, of buildings which were struck and severely -injured by lightning though provided with lightning conductors, that a -lightning rod affords a very imperfect protection to life and property. -But such an idea would be entirely at variance with the evidence at -hand on the subject. In all the cases to which I have referred, and in -many others which might easily have been cited, the damage was done -simply because the lightning rods were deficient in one or more of the -conditions on which I have so much insisted. Where these conditions are -fulfilled, the lightning flash will either not come down at all upon -the building, or, if it do come, it will be carried harmless to the -earth.</p> - -<p>Perhaps there is no one fact that so forcibly brings home to the -mind the complete protection afforded by lightning conductors as the -change which followed their introduction into the Royal Navy. I have -already told you that in former times the damage done by lightning to -ships of the Royal Navy was a regular source of expenditure, amounting -every year to several thousand pounds sterling. But, after the general -adoption of lightning conductors about forty years ago, through the -indefatigable exertions of Sir William Snow Harris, this source of -expenditure absolutely disappeared, and injury to life and property has -long been practically unknown in Her Majesty’s Fleet.</p> - -<p>I should say, however, that the trial of lightning conductors in the -Navy, though it lasted long enough to prove their perfect efficiency, -has almost come to an end in our own days. The great iron monsters -which in recent times have taken the place of the wooden ships of -Old England are quite independent of lightning rods in the common -sense of the word. Their ponderous masts are virtually lightning rods -of colossal dimensions, and their unsightly hulls are, so to speak, -earth-plates of enormous size in perfect electrical contact with the -ocean. To add to such structures lightning conductors of the common -kind would be nothing better than “wasteful and ridiculous excess.”</p> - -<p>As regards buildings on land, I may refer to the little province -of Schleswig-Holstein, of which I have already spoken to you. From -some cause or other this small peninsula is singularly exposed to -thunderstorms, and of late years it has been more abundantly provided -with lightning conductors than, perhaps, any other district of equal -extent in Europe. Now, as a simple illustration of the protection -afforded by these lightning conductors, I may mention that, on the -26th of May, 1878, a violent thunderstorm burst over the little town -of Utersen. Five several flashes of lightning fell in different parts -of the town, but not the slightest harm was done, each flash being -safely carried to earth by a lightning conductor. Further, it<span class="pagenum" id="Page_52">[Pg 52]</span> appears -from the records of the fire insurance company that, out of 552 -buildings injured by lightning during a period of eight years—from -1870 to 1878—only four had lightning conductors; and in these four -cases it was found, on examination, that the lightning conductors were -defective.<a id="FNanchor_39" href="#Footnote_39" class="fnanchor">[39]</a></p> - -<p>It would be easy to multiply evidence on this subject. But as I have -already trespassed, I fear, too far on your patience, I will content -myself with saying, in conclusion, that according to all the highest -authorities, both practical and theoretical, any structure provided -with a lightning conductor properly fitted up in conformity with the -principles I have set before you to-day is perfectly secure against -lightning. The lightning, indeed, may fall upon it, but it will pass -harmless to the earth; and the experience of more than a hundred years -has fully justified the simple and modest words of the great inventor -of lightning conductors: “It has pleased God, in His goodness to -mankind, at length to discover to them the means of securing their -habitations and other buildings from mischief by thunder and lightning.”</p> -<hr class="r5" /> - -<h3>NOTE I.</h3> - -<p class="center caption">ON THE LIGHTNING CONDUCTOR AT BEREHAVEN.<a id="FNanchor_40" href="#Footnote_40" class="fnanchor">[40]</a></p> - -<div class="blockquot"> - -<p>It is satisfactory to know that the lightning conductor referred to -in my lecture as attached to the lighthouse at Berehaven has been -put in good order under the best scientific guidance. The following -interesting letter from Professor Tyndall, which appeared in the -<i>Times</i>, August 31, 1887, gives the history of the matter very -clearly, and fully bears out the views put forward in my lecture:</p> - -<p>“Your recent remarks on thunderstorms and their effects induce me to -submit to you the following facts and considerations. Some years ago -a rock lighthouse on the coast of Ireland was struck and damaged by -lightning. An engineer was sent down to report on the occurrence; -and, as I then held the honorable and responsible post of scientific -adviser to the Trinity House and Board of Trade, the report was -submitted to me. The lightning conductor had been carried down the -lighthouse tower, its lower extremity being carefully embedded in a -stone perforated to receive it. If the object had been to invite the -lightning to strike the tower, a better arrangement could hardly have -been adopted.</p> - -<p>“I gave directions to have the conductor immediately prolonged, and -to have added to it a large terminal plate of copper, which was to be -completely submerged in the sea. The obvious convenience of a chain as -a prolongation of the conductor caused the authorities in Ireland to -propose it; but I was obliged to veto the adoption of the chain. The -contact of link with link is never perfect. I had, moreover, beside -me a portion of a chain cable through which a lightning discharge had -passed, the electricity in passing from link to link encountering -a resistance sufficient to enable it to partially fuse the chain. -The abolition of resistance is absolutely necessary in connecting -a lightning conductor with the earth, and this is done by closely -embedding in the earth a plate of good conducting material and of -large area. The largeness of area makes atonement for the imperfect -conductivity of earth. The plate, in fact, constitutes<span class="pagenum" id="Page_53">[Pg 53]</span> a wide door -through which the electricity passes freely into the earth, its -disruptive and damaging effects being thereby avoided.</p> - -<p>“These truths are elementary, but they are often neglected. I watched -with interest some time ago the operation of setting up a lightning -conductor on the house of a neighbor of mine in the country. The -wire rope which formed part of the conductor was carried down the -wall and comfortably laid in the earth below without any terminal -plate whatever. I expostulated with the man who did the work, but -he obviously thought he knew more about the matter than I did. I am -credibly informed that this is a common way of dealing with lightning -conductors by ignorant practitioners, and the Bishop of Winchester’s -palace at Farnham has been mentioned to me as an edifice ‘protected’ -in this fashion. If my informant be correct, the ‘protection’ is a -mockery, a delusion, and a snare.”</p> -</div> -<hr class="r5" /> - -<h3>NOTE II.</h3> - -<p class="center caption">BOOKS OF REFERENCE.</p> - -<div class="blockquot"> - -<p>As some of my readers may wish to pursue the study of lightning and -lightning conductors beyond the limits to which a popular lecture -must, of necessity, be confined, I subjoin a list of the books which -I think they would be likely to find most useful for the purpose. -Among ordinary text-books on physics—Jamin, Cours de Physique, -<abbr title="volume">vol.</abbr> i., <abbr title="pages">pp.</abbr> 470-494; Mascart, Traité d’Electricité Statique, <abbr title="volume">vol.</abbr> -ii., <abbr title="pages">pp.</abbr> 555-579; De Larive, A Treatise on Electricity, in three -volumes, London, 1853-8, <abbr title="volume">vol.</abbr> iii., <abbr title="pages">pp.</abbr> 90-201; Daguin, Traité -de Physique, <abbr title="volume">vol.</abbr> iii., <abbr title="pages">pp.</abbr> 209-280; Riess, Die Lehre von der -Reibungs-Elektricität, <abbr title="volume">vol.</abbr> ii., <abbr title="pages">pp.</abbr> 494-564; Müller-Pouillet, -Lehrbuch der Physik, Braunschweig, 1881, <abbr title="volume">vol.</abbr> iii., <abbr title="pages">pp.</abbr> 210-225; -Scott, Elementary Meteorology, <abbr title="chapter">chap.</abbr> x. Of the numerous special -treatises and detached papers on the subject, I would recommend -Instruction sur les Paratonnerres adopté par l’Académie des Sciences, -Part i., 1823, Part ii., 1854, Part iii., 1867, Paris, 1874; Arago, -Sur le Tonnerre, Paris, 1837; also his Meteorological Essays, -translated by Sabine, London, 1855; Sir William Snow Harris, On the -Nature of Thunderstorms, London, 1843; also by the same writer, -A Treatise on Frictional Electricity, London, 1867; and various -papers on lightning conductors, from 1822 to 1859; Tomlinson, The -Thunderstorm, London, 1877; Anderson, Lightning Conductors, London, -1880; Holtz, Ueber die Theorie, die Anlage, und die Prüfung der -Blitzableiter, Greifswald, 1878; Weber, Berichte über Blitzschläge -in der Provinz Schleswig-Holstein, Kiel, 1880-1; Tait, A Lecture -on Thunderstorms, delivered in the City Hall, Glasgow, in 1880, -Nature, <abbr title="volume">vol.</abbr> xxii.; Report of the Lightning Rod Conference, London, -1882. This last-mentioned volume comes to us with very high -authority, representing, as it does, the joint labors of several -eminent scientific men selected from the following societies: The -Meteorological Society, the Royal Institute of British Architects, the -Society of Telegraph Engineers and Electricians, the Physical Society.</p> - -<p>Since the above was in print, two lectures given before the Society -of Arts by Professor Oliver Lodge, F. R. S., have appeared in the -<i>Electrician</i>, June and July, 1888, in which some new views are -put forward respecting lightning conductors, that seem deserving of -careful consideration.</p> -</div> - -<p><span class="pagenum" id="Page_55">[Pg 55]</span></p> - - -<div class="footnotes"><h3>FOOTNOTES:</h3> - - - -<div class="footnote"> - -<p><a id="Footnote_17" href="#FNanchor_17" class="label">[17]</a> The Thunderstorm, by Charles Tomlinson, F. R. S., Third -Edition, <abbr title="pages">pp.</abbr> 153-4.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_18" href="#FNanchor_18" class="label">[18]</a> Two Lectures on Atmospheric Electricity and Protection -from Lightning, published at the end of his Treatise on Frictional -Electricity, <abbr title="page">p.</abbr> 273.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_19" href="#FNanchor_19" class="label">[19]</a> See Report of Lightning Rod Conference, <abbr title="page">p.</abbr> 119.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_20" href="#FNanchor_20" class="label">[20]</a> <i>Loco citato.</i></p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_21" href="#FNanchor_21" class="label">[21]</a> Sir William Snow Harris, <i>loco citato</i>, <abbr title="page">p.</abbr> 274.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_22" href="#FNanchor_22" class="label">[22]</a> <i>Id.</i>, <abbr title="page">p.</abbr> 275.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_23" href="#FNanchor_23" class="label">[23]</a> The Thunderstorm, by Charles Tomlinson, F.R.S., Third -Edition, <abbr title="page">p.</abbr> 172.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_24" href="#FNanchor_24" class="label">[24]</a> See for these facts, Anderson, Lightning Conductors, -<abbr title="page">p.</abbr> 197; Tomlinson, The Thunderstorm, <abbr title="pages">pp.</abbr> 167-9; Harris, <i>loco -citato</i>, <abbr title="pages">pp.</abbr> 273-4.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_25" href="#FNanchor_25" class="label">[25]</a> See Anderson, Lightning Conductors, <abbr title="pages">pp.</abbr> 170-5.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_26" href="#FNanchor_26" class="label">[26]</a> The Thunderstorm, <abbr title="pages">pp.</abbr> 158-9. See also an account of four -persons who were struck on the Matterhorn, in July, 1869, all of whom -were hurt, and none killed: Whymper’s Scrambles Among the Alps, <abbr title="pages">pp.</abbr> -414, 415.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_27" href="#FNanchor_27" class="label">[27]</a> See Philosophical Transactions of the Royal Society, -1773, <abbr title="page">p.</abbr> 42, and 1778, part i., <abbr title="page">p.</abbr> 232; Anderson’s Lightning -Conductors, <abbr title="pages">pp.</abbr> 40-2; Lighting Rod Conference, <abbr title="pages">pp.</abbr> 76-9.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_28" href="#FNanchor_28" class="label">[28]</a> See A Lecture on Thunderstorms, by Professor Tait of -Edinburgh, published in Nature, <abbr title="volume">vol.</abbr> xxii., <abbr title="page">p.</abbr> 365.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_29" href="#FNanchor_29" class="label">[29]</a> Report of the Lightning Rod Conference, <abbr title="page">p.</abbr> 4.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_30" href="#FNanchor_30" class="label">[30]</a> The dimensions here set forth are greater in some -respects than those “recommended as a minimum” in the report of the -Lightning Rod Conference, page 6. But it will be observed by those who -consult the report that the minimum recommended is just the size which, -in the preceding paragraph of the report, is said to have been actually -melted by a flash of lightning; and, therefore, it seems not to be a -very safe minimum. It will be also seen that there is some confusion -in the figures given, and that they contradict one another. For the -dimensions of iron rods, see the instructions adopted by the Academy of -Science, Paris, May 20, 1875; Lightning Rod Conference, <abbr title="pages">pp.</abbr> 67-8.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_31" href="#FNanchor_31" class="label">[31]</a> See letter of <abbr title="mister">Mr.</abbr> R. S. Newall, F. R. S., in the -<i>Times</i>, May 30, 1879.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_32" href="#FNanchor_32" class="label">[32]</a> See Nature, June 12, 1879, <abbr title="volume">vol.</abbr> xx., <abbr title="page">p.</abbr> 146.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_33" href="#FNanchor_33" class="label">[33]</a> See letter of <abbr title="mister">Mr.</abbr> Tomes in Nature, <abbr title="volume">vol.</abbr> xx., <abbr title="page">p.</abbr> 145; also -Lightning Rod Conference, <abbr title="pages">pp.</abbr> 210-15.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_34" href="#FNanchor_34" class="label">[34]</a> See Anderson, Lightning Conductors, <abbr title="pages">pp.</abbr> 208-10.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_35" href="#FNanchor_35" class="label">[35]</a> See Lightning Rod Conference, <abbr title="pages">pp.</abbr> 208-10; see also the -note at the end of this Lecture, <a href="#Page_52"><abbr title="page">p.</abbr> 52</a>.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_36" href="#FNanchor_36" class="label">[36]</a> Lecture on Thunderstorms, Nature, <abbr title="volume">vol.</abbr> xxii., <abbr title="pages">pp.</abbr> 365, -437. See, also, a very interesting paper by the late Professor J. Clerk -Maxwell, read before the British Association at Glasgow in 1876, and -reprinted in the report of the Lightning Rod Conference, <abbr title="pages">pp.</abbr> 109, 110.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_37" href="#FNanchor_37" class="label">[37]</a> Nature, <abbr title="volume">vol.</abbr> xxxi., <abbr title="page">p.</abbr> 459.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_38" href="#FNanchor_38" class="label">[38]</a> See further information on this interesting subject in -the Report of the Lightning Rod Conference, <abbr title="pages">pp.</abbr> 233-5.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_39" href="#FNanchor_39" class="label">[39]</a> See “Die Theorie, die Anlage, und die Prüfung der -Blitzableiter,” von Doctor W. Holtz, Griefswald, 1878.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_40" href="#FNanchor_40" class="label">[40]</a> See <a href="#Page_40">page 44</a>.</p> - -</div> -</div> -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_54">[Pg 54]</span></p> - -<h2 class="nobreak" id="APPENDIX">APPENDIX.<br /><span class="small">RECENT CONTROVERSY ON LIGHTNING CONDUCTORS.</span></h2> -</div> - - -<p>The lecture on lightning conductors contained in this volume fairly -represents, I think, the theory hitherto received on the subject. It -is, moreover, entirely in accord with the report of the Lightning Rod -Conference, brought out in 1883, by a committee of most eminent men, -representing several branches of science, who were specially chosen to -consider this question some ten years ago.</p> - - -<p><b>Lectures of Professor Lodge.</b>—But, in the month of March, 1888, -two lectures were given before the Society of Arts, in London, by -Professor Oliver Lodge, in which this theory was directly challenged, -and attacked with cogent arguments, supported by striking and original -experiments. These lectures gave rise to an animated controversy, which -culminated in a formal discussion at the recent meeting of the British -Association in Bath. The discussion was carried on with great spirit, -and most of the leading representatives of physical and mechanical -science took an active part in it. The greater portion of this volume -was printed off before the meeting of the British Association took -place. But the discussion on the theory of lightning conductors seemed -to me so interesting and important that I thought it right, in the form -of an Appendix, to give some account of the questions at issue, and of -the opinions expressed upon them.</p> - -<p>Professor Lodge maintains<a id="FNanchor_41" href="#Footnote_41" class="fnanchor">[41]</a> that the received theory of lightning -rods is open to two objections. First, it takes account only of the -conducting power of the lightning rod, and takes no account of the -phenomenon known as self-induction, or electrical inertia. Secondly, -it assumes that the whole substance of a lightning rod acts as a -conductor, in all cases of lightning discharge; whereas there is reason -to believe that, in many cases, it is only a thin outer shell that -really comes into action. I will deal with these two points separately.</p> - - -<p><b>The Effect of Self-Induction.</b>—When an electric discharge -begins to pass through a conductor, a momentary back electro-motive -force is developed in the conductor, which obstructs its passage. -This phenomenon is called by some self-induction, by others -electrical inertia; but its existence is admitted by all. Now, when -a flash of lightning, so to say, falls on a lightning rod, the back -electro-motive<span class="pagenum" id="Page_56">[Pg 56]</span> force developed is very considerable; and it may -offer so great an obstruction that the discharge will find an easier -passage by some other route, such as the stone walls and woodwork, and -furniture of the building.</p> - -<p>According to this view, the obstruction which a flash of lightning -encounters in a conductor consists partly of the resistance of the -conductor, in the ordinary sense of the word resistance, and partly of -the back electro-motive force due to self-induction. The sum of these -two Professor Lodge calls the <em>impedance</em> of the lightning rod; -and he considers that the impedance may be enormously great, even when -the resistance, in the ordinary sense, is comparatively small.</p> - -<p>In support of this view he has devised the following extremely -ingenious and remarkable experiment. A large Leyden jar, L, was -arranged in such a manner that, while it received a steady charge from -an electrical machine, it discharged itself, at intervals, across the -air space at A, between two brass balls. The discharge had then two -alternative paths before it; one through a conducting wire, C, the -other across a second air space, between two brass balls at B. During -the experiment, the two balls at A were kept at a fixed distance of one -inch apart; but the distance between the two balls at B was varied. -The conductor, C, used in the first instance, was a stout copper wire, -about forty feet long, and having a resistance of only one-fortieth of -an ohm.</p> - -<p class="center p2"><span class="figcenter" id="img014"> - <img src="images/014.jpg" class="w50" alt="INDUCTION EFFECT OF LEYDEN JAR DISCHARGE." /> -</span></p> -<p class="center caption">INDUCTION EFFECT OF LEYDEN JAR DISCHARGE.<br />M Electrical Machine.<br /> -L Leyden Jar.<br /> -A B Air Spaces between Brass Knobs.<br /> -C Conducting Wire.<br /></p> - -<p>It was found that, so long as the distance between the B knobs was less -than 1.43 inches, all the discharges passed across between the<span class="pagenum" id="Page_57">[Pg 57]</span> knobs, -in the form of a spark. When the distance exceeded 1.43 inches, all -the discharges passed through the conductor, C, and no spark appeared -between the balls at B. And when the distance was exactly 1.43 inches, -the discharge sometimes took place between the knobs, and sometimes -followed the conductor, C. The interpretation given to these facts -is that the obstruction offered by the conductor C was about equal -to the resistance of 1.43 inches of air; and it is proposed to call -this distance, under the conditions of the experiment, the <em>critical -distance</em>.</p> - -<p>Coming now to the application of these results, Professor Lodge argues -that the conductor C, in his experiment, represents a lightning rod -of unimpeachable excellence; and yet, in certain cases, the discharge -refuses to follow the conductor, and prefers to leap across a -considerable space of air, notwithstanding the enormous resistance it -there encounters. In like manner, he says, a flash of lightning may, -in certain cases, leave a lightning rod fitted up in the most orthodox -manner, and force its way to earth through resisting masses of mason -work and such chance conductors as may come across its path.</p> - -<p>This conclusion, he admits, is altogether at variance with the received -views on the subject; but he contends that it is perfectly in accord -with the scientific theory of an electrical discharge. The moment -the discharge begins to pass in the conductor, it encounters the -obstruction due to self-induction; and this obstruction is so great -that the bad conductors offer, on the whole, an easier path to earth.</p> - - -<p><b>Variation of the Experiment.</b>—When the experiment was varied -by substituting a thin iron wire for the stout copper wire at first -employed, a very curious result was obtained. The wire chosen was of -the same length as the copper, but had a resistance about 1,300 times -as great; its resistance being, in fact, 33.3 ohms. Nevertheless, in -this experiment, when the B knobs were at a distance of 1.43 inches, -no spark passed, which showed that the discharge always followed the -line of the conductor, and therefore that the conductor offered less -obstruction than 1.43 inches of air. The knobs were then brought -gradually nearer and nearer; and it was not until the distance was -considerably reduced that the sparks began to pass between them. When -the distance was exactly 1.03 inches, the discharge sometimes passed -between the knobs, and sometimes through the conductor; this was, -therefore, the <em>critical distance</em>, in the case of the iron wire. -Thus it appeared that the obstruction offered to the discharge by -the iron wire was much less than that offered by the copper, the one -being equal to a resistance of only 1.03 inches of air, the other to a -resistance of 1.43 inches.</p> - -<p>It does not appear that Professor Lodge undertakes to offer any -satisfactory explanation of this result. He has come to the conclusion, -from his various experiments, that, in the case of a sudden<span class="pagenum" id="Page_58">[Pg 58]</span> discharge, -difference of conducting power between fairly good conductors is a -matter of practically no account; and that difference of sectional -area is a matter of only trifling account. But he does not see why -a thin iron wire should have a <em>smaller</em> impedance than a much -thicker wire of copper. He proposes to repeat the experiments so as to -confirm or to modify the result, which for the present seems to him -anomalous.<a id="FNanchor_42" href="#Footnote_42" class="fnanchor">[42]</a></p> - - -<p><b>The Outer Shell only of a Lightning Rod Acts as a Conductor.</b>—As -a consequence of self-induction or electrical inertia, Professor -Lodge contends that a lightning discharge in a conductor consists of -a series of oscillations. These oscillations follow one another with -extraordinary rapidity—there may be a hundred thousand in a second, -there may be a million. Now it has been shown that, when a current -starts in a conductor, it does not start at once all through its -section; it begins on the outside, and then gradually, but rapidly, -penetrates to the interior. From this he infers that the extremely -rapid oscillations of a lightning discharge have not time to penetrate -to the interior of a conductor. The electricity keeps surging to -and fro in the superficial layer or outer shell, while the interior -substance of the rod remains inert and takes no part in the action. A -conductor, therefore, will be most efficient for carrying off a flash -of lightning if it present the greatest possible amount of surface; a -thin, flat tape will be more efficient than a rod of the same mass; and -a number of detached wires more efficient than a solid cylinder. As for -existing lightning conductors, the greater part of their mass would, -in many cases, have no efficacy whatever in carrying off a flash of -lightning.</p> - - -<p><b>The Discussion.</b>—The discussion at the meeting of the British -Association was opened by <abbr title="mister">Mr.</abbr> William H. Preece, F.R.S., Electrician -to the Post Office, who claimed to have 500,000 lightning conductors -under his control. He expressed his conviction that a lightning rod, -properly erected and duly maintained, was a perfect protection against -injury from lightning; and in support of this conviction he urged -very strongly the report of the Lightning Rod Conference. This report -represented the mature judgment of the most eminent scientific men, -who had devoted years to the study of the question; and he wished -particularly to bring before the meeting their clear and decisive -assertion—an assertion he was there to defend—that “there is no -authentic case on record where a properly constructed conductor failed -to do its duty.”</p> - -<p>The new views put forward by Professor Lodge were based, in great -measure, on his theory that a lightning discharge consisted of a series -of rapid oscillations. But this theory should be received with great -caution. It seemed to be nothing more than a deduction from<span class="pagenum" id="Page_59">[Pg 59]</span> certain -mathematical formulas, and was not supported by any solid basis of -observation or experiment. Besides, there were many facts against -it. They all knew that a flash of lightning magnetized steel bars, -deranged the compasses of ships at sea, and transmitted signals on -telegraph wires. But such effects could not be produced by a series of -oscillations, which, being equal and opposite, would neutralize each -other. It was alleged that these rapid oscillations occurred in the -discharge of a Leyden jar. That might be true, and probably was true; -but they were not dealing with Leyden jars, they were dealing with -flashes of lightning. If there was any analogy between the discharge of -a Leyden jar and a flash of lightning, it was to be found, not in the -external discharge employed by Professor Lodge in his experiments, but -in the bursting of the glass cylinder between the two coatings of the -jar.</p> - -<p>Lord Rayleigh thought the experiments of Professor Lodge were likely -to have important practical applications to lightning conductors. But -though these experiments were valuable as suggestions, they did not -furnish a sufficient ground for adopting any new system of protection. -It was only by experience with lightning conductors themselves that the -question could be finally settled.</p> - -<p>Sir William Thomson hoped for great fruit from the further -investigation of self-induction in the case of sudden electrical -discharges. He warmly encouraged Professor Lodge to continue his -researches; but he expressed no decided opinion on the question at -issue. Incidentally he observed that the best security for a gun-powder -magazine was an iron house; no lightning conductor at all, but an iron -roof, iron walls, and an iron floor. Wooden boards should, of course, -be placed over the floor to prevent the danger of sparks from people -walking on sheet-iron. This iron magazine might be placed on a dry -granite rock, or on wet ground; it might even be placed on a foundation -under water; it might be placed anywhere they pleased; no matter what -the surroundings were, the interior would be safe. He thought that was -an important practical conclusion which might safely be drawn from the -consideration of these electrical oscillations and the experiments -regarding them.</p> - -<p>Professor Rowland, of the Johns Hopkins University, America, said that -the question seemed to be whether the experiment of Professor Lodge -actually represented the case of lightning. He was very much disposed -to think it did not. In the experiment almost the whole circuit -consisted of good conductors; whereas, in the case of lightning, the -path of the discharge was, for the most part, through the air, and -therefore it might be an entirely different phenomenon. The air being a -very bad conductor, a flash of lightning might, perhaps, not consist of -oscillations, but rather of a single swing. Moreover, it was not at all -clear that the length of the spark, in the experiment, could<span class="pagenum" id="Page_60">[Pg 60]</span> be taken -as a measure of the obstruction offered by the conductor. Professor -George Forbes was greatly impressed with the beauty and significance of -Professor Lodge’s experiments, but he did not think the result so clear -that they should be warranted in abandoning the principles laid down by -the Lightning Rod Conference.</p> - -<p>M. de Fonvielle, of Paris, supported the views of <abbr title="mister">Mr.</abbr> Preece. He cited -the example of Paris, where they had erected a sufficient number -of lightning conductors, according to the received principles, and -calamities from lightning were practically unknown. He suggested that -the Eiffel Tower, which they were now building, and which would be -raised to the height of a thousand feet, would furnish an unrivalled -opportunity for experiments on lightning conductors.</p> - -<p>Sir James Douglass, Chief Engineer to the Corporation of Trinity House, -had a large experience with lighthouse towers. The lightning rods on -these towers had been erected and maintained during the last fifty -years entirely according to the advice of Faraday. They never had a -serious accident; and such minor accidents as did occur from time to -time were always traced to some defect in the conductor. They had now -established a more rigid system of inspection, and he, for one, should -feel perfectly safe in any tower where this system was carried out.</p> - -<p><abbr title="mister">Mr.</abbr> Symons, F.R.S., Secretary to the Meteorological Society, had taken -part in a discussion on lightning conductors as long ago as 1859. It -had been a hobby with him all his life to investigate the circumstances -of every case he came across in which damage was done by lightning, -and the general impression left by his investigations entirely -coincided with the views just expressed by Sir James Douglass. He had -been a member of the Lightning Rod Conference, and was the editor of -their report; and he wished to enter his protest against the idea of -rejecting all that had hitherto been done in connection with lightning -conductors on the strength of mere laboratory experiments.</p> - -<p>Professor Lodge, in reply, said he could perfectly understand the -position of those who held that a lightning rod properly fitted up -never failed to do its duty, because, whenever it failed, they said -it was not properly fitted up. The great resource in such cases was -to ascribe the failure to bad earth contact. He thought a good earth -contact was a very good thing, but he could not understand why such -extraordinary importance should be attached to it. A lightning rod -had two ends—an earth end and a sky end—and he did not see why good -contact was more necessary at one end than at the other. If a few sharp -points sticking out from the conductor were sufficient for a good sky -contact, why were they not sufficient also for a good earth contact?</p> - -<p>Besides, though a bad earth contact might explain why a certain amount -of disruption should take place at the earth where the bad<span class="pagenum" id="Page_61">[Pg 61]</span> contact -existed, he did not see how it accounted for the flash shooting off -sideways half-way down the conductor. Again, what does a bad earth -contact mean? If an electrical engineer finds a resistance of a -hundred ohms, he will rightly pronounce the earth contact to be very -bad indeed. But why should the lightning flash leave a conductor -with a resistance of a hundred ohms in order to follow a line of -non-conductors where it encounters a resistance of many thousand ohms?</p> - -<p>He accepted the statement of <abbr title="mister">Mr.</abbr> Preece that his whole theory depended -on the existence of oscillations in the lightning discharge; but there -was good reason to believe they existed, because they were proved to -exist in the discharge of a Leyden jar. <abbr title="mister">Mr.</abbr> Preece objected that an -oscillating discharge could not produce magnetic effects, as a flash of -lightning was known to do. He confessed he was unable to explain how -an oscillating discharge produced such effects;<a id="FNanchor_43" href="#Footnote_43" class="fnanchor">[43]</a> but that it could -produce them there was no doubt whatever, for the discharge of a Leyden -jar produces magnetic effects, and we have ocular demonstration that -the discharge of a Leyden jar is an oscillating discharge.</p> - -<p>As to the assurances we had received from electrical engineers that -a properly fitted lightning conductor never fails, he should like to -ask them how the Hotel de Ville, in Brussels, had been set on fire by -lightning on the 1st of last June. The system of lightning conductors -on this building had been erected in accordance with the received -theory, and had been held up by writers on the subject as the most -perfect in Europe. Unless some explanation were forthcoming to account -for its failure, we could no longer regard lightning conductors as a -perfect security against danger.</p> - -<p>The President of Section A, Professor Fitzgerald, in bringing the -discussion to a close, observed that one result of this meeting would -be to give a new interest to the phenomena of static electricity and -its practical applications. He was inclined himself to think that the -experiments of Professor Lodge were not quite analogous to the case of -a flash of lightning. In comparing the discharge of a Leyden jar with -a flash of lightning they should look for the analogy, not so much in -the external discharge through a series of conductors, but rather, -as <abbr title="mister">Mr.</abbr> Preece had observed, in the bursting of the glass between the -two coatings of the jar. As regarded the oscillations in a Leyden jar -discharge, he did not think such oscillations were at all necessary -to account for the phenomena observed in the experiments. Many of -the results which Professor Lodge seemed to think would require some -millions of oscillations per second would be produced by a single -discharge lasting for a millionth of a second. Improvements, perhaps, -were possible in our present system of lightning conductors,<span class="pagenum" id="Page_62">[Pg 62]</span> but -practical experience had shown, however we might reason on the matter, -that, on the whole, lightning conductors had been a great protection to -mankind from the dangers of lightning.</p> - - -<p><b>Summary.</b>—I will now try to sum up the results of this -interesting discussion, and state briefly the conclusions which, as it -seems to me, may be deduced from it. First, I would remind my readers -that a lightning rod has two functions to fulfill. Its first function -is to promote a gradual, but rapid, discharge of electricity according -as it is developed, and thus to prevent such an accumulation as would -lead to a flash of lightning. Its second function is to convey the -flash of lightning, when it does come, harmless to the earth. Now, the -new views advanced by Professor Lodge in no way impugn the efficiency -of lightning rods as regards their first function; and it is evident -that the greater the number of lightning rods distributed over a given -area, the more perfectly will this function be fulfilled. This is a -point of great practical importance which seemed to me, in some degree, -lost sight of during the progress of the discussion.</p> - -<p>Secondly, it was practically admitted by the highest authorities that -the experiments and reasoning of Professor Lodge afford good grounds -for reconsidering the received theory of lightning conductors as -regards their second function—that of carrying the lightning flash -harmless to the earth. But there was undoubtedly a general feeling -that it would be rash to set aside, all at once, the received theory -on the strength of laboratory experiments made under conditions widely -different from those which actually exist in a lightning discharge. -Experiments are wanted on a larger scale; and, if possible, experiments -with lightning rods themselves.</p> - -<p>Thirdly, the testimony of electrical engineers who have had large -experience with lightning conductors seems almost unanimous that a -lightning conductor erected and maintained in accordance with the -conditions prescribed by the Lightning Rod Conference gives perfect -protection. It was certainly unfortunate that the Hotel de Ville, in -Brussels, which was reputed the best protected building in Europe, -should have been damaged by lightning just two months before the -discussion took place; but no certain conclusion can be drawn from -this catastrophe until we know exactly the conditions under which it -occurred.</p> - -<p>So the matter stands, awaiting further investigation.</p> - - -<div class="footnotes"><h3>FOOTNOTES:</h3> - -<div class="footnote"> - -<p><a id="Footnote_41" href="#FNanchor_41" class="label">[41]</a> See his Lectures, published in the <i>Electrician</i>, -June 22, June 29, July 6, and July 13, 1888.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_42" href="#FNanchor_42" class="label">[42]</a> See paper read at the meeting of the British Association, -in Bath, 1888, published in the <i>Electrician</i>, page 607. September -14.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_43" href="#FNanchor_43" class="label">[43]</a> See a very ingenious hypothesis, to account for this -phenomenon, suggested by Professor Ewing in the <i>Electrician</i>, <abbr title="page">p.</abbr> -712. October 5, 1888.</p> - -</div> -</div> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter transnote"> -<h2 class="nobreak" id="Transcribers_Notes">Transcriber’s Notes</h2> - - -<p>Errors and omissions in punctuation have been corrected.</p> - -<p><a href="#Page_11">Page 11</a>: “continuous inpression” changed to “continuous impression”</p> - -<p><a href="#Page_41">Page 41</a>: “full conconducting power” changed to “full conducting power”</p> - -<p><a href="#Page_44">Page 44</a>: “it base” changed to “its base”</p> - -<p><a href="#Page_58">Page 58</a>: “follow one an-another” changed to “follow one another”</p> - -</div> -<div style='display:block; margin-top:4em'>*** END OF THE PROJECT GUTENBERG EBOOK LIGHTNING, THUNDER AND LIGHTNING CONDUCTORS ***</div> -<div style='text-align:left'> - -<div style='display:block; margin:1em 0'> -Updated editions will replace the previous one—the old editions will -be renamed. -</div> - -<div style='display:block; margin:1em 0'> -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. 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