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right: 84%; text-align: right;} - -.fnanchor { - vertical-align: super; - font-size: .8em; - text-decoration: - none; -} - - -/* Transcriber's notes */ -.transnote { - background-color: #E6E6FA; - color: black; - font-size:90%; - padding:0.5em; - margin-top:5em; - margin-bottom:5em; - font-family:sans-serif, serif; -} - -.transnote p {text-indent: 0em;} - - </style> - </head> - -<body> - - -<pre> - -Project Gutenberg's Practical Talks by an Astronomer, by Harold Jacoby - -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'll have -to check the laws of the country where you are located before using this ebook. - - - -Title: Practical Talks by an Astronomer - -Author: Harold Jacoby - -Release Date: October 29, 2016 [EBook #53396] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK PRACTICAL TALKS BY AN ASTRONOMER *** - - - - -Produced by Chris Curnow, John Campbell and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - - - - - -</pre> - - - -<div class="transnote"> -<p><strong>TRANSCRIBER'S NOTE</strong></p> - -<p>Obvious typographical errors and punctuation errors have been -corrected after careful comparison with other occurrences within -the text and consultation of external sources.</p> - -<p>More detail can be found at <a href="#TN">the end of the book</a>.</p> -</div> - - -<hr class="chap pg-brk" /> -<p class="p6" /> - -<h1>PRACTICAL TALKS BY<br /> -AN ASTRONOMER</h1> - -<p class="p6" /> - -<div class="figcenter pg-brk"> -<a name="FP" id="FP"></a> -<br /> -<img src="images/i_frontis.jpg" width="525" alt="" /> -<div class="caption"> - -The Moon. First Quarter.<br /> - -<span class="fs80">Photographed by Loewy and Puiseux, February 13, 1894.</span></div> -</div> - - -<hr class="chap" /> - -<div class="tpage"> -<br /><br /> -<p><span class="xxl">PRACTICAL TALKS BY<br /><br /> -AN ASTRONOMER</span></p> - -<br /><br /> -<p><span class="small">BY</span><br /><br /> -<span class="large">HAROLD JACOBY</span><br /> -<span class="xs">ADJUNCT PROFESSOR OF ASTRONOMY IN<br /> -COLUMBIA UNIVERSITY</span></p> -<br /><br /> -<p><span class="small">ILLUSTRATED</span></p> -<br /><br /> -<p><span class="medium">NEW YORK</span><br /><br /> -<span class="large lsp">CHARLES SCRIBNER'S SONS</span><br /><br /> -<span class="medium">1902</span></p> -</div> - - -<hr class="chap pg-brk" /> -<p class="p6" /> - -<p class="pfs90"><span class="smcap">Copyright, 1902, by</span><br /> -CHARLES SCRIBNER'S SONS</p> -<hr class="r5a" /> -<p class="pfs90">Published, April, 1902</p> -<p class="p6" /> - - -<p class="pfs60">TROW DIRECTORY<br /> -PRINTING AND BOOKBINDING COMPANY<br /> -NEW YORK</p> - - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_v" id="Page_v">[Pg v]</a></span></p> - -<h2><a name="PREFACE" id="PREFACE">PREFACE</a></h2> - - -<p>The present volume has not been designed as -a systematic treatise on astronomy. There are -many excellent books of that kind, suitable for -serious students as well as the general reader; -but they are necessarily somewhat dry and unattractive, -because they must aim at completeness. -Completeness means detail, and detail -means dryness.</p> - -<p>But the science of astronomy contains subjects -that admit of detached treatment; and as many -of these are precisely the ones of greatest general -interest, it has seemed well to select several, and -describe them in language free from technicalities. -It is hoped that the book will thus prove useful -to persons who do not wish to give the time -required for a study of astronomy as a whole, but -who may take pleasure in devoting a half-hour<span class="pagenum"><a name="Page_vi" id="Page_vi">[vi]</a></span> -now and then to a detached essay on some special -topic.</p> - -<p>Preparation of the book in this form has made -it suitable for prior publication in periodicals; -and the several essays have in fact all been -printed before. But the intention of collecting -them into a book was kept in mind from the -first; and while no attempt has been made at -consecutiveness, it is hoped that nothing of -merely ephemeral value has been included.</p> - - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_vii" id="Page_vii">[vii]</a></span></p> -<p class="p4" /> - -<h2><a name="CONTENTS" id="CONTENTS">CONTENTS</a></h2> - - -<div class="center smcap"> -<table border="0" cellpadding="4" cellspacing="2" width="80%" summary=""> -<tr><td class="tdl"></td><td class="tdr xs">PAGE</td></tr> -<tr><td class="tdl">Navigation at Sea</td><td class="tdr"><a href="#Page_1">1</a></td></tr> -<tr><td class="tdl">The Pleiades</td><td class="tdr"><a href="#Page_10">10</a></td></tr> -<tr><td class="tdl">The Pole-Star</td><td class="tdr"><a href="#Page_18">18</a></td></tr> -<tr><td class="tdl">Nebulæ</td><td class="tdr"><a href="#Page_27">27</a></td></tr> -<tr><td class="tdl">Temporary Stars</td><td class="tdr"><a href="#Page_37">37</a></td></tr> -<tr><td class="tdl">Galileo</td><td class="tdr"><a href="#Page_47">47</a></td></tr> -<tr><td class="tdl">The Planet of 1898</td><td class="tdr"><a href="#Page_58">58</a></td></tr> -<tr><td class="tdl">How to Make a Sun-Dial</td><td class="tdr"><a href="#Page_69">69</a></td></tr> -<tr><td class="tdl">Photography in Astronomy</td><td class="tdr"><a href="#Page_81">81</a></td></tr> -<tr><td class="tdl">Time Standards of the World</td><td class="tdr"><a href="#Page_111">111</a></td></tr> -<tr><td class="tdl">Motions of the Earth's Pole</td><td class="tdr"><a href="#Page_131">131</a></td></tr> -<tr><td class="tdl">Saturn's Rings</td><td class="tdr"><a href="#Page_140">140</a></td></tr> -<tr><td class="tdl">The Heliometer</td><td class="tdr"><a href="#Page_152">152</a></td></tr> -<tr><td class="tdl">Occultations</td><td class="tdr"><a href="#Page_161">161</a></td></tr> -<tr><td class="tdl">Mounting Great Telescopes</td><td class="tdr"><a href="#Page_170">170</a></td></tr> -<tr><td class="tdl">The Astronomer's Pole</td><td class="tdr"><a href="#Page_184">184</a></td></tr> -<tr><td class="tdl">The Moon Hoax</td><td class="tdr"><a href="#Page_199">199</a></td></tr> -<tr><td class="tdl">The Sun's Destination</td><td class="tdr"><a href="#Page_210">210</a></td></tr> -</table></div> - - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_viii" id="Page_viii">[viii]</a></span><br /> - <span class="pagenum"><a name="Page_ix" id="Page_ix">[ix]</a></span></p> -<p class="p4" /> - -<h2><a name="ILLUSTRATIONS" id="ILLUSTRATIONS">ILLUSTRATIONS</a></h2> - -<div class="center"> -<table border="0" cellpadding="4" cellspacing="0" summary=""> -<tr><td class="tdlv"><span class="smcap">The Moon. First Quarter</span><br /><em>Photographed by Loewy and Puiseux, February 13, 1894.</em></td><td class="tdr"><a href="#FP"><em>Frontispiece</em></a></td></tr> -<tr><td class="tdlv"></td><td class="tdr xs">FACING<br />PAGE </td></tr> -<tr><td class="tdlv"><span class="smcap">Spiral Nebula in Constellation Leo</span><br /><em>Photographed by Keeler, February 24, 1900.</em></td><td class="tdr"><a href="#P_26">26</a></td></tr> -<tr><td class="tdlv"><span class="smcap">Nebula in Andromeda</span><br /><em>Photographed by Barnard, November 21, 1892.</em></td><td class="tdr"><a href="#P_28">28</a></td></tr> -<tr><td class="tdlv"><span class="smcap">The "Dumb-Bell" Nebula</span><br /><em>Photographed by Keeler, July 31, 1899.</em></td><td class="tdr"><a href="#P_34">34</a></td></tr> -<tr><td class="tdlv"><span class="smcap">Star-Field in Constellation Monoceros</span><br /><em>Photographed by Barnard, February 1, 1894.</em></td><td class="tdr"><a href="#P_84">84</a></td></tr> -<tr><td class="tdlv"><span class="smcap">Solar Corona. Total Eclipse</span><br /><em>Photographed by Campbell, January 22, 1898; Jeur, India.</em></td><td class="tdr"><a href="#P_108">108</a></td></tr> -<tr><td class="tdlv"><span class="smcap">Forty-Inch Telescope, Yerkes Observatory</span></td><td class="tdr"><a href="#P_170">170</a></td></tr> -<tr><td class="tdlv"><span class="smcap">Yerkes Observatory, University of Chicago</span></td><td class="tdr"><a href="#P_176">176</a></td></tr> -</table></div> - - -<hr class="chap pg-brk" /> -<p><span class="pagenum"><a name="Page_1" id="Page_1">[Pg 1]</a></span></p> - -<p class="p4" /> -<p class="pfs150">PRACTICAL TALKS<br /> -BY AN ASTRONOMER</p> - -<h2 class="no-brk"><a name="NAVIGATION_AT_SEA" id="NAVIGATION_AT_SEA"></a><a href="#CONTENTS">NAVIGATION AT SEA</a></h2> - - -<p>A short time ago the writer had occasion to -rummage among the archives of the Royal Astronomical -Society in London, to consult, if possible, -the original manuscripts left by one Stephen -Groombridge, an English astronomer of the good -old days, who died in 1832. It was known that -they had been filed away about a generation ago, -by the late Sir George Airy, who was Astronomer -Royal of England between the years 1835 and -1881. After a long search, a large and dusty -box was found and opened. It was filled with -documents, of which the topmost was in Sir -George's own handwriting, and began substantially -as follows:</p> - -<p>"List of articles within this box.</p> - -<p class="noindent pad6"> -"No. 1, This list,<br /> -"No. 2, etc., etc."</p> - -<p><span class="pagenum"><a name="Page_2" id="Page_2">[2]</a></span></p> - -<p>Astronomical precision can no further go: he -had listed even the list itself. Truly, Airy was -rightly styled "prince of precisians." A worthy -Astronomer Royal was he, to act under the -royal warrant of Charles II., who established the -office in 1675. Down to this present day that -warrant still makes it the duty of His Majesty's -Astronomer "to apply himself with the most -exact care and diligence to the rectifying of the -tables of the motions of the heavens and the -places of the fixed stars, in order to find out the -so much desired longitude at sea, for the perfecting -the art of navigation."</p> - -<p>The "so much desired longitude at sea" is, -indeed, a vastly important thing to a maritime -nation like England. And only in comparatively -recent years has it become possible and easy for -vessels to be navigated with safety and convenience -upon long voyages. The writer was well -acquainted with an old sea-captain of New York, -who had commanded one of the earliest transatlantic -steamers, and who died only a few years ago. -He had a goodly store of ocean yarn, fit and -ready for the spinning, if he could but find some<span class="pagenum"><a name="Page_3" id="Page_3">[3]</a></span>one -who, like himself, had known and loved the -ocean. In his early sea-going days, only the -wealthiest of captains owned chronometers. This -instrument is now considered indispensable in -navigation, but at that time it was a new invention, -very rare and costly. Upon a certain -voyage from England to Rio Janeiro, in South -America, the old captain could remember the -following odd method of navigation: The ship -was steered by compass to the southward and -westward, more or less, until the skipper's -antique quadrant showed that they had about -reached the latitude of Rio. Then they swung -her on a course due west by compass, and away -she went for Rio, relying on the lookout man forward -to keep the ship from running ashore. For -after a certain lapse of time, being ignorant of the -longitude, they could not know whether they -would "raise" the land within an hour or in six -weeks. We are glad of an opportunity to put -this story on record, for the time is not far distant -when there will be no man left among the living -who can remember how ships were taken across -the seas in the good old days before chronometers.</p> - -<p><span class="pagenum"><a name="Page_4" id="Page_4">[4]</a></span></p> - -<p>Anyone who has ever been a passenger on a -great transatlantic liner of to-day knows what an -important, imposing personage is the brass-bound -skipper. A very different creature is he on the -deck of his ship from the modest seafaring man we -meet on land, clad for the time being in his shore-going -togs. But the captain's dignity is not all -brass buttons and gold braid. He has behind -him the powerful support of a deep, delightful -mystery. He it is who "takes the sun" at -noon, and finds out the ship's path at sea. And -in truth, regarded merely as a scientific experiment, -the guiding of a vessel across the unmarked -trackless ocean has few equals within the whole -range of human knowledge. It is our purpose -here to explain quite briefly the manner in which -this seeming impossibility is accomplished. We -shall not be able to go sufficiently into details to -enable him who reads to run and navigate a magnificent -steamer. But we hope to diminish somewhat -that small part of the captain's vast dignity -which depends upon his mysterious operations -with the sextant.</p> - -<p>To begin, then, with the sextant itself. It is<span class="pagenum"><a name="Page_5" id="Page_5">[5]</a></span> -nothing but an instrument with which we can -measure how high up the sun is in the sky. Now, -everyone knows that the sun slowly climbs the -sky in the morning, reaches its greatest height at -noon, and then slowly sinks again in the afternoon. -The captain simply begins to watch the -sun through the sextant shortly before noon, -and keeps at it until he discovers that the sun is -just beginning to descend. That is the instant of -noon on the ship. The captain quickly glances -at the chronometer, or calls out "noon" to an -officer who is near that instrument. And so the -error of the chronometer becomes known then -and there without any further astronomical calculations -whatever. Navigators can also find the -chronometer error by sextant observations when -the sun is a long way from noon. The methods -of doing this are somewhat less simple than for -the noon observation; they belong to the details -of navigation, into which we cannot enter here.</p> - -<p>Incidentally, the captain also notes with the -sextant how high the sun was in the sky at the -noon observation. He has in his mysterious -"chart-room" some printed astronomical tables,<span class="pagenum"><a name="Page_6" id="Page_6">[6]</a></span> -which tell him in what terrestrial latitude the sun -will have precisely that height on that particular -day of the year. Thus the terrestrial latitude becomes -known easily enough, and if only the captain -could get his longitude too, he would know -just where his ship was that day at noon.</p> - -<p>We have seen that the sextant observations -furnish the error of the chronometer according to -ship's time. In other words, the captain is in -possession of the correct local time in the place -where the ship actually is. Now, if he also had -the correct time at that moment of some well-known -place on shore, he would know the difference -in time between that place on shore and the -ship. But every traveller by land or sea is aware -that there are always differences of time between -different places on the earth. If a watch be -right on leaving New York, for instance, it will -be much too fast on arriving at Chicago or San -Francisco; the farther you go the larger becomes -the error of your watch. In fact, if you could -find out how much your watch had gone into -error, you would in a sense know how far east or -west you had travelled.</p> - -<p><span class="pagenum"><a name="Page_7" id="Page_7">[7]</a></span></p> - -<p>Now the captain's chronometer is set to correct -"Greenwich time" on shore before the ship -leaves port. His observations having then told -him how much this is wrong on that particular -day, and in that particular spot where the ship is, -he knows at once just how far he has travelled -east or west from Greenwich. In other words, -he knows his "longitude from Greenwich," for -longitude is nothing more than distance from -Greenwich in an east-and-west direction, just as -latitude is only distance from the equator measured -in a north-and-south direction. Greenwich -observatory is usually selected as the beginning -of things for measuring longitudes, because it is -almost the oldest of existing astronomical establishments, -and belongs to the most prominent -maritime nation, England.</p> - -<p>One of the most interesting bits of astronomical -history was enacted in connection with this -matter of longitude. From what has been said, -it is clear that the ship's longitude will be obtained -correctly only if the chronometer has kept -exact time since the departure of the ship from -port. Even a very small error of the chronom<span class="pagenum"><a name="Page_8" id="Page_8">[8]</a></span>eter -will throw out the longitude a good many -miles, and we can understand readily that it must -be difficult in the extreme to construct a mechanical -contrivance capable of keeping exact time -when subjected to the rolling and pitching of a -vessel at sea.</p> - -<p>It was as recently as the year 1736 that the -first instrument capable of keeping anything like -accurate time at sea was successfully completed. -It was the work of an English watchmaker -named John Harrison, and is one of the few -great improvements in matters scientific which -the world owes to a desire for winning a money -prize. It appears that in 1714 a committee was -appointed by the House of Commons, with no -less a person than Sir Isaac Newton himself as -one of its members, to consider the desirability -of offering governmental encouragement for the -invention of some means of finding the longitude -at sea. Finally, the British Government offered -a reward of $50,000 for an instrument which -would find the longitude within sixty miles; -$75,000, if within forty miles, and $100,000, if -within thirty miles. Harrison's chronometer was<span class="pagenum"><a name="Page_9" id="Page_9">[9]</a></span> -finished in 1736, but he did not receive the final -payment of his prize until 1764.</p> - -<p>We shall not enter into a detailed account of -the vexatious delays and official procedures to -which he was forced to submit during those -twenty-eight long years. It is a matter of satisfaction -that Harrison lived to receive the money -which he had earned. He had the genius to -plan and master intricate mechanical details, but -perhaps he lacked in some degree the ability of -tongue and pen to bring them home to others. -This may be the reason he is so little known, -though it was his fortune to contribute so large -and essential a part to the perfection of modern -navigation. Let us hope this brief mention may -serve to recall his memory from oblivion even for -a fleeting moment; that we may not have written -in vain of that longitude to which his life was -given.</p> - - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_10" id="Page_10">[10]</a></span></p> - -<h2><a name="THE_PLEIADES" id="THE_PLEIADES"></a><a href="#CONTENTS">THE PLEIADES</a></h2> - - -<p>Famed in legend; sung by early minstrels of -Persia and Hindustan;</p> - -<p class="pfs80"> -"—like a swarm of fire-flies tangled in a silver braid";<br /> -</p> - -<p class="noindent">yonder distant misty little cloud of Pleiades has -always won and held the imagination of men. -But it was not only for the inspiration of poets, -for quickening fancy into song, that the seven -daughters of Atlas were fixed upon the firmament. -The problems presented by this group of -stars to the unobtrusive scientific investigator are -among the most interesting known to astronomy. -Their solution is still very incomplete, but what -we have already learned may be counted justly -among the richest spoils brought back by science -from the stored treasure-house of Nature's -secrets.</p> - -<p>The true student of astronomy is animated by -no mere vulgar curiosity to pry into things hidden. -If he seeks the concealed springs that<span class="pagenum"><a name="Page_11" id="Page_11">[11]</a></span> -move the complex visible mechanism of the -heavens, he does so because his imagination is -roused by the grandeur of what he sees; and -deep down within him stirs the true love of the -artist for his art. For it is indeed a fine art, that -science of astronomy.</p> - -<p>It can have been no mere chance that has -massed the Pleiades from among their fellow -stars. Men of ordinary eyesight see but a half-dozen -distinct objects in the cluster; those of -acuter vision can count fourteen; but it is not -until we apply the space-penetrating power of -the telescope that we realize the extraordinary -scale upon which the system of the Pleiades is -constructed. With the Paris instrument Wolf in -1876 catalogued 625 stars in the group; and the -searching photographic survey of Henry in 1887 -revealed no less than 2,326 distinct stars within -and near the filmy gauze of nebulous matter always -so conspicuous a feature of the Pleiades.</p> - -<p>The means at our disposal for the study of -stellar distances are but feeble. Only in the case -of a very small number of stars have we been -able to obtain even so much as an approximate<span class="pagenum"><a name="Page_12" id="Page_12">[12]</a></span> -estimate of distance. The most powerful observational -machinery, though directed by the tried -skill of experience, has not sufficed to sound the -profounder depths of space. The Pleiad stars -are among those for which no measurement of -distance has yet been made, so that we do not -know whether they are all equally far away from -us. We see them projected on the dark background -of the celestial vault; but we cannot tell -from actual measurement whether they are all -situated near the same point in space. It may be -that some are immeasurably closer to us than are -the great mass of their companions; possibly we -look through the cluster at others far behind it, -clinging, as it were, to the very fringe of the visible -universe.</p> - -<p>Farther on we shall find evidence that something -like this really is the case. But under no -circumstances is it reasonable to suppose that the -whole body of stars can be strung out at all sorts -of distances near a straight line pointing in the -direction of the visible cluster. Such a distribution -may perhaps remain among the possibilities, -so long as we cannot measure directly the actual<span class="pagenum"><a name="Page_13" id="Page_13">[13]</a></span> -distances of the individual stars. But science -never accepts a mere possibility against which we -can marshal strong circumstantial evidence. We -may conclude on general principles that the -gathering of these many objects into a single -close assemblage denotes community of origin -and interests.</p> - -<p>The Pleiades then really belong to one another. -What is the nature of their mutual tie? -What is their mystery, and can we solve it? -The most obvious theory is, of course, suggested -by what we know to be true within our own -solar system. We owe to Newton the beautiful -conception of gravitation, that unique law by -means of which astronomers have been enabled -to reduce to perfect order the seeming -tangle of planetary evolutions. The law really -amounts, in effect, to this: All objects suspended -within the vacancy of space attract or pull one -another. How they can do this without a visible -connecting link between them is a mystery -which may always remain unsolved. But mystery -as it is, we must accept it as an ascertained -fact. It is this pull of gravitation that holds to<span class="pagenum"><a name="Page_14" id="Page_14">[14]</a></span>gether -the sun and planets, forcing them all to -follow out their due and proper paths, and so to -continue throughout an unbroken cycle until the -great survivor, Time, shall be no more.</p> - -<p>This same gravitational attraction must be at -work among the Pleiades. They, too, like ourselves, -must have bounds and orbits set and -interwoven, revolutions and gyrations far more -complex than the solar system knows. The -visual discovery of such motion of rotation -among the Pleiades may be called one of the -pressing problems of astronomy to-day. We -feel sure that the time is ripe, and that the discovery -is actually being made at the present moment: -for a generation of men is not too great a -period to call a moment, when we have to deal -with cosmic time.</p> - -<p>It is indeed the lack of observations extending -through sufficient centuries that stays our hand -from grasping the coveted result. The Pleiades -are so far from us that we cannot be sure of -changes among them. Magnitudes are always -relative. It matters not how large the actual -movements may be; if they are extremely small<span class="pagenum"><a name="Page_15" id="Page_15">[15]</a></span> -in comparison with our distance, they must -shrink to nothingness in our eyes. Trembling -on the verge of invisibility, elusive, they are in -that borderland where science as yet but feels her -way, though certain that the way is there.</p> - -<p>The foundations of exact modern knowledge -of the group were laid by Bessel about 1840. -With the modesty characteristic of the great, he -says quite simply that he has made a number of -measures of the Pleiades, thinking that the time -may come when astronomers will be able to find -some evidence of motion. In this unassuming -way he prefaces what is still the classic model of -precision and thoroughness in work of this kind. -Bessel cleared the ground for a study of inter-stellar -motion within the close star-clusters; and -it is probable that only by such study may we -hope to demonstrate the universality of the law -of gravitation in cosmic space.</p> - -<p>Bessel's acuteness in forecasting the direction -of coming research was amply verified by the -work of Elkin in 1885 at Yale College. Provided -with a more modern instrument, but similar -to Bessel's, Elkin was able to repeat his<span class="pagenum"><a name="Page_16" id="Page_16">[16]</a></span> -observations with a slight increase of precision. -Motions in the interval of forty-five years, sufficiently -great to hint at coming possibilities, were -shown conclusively to exist. Six stars at all -events have been fairly excluded from the group -on account of their peculiar motions shown by -Elkin's research. It is possible that they are -merely seen in the background through the interstices -of the cluster itself, or they may be suspended -between us and the Pleiades, in either -case having no real connection with the group. -Finally, these observations make it reasonably -certain that many of the remaining mass of stars -really constitute a unit aggregation in space. -Astronomers of a coming generation will again -repeat the Besselian work. At present we have -been able to use his method only for the separation -from the true Pleiades of chance stars that -happen to lie in the same direction. Let us hope -that man shall exist long enough upon this earth -to see the clustered stars themselves begin and -carry out such gyrations as gravitation imposes.</p> - -<p>These will doubtless be of a kind not even -suggested by the lesser complexities of our solar<span class="pagenum"><a name="Page_17" id="Page_17">[17]</a></span> -system. For the most wonderful thing of all -about the Pleiades seems to point to an intricacy -of structure whose details may be destined to -shake the confidence of the profoundest mathematician. -There is an extraordinary nebulous -condensation that seems to pervade the entire -space occupied by the stellar constituents of the -group. The stars are swimming in a veritable -sea of luminous cloud. There are filmy tenuous -places, and again condensing whirls of material -doubtless still in the gaseous or plastic stage. -Most noticeable of all are certain almost straight -lines of nebula that connect series of stars. In -one case, shown upon a photograph made by -Henry at Paris, six stars are strung out upon -such a hazy line. We might give play to fancy, -and see in this the result of some vast eruption -of gaseous matter that has already begun to -solidify here and there into stellar nuclei. But -sound science gives not too great freedom to -mere speculative theories. Her duty has been -found in quiet research, and her greatest rewards -have flowed from imaginative speculation, only -when tempered by pure reason.</p> - - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_18" id="Page_18">[18]</a></span></p> - -<h2><a name="THE_POLE-STAR" id="THE_POLE-STAR"></a><a href="#CONTENTS">THE POLE-STAR</a></h2> - - -<p>One of the most brilliant observations of the -last few years is Campbell's recent discovery of -the triple character of this star. Centuries and -centuries ago, when astronomy, that venerable -ancient among the sciences, was but an infant, -the pole-star must have been considered the very -oldest of observed heavenly bodies. In the beginning -it was the only sure guide of the navigator -at night, just as to this day it is the foundation-stone -for all observational stellar astronomy -of precision. There has never been a time in -the history of astronomy when the pole-star -might not have been called the most frequently -measured object in the sky of night. So it is -indeed strange that we should find out something -altogether new about it after all these ages -of study.</p> - -<p>But the importance of the discovery rests -upon a surer foundation than this. The method<span class="pagenum"><a name="Page_19" id="Page_19">[19]</a></span> -by which it has been made is almost a new one -in the science. A generation ago, men thought -the "perfect science," for so we love to call -astronomy, could advance only by increasing a -little the exact precision of observation. The -citadel of perfect truth might be more closely invested; -the forces of science might push forward -step by step; the machinery of research might -be strengthened, but that a new engine of investigation -would be discovered capable of penetrating -where no telescope can ever reach, this, indeed, -seemed far beyond the liveliest hope of -science. Even the discoverer of the spectroscope -could never have dreamed of its possibilities, -could never have foreseen its successes, its -triumphs.</p> - -<p>The very name of this instrument suggests -mystery to the popular mind. It is set down at -once among the things too difficult, too intricate, -too abstruse to understand. Yet in its essentials -there is nothing about the spectroscope that cannot -be made clear in a few words. Even the -modern "undulatory theory" of light itself is -terrible only in the length of its name. Any<span class="pagenum"><a name="Page_20" id="Page_20">[20]</a></span>one -who has seen the waves of ocean roll, roll, -and ever again roll in upon the shore, can form a -very good notion of how light moves. 'Tis just -such a series of rolling waves; started perhaps -from some brilliant constellation far out upon the -confining bounds of the visible universe, or perhaps -coming from a humble light upon the student's -table; yet it is never anything but a -succession of rolling waves. Only, unlike the -waves of the sea, light waves are all excessively -small. We should call one whose length was a -twenty-thousandth of an inch a big one!</p> - -<p>Now the human eye possesses the property of -receiving and understanding these little waves. -The process is an unconscious one. Let but a -set of these tiny waves roll up, as it were, out of -the vast ocean of space and impinge upon the eye, -and all the phenomena of light and color become -what we call "visible." We see the light.</p> - -<p>And how does all this find an application in -astronomy? Not to enter too much into technical -details, we may say that the spectroscope is an instrument -which enables us to measure the length -of these light waves, though their length is so<span class="pagenum"><a name="Page_21" id="Page_21">[21]</a></span> -exceedingly small. The day has indeed gone by -when that which poets love to call the Book of -Nature was printed in type that could be read -by the eye unaided. Telescope, microscope, and -spectroscope are essential now to him who would -penetrate any of Nature's secrets. But measurements -with a telescope, like eye observations, are -limited strictly to determining the directions in -which we see the heavenly bodies. Ever since -the beginning of things, when old Hipparchus and -Ulugh Beg made the first rude but successful attempts -to catalogue the stars, the eye and telescope -have been able to measure only such directions. -We aim the telescope at a star, and record the -direction in which it was pointed. Distances in -astronomy can never be measured directly. All -that we know of them has been obtained by calculations -based upon the Newtonian law of gravitation -and observations of directions.</p> - -<p>Now the spectroscope seems to offer a sort of -exception to this rule. Suppose we can measure -the wave-lengths of the light sent us from a star. -Suppose again that the star is itself moving swiftly -toward us through space, while continually set<span class="pagenum"><a name="Page_22" id="Page_22">[22]</a></span>ting -in motion the waves of light that are ultimately -to reach the waiting astronomer. Evidently -the light waves will be crowded together -somewhat on account of the star's motion. More -waves per second will reach us than would be received -from a star at rest. It is as though the -light waves were compressed or shortened a little. -And if the star is leaving us, instead of coming -nearer, opposite effects will occur. We have then -but to compare spectroscopically starlight with -some artificial source of light in the observatory in -order to find out whether the star is approaching -us or receding from us. And by a simple process -of calculation this stellar motion can be obtained -in miles per second. Thus we can now actually -measure directly, in a certain sense, linear speed -in stellar space, though we are still without the -means of getting directly at stellar distances.</p> - -<p>But the most wonderful thing of all about these -spectroscopic measures is the fact that it makes -no difference whatever how far away is the star -under observation. What we learn through the -spectroscope comes from a study of the waves -themselves, and it is of no consequence how far<span class="pagenum"><a name="Page_23" id="Page_23">[23]</a></span> -they have travelled, or how long they have been -a-coming. For it must not be supposed that -these waves consume no time in passing from a -distant star to our own solar system. It is true -that they move exceeding fast; certainly 180,000 -miles per second may be called rapid motion. -But if this cosmic velocity of light is tremendous, -so also are cosmic distances correspondingly vast. -Light needs to move quickly coming from a star, -for even at the rate of motion we have mentioned -it requires many years to reach us from some of -the more distant constellations. It has been well -said that an observer on some far-away star, if -endowed with the power to see at any distance, -however great, might at this moment be looking -on the Crusaders proceeding from Europe against -the Saracen at Jerusalem. For it is quite possible -that not until now has the light which would -make the earth visible had time to reach him. -Yet distant as such an observer might be, light -from the star on which he stood could be measured -in the spectroscope, and would infallibly tell -us whether the earth and star are approaching in -space or gradually drawing farther asunder.</p> - -<p><span class="pagenum"><a name="Page_24" id="Page_24">[24]</a></span></p> - -<p>The pole-star is not one of the more distant -stellar systems. We do not know how far it is -from us very exactly, but certainly not less than -forty or fifty years are necessary for its light to -reach us. The star might have gone out of existence -twenty years ago, and we not yet know -of it, for we would still be receiving the light -which began its long journey to us about 1850 or -1860. But no matter what may be its distance, -Campbell found by careful observations, made in -the latter part of 1896, that the pole-star was then -approaching the earth at the rate of about twelve -miles per second. So far there was nothing especially -remarkable. But in August and September -of the present year twenty-six careful determinations -were made, and these showed that now the -rate of approach varied between about five and -nine miles per second. More astonishing still, -there was a uniform period in the changes of -velocity. In about four days the rate of motion -changed from about five to nine miles and back -again. And this variation kept on with great -regularity. Every successive period of four days -saw a complete cycle of velocity change forward<span class="pagenum"><a name="Page_25" id="Page_25">[25]</a></span> -and back between the same limits. There can be -but one reasonable explanation. This star must -be a double, or "binary" star. The two components, -under the influence of powerful mutual -gravitational attraction, must be revolving in a -mighty orbit. Yet this vast orbit, as a whole, with -the two great stars in it, must be approaching our -part of the universe all the time. For the spectroscope -shows the velocity of approach to increase -and diminish, indeed, but it is always present. -Here, then, is this great stellar system, having a -four-day revolution of its own, and yet swinging -rapidly through space in our direction. Nor is -this all. One of the component stars must be -nearly or quite dark; else its presence would infallibly -be detected by our instruments.</p> - -<p>And now we come to the most astonishing -thing of all. How comes it that the average -rate of approach of the "four-day system," as a -whole, changed between 1896 and 1899? In -1896 only this velocity of the whole system was -determined, the four-day period remaining undiscovered -until the more numerous observations of -<span class="pagenum"><a name="Page_26" id="Page_26">[26]</a></span>1899. But even without considering the four-day -period, the changing velocity of the entire -system offers one of those problems that exact -science can treat only by the help of the imagination. -There must be some other great centre of -attraction, some cosmic giant, holding the visible -double pole-star under its control. Thus, that -which we see, and call the pole-star, is in reality -threading its path about the third and greatest -member of the system, itself situated in space, we -know not where.</p> - -<p><span class="pagenum"><a name="Page_27" id="Page_27">[27]</a></span></p> - -<div class="figcenter"> -<a name="P_26" id="P_26"></a> -<br /> -<img src="images/i_026fp.jpg" width="600" alt="" /> -<div class="caption"> -Spiral Nebula in Constellation Leo.<br /> - -<span class="fs80">Photographed by Keeler, February 24, 1900.<br /> -Exposure, three hours, fifty minutes.</span></div> -</div> - - -<hr class="chap" /> -<h2><a name="NEBULAE" id="NEBULAE"></a><a href="#CONTENTS">NEBULÆ</a></h2> - - -<p>Scattered about here and there among the -stars are certain patches of faint luminosity called -by astronomers Nebulæ. These "little clouds" -of filmy light are among the most fascinating of -all the kaleidoscopic phenomena of the heavens; -for it needs but a glance at one of them to give -the impression that here before us is the stuff of -which worlds are made. All our knowledge of -Nature leads us to expect in her finished work -the result of a series of gradual processes of development. -Highly organized phenomena such -as those existing in our solar system did not -spring into perfection in an instant. Influential -forces, easy to imagine, but difficult to define, -must have directed the slow, sure transformation -of elemental matter into sun and planets, things -and men. Therefore a study of those forces -and of their probable action upon nebular -material has always exerted a strong attraction<span class="pagenum"><a name="Page_28" id="Page_28">[28]</a></span> -upon the acutest thinkers among men of exact -science.</p> - -<p>Our knowledge of the nebulæ is of two kinds—that -which has been ascertained from observation -as to their appearance, size, distribution, and distance; -and that which is based upon hypotheses -and theoretical reasoning about the condensation -of stellar systems out of nebular masses. It so -happens that our observational material has received -a very important addition quite recently -through the application of photography to the -delineation of nebulæ, and this we shall describe -farther on.</p> - -<p>Two nebulæ only are visible to the unaided -eye. The brighter of these is in the constellation -Andromeda; it is of oval or elliptical shape, and -has a distinct central condensation or nucleus. -Upon a photograph by Roberts it appears to -have several concentric rings surrounding the -nebula proper, and gives the general impression -of a flat round disk foreshortened into an oval -shape on account of the observer's position not -being square to the surface of the disk. Very -recent photographs of this nebula, made with the -<span class="pagenum"><a name="Page_29" id="Page_29">[29]</a></span>three-foot reflecting telescope of the Lick Observatory, -bring out the fact that it is really spiral -in form, and that the outlying nebulous rings are -only parts of the spires in a great cosmic whorl.</p> - -<div class="figcenter"> -<a name="P_28" id="P_28"></a> -<br /> -<img src="images/i_028fp.jpg" width="500" alt="" /> -<div class="caption"> -Nebula in Andromeda.<br /> - -<span class="fs80">Lower object in the photograph is a Comet.<br /> -Photographed by Barnard, November 21, 1892.</span></div> -</div> - -<p>This Andromeda nebula is the one in which -the temporary star of 1885 appeared. It blazed -up quite suddenly near the apparent centre of the -nebula, and continued in view for six months, -fading finally beyond the reach of our most -powerful telescopes. There can be little doubt -that the star was actually in the nebula, and not -merely seen through it, though in reality situated -in the extreme outlying part of space at a distance -immeasurably greater than that separating us from -the nebula itself. Such an accidental superposition -of nebula and star might even be due to -sudden incandescence of a new star between us -and the nebula. In such a case we should see -the star projected upon the surface of the nebula, -so that the superposition would be identical with -that actually observed. Therefore, while it is, -indeed, possible that the star may have been either -far behind the nebula or in front of it, we must -accept as more probable the supposition that<span class="pagenum"><a name="Page_30" id="Page_30">[30]</a></span> -there was a real connection between the two. In -that case there is little doubt that we have actually -observed one of those cataclysms that mark -successive steps of cosmic evolution. We have -no thoroughly satisfactory theory to account for -such an explosive catastrophe within the body of -the nebula itself.</p> - -<p>The other naked-eye nebula is in the constellation -Orion. In the telescope it is a more striking -object, perhaps, than the Andromeda nebula; -for it has no well-defined geometrical form, but -consists of an immense odd-shaped mass of light -enclosing and surrounding a number of stars. It -is unquestionably of a very complicated structure, -and is, therefore, less easily studied and explained -than the nebulæ of simpler form. There is no -doubt that the Orion nebula is composed of luminous -gas, and is not merely a cluster of small -stars too numerous and too near together to be -separated from each other, even in our most -powerful telescopes. It was, indeed, supposed, -until about forty years ago, that all the nebulæ -are simply irresolvable star-clusters; but we now -have indisputable evidence, derived from the<span class="pagenum"><a name="Page_31" id="Page_31">[31]</a></span> -spectroscope, that many nebulæ are composed of -true gases, similar to those with which we experiment -in chemical laboratories. This spectroscopic -proof of the gaseous character of nebulæ is -one of the most important discoveries contributed -by that instrument to our small stock of -facts concerning the structure of the sidereal universe.</p> - -<p>Coming now to the smaller nebulæ, we find a -great diversity of form and appearance. Some -are ring-shaped, perhaps having a less brilliant -nebulosity within the ring. Many show a central -condensation of disk-like appearance (planetary -nebulæ), or have simply a star at the centre -(nebulous stars). Altogether about ten thousand -such objects have been catalogued by successive -generations of astronomers since the invention of -the telescope, and most of these have been reported -as oval in form. Now we have already -referred to the important addition to our knowledge -of the nebulæ obtained by recent photographic -observations; and this addition consists in -the discovery that most of these oval nebulæ are -in reality spirals. Indeed, it appears that the spiral<span class="pagenum"><a name="Page_32" id="Page_32">[32]</a></span> -type is the normal type, and that nebulæ of irregular -or other forms are exceptions to the general -rule. Even the great Andromeda nebula, as -we have seen, is now recognized as a spiral.</p> - -<p>The instrument with which its convolute -structure was discovered is a three-foot reflecting -telescope, made by Common of England, and now -mounted at the Lick Observatory, in California. -The late Professor Keeler devoted much of his -time to photographing nebulæ during the last year -or two. He was able to establish the important -fact just mentioned, that most nebulæ formerly -thought to be mere ovals, turn out to be spiral -when brought under the more searching scrutiny -of the photographic plate applied at the focus of -a telescope of great size, and with an exposure to -the feeble nebular light extending through three -or four consecutive hours.</p> - -<p>Many of the spirals have more than a single -volute. It is as though one were to attach a -number of very flexible rods to an axle, like -spokes of a wheel without a rim and then revolve -the axle rapidly. The flexible rods would bend -under the rapid rotation, and form a series of<span class="pagenum"><a name="Page_33" id="Page_33">[33]</a></span> -spiral curves not unlike many of these nebulæ. -Indeed, it is impossible to escape the conviction -that these great celestial whorls are whirling -around an axis. And it is most important in -the study of the growth of worlds, to recognize -that the type specimen is a revolving spiral. -Therefore, the rotating flattened globe of incandescent -matter postulated by Laplace's nebular -hypothesis would make of our solar system an -exceptional world, and not a type of stellar evolution -in general.</p> - -<p>Keeler's photographs have taught us one thing -more. Scarcely is there a single one of his negatives -that does not show nebulæ previously uncatalogued. -It is estimated that if this process of -photography could be extended so as to cover the -entire sky, the whole number of nebulæ would -add up to the stupendous total of 120,000; and -of these the great majority would be spiral.</p> - -<p>When we approach the question of the distribution -of nebulæ in different parts of the sky, as -shown by their catalogued positions, we are met -by a curious fact. It appears that the region in -the neighborhood of the Milky Way is espe<span class="pagenum"><a name="Page_34" id="Page_34">[34]</a></span>cially -poor in nebulæ, whereas these objects seem -to cluster in much larger numbers about those -points in the sky that are farthest from the -Milky Way. But we know that the Milky -Way is richer in stars than any other part of the -sky, since it is, in fact, made up of stellar bodies -clustered so closely that it is wellnigh impossible -to see between them in the denser portions. -Now, it cannot be the result of chance that the -stars should tend to congregate in the Milky -Way, while the nebulæ tend to seek a position as -far from it as possible. Whatever may be the -cause, we must conclude that the sidereal system, -as we see it, is in general constructed upon a single -plan, and does not consist of a series of universes -scattered at random throughout space. If -we are to suppose that nebulæ turn into stars as -a result of condensation or any other change, -then it is not astonishing to find a minimum of -nebulæ where there is a maximum of stars, since -the nebulæ will have been consumed, as it were, -in the formation of the stars.</p> - -<div class="figcenter"> -<a name="P_34" id="P_34"></a> -<br /> -<img src="images/i_034fp.jpg" width="500" alt="" /> -<div class="caption"> -The "Dumb-Bell" Nebula.<br /> - -<span class="fs80">Photographed by Keeler, July 31, 1899.<br /> -Exposure, three hours.</span></div> -</div> - -<p>It is never advisable to push philosophical -speculation very far when supported by too slender -<span class="pagenum"><a name="Page_35" id="Page_35">[35]</a></span>a basis of fact. But if we are to regard the -visible universe as made up on the whole of a -single system of bodies, we may well ask one or -two questions to be answered by speculative theory. -We have said the stars are not uniformly -distributed in space. Their concentration in the -Milky Way, forming a narrow band dividing the -sky into two very nearly equal parts, must be -due to their being actually massed in a thin disk -or ring of space within which our solar system is -also situated. This thin disk projected upon the -sky would then appear as the narrow star-band -of the Milky Way. Now, suppose this disk has -an axis perpendicular to itself, and let us imagine -a rotation of the whole sidereal system about -that axis. Then the fact that the visible nebulæ -are congregated far from the Milky Way -means that they are actually near the imaginary -axis.</p> - -<p>Possibly the diminished velocity of motion -near the axis may have something to do with -the presence of the nebulæ there. Possibly the -nebulæ themselves have axes perpendicular to -the plane of the Milky Way. If so, we should<span class="pagenum"><a name="Page_36" id="Page_36">[36]</a></span> -see the spiral nebulæ near the Milky Way edgewise, -and those far from it without foreshortening. -Thus, the paucity of nebulæ near the -Milky Way may be due in part to the increased -difficulty of seeing them when looked at edgewise. -Indeed, there is no limit to the possibilities -of hypothetical reasoning about the nebular -structure of our universe; unfortunately, the -whole question must be placed for the present -among those intensely interesting cosmic problems -awaiting elucidation, let us hope, in this -new century.</p> - - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_37" id="Page_37">[37]</a></span></p> - -<h2><a name="TEMPORARY_STARS" id="TEMPORARY_STARS"></a><a href="#CONTENTS">TEMPORARY STARS</a></h2> - - -<p>Nothing can be more erroneous than to suppose -that the stellar multitude has continued unchanged -throughout all generations of men. -"Eternal fires" poets have called the stars; yet -they burn like any little conflagration on the -earth; now flashing with energy, brilliant, incandescent, -and again sinking into the dull glow of -smouldering half-burned ashes. It is even probable -that space contains many darkened orbs, stars -that may have risen in constellations to adorn -the skies of prehistoric time—now cold, unseen, -unknown. So far from dealing with an unvarying -universe, it is safe to say that sidereal -astronomy can advance only by the discovery of -change. Observational science watches with untiring -industry, and night hides few celestial events -from the ardent scrutiny of astronomers. Old -theories are tested and newer ones often perfected -by the detection of some slight and previously<span class="pagenum"><a name="Page_38" id="Page_38">[38]</a></span> -unsuspected alteration upon the face of the sky. -The interpretation of such changes is the most -difficult task of science; it has taxed the acutest -intellects among men throughout all time.</p> - -<p>If, then, changes can be seen among the stars, -what are we to think of the most important change -of all, the blazing into life of a new stellar system? -Fifteen times since men began to write their -records of the skies has the birth of a star been -seen. Surely we may use this term when we -speak of the sudden appearance of a brilliant luminary -where nothing visible existed before. But -we shall see further on that scientific considerations -make it highly probable that the phenomenon -in question does not really involve the creation -of new matter. It is old material becoming -suddenly luminous for some hidden reason. In -fact, whenever a new object of great brilliancy has -been discovered, it has been found to lose its light -again quite soon, ending either in total extinction -or at least in comparative darkness. It is for this -reason that the name "temporary star" has been -applied to cases of this kind.</p> - -<p>The first authenticated instance dates from the<span class="pagenum"><a name="Page_39" id="Page_39">[39]</a></span> -year 134 <span class="smcap">B.C.</span>, when a new star appeared in the -constellation Scorpio. It was this star that led -Hipparchus to construct his stellar catalogue, the -first ever made. It occurred to him, of course, -that there could be but one way to make sure in -the future that any given object discovered in the -sky was new; it was necessary to make a complete -list of everything visible in his day. Later -astronomers need then only compare Hipparchus's -catalogue with the heavens from time to -time in order to find out whether anything unknown -had appeared. This work of Hipparchus -became the foundation of sidereal study, and led -to most important discoveries of various kinds.</p> - -<p>But no records remain concerning his new star -except the bare fact of its appearance in Scorpio. -Hipparchus's published works are all lost. We -do not even know the exact place of his birth, -and as for those two dates of entry and exit that -history attaches to great names—we have them -not. Yet he was easily the first astronomer of -antiquity, one of the first of all time; and we -know of him only from the writings of Ptolemy, -who lived three hundred years after him.</p> - -<p><span class="pagenum"><a name="Page_40" id="Page_40">[40]</a></span></p> - -<p>More than five centuries elapsed before another -temporary star was entered in the records of -astronomy. This happened in the year 389 <span class="fs70">A.D.</span>, -when a star appeared in Aquila; and of this one -also we know nothing further. But about twelve -centuries later, in November, 1572, a new and -brilliant object was found in the constellation -Cassiopeia. It is known as Tycho's star, since it -was the means of winning for astronomy a man -who will always take high rank in her annals, -Tycho Brahe, of Denmark. When he first saw -this star, it was already very bright, equalling even -Venus at her best; and he continued a careful -series of observations for sixteen months, when it -faded finally from his view. The position of the -new star was measured with reference to other stars -in the constellation Cassiopeia, and the results of -Tycho's observations were finally published by -him in the year 1573. It appears that much urging -on the part of friends was necessary to induce -him to consent to this publication, not because of -a modest reluctance to rush into print, but for the -reason that he considered it undignified for a nobleman -of Denmark to be the author of a book!</p> - -<p><span class="pagenum"><a name="Page_41" id="Page_41">[41]</a></span></p> - -<p>An important question in cosmic astronomy is -opened by Tycho's star. Did it really disappear -from the heavens when he saw it no more, or had -its lustre simply been reduced below the visual -power of the unaided eye? Unfortunately, -Tycho's observations of the star's position in the -constellation were necessarily crude. He possessed -no instruments of precision such as we now -have at our disposal, and so his work gives us -only a rather rough approximation of the true -place of the star. A small circle might be imagined -on the sky of a size comparable with the -possible errors of Tycho's observations. We -could then say with certainty that his star must -have been situated somewhere within that little -circle, but it is impossible to know exactly where.</p> - -<p>It happens that our modern telescopes reveal -the existence of several faint stars within the -space covered by such a circle. Any one of these -would have been too small for Tycho to see, and, -therefore, any one of them may be his once brilliant -luminary reduced to a state of permanent or temporary -semi-darkness. These considerations are, -indeed, of great importance in explaining the<span class="pagenum"><a name="Page_42" id="Page_42">[42]</a></span> -phenomena of temporary stars. If Tycho had -been able to leave us a more exact determination -of his star's place in the sky, and even if our most -powerful instruments could not show anything in -that place to-day, we might nevertheless theorize -on the supposition that the object still exists, but -has reached a condition almost entirely dark.</p> - -<p>Indeed, the latest theory classes temporary stars -among those known as variable. For many stars -are known to undergo quite decided changes in -brilliancy; possibly inconstancy of light is the rule -rather than the exception. But while such changes, -when they exist, are too small to be perceptible -in most cases, there is certainly a large number of -observable variables, subject to easily measurable -alterations of light. Astronomers prefer to see in -the phenomena of temporary stars simple cases of -variation in which the increase of light is sudden, -and followed by a gradual diminution. Possibly -there is then a long period of comparative or even -complete darkness, to be followed as before by a -sudden blazing up and extinction. No temporary -star, however, has been observed to reappear in the -same celestial place where once had glowed its<span class="pagenum"><a name="Page_43" id="Page_43">[43]</a></span> -sudden outburst. But cases are not wanting -where incandescence has been both preceded and -followed by a continued existence, visible though -not brilliant.</p> - -<p>For such cases as these it is necessary to come -down to modern records. We cannot be sure -that some faint star has been temporarily brilliant, -unless we actually see the conflagration itself, or -are able to make the identity of the object's precise -location in the sky before and after the event -perfectly certain by the aid of modern instruments -of precision. But no one has ever seen -the smouldering fires break out. Temporary -stars have always been first noticed only after -having been active for hours if not for days. So -we must perforce fall back on instrumental identification -by determinations of the star's exact -position upon the celestial vault.</p> - -<p>Some time between May 10th and 12th in the -year 1866 the ninth star in the list of known -"temporaries" appeared. It possessed very -great light-giving power, being surpassed in brilliancy -by only about a score of stars in all the -heavens. It retained a maximum luminosity only<span class="pagenum"><a name="Page_44" id="Page_44">[44]</a></span> -three or four days, and in less than two months -had diminished to a point somewhere between the -ninth and tenth "magnitudes." In other words, -from a conspicuous star, visible to the naked eye, -it had passed beyond the power of anything less -than a good telescope. Fortunately, we had excellent -star-catalogues before 1866. These were -at once searched, and it was possible to settle -quite definitely that a star of about the ninth or -tenth magnitude had really existed before 1866 -at precisely the same point occupied by the new -one. Needless to say, observations were made of -the new star itself, and afterward compared with -later observations of the faint one that still occupies -its place. These render quite certain the -identity of the temporary bright star with the -faint ones that preceded and followed it.</p> - -<p>Such results, on the one hand, offer an excellent -vindication of the painstaking labor expended -on the construction of star-catalogues, and, on the -other, serve to elucidate the mystery of temporary -stars. Nothing can be more plausible than -to explain by analogy those cases in which no -previous or subsequent existence has been ob<span class="pagenum"><a name="Page_45" id="Page_45">[45]</a></span>served. -It is merely necessary to suppose that, -instead of varying from the ninth or tenth magnitude, -other temporary objects have begun and -ended with the twentieth; for the twentieth magnitude -would be beyond the power of our best -instruments.</p> - -<p>Nor is the star of 1866 an isolated instance. -Ten years later, in 1876, a temporary star blazed -up to about the second magnitude, and returned -to invisibility, so far as the naked eye is concerned, -within a month, having retained its greatest brilliancy -only one or two days. This star is still -visible as a tiny point of light, estimated to be of -the fifteenth magnitude. Whether it existed -prior to its sudden outburst can never be known, -because we do not possess catalogues including -the generality of stars as faint as this one must -have been. But at all events, the continued existence -of the object helps to place the temporary -stars in the class of variables.</p> - -<p>The next star, already mentioned under "nebula," -was first seen in 1885. It was in one respect -the most remarkable of all, for it appeared -almost in the centre of the great nebula in the<span class="pagenum"><a name="Page_46" id="Page_46">[46]</a></span> -constellation Andromeda. It was never very -bright, reaching only the sixth magnitude or -thereabouts, was observed during a period of only -six months, and at the end of that time had faded -beyond the reach of our most powerful glasses. -It is a most impressive fact that this event occurred -within the nebula. Whatever may be the -nature of the explosive catastrophe to which the -temporary stars owe their origin, we can now say -with certainty that not even those vast elemental -luminous clouds men call nebulæ are free from -danger.</p> - -<p>The last outburst on our records was first noticed -February 22, 1901. The star appeared in -the constellation Perseus, and soon reached the -first magnitude, surpassing almost every other -star in the sky. It has been especially remarkable -in that it has become surrounded by a nebulous -mass in which are several bright condensations -or nuclei; and these seem to be in very -rapid motion. The star is still under observation -(January, 1902).</p> - - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_47" id="Page_47">[47]</a></span></p> - -<h2><a name="GALILEO" id="GALILEO"></a><a href="#CONTENTS">GALILEO</a></h2> - - -<p>Among the figures that stand out sharply -upon the dim background of old-time science, -there is none that excites a keener interest than -Galileo. Most people know him only as a distinguished -man of learning; one who carried on -a vigorous controversy with the Church on matters -scientific. It requires some little study, -some careful reading between the lines of astronomical -history, to gain acquaintance with the -man himself. He had a brilliant, incisive wit; -was a genuine humorist; knew well and loved -the amusing side of things; and could not often -forego a sarcastic pleasantry, or deny himself the -pleasure of argument. Yet it is more than -doubtful if he ever intended impertinence, or -gave willingly any cause of quarrel to the -Church.</p> - -<p>His acute understanding must have seen that -there exists no real conflict between science and<span class="pagenum"><a name="Page_48" id="Page_48">[48]</a></span> -religion; for time, in passing, has made common -knowledge of this truth, as it has of many things -once hidden. When we consider events that occurred -three centuries ago, it is easy to replace -excited argument with cool judgment; to remember -that those were days of violence and -cruelty; that public ignorance was of a density -difficult to imagine to-day; and that it was universally -considered the duty of the Church to -assume an authoritative attitude upon many -questions with which she is not now required to -concern herself in the least. Charlatans, unbalanced -theorists, purveyors of scientific marvels, -were all liable to be passed upon definitely by -the Church, not in a spirit of impertinent interference, -but simply as part of her regular duties.</p> - -<p>If the Church's judgment in such matters was -sometimes erroneous; if her interference now -and again was cruel, the cause must be sought in -the manners and customs of the time, when persecution -rioted in company with ignorance, and -violence was the law. Perhaps even to-day it -would not be amiss to have a modern scientific -board pass authoritatively upon novel discov<span class="pagenum"><a name="Page_49" id="Page_49">[49]</a></span>eries -and inventions, so as to protect the public -against impostors as the Church tried to do of -old.</p> - -<p>Galileo was born at Pisa in 1564, and his long -life lasted until 1642, the very year of Newton's -birth. His most important scientific discoveries -may be summed up in a few words; he was the -first to use a telescope for examining the heavenly -bodies; he discovered mountains on the -moon; the satellites of Jupiter; the peculiar -appearance of Saturn which Huygens afterward -explained as a ring surrounding the ball of the -planet; and, finally, he found black spots on the -sun's disk. These discoveries, together with his -remarkable researches in mechanical science, constitute -Galileo's claim to immortality as an investigator. -But, as we have said, it is not our -intention to consider his work as a series of scientific -discoveries. We shall take a more interesting -point of view, and deal with him rather as -a human being who had contracted the habit of -making scientific researches.</p> - -<p>What must have been his feelings when he -first found with his "new" telescope the satel<span class="pagenum"><a name="Page_50" id="Page_50">[50]</a></span>lites -of Jupiter? They were seen on the night -of January 7, 1610. He had already viewed the -planet through his earlier and less powerful -glass, and was aware that it possessed a round -disk like the moon, only smaller. Now he saw -also three objects that he took to be little stars -near the planet. But on the following night, as -he says, "drawn by what fate I know not," the -tube was again turned upon the planet. The -three small stars had changed their positions, and -were now all situated to the west of Jupiter, -whereas on the previous night two had been on -the eastern side. He could not explain this -phenomenon, but he recognized that there was -something peculiar at work. Long afterward, in -one of his later works, translated into quaint old -English by Salusbury, he declared that "one sole -experiment sufficeth to batter to the ground a -thousand probable Arguments." This was already -the guiding principle of his scientific activity, -a principle of incomparable importance, and -generally credited to Bacon. Needless to say, -Jupiter was now examined every night.</p> - -<p>The 9th was cloudy, but on the 10th he again<span class="pagenum"><a name="Page_51" id="Page_51">[51]</a></span> -saw his little stars, their number now reduced to -two. He guessed that the third was behind the -planet's disk. The position of the two visible -ones was altogether different from either of the -previous observations. On the 11th he became -sure that what he saw was really a series of satellites -accompanying Jupiter on his journey -through space, and at the same time revolving -around him. On the 12th, at 3 <span class="fs70">A.M.</span>, he actually -saw one of the small objects emerge from -behind the planet; and on the 13th he finally -saw four satellites. Two hundred and eighty-two -years were destined to pass away before any -human eye should see a fifth. It was Barnard in -1892 who followed Galileo.</p> - -<p>To understand the effect of this discovery -upon Galileo requires a person who has himself -watched the stars, not, as a dilettante, seeking -recreation or amusement, but with that deep reverence -that comes only to him who feels—nay, -knows—that in the moment of observation just -passed he too has added his mite to the great -fund of human knowledge. Galileo's mummied -forefinger still points toward the stars from its<span class="pagenum"><a name="Page_52" id="Page_52">[52]</a></span> -little pedestal of wood in the <i lang="it" xml:lang="it">Museo</i> at Florence, -a sign to all men that he is unforgotten. But -Galileo knew on that 11th of January, 1610, that -the memory of him would never fade; that the -very music of the spheres would thenceforward -be attuned to a truer note, if any would but -hearken to the Jovian harmony. For he recognized -at once that the visible revolution of these -moons around Jupiter, while that planet was -himself visibly travelling through space, must -deal its death-blow to the old Ptolemaic system -of the universe. Here was a great planet, the -centre of a system of satellites, and yet not -the centre of the universe. Surely, then, the -earth, too, might be a mere planet like Jupiter, -and not the supposed motionless centre of all -things.</p> - -<p>The satellite discovery was published in 1610 -in a little book called "Sidereus Nuncius," usually -translated "The Sidereal Messenger." It seems -to us, however, that the word "messenger" is -not strong enough; surely in Papal Italy a <i lang="la" xml:lang="la">nuncius</i> -was more than a mere messenger. He was -clothed with the very highest authority, and we<span class="pagenum"><a name="Page_53" id="Page_53">[53]</a></span> -think it probable that Galileo's choice of this -word in the title of his book means that he -claimed for himself similar authority in science. -At all events, the book made him at once a great -reputation and numerous enemies.</p> - -<p>But it was not until 1616 that the Holy Office -(Inquisition) issued an edict ordering Galileo to -abandon his opinion that the earth moved, and at -the same time placed Copernicus's <cite lang="la" xml:lang="la">De Revolutionibus</cite> -and two other books advocating that doctrine -on the "Index Librorum Prohibitorum," -or list of books forbidden by the Church. These -volumes remained in subsequent editions of the -"Index" down to 1821, but they no longer -appear in the edition in force to-day.</p> - -<p>Galileo's most characteristic work is entitled -the "Dialogue on the Two Chief Systems of the -World." It was not published until 1632, -although the idea of the book was conceived -many years earlier. In it he gave full play to his -extraordinary powers as a true humorist, a <i lang="fr" xml:lang="fr">fine -lame</i> among controversialists, and a genuine -man of science, valuing naked truth above all -other things. As may be imagined, it was no<span class="pagenum"><a name="Page_54" id="Page_54">[54]</a></span> -small matter to obtain the authorities' consent to -this publication. Galileo was already known to -hold heretical opinions, and it was suspected that -he had not laid them aside when commanded to -do so by the edict of 1616. But perhaps Galileo's -introduction to the "Dialogue" secured the -censor's <i lang="la" xml:lang="la">imprimatur</i>; it is even suspected that -the Roman authorities helped in the preparation -of this introduction. Fortunately, we have a delightful -contemporary translation into English, -by Thomas Salusbury, printed at London by -Leybourne in 1661. We have already quoted -from this translation, and now add from the same -work part of Galileo's masterly preface to the -"Dialogue":</p> - -<p>"Judicious reader, there was published some -years since in <em>Rome</em> a salutiferous Edict, that, for -the obviating of the dangerous Scandals of the -Present Age, imposed a reasonable Silence upon -the Pythagorean (Copernican) opinion of the -Mobility of the Earth. There want not such -as unadvisedly affirm, that the Decree was not -the production of a sober Scrutiny, but of an -ill-formed passion; and one may hear some<span class="pagenum"><a name="Page_55" id="Page_55">[55]</a></span> -mutter that Consultors altogether ignorant of -Astronomical observations ought not to clipp -the wings of speculative wits with rash prohibitions."</p> - -<p>Galileo first states his own views, and then -pretends that he will oppose them. He goes on -to say that he believes in the earth's immobility, -and takes "the contrary only for a mathematical -<i lang="it" xml:lang="it">Capriccio</i>," as he calls it; something to be considered, -because possessing an academical interest, -but on no account having a real existence. Of -course any one (even a censor) ought to be able -to see that it is the Capriccio, and not its opposite, -that Galileo really advocates. Three persons -appear in the "Dialogue": Salviati, who believes -in the Copernican system; Simplicio, of -suggestive name, who thinks the earth cannot -move; and, finally, Sagredus, a neutral gentleman -of humorous propensities, who usually begins -by opposing Salviati, but ends by being convinced. -He then helps to punish poor Simplicio, -who is one of those persons apparently incapable -of comprehending a reasonable argument. Here -is an interesting specimen of the "Dialogue"<span class="pagenum"><a name="Page_56" id="Page_56">[56]</a></span> -taken from Salusbury's translation: Salviati refers -to the argument, then well known, that the earth -cannot rotate on its axis, "because of the impossibility -of its moving long without wearinesse." -Sagredus replies: "There are some kinds of -animals which refresh themselves after wearinesse -by rowling on the earth; and that therefore there -is no need to fear that the Terrestrial Globe -should tire, nay, it may be reasonably affirmed -that it enjoyeth a perpetual and most tranquil repose, -keeping itself in an eternal rowling." Salviati's -comment on this sally is, "You are too -tart and satyrical, Sagredus."</p> - -<p>There is no doubt that the "Dialogue" finished -the Ptolemaic theory, and made that of -Copernicus the only possible one. At all events, -it brought about the well-known attack upon -Galileo from the authorities of the Holy Office. -We shall not recount the often-told tale of his -recantation. He was convicted (very rightly) of -being a Copernican, and was forced to abjure that -doctrine. Galileo's life may be summed up as -one of those through which the world has been -made richer. A clean-cutting analytic wit, never<span class="pagenum"><a name="Page_57" id="Page_57">[57]</a></span> -becoming dull: heated again and again in the -fierce blaze of controversy, it was allowed to cool -only that it might acquire a finer temper, to -pierce with fatal certainty the smallest imperfections -in the armor of his adversaries.</p> - - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_58" id="Page_58">[58]</a></span></p> - -<h2><a name="THE_PLANET_OF_1898" id="THE_PLANET_OF_1898"></a><a href="#CONTENTS">THE PLANET OF 1898</a></h2> - - -<p>The discovery of a new and important planet -usually receives more immediate popular attention -and applause than any other astronomical -event. Philosophers are fond of referring to our -solar system as a mere atom among the countless -universes that seem to be suspended within the -profound depths of space. They are wont to -point out that this solar system, small and insignificant -as a whole in comparison with many -of the stellar worlds, is, nevertheless, made up of -a large number of constituent planets; and these -in turn are often accompanied with still smaller -satellites, or moons. Thus does Nature provide -worlds within worlds, and it is not surprising -that public attention should be at once attracted -by any new member of our sun's own special -family of planets. The ancients were acquainted -with only five of the bodies now counted as -planets, viz.: Mercury, Venus, Mars, Jupiter,<span class="pagenum"><a name="Page_59" id="Page_59">[59]</a></span> -and Saturn. The dates of their discovery are -lost in antiquity. To these Uranus was added -in 1781 by a brilliant effort of the elder Herschel. -We are told that intense popular excitement -followed the announcement of Herschel's first -observation: he was knighted and otherwise -honored by the English King, and was enabled -to lay a secure foundation for the future distinguished -astronomical reputation of his family.</p> - -<p>Herschel's discovery quickened the restless -activity of astronomers. Persistent efforts were -made to sift the heavens more and more closely, -with the strengthened hope of adding still further -to our planetary knowledge. An association -of twenty-four enthusiastic German astronomers -was formed for the express purpose of hunting -planets. But it fell to the lot of an Italian, -Piazzi, of Palermo, to find the first of that -series of small bodies now known as the asteroids -or minor planets. He made the discovery at the -very beginning of our century, January 1, 1801.</p> - -<p>But news travelled slowly in those days, and it -was not until nearly April that the German observers -heard from Piazzi. In the meantime, he had<span class="pagenum"><a name="Page_60" id="Page_60">[60]</a></span> -himself been prevented by illness from continuing -his observations. Unfortunately, the planet -had by this time moved so near the sun, on account -of its own motions and those of the earth, -that it could no longer be observed. The bright -light of the sun made observations of the new -body impossible; and it was feared that, owing -to lack of knowledge of the planet's orbit, astronomers -would be unable to trace it. So there -seemed, indeed, to be danger of an almost irreparable -loss to science. But in scientific, as in -other human emergencies, someone always appears -at the proper moment. A very young -mathematician at Göttingen, named Gauss, attacked -the problem, and was able to devise a -method of predicting the future course of the -planet on the sky, using only the few observations -made by Piazzi himself. Up to that time -no one had attempted to compute a planetary -orbit, unless he had at his disposal a series of -observations extending throughout the whole -period of the planet's revolution around the sun. -But the Piazzi planet offered a new problem in -astronomy. It had become imperatively neces<span class="pagenum"><a name="Page_61" id="Page_61">[61]</a></span>sary -to obtain an orbit from a few observations -made at nearly the same date. Gauss's work was -signally triumphant, for the planet was actually -found in the position predicted by him, as soon -as a change in the relative places of the planet -and earth permitted suitable observations to be -made.</p> - -<p>But after all, Piazzi's planet belongs to a class -of quite small bodies, and is by no means as interesting -as Herschel's discovery, Uranus. Yet -even this must be relegated to second rank -among planetary discoveries. On September 23, -1846, the telescope of the Berlin Observatory -was directed to a certain point on the sky for a -very special reason. Galle, the astronomer of -Berlin, had received a letter from Leverrier, of -Paris, telling him that if he would look in a certain -direction he would detect a new and large planet.</p> - -<p>Leverrier's information was based upon a mathematical -calculation. Seated in his study, with -no instruments but pen and paper, he had slowly -figured out the history of a world as yet unseen. -Tiny discrepancies existed in the observed -motions of Herschel's planet Uranus. No man<span class="pagenum"><a name="Page_62" id="Page_62">[62]</a></span> -had explained their cause. To Leverrier's acute -understanding they slowly shaped themselves -into the possible effects of attraction emanating -from some unknown planet exterior to Uranus. -Was it conceivable that these slight tremulous -imperfections in the motion of a planet could be -explained in this way? Leverrier was able to -say confidently, "Yes." But we may rest assured -that Galle had but small hopes that upon -his eye first, of all the myriad eyes of men, would -fall a ray of the new planet's light. Careful -and methodical, he would neglect no chance of -advancing his beloved science. He would look.</p> - -<p>Only one who has himself often seen the morning's -sunrise put an end to a night's observation -of the stars can hope to appreciate what Galle's -feelings must have been when he saw the planet. -To his trained eye it was certainly recognizable -at once. And then the good news was sent on -to Paris. We can imagine Leverrier, the cool -calculator, saying to himself: "Of course he -found it. It was a mathematical certainty." -Nevertheless, his satisfaction must have been -of the keenest. No triumphs give a pleasure<span class="pagenum"><a name="Page_63" id="Page_63">[63]</a></span> -higher than those of the intellect. Let no one -imagine that men who make researches in the -domain of pure science are under-paid. They -find their reward in pleasure that is beyond any -price.</p> - -<p>The Leverrier planet was found to be the last -of the so-called major planets, so far as we can -say in the present state of science. It received -the name Neptune. Observers have found no -other member of the solar system comparable in -size with such bodies as Uranus and Neptune. -More than one eager mathematician has tried to -repeat Leverrier's achievement, but the supposed -planet was not found. It has been said that figures -never lie; yet such is the case only when -the computations are correctly made. People -are prone to give to the work of careless or incompetent -mathematicians the same degree of -credence that is really due only to masters of the -craft. It requires the test of time to affix to any -man's work the stamp of true genius.</p> - -<p>While, then, we have found no more large -planets, quite a group of companions to Piazzi's -little one have been discovered. They are all<span class="pagenum"><a name="Page_64" id="Page_64">[64]</a></span> -small, probably never exceeding about 400 miles -in diameter. All travel around the sun in orbits -that lie wholly within that of Jupiter and are exterior -to that of Mars. The introduction of astronomical -photography has given a tremendous -impetus to the discovery of these minor planets, -as they are called. It is quite interesting to examine -the photographic process by which such -discoveries are made possible and even easy. -The matter will not be difficult to understand if -we remember that all the planets are continually -changing their places among the other stars. For -the planets travel around the sun at a comparatively -small distance. The great majority of the -stars, on the contrary, are separated from the sun -by an almost immeasurable space. As a result, -they do not seem to move at all among themselves, -and so we call them fixed stars: they may, -indeed, be in motion, but their great distance prevents -our detecting it in a short period of time.</p> - -<p>Now, stellar photographs are made in much -the same way as ordinary portraits. Only, instead -of using a simple camera, the astronomer -exposes his photographic plate at the eye-end of<span class="pagenum"><a name="Page_65" id="Page_65">[65]</a></span> -a telescope. The sensitive surface of the plate is -substituted for the human eye. We then find -on the picture a little dot corresponding to every -star within the photographed region of the sky. -But, as everyone knows, the turning of the -earth on its axis makes the whole heavens, including -the sun, moon, and stars, rise and set -every day. So the stars, when we photograph -them, are sure to be either climbing up in the -eastern sky or else slowly creeping down in the -western. And that makes astronomical photography -very different from ordinary portrait work.</p> - -<p>The stars correspond to the sitter, but they -don't sit still. For this reason it is necessary to -connect the telescope with a mechanical contrivance -which makes it turn round like the hour-hand -of an ordinary clock. The arrangement is -so adjusted that the telescope, once aimed at the -proper object in the sky, will move so as to remain -pointed exactly the same during the whole -time of the photographic exposure. Thus, while -the light of any star is acting on the plate, such -action will be continuous at a single point. -Consequently, the finished picture will show the<span class="pagenum"><a name="Page_66" id="Page_66">[66]</a></span> -star as a little dot; while without this arrangement, -the star would trail out into a line instead -of a dot. Now we have seen that the -planets are all moving slowly among the fixed -stars. So if we make a star photograph in a part -of the sky where a planet happens to be, the -planet will make a short line on the plate; -whereas, if the planet remained quite unmoved -relatively to the stars it would give a dot like -the star dots. The presence of a line, therefore, -at once indicates a planet.</p> - -<p>This method of planet-hunting has proved -most useful. More than 400 small planets similar -to Piazzi's have been found, though never -another one like Uranus and Neptune. As -we have said, all these little bodies lie between -Mars and Jupiter. They evidently belong to a -group or family, and many astronomers have -been led to believe that they are but fragments -of a former large planet.</p> - -<p>In August, 1898, however, one was found by -Witt, of Berlin, which will probably occupy a -very prominent place in the annals of astronomy. -For this planet goes well within the orbit of<span class="pagenum"><a name="Page_67" id="Page_67">[67]</a></span> -Mars, and this will bring it at times very close -to the earth. In fact, when the motions of the -new planet and the earth combine to bring them -to their positions of greatest proximity, the new -planet will approach us closer than any other -celestial body except our own moon. Witt -named his new planet Eros. Its size, though -small, may prove to be sufficient to bring it within -the possibilities of naked-eye observation at the -time of closest approach to the earth.</p> - -<p>To astronomers the great importance of this -new planet is due to the following circumstance: -For certain reasons too technical to be stated -here in detail, the distance from the earth to any -planet can be determined with a degree of precision -which is greatest for planets that are near -us. Thus in time we shall learn the distance of -Eros more accurately than we know any other -celestial distance. From this, by a process of -calculation, the solar distance from the earth is -determinable. But the distance from earth to -sun is the fundamental astronomical unit of measure; -so that Witt's discovery, through its effect -on the unit of measure, will doubtless influence<span class="pagenum"><a name="Page_68" id="Page_68">[68]</a></span> -every part of the science of astronomy. Here -we have once more a striking instance of the -reward sure to overtake the diligent worker in -science—a whole generation of men will doubtless -pass away before we shall have exhausted -the scientific advantages to be drawn from Witt's -remarkable observation of 1898.</p> - - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_69" id="Page_69">[69]</a></span></p> - -<h2><a name="HOW_TO_MAKE_A_SUN-DIAL" id="HOW_TO_MAKE_A_SUN-DIAL"></a><a href="#CONTENTS">HOW TO MAKE A SUN-DIAL</a><a name="FNanchor_A_1" id="FNanchor_A_1"></a><a href="#Footnote_A_1" class="fnanchor">[A]</a></h2> - - -<p>Long before clocks and watches had been invented, -people began to measure time with sun-dials. -Nowadays, when almost everyone has a -watch in his pocket, and can have a clock, too, -on the mantel-piece of every room in the house, -the sun-dial has ceased to be needed in ordinary -life. But it is still just as interesting as ever to -anyone who would like to have the means of -getting time direct from the sun, the great hour-hand -or timekeeper of the sky. Any person -who is handy with tools can make a sun-dial -quite easily, by following the directions given -below.</p> - -<p>In the first place, you must know that the -sun-dial gives the time by means of the sun's -shadow. If you stick a walking-cane up in the -sand on a bright, sunshiny day, the cane has a<span class="pagenum"><a name="Page_70" id="Page_70">[70]</a></span> -long shadow that looks like a dark line on the -ground. Now if you watch this shadow carefully, -you will see that it does not stay in the -same place all day. Slowly but surely, as the -sun climbs up in the sky, the shadow creeps -around the cane. You can see quite easily that -if the cane were fastened in a board floor, and if -we could mark on the floor the places where the -shadow was at different hours of the day, we -could make the shadow tell us the time just like -the hour-hand of a clock. A sun-dial is just -such an arrangement as this, and I will show you -how to mark the shadow places exactly, so as to -tell the right time without any trouble whenever -the sun shines.</p> - -<p>If you were to watch very carefully such an -arrangement as a cane standing in a board floor, -you would not find the creeping shadow in just -the same place at the same time every day. If -you marked the place of the shadow at exactly -ten o'clock by your watch some morning, and -then went back another day at ten, you would -not find the shadow on the old mark. It would -not get very far from it in a day or two, but in a<span class="pagenum"><a name="Page_71" id="Page_71">[71]</a></span> -month or so it would be quite a distance away. -Now, of course, a sun-dial would be of no use if -it did not tell the time correctly every day; and -in fact, it is not easy to make a dial when the -shadow is cast by a stick standing straight up. -But we can get over this difficulty very well by -letting the shadow be cast by a stick that leans -over toward the floor just the right amount, as I -will explain in a moment. Of course, we should -not really use the floor for our sun-dial. It is -much better to mark out the hour-lines, as they -are called, on a smooth piece of ordinary white -board, and then, after the dial is finished, it can -be screwed down to a piazza floor or railing, or it -can be fastened on a window-sill. It ought to -be put in a place where the sun can get at it -most of the time, because, of course, you cannot -use the sun-dial when the sun is not shining on -it. If the dial is set on a window-sill (of a city -house, for instance) you must choose a south -window if you can, so as to get the sun nearly all -day. If you have to take an east window, you -can use the dial in the morning only, and in a -west window only in the afternoon. Sometimes<span class="pagenum"><a name="Page_72" id="Page_72">[72]</a></span> -it is best not to try to fasten the dial to its -support with screws, but just to mark its place, -and then set it out whenever you want to use -it. For if the dial is made of wood, and not -painted, it might be injured by rain or snow -in bad weather if left out on a window-sill or -piazza.</p> - -<div class="figcenter"> -<a name="FIG_1" id="FIG_1"></a> -<img src="images/i_072.jpg" width="600" alt="" /> -<div class="caption"> -Fig. 1.</div> -</div> - -<p>It is not quite easy to fasten a little stick to a -board so that it will lean over just right. So it -is better not to use a stick or a cane in the way -I have described, but instead to use a piece of -board cut to just the right shape.</p> - -<p>Fig. 1 shows what a sun-dial should look like.<span class="pagenum"><a name="Page_73" id="Page_73">[73]</a></span> -The lines to show the shadow's place at the different -hours of the day will be marked on the -board ABCD, and this will -be put flat on the window-sill -or piazza floor. The three-cornered -piece of board <em>abc</em> -is fastened to the bottom-board -ABCD by screws going -through ABCD from underneath. The edge <em>ab</em> -of the three-cornered board <em>abc</em> then takes the -place of the leaning stick or cane, and the time is -marked by the shadow cast by the edge <em>ab</em>. Of -course, it is important that this edge should be -straight and perfectly flat and even. If you are -handy with tools, you can make it quite easily, -but if not, you can mark the right shape on a -piece of paper very carefully, and take it to a -carpenter, who can cut the board according to -the pattern you have marked on the paper.</p> - -<div class="figright"> -<a name="FIG_2" id="FIG_2"></a> -<img src="images/i_073.jpg" width="200" alt="" /> -<div class="caption"> -Fig. 2.</div> -</div> - -<p>Now I must tell you how to draw the shape -of the three-cornered board <em>abc</em>. Fig. 2 shows -how it is done. The side <em>ac</em> should always be -just five inches long. The side <em>bc</em> is drawn at -right angles to <em>ac</em>, which you can do with an or<span class="pagenum"><a name="Page_74" id="Page_74">[74]</a></span>dinary -carpenter's square. The length of <em>bc</em> depends -on the place for which the dial is made. -The following table gives the length of <em>bc</em> for -various places in the United States, and, after you -have marked out the length of <em>bc</em>, it is only -necessary to complete the three-cornered piece by -drawing the side <em>ab</em> from <em>a</em> to <em>b</em>.</p> - -<div class="clear"><a name="TABLE_1" id="TABLE_1"></a></div> -<p class="p2 pfs80"><span class="smcap">Table Showing the Length of the Side</span> <em>bc</em>.</p> - -<div class="center fs80"> -<br /> -<table border="0" cellpadding="2" cellspacing="0" width="95%" summary=""> -<tr><td class="tdlbt" colspan="7"></td></tr> -<tr><td class="tdlx" rowspan="2">Place.</td><td></td><td class="tdc"><em>b c</em></td><td class="tdlbl tdlbr wd1"></td><td class="tdlx" rowspan="2">Place.</td><td></td><td class="tdc"><em>b c</em></td></tr> -<tr><td class="tdc wd10 pad1" colspan="2">Inches.</td><td class="tdlbl tdlbr"></td><td class="tdc wd10 pad1" colspan="2">Inches.</td></tr> -<tr><td class="tdlbt" colspan="3"></td><td class="tdlbl tdlbr"></td><td class="tdlbt" colspan="3"></td></tr> -<tr><td class="tdl">Albany</td><td class="tdl wd1">4</td><td class="tdc">11-16</td><td class="tdlbl tdlbr"></td><td class="tdl">New York</td><td class="tdl wd1">4</td><td class="tdc">3-8</td></tr> -<tr><td class="tdl">Baltimore</td><td class="tdl">4</td><td class="tdc">1-16</td><td class="tdlbl tdlbr"></td><td class="tdl">Omaha</td><td class="tdl">4</td><td class="tdc">3-8</td></tr> -<tr><td class="tdl">Boston</td><td class="tdl">4</td><td class="tdc">1-2</td><td class="tdlbl tdlbr"></td><td class="tdl">Philadelphia</td><td class="tdl">4</td><td class="tdc">3-16</td></tr> -<tr><td class="tdl">Buffalo</td><td class="tdl">4</td><td class="tdc">11-16</td><td class="tdlbl tdlbr"></td><td class="tdl">Pittsburg</td><td class="tdl">4</td><td class="tdc">3-8</td></tr> -<tr><td class="tdl">Charleston</td><td class="tdl">3</td><td class="tdc">1-4</td><td class="tdlbl tdlbr"></td><td class="tdl">Portland, Me</td><td class="tdl">4</td><td class="tdc">13-16</td></tr> -<tr><td class="tdl">Chicago</td><td class="tdl">4</td><td class="tdc">1-2</td><td class="tdlbl tdlbr"></td><td class="tdl">Richmond</td><td class="tdl">3</td><td class="tdc">15-16</td></tr> -<tr><td class="tdl">Cincinnati</td><td class="tdl">4</td><td class="tdc">1-16</td><td class="tdlbl tdlbr"></td><td class="tdl">Rochester</td><td class="tdl">4</td><td class="tdc">11-16</td></tr> -<tr><td class="tdl">Cleveland</td><td class="tdl">4</td><td class="tdc">1-2</td><td class="tdlbl tdlbr"></td><td class="tdl">San Diego</td><td class="tdl">3</td><td class="tdc">1-4</td></tr> -<tr><td class="tdl">Denver</td><td class="tdl">4</td><td class="tdc">3-16</td><td class="tdlbl tdlbr"></td><td class="tdl">San Francisco</td><td class="tdl">3</td><td class="tdc">15-16</td></tr> -<tr><td class="tdl">Detroit</td><td class="tdl">4</td><td class="tdc">1-2</td><td class="tdlbl tdlbr"></td><td class="tdl">Savannah</td><td class="tdl">3</td><td class="tdc">1-8</td></tr> -<tr><td class="tdl">Indianapolis</td><td class="tdl">4</td><td class="tdc">1-16</td><td class="tdlbl tdlbr"></td><td class="tdl">St. Louis</td><td class="tdl">3</td><td class="tdc">15-16</td></tr> -<tr><td class="tdl">Kansas City</td><td class="tdl">3</td><td class="tdc">15-16</td><td class="tdlbl tdlbr"></td><td class="tdl">St. Paul</td><td class="tdl">5</td></tr> -<tr><td class="tdl">Louisville</td><td class="tdl">3</td><td class="tdc">15-16</td><td class="tdlbl tdlbr"></td><td class="tdl">Seattle</td><td class="tdl">5</td><td class="tdc">9-16</td></tr> -<tr><td class="tdl">Milwaukee</td><td class="tdl">3</td><td class="tdc">11-16</td><td class="tdlbl tdlbr"></td><td class="tdl">Washington, D. C.</td><td class="tdl">4</td><td class="tdc">1-16</td></tr> -<tr><td class="tdl">New Orleans</td><td class="tdl">2</td><td class="tdc">7-8</td><td class="tdlbl tdlbr"></td></tr> -<tr><td class="tdlbt" colspan="7"></td></tr> -</table><br /></div> - -<p>If you wish to make a dial for a place not given -in the table, it will be near enough to use the distance -<em>bc</em> as given for the place nearest to you. -But in selecting the nearest place from the table, -please remember to take that one of the cities -mentioned which is nearest to you in a north-and-south<span class="pagenum"><a name="Page_75" id="Page_75">[75]</a></span> -direction. It does not matter how far away -the place is in an east-and-west direction. So, instead -of taking the place that is nearest to you on -the map in a straight line, take the place to which -you could travel by going principally east or west, -and very little north or south. The figure drawn -is about the right shape for New York. The -board used for the three-cornered piece should be -about one-half inch thick. But if you are making -a window-sill dial, you may prefer to have it -smaller than I have described. You can easily -have it half as big by making all the sizes and -lines in half-inches where the table calls for -inches.</p> - -<p>After you have marked out the dimensions for -the three-cornered piece that is to throw the -shadow, you can prepare the dial itself, with the -lines that mark the place of the shadow for every -hour of the day. This you can do in the manner -shown in Fig. 3. Just as in the case of the three-cornered -piece, you can draw the dial with a pencil -directly on a smooth piece of white board, about -three-quarters of an inch thick, or you can mark -it out on a paper pattern and transfer it afterward<span class="pagenum"><a name="Page_76" id="Page_76">[76]</a></span> -to the board. Perhaps it will be as well to begin -by drawing on paper, as any mistakes can then -be corrected before you commence to mark your -wood.</p> - -<div class="figcenter"> -<a name="FIG_3" id="FIG_3"></a> -<img src="images/i_076.jpg" width="600" alt="" /> -<div class="caption"> -Fig. 3.</div> -</div> - -<p>In the first place you must draw a couple of -lines MN and M′N′, eight inches long, and just -far enough apart to fit the edge of your three-cornered -shadow-piece. You will remember I -told you to make that one-half inch thick, so -your two lines will also be one-half inch apart. -Now draw the two lines NO and N′O′ square<span class="pagenum"><a name="Page_77" id="Page_77">[77]</a></span> -with MN and M′N′, and make the distances -NO and N′O′ just five inches each. The lines -OK, O′K′, and the other lines forming the outer -border of the dial, are then drawn just as shown, -OK and O′K′ being just eight inches long, the -same as MN and M′N′. The lower lines in -the figure, which are not very important, are to -complete the squares. You must mark the lines -NO and N′O′ with the figures VI, these being -the lines reached by the shadow at six o'clock in -the morning and evening. The points where the -VII, VIII, and other hour-lines cut the lines -OK, O′K′, MK, and M′K′ can be found from -the table on <a href="#Page_78">page 78</a>.</p> - -<p>In using the table you will notice that the line -IX falls sometimes on one side of the corner K, -and sometimes on the other. Thus for Albany -the line passes seven and seven-sixteenth inches -from O, while for Charleston it passes four and -three-eighth inches from M. For Baltimore it -passes exactly through the corner K.</p> - -<p><span class="pagenum"><a name="Page_78" id="Page_78">[78]</a></span></p> - -<div><a name="TABLE_2" id="TABLE_2"></a></div> -<p class="p2 pfs80"><span class="smcap">Table Showing How to Mark the Hour-lines.</span></p> - -<div class="center fs80"> -<br /> -<table border="0" cellpadding="4" cellspacing="0" width="95%" summary=""> -<tr><td class="tdlbt" colspan="8"></td></tr> -<tr><td class="tdc" rowspan="3">Place.</td><td class="tdcbl tdpp" colspan="3">Distance from O to the line marked</td> - <td class="tdlbl tdlbr"></td><td class="tdc tdpp" colspan="3">Distance from M to the line marked</td></tr> -<tr><td class="tdlbt" colspan="3"></td><td class="tdlbl tdlbr"></td><td class="tdlbt" colspan="3"></td></tr> -<tr><td class="tdcbl tdpp">VII.</td><td class="tdcbl tdpp">VIII.</td><td class="tdcbl tdpp">IX.</td> - <td class="tdlbl tdlbr"></td><td class="tdc tdpp">IX.</td><td class="tdcbl tdpp">X.</td><td class="tdcbl tdpp">XI.</td></tr> -<tr><td class="tdlbt" colspan="4"></td><td class="tdlbl tdlbr"></td><td class="tdlbt" colspan="3"></td></tr> -<tr><td class="tdl"></td><td class="tdlbl">Inches.</td><td class="tdlbl">Inches.</td><td class="tdlbl">Inches.</td> - <td class="tdlbl tdlbr"></td><td class="tdl">Inches.</td><td class="tdlbl">Inches.</td><td class="tdlbl">Inches.</td></tr> -<tr><td class="tdl">Albany</td><td class="tdlbl">1 15-16</td><td class="tdlbl">4 3-16</td><td class="tdlbl">7 7-16</td> - <td class="tdlbl tdlbr wd1"></td><td class="tdl"></td><td class="tdlbl">3 1-16</td><td class="tdlbl">1 7-16</td></tr> -<tr><td class="tdl">Baltimore</td><td class="tdlbl">2 1-8</td><td class="tdlbl">4 11-16</td><td class="tdlbl">8</td> - <td class="tdlbl tdlbr"></td><td class="tdl"></td><td class="tdlbl">2 7-8</td><td class="tdlbl">1 7-16</td></tr> -<tr><td class="tdl">Boston</td><td class="tdlbl">2</td><td class="tdlbl">4 5-16</td><td class="tdlbl">7 7-16</td> - <td class="tdlbl tdlbr"></td><td class="tdl"></td><td class="tdlbl">3 1-16</td><td class="tdlbl">1 7-16</td></tr> -<tr><td class="tdl">Buffalo</td><td class="tdlbl">1 15-16</td><td class="tdlbl">4 3-16</td><td class="tdlbl">7 7-16</td> - <td class="tdlbl tdlbr"></td><td class="tdl"></td><td class="tdlbl">3 1-16</td><td class="tdlbl">1 7-16</td></tr> -<tr><td class="tdl">Charleston</td><td class="tdlbl">2 7-16</td><td class="tdlbl">5 3-8</td><td class="tdlbl"></td> - <td class="tdlbl tdlbr"></td><td class="tdl">4 3-8</td><td class="tdlbl">2 1-2</td><td class="tdlbl">1 1-8</td></tr> -<tr><td class="tdl">Chicago</td><td class="tdlbl">2</td><td class="tdlbl">4 5-16</td><td class="tdlbl">7 7-16</td> - <td class="tdlbl tdlbr"></td><td class="tdl"></td><td class="tdlbl">3 1-16</td><td class="tdlbl">1 7-16</td></tr> -<tr><td class="tdl">Cincinnati</td><td class="tdlbl">2 1-8</td><td class="tdlbl">4 11-16</td><td class="tdlbl">8</td> - <td class="tdlbl tdlbr"></td><td class="tdl"></td><td class="tdlbl">2 7-8</td><td class="tdlbl">1 7-16</td></tr> -<tr><td class="tdl">Cleveland</td><td class="tdlbl">2</td><td class="tdlbl">4 5-16</td><td class="tdlbl">7 7-16</td> - <td class="tdlbl tdlbr"></td><td class="tdl"> —</td><td class="tdlbl">3 1-16</td><td class="tdlbl">1 7-16</td></tr> -<tr><td class="tdl">Denver</td><td class="tdlbl">2 1-8</td><td class="tdlbl">4 1-2</td><td class="tdlbl">7 11-16</td> - <td class="tdlbl tdlbr"></td><td class="tdl"></td><td class="tdlbl">2 7-8</td><td class="tdlbl">1 7-16</td></tr> -<tr><td class="tdl">Detroit</td><td class="tdlbl">2</td><td class="tdlbl">4 5-16</td><td class="tdlbl">7 7-16</td> - <td class="tdlbl tdlbr"></td><td class="tdl"></td><td class="tdlbl">3 1-16</td><td class="tdlbl">1 7-16</td></tr> -<tr><td class="tdl">Indianapolis</td><td class="tdlbl">2 1-8</td><td class="tdlbl">4 11-16</td><td class="tdlbl">8</td> - <td class="tdlbl tdlbr"></td><td class="tdl"></td><td class="tdlbl">2 7-8</td><td class="tdlbl">1 7-16</td></tr> -<tr><td class="tdl">Kansas City</td><td class="tdlbl">2 1-4</td><td class="tdlbl">4 11-16</td><td class="tdlbl">8</td> - <td class="tdlbl tdlbr"></td><td class="tdl"></td><td class="tdlbl">2 7-8</td><td class="tdlbl">1 5-16</td></tr> -<tr><td class="tdl">Louisville</td><td class="tdlbl">2 1-4</td><td class="tdlbl">4 11-16</td><td class="tdlbl">8</td> - <td class="tdlbl tdlbr"></td><td class="tdl"></td><td class="tdlbl">2 7-8</td><td class="tdlbl">1 5-16</td></tr> -<tr><td class="tdl">Milwaukee</td><td class="tdlbl">1 15-16</td><td class="tdlbl">4 3-16</td><td class="tdlbl">7 7-16</td> - <td class="tdlbl tdlbr"></td><td class="tdl"></td><td class="tdlbl">3 1-16</td><td class="tdlbl">1 7-16</td></tr> -<tr><td class="tdl">New Orleans</td><td class="tdlbl">2 11-16</td><td class="tdlbl">5 3-4</td><td class="tdlbl"></td> - <td class="tdlbl tdlbr"></td><td class="tdl">4 1-16</td><td class="tdlbl">2 5-16</td><td class="tdlbl">1 1-8</td></tr> -<tr><td class="tdl">New York</td><td class="tdlbl">2</td><td class="tdlbl">4 5-16</td><td class="tdlbl">7 11-16</td> - <td class="tdlbl tdlbr"></td><td class="tdl"></td><td class="tdlbl">3 1-16</td><td class="tdlbl">1 7-16</td></tr> -<tr><td class="tdl">Omaha</td><td class="tdlbl">2</td><td class="tdlbl">4 5-16</td><td class="tdlbl">7 11-16</td> - <td class="tdlbl tdlbr"></td><td class="tdl"></td><td class="tdlbl">3 1-16</td><td class="tdlbl">1 7-16</td></tr> -<tr><td class="tdl">Philadelphia</td><td class="tdlbl">2 1-8</td><td class="tdlbl">4 1-2</td><td class="tdlbl">7 11-16</td> - <td class="tdlbl tdlbr"></td><td class="tdl"></td><td class="tdlbl">2 7-8</td><td class="tdlbl">1 7-16</td></tr> -<tr><td class="tdl">Pittsburg</td><td class="tdlbl">2</td><td class="tdlbl">4 5-16</td><td class="tdlbl">7 11-16</td> - <td class="tdlbl tdlbr"></td><td class="tdl"></td><td class="tdlbl">3 1-16</td><td class="tdlbl">1 7-16</td></tr> -<tr><td class="tdl">Portland, Me</td><td class="tdlbl">1 15-16</td><td class="tdlbl">4 3-16</td><td class="tdlbl">7 1-8</td> - <td class="tdlbl tdlbr"></td><td class="tdl"></td><td class="tdlbl">3 3-16</td><td class="tdlbl">1 1-2</td></tr> -<tr><td class="tdl">Richmond</td><td class="tdlbl">2 1-4</td><td class="tdlbl">4 11-16</td><td class="tdlbl">8</td> - <td class="tdlbl tdlbr"></td><td class="tdl"></td><td class="tdlbl">2 7-8</td><td class="tdlbl">1 5-16</td></tr> -<tr><td class="tdl">Rochester</td><td class="tdlbl">1 15-16</td><td class="tdlbl">4 3-16</td><td class="tdlbl">7 7-16</td> - <td class="tdlbl tdlbr"></td><td class="tdl"></td><td class="tdlbl">3 1-16</td><td class="tdlbl">1 7-16</td></tr> -<tr><td class="tdl">San Diego</td><td class="tdlbl">2 7-16</td><td class="tdlbl">5 3-8</td><td class="tdlbl"></td> - <td class="tdlbl tdlbr"></td><td class="tdl">4 3-8</td><td class="tdlbl">2 1-2</td><td class="tdlbl">1 1-8</td></tr> -<tr><td class="tdl">San Francisco</td><td class="tdlbl">2 1-4</td><td class="tdlbl">4 11-16</td><td class="tdlbl">8</td> - <td class="tdlbl tdlbr"></td><td class="tdl"></td><td class="tdlbl">2 7-8</td><td class="tdlbl">1 5-16</td></tr> -<tr><td class="tdl">Savannah</td><td class="tdlbl">2 9-16</td><td class="tdlbl">5 9-16</td><td class="tdlbl"></td> - <td class="tdlbl tdlbr"></td><td class="tdl">4 1-4</td><td class="tdlbl">2 1-2</td><td class="tdlbl">1 1-8</td></tr> -<tr><td class="tdl">St. Louis</td><td class="tdlbl">2 1-4</td><td class="tdlbl">4 11-16</td><td class="tdlbl">8</td> - <td class="tdlbl tdlbr"></td><td class="tdl"></td><td class="tdlbl">2 7-8</td><td class="tdlbl">1 5-16</td></tr> -<tr><td class="tdl">St. Paul</td><td class="tdlbl">1 15-16</td><td class="tdlbl">4 1-16</td><td class="tdlbl">7 1-8</td> - <td class="tdlbl tdlbr"></td><td class="tdl"></td><td class="tdlbl">3 3-16</td><td class="tdlbl">1 1-2</td></tr> -<tr><td class="tdl">Seattle</td><td class="tdlbl">1 13-16</td><td class="tdlbl">3 15-16</td><td class="tdlbl">6 5-8</td> - <td class="tdlbl tdlbr"></td><td class="tdl"></td><td class="tdlbl">3 3-8</td><td class="tdlbl">1 1-2</td></tr> -<tr><td class="tdl">Washington, D. C.</td><td class="tdlbl">2 1-8</td><td class="tdlbl">4 11-16</td><td class="tdlbl">8</td> - <td class="tdlbl tdlbr"></td><td class="tdl"></td><td class="tdlbl">2 7-8</td><td class="tdlbl">1 7-16</td></tr> -<tr><td class="tdlbt" colspan="8"></td></tr> -</table><br /></div> - -<p>The distance for the line marked V from O′ is -just the same as the distance from O to VII. -Similarly, IV corresponds to VIII, III to IX, -II to X, and I to XI. The number XII is -marked at MM′ as shown. If you desire to add -lines (not shown in Fig. 3 to avoid confusion) for -hours earlier than six in the morning, it is merely -necessary to mark off a distance on the line KO, -below the point O, and equal to the distance from -O to VII. This will give the point where the<span class="pagenum"><a name="Page_79" id="Page_79">[79]</a></span> -5 <span class="fs70">A.M.</span> shadow line drawn from N cuts the line -KO. A corresponding line for 7 <span class="fs70">P.M.</span> can be -drawn from N′ on the other side of the figure.</p> - -<p>After you have marked out the dial very carefully, -you must fasten the three-cornered shadow-piece -to it in such a way that the whole instrument -will look like <a href="#FIG_1">Fig. 1</a>. The edge <em>ac</em> (Fig. -2) goes on NM (Fig. 3). The point <em>a</em> (Fig. 2) -must come exactly on N (Fig. 3); and as the -lines NM (Fig. 3) and N′M′ (Fig. 3) have been -made just the right distance apart to fit the -thickness of the three-cornered piece <em>abc</em> (Fig. -2), everything will go together just right. The -point <em>c</em> (Fig. 2) will not quite reach to M (Fig. 3), -but will be on the line NM (Fig. 3) at a distance -of three inches from M. The two pieces of -wood will be fastened together with three screws -going through the bottom-board ABCD (Figs. -1 and 3) and into the edge <em>ac</em> (Fig. 2) of the -three-cornered piece. The whole instrument will -then look something like <a href="#FIG_1">Fig. 1</a>.</p> - -<p>After you have got your sun-dial put together, -you need only set it in the sun in a level place, -on a piazza or window-sill, and turn it round<span class="pagenum"><a name="Page_80" id="Page_80">[80]</a></span> -until it tells the right time by the shadow. You -can get your local time from a watch near enough -for setting up the dial. Once the dial is set right -you can screw it down or mark its position, and -it will continue to give correct solar time every -day in the year.</p> - -<p>If you wish to adjust the dial very closely, -you must go out some fine day and note the error -of the dial by a watch at about ten in the -morning, and at noon, and again at about two in -the afternoon. If the error is the same each time, -the dial is rightly set. If not, you must try, by -turning the dial slightly, to get it so placed that -your three errors will be nearly the same. When -you have got them as nearly alike as you can, the -dial will be sufficiently near right. The solar or -dial time may, however, differ somewhat from -ordinary watch time, but the difference will never -be great enough to matter, when we remember -that sun-dials are only rough timekeepers after -all, and useful principally for amusement.</p> - - -<div class="footnotes"><h3>FOOTNOTE:</h3> - -<div class="footnote"> - -<p><a name="Footnote_A_1" id="Footnote_A_1"></a><a href="#FNanchor_A_1"><span class="label">[A]</span></a> This chapter is especially intended for boys and girls and others who like -to make things with carpenters' tools.</p></div></div> - - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_81" id="Page_81">[81]</a></span></p> - -<h2><a name="PHOTOGRAPHY_IN_ASTRONOMY" id="PHOTOGRAPHY_IN_ASTRONOMY"></a><a href="#CONTENTS">PHOTOGRAPHY IN ASTRONOMY</a></h2> - - -<p>New highways of science have been monumented -now and again by the masterful efforts of -genius, working single-handed; but more often -it is slow-moving time that ripens discovery, and, -at the proper moment, opens some new path to -men whose intellectual power is but willingness -to learn. So the annals of astronomical photography -do not recount the achievements of extraordinary -genius. It would have been strange, -indeed, if the discovery of photography had not -been followed by its application to astronomy.</p> - -<p>The whole range of chemical science contains -no experiment of greater inherent interest than -the development of a photographic plate. Let -but the smallest ray of light fall upon its -strangely sensitive surface, and some subtle invisible -change takes place. It is then merely necessary -to plunge the plate into a properly prepared -chemical bath, and the gradual process of<span class="pagenum"><a name="Page_82" id="Page_82">[82]</a></span> -developing the picture begins. Slowly, very -slowly, the colorless surface darkens wherever -light has touched it. Let us imagine that the -exposure has been made with an ordinary lens -and camera, and that it is a landscape seeming to -grow beneath the experimenter's eyes. At first -only the most conspicuous objects make their -appearance. But gradually the process extends, -until finally every tiny detail is reproduced with -marvellous fidelity to the original. The photographic -plate, when developed in this way, is -called a "negative." For in Nature luminous -points, or sources of light, are bright, while the -developing negative turns dark wherever light -has acted. Thus the negative, while true to Nature, -reproduces everything in a reversed way; -bright things are dark, and shadows appear light. -For ordinary purposes, therefore, the negative has -to be replaced by a new photograph made by copying -it again photographically. In this way it is -again reversed, giving us a picture corresponding -correctly to the facts as seen. Such a copy from -a negative is what is ordinarily called a photograph; -technically, it is known as a "positive."</p> - -<p><span class="pagenum"><a name="Page_83" id="Page_83">[83]</a></span></p> - -<p>One of the remarkable things about the sensitive -plate is its complete indifference to the distance -from which the light comes. It is ready -to yield obediently to the ray of some distant -star that may have journeyed, as it were, from -the very vanishing point of space, or to the -bright glow of an electric light upon the photographer's -table. This quality makes its use especially -advantageous in astronomy, since we can -gain knowledge of remote stars only by a study -of the light they send us. In such study the -photographic plate possesses a supreme advantage -over the human eye. If the conditions of -weather and atmosphere are favorable, an observer -looking through an ordinary telescope -will see nearly as much at the first glance as he -will ever see. Attentive and continued study will -enable him to fix details upon his memory, and -to record them by means of drawings and diagrams. -Occasional moments of especially undisturbed -atmospheric conditions will allow him -to glimpse faint objects seldom visible. But on -the whole, telescopic astronomers add little to -their harvest by continued husbandry in the<span class="pagenum"><a name="Page_84" id="Page_84">[84]</a></span> -same field of stars. Photography is different. -The effect of light upon the sensitive surface of -the plate is strictly cumulative. If a given star -can bring about a certain result when it has been -allowed to act upon the plate for one minute, -then in two or three minutes it will accomplish -much more. Perhaps a single minute's exposure -would have produced a mark scarcely perceptible -upon the developed negative. In that case, -three or four minutes would give us a perfectly -well defined black image of the star.</p> - -<div class="figcenter"> -<a name="P_84" id="P_84"></a> -<br /> -<img src="images/i_084fp.jpg" width="500" alt="" /> -<div class="caption"> -Star-Field in Constellation Monoceros.<br /> -<span class="fs80">Photographed by Barnard, February 1, 1894.<br /> -Exposure, three hours.</span></div> -</div> - -<p>Thus, by lengthening the exposure we can -make the fainter stars impress themselves upon -the plate. If their light is not able to produce -the desired effect in minutes, we can let its action -accumulate for hours. In this manner it becomes -possible and easy to photograph objects -so faint that they have never been seen, even -with our most powerful telescopes. This -achievement ranks high among those which -make astronomy appeal so strongly to the imagination. -Scientific men are not given to fancies; -nor should they be. But the first long-exposure -photograph must have been an exciting thing. -<span class="pagenum"><a name="Page_85" id="Page_85">[85]</a></span>After coming from the observatory, the chemical -development was, of course, made in a dark -room, so that no additional light might harm the -plate until the process was complete. Carrying -it out then into the light, that early experimenter -cannot but have felt a thrill of triumph; -for his hand held a true picture of dim stars to -the eye unlighted, lifted into view as if by magic.</p> - -<p>Plates have been thus exposed as long as -twenty-five hours, and the manner of doing it is -very interesting. Of course, it is impossible to -carry on the work continuously for so long a -period, since the beginning of daylight would -surely ruin the photograph. In fact, the astronomer -must stop before even the faintest streak of -dawn begins to redden the eastern sky. Moreover, -making astronomical negatives requires excessively -close attention, and this it is impossible -to give continuously during more than a few -hours. But the exposure of a single plate can be -extended over several nights without difficulty. -It is merely necessary to close the plate-holder -with a "light-tight" cover when the first night's -work is finished. To begin further exposure of<span class="pagenum"><a name="Page_86" id="Page_86">[86]</a></span> -the same plate on another night, we simply aim -the photographic telescope at precisely the same -point of the sky as before. The light-tight -plate-holder being again opened, the exposure -can go on as if there had been no interruption.</p> - -<p>Astronomers have invented a most ingenious -device for making sure that the telescope's aim -can be brought back again to the same point with -great exactness. This is a very important matter; -for the slightest disturbance of the plate before -the second or subsequent portions of the exposure -would ruin everything. Instead of a very -complete single picture, we should have two partial -ones mixed up together in inextricable confusion.</p> - -<p>To prevent this, photographic telescopes are -made double, not altogether unlike an opera-glass. -One of the tubes is arranged for photography -proper, while the other is fitted with lenses suitable -for an ordinary visual telescope. The two tubes -are made parallel. Thus the astronomer, by looking -through the visual glass, can watch objects in -the heavens even while they are being photographed. -The visual half of the instrument is -provided with a pair of very fine cross-wires mov<span class="pagenum"><a name="Page_87" id="Page_87">[87]</a></span>able -at will in the field of view. These can be -made to bisect some little star exactly, before beginning -the first night's work. Afterward, everything -about the instrument having been left unchanged, -the astronomer can always assure himself -of coming back to precisely the same point of the -sky, by so adjusting the instrument that the same -little star is again bisected.</p> - -<p>It must not be supposed, however, that the -entire instrument remains unmoved, even during -the whole of a single night's exposure. For in -that case, the apparent motion of the stars as they -rise or set in the sky would speedily carry them -out of the telescope's field of view. Consequently, -this motion has to be counteracted by shifting -the telescope so as to follow the stars. This -can be accomplished accurately and automatically -by means of clock-work mechanism. Such contrivances -have already been applied in the past to -visual telescopes, because even then they facilitated -the observer's work. They save him the -trouble of turning his instrument every few minutes, -and allow him to give his undivided attention -to the actual business of observation.</p> - -<p><span class="pagenum"><a name="Page_88" id="Page_88">[88]</a></span></p> - -<p>For photographic purposes the telescope needs -to "follow" the stars far more accurately than in -the older kind of observing with the eye. Nor is -it possible to make a clock that will drive the instrument -satisfactorily and quite automatically. -But by means of the second or visual telescope, -astronomers can always ascertain whether the -clock is working correctly at any given moment. -It requires only a glance at the little star bisected -by the cross-wires, and, if there has been the -slightest imperfection in the following by clock-work, -the star will no longer be cut exactly by -the wires.</p> - -<p>The astronomer can at once correct any error -by putting in operation a very ingenious mechanical -device sometimes called a "mouse-control." -He need only touch an electric button, -and a signal is sent into the clock-work. -Instantly there is a shifting of the mechanism. -For one of the regular driving wheels is substituted, -temporarily, another having an <em>extra tooth</em>. -This makes the clock run a little faster so long -as the electric current passes. In a similar way, -by means of another button, the clock can be<span class="pagenum"><a name="Page_89" id="Page_89">[89]</a></span> -made to run slower temporarily. Thus by -watching the cross-wires continuously, and manipulating -his two electric buttons, the photographic -astronomer can compel his telescope to -follow exactly the object under observation, and -he can make certain of obtaining a perfect negative.</p> - -<p>These long-exposure plates are intended especially -for what may be called descriptive astronomy. -With them, as we have seen, advantage is -taken of cumulative light-effects on the sensitive -plate, and the telescope's light-gathering and -space-penetrating powers are vastly increased. -We are enabled to carry our researches far beyond -the confines of the old visible universe. -Extremely faint objects can be recorded, even -down to their minutest details, with a fidelity unknown -to older visual methods. But at present -we intend to consider principally applications -of photography in the astronomy of measurement, -rather than the descriptive branch of our -subject. Instead of describing pictures made -simply to see what certain objects look like in -the sky, we shall consider negatives intended for<span class="pagenum"><a name="Page_90" id="Page_90">[90]</a></span> -precise measurement, with all that the word precision -implies in celestial science.</p> - -<p>Taking up first the photography of stars, we -must begin by mentioning the work of Rutherfurd -at New York. More than thirty years ago -he had so far perfected methods of stellar photography -that he was able to secure excellent -pictures of stars as faint as the ninth magnitude. -In those days the modern process of dry-plate -photography had not been invented. To-day, -plates exposed in the photographic telescope are -made of glass covered with a perfectly dry film -of sensitized gelatine. But in the old wet-plate -process the sensitive film was first wetted with a -chemical solution; and this solution could not be -allowed to dry during the exposure. Consequently, -Rutherfurd was limited to exposures a -few minutes in length, while nowadays, as we -have said, their duration can be prolonged at will.</p> - -<p>When we add to this the fact that the old -plates were far less sensitive to light than those -now available, it is easy to see what were the difficulties -in the way of photographing faint stars -in Rutherfurd's time. Nor did he possess the<span class="pagenum"><a name="Page_91" id="Page_91">[91]</a></span> -modern ingenious device of a combined visual -and photographic instrument. He had no electric -controlling apparatus. In fact, the younger -generation of astronomers can form no adequate -idea of the patience and personal skill Rutherfurd -must have had at his command. For he -certainly did produce negatives that are but little -inferior to the best that can be made to-day. -His only limitation was that he could not -obtain images of stars much below the ninth -magnitude.</p> - -<p>To understand just what is meant here by the -ninth magnitude, it is necessary to go back in imagination -to the time of Hipparchus, the father -of sidereal astronomy. (See <a href="#Page_39">page 39</a>.) He -adopted the convenient plan of dividing all the -stars visible to the naked eye (of course, he had -no telescope) into six classes, according to their -brilliancy. The faintest visible stars were put in -the sixth class, and all the others were assigned -somewhat arbitrarily to one or the other of the -brighter classes.</p> - -<p>Modern astronomers have devised a more scientific -system, which has been made to conform<span class="pagenum"><a name="Page_92" id="Page_92">[92]</a></span> -very nearly to that of Hipparchus, just as it has -come down to us through the ages. We have -adopted a certain arbitrary degree of luminosity -as the standard "first-magnitude"; compared with -sunlight, this may be represented roughly by a -fraction of which the numerator is 1, and the denominator -about eighty thousand millions. The -standard second-magnitude star is one whose -light, compared with a first-magnitude, may be -represented approximately by the fraction ⅖. -The third magnitude, in turn, may be compared -with the second by the same fraction ⅖; and so -the classification is extended to magnitudes below -those visible to the unaided eye. Each magnitude -compares with the one above it, as the light -of two candles would compare with the light of -five.</p> - -<p>Rutherfurd did not stop with mere photographs. -He realized very clearly the obvious -truth that by making a picture of the sky we -simply change the scene of our operations. -Upon the photograph we can measure that which -we might have studied directly in the heavens; -but so long as they remain unmeasured, celestial<span class="pagenum"><a name="Page_93" id="Page_93">[93]</a></span> -pictures have a potential value only. Locked -within them may lie hidden some secret of our -universe. But it will not come forth unsought. -Patient effort must precede discovery, in photography, -as elsewhere in science. There is no -royal road. Rutherfurd devised an elaborate -measuring-machine in which his photographs -could be examined under the microscope with -the most minute exactness. With this machine -he measured a large number of his pictures; and -it has been shown quite recently that the results -obtained from them are comparable in accuracy -with those coming from the most highly accredited -methods of direct eye-observation.</p> - -<p>And photographs are far superior in ease of manipulation. -Convenient day-observing under the -microscope in a comfortable astronomical laboratory -is substituted for all the discomforts of a -midnight vigil under the stars. The work of -measurement can proceed in all weathers, whereas -formerly it was limited strictly to perfectly clear -nights. Lastly, the negatives form a permanent -record, to which we can always return to correct -errors or re-examine doubtful points.</p> - -<p><span class="pagenum"><a name="Page_94" id="Page_94">[94]</a></span></p> - -<p>Rutherfurd's stellar work extended down to -about 1877, and included especially parallax determinations -and the photography of star-clusters. -Each of these subjects is receiving close attention -from later investigators, and, therefore, merits -brief mention here. Stellar parallax is in one -sense but another name for stellar distance. Its -measurement has been one of the important -problems of astronomy for centuries, ever since -men recognized that the Copernican theory of -our universe requires the determination of stellar -distances for its complete demonstration.</p> - -<p>If the earth is swinging around the sun once a -year in a mighty path or orbit, there must be -changes of its position in space comparable in size -with the orbit itself. And the stars ought to shift -their apparent places on the sky to correspond -with these changes in the terrestrial observer's -position. The phenomenon is analogous to what -occurs when we look out of a room, first through -one window, and then through another. Any -object on the opposite side of the street will be -seen in a changed direction, on account of the -observer's having shifted his position from one<span class="pagenum"><a name="Page_95" id="Page_95">[95]</a></span> -window to the other. If the object seemed to -be due north when seen from the first window, -it will, perhaps, appear a little east of north from -the other. But this change of direction will be -comparatively small, if the object under observation -is very far away, in comparison with the distance -between the two windows.</p> - -<p>This is what occurs with the stars. The earth's -orbit, vast as it is, shrinks into almost absolute -insignificance when compared with the profound -distances by which we are sundered from even -the nearest fixed stars. Consequently, the shifting -of their positions is also very small—so -small as to be near the extreme limit separating -that which is measurable from that which is beyond -human ken.</p> - -<p>Photography lends itself most readily to a -study of this matter. Suppose a certain star is -suspected of "having a parallax." In other -words, we have reason to believe it near enough -to admit of a successful measurement of distance. -Perhaps it is a very bright star; and, other -things being equal, it is probably fair to assume -that brightness signifies nearness. And astrono<span class="pagenum"><a name="Page_96" id="Page_96">[96]</a></span>mers -have certain other indications of proximity -that guide them in the selection of proper objects -for investigation, though such evidence, of course, -never takes the place of actual measurement.</p> - -<p>The star under examination is sure to have near -it on the sky a number of stars so very small -that we may safely take them to be immeasurably -far away. The parallax star is among them, -but not of them. We see it projected upon the -background of the heavens, though it may in -reality be quite near us, astronomically speaking. -If this is really so, and the star, therefore, subject -to the slight parallactic shifting already mentioned, -we can detect it by noting the suspected -star's position among the surrounding small -stars. For these, being immeasurably remote, -will remain unchanged, within the limits of our -powers of observation, and thus serve as points -of reference for marking the apparent shifting of -the brighter star we are actually considering.</p> - -<p>We have merely to photograph the region at -various seasons of the year. Careful examination -of the photographs under the microscope -will then enable us to measure the slightest dis<span class="pagenum"><a name="Page_97" id="Page_97">[97]</a></span>placement -of the parallax star. From these -measures, by a process of calculation, astronomers -can then obtain the star's distance. It will -not become known in miles; we shall only ascertain -how many times the distance between the -earth and sun would have to be laid down like a -measuring-rod, in order to cover the space separating -us from the star: and the subsequent -evaluation of this distance "earth to sun" in -miles is another important problem in whose solution -photography promises to be most useful.</p> - -<p>The above method of measuring stellar distance -is, of course, subject to whatever slight uncertainty -arises from the assumption that the small stars -used for comparison are themselves beyond the -possibility of parallactic shifting. But astronomy -possesses no better method. Moreover, -the number of small stars used in this way is, of -course, much larger in photography than it ever -can be in visual work. In the former process, -all surrounding stars can be photographed at -once; in the latter each star must be measured -separately, and daylight soon intervenes to impose -a limit on numbers. Usually only two can<span class="pagenum"><a name="Page_98" id="Page_98">[98]</a></span> -be used; so that here photography has a most -important advantage. It minimizes the chance -of our parallax being rendered erroneous, by the -stars of comparison not being really infinitely -remote. This might happen, perhaps, in the -case of one or two; but with an average result -from a large number we know it to be practically -impossible.</p> - -<p>Cluster work is not altogether unlike "parallax -hunting" in its preliminary stage of securing -the photographic observations. The object is to -obtain an absolutely faithful picture of a star -group, just as it exists in the sky. We have -every reason to suppose that a very large number -of stars condensed into one small spot upon -the heavens means something more than chance -aggregation. The Pleiades group (<a href="#Page_10">page 10</a>) contains -thousands of massive stars, doubtless held -together by the force of their mutual gravitational -attraction. If this be true, there must be -complex orbital motion in the cluster; and, as -time goes on, we should actually see the separate -components change their relative positions, -as it were, before our eyes. The details of such<span class="pagenum"><a name="Page_99" id="Page_99">[99]</a></span> -motion upon the great scale of cosmic space offer -one of the many problems that make astronomy -the grandest of human sciences.</p> - -<p>We have said that time must pass before we -can see these things; there may be centuries of -waiting. But one way exists to hurry on the -perfection of our knowledge; we must increase -the precision of observations. Motions that -would need the growth of centuries to become -visible to the older astronomical appliances, -might yield in a few decades to more delicate -observational processes. Here photography is -most promising. Having once obtained a surpassingly -accurate picture of a star-cluster, we -can subject it easily to precise microscopic measurement. -The same operations repeated at a -later date will enable us to compare the two -series of measures, and thus ascertain the motions -that may have occurred in the interval. -The Rutherfurd photographs furnish a veritable -mine of information in researches of this kind; -for they antedate all other celestial photographs -of precision by at least a quarter-century, and -bring just so much nearer the time when definite<span class="pagenum"><a name="Page_100" id="Page_100">[100]</a></span> -knowledge shall replace information based on -reasoning from probabilities.</p> - -<p>Rutherfurd's methods showed the advantages -of photography as applied to individual star-clusters. -It required only the attention of some -astronomer disposing of large observational facilities, -and accustomed to operations upon a great -scale, to apply similar methods throughout the -whole heavens. In the year 1882 a bright -comet was very conspicuous in the southern -heavens. It was extensively observed from the -southern hemisphere, and especially at the British -Royal Observatory at the Cape of Good Hope.</p> - -<p>Gill, director of that institution, conceived the -idea that this comet might be bright enough -to photograph. At that time, comet photography -had been attempted but little, if at all, and it -was by no means sure that the experiment would -be successful. Nor was Gill well acquainted with -the work of Rutherfurd; for the best results of -that astronomer had lain dormant many years. -He was one of those men with whom personal -modesty amounts to a fault. Loath to put himself -forward in any way, and disliking to rush<span class="pagenum"><a name="Page_101" id="Page_101">[101]</a></span> -into print, Rutherfurd had given but little publicity -to his work. This peculiarity has, doubtless, -delayed his just reputation; but he will lose -nothing in the end from a brief postponement. -Gill must, however, be credited with more penetration -than would be his due if Rutherfurd had -made it possible for others to know that he had -anticipated many of the newer ideas.</p> - -<p>However this may be, the comet was photographed -with the help of a local portrait photographer -named Allis. When Gill and Allis fastened -a simple portrait camera belonging to the -latter upon the tube of one of the Cape telescopes, -and pointed it at the great comet, they -little thought the experiment would lead to one -of the greatest astronomical works ever attempted -by men. Yet this was destined to occur. -The negative they obtained showed an -excellent picture of the comet; but what was -more important for the future of sidereal astronomy, -it was also quite thickly dotted with little -black points corresponding to stars. The extraordinary -ease with which the whole heavens -could be thus charted photographically was<span class="pagenum"><a name="Page_102" id="Page_102">[102]</a></span> -brought home to Gill as never before. It was -this comet picture that interested him in the application -of photography to star-charting; and -without his interest the now famous astro-photographic -catalogue of the heavens would probably -never have been made.</p> - -<p>After considerable preliminary correspondence, -a congress of astronomers was finally called to -meet at Paris in 1887. Representatives of the -principal observatories and civilized governments -were present. They decided that the end of the -nineteenth century should see the making of a -great catalogue of all the stars in the sky, upon a -scale of completeness and precision surpassing -anything previously attempted. It is impossible -to exaggerate the importance of such a work; -for upon our star-catalogues depends ultimately -the entire structure of astronomical science.</p> - -<p>The work was far too vast for the powers of -any observatory alone. Therefore, the whole -sky, from pole to pole, was divided into eighteen -belts or zones of approximately equal area; and -each of these was assigned to a single observatory -to be photographed. A series of telescopes<span class="pagenum"><a name="Page_103" id="Page_103">[103]</a></span> -was specially constructed, so that every part of -the work should be done with the same type of -instrument. As far as possible, an attempt was -made to secure uniformity of methods, and particularly -a uniform scale of precision. To cover -the entire sky upon the plan proposed no less -than 44,108 negatives are required; and most -of these have now been finished. The further -measurement of the pictures and the drawing up -of a vast printed star-catalogue are also well under -way. One of the participating observatories, -that at Potsdam, Germany, has published the first -volume of its part of the catalogue. It is estimated -that this observatory alone will require -twenty quarto volumes to contain merely the -final results of its work on the catalogue. Altogether -not less than two million stars will find a -place in this, our latest directory of the heavens.</p> - -<p>Such wholesale methods of attacking problems -of observational astronomy are particularly characteristic -of photography. The great catalogue -is, perhaps, the best illustration of this tendency; -but of scarcely smaller interest, though less important -in reality, is the photographic method of<span class="pagenum"><a name="Page_104" id="Page_104">[104]</a></span> -dealing with minor planets. We have already -said (<a href="#Page_63">page 63</a>) that in the space between the orbits -of Mars and Jupiter several hundred small bodies -are moving around the sun in ordinary planetary -orbits. These bodies are called asteroids, or -minor planets. The visual method of discovering -unknown members of this group was painfully -tedious; but photography has changed -matters completely, and has added immensely -to our knowledge of the asteroids.</p> - -<p>Wolf, of Heidelberg, first made use of the -new process for minor-planet discovery. His -method is sufficiently ingenious to deserve brief -mention again. A photograph of a suitable region -of the sky was made with an exposure lasting -two or three hours. Throughout all this -time the instrument was manipulated so as to -follow the motion of the heavens in the way we -have already explained, so that each star would -appear on the negative as a small, round, black -dot.</p> - -<p>But if a minor planet happened to be in the -region covered by the plate, its photographic -image would be very different. For the orbital<span class="pagenum"><a name="Page_105" id="Page_105">[105]</a></span> -motion of the planet about the sun would make -it move a little among the stars even in the two -or three hours during which the plate was exposed. -This motion would be faithfully reproduced -in the picture, so that the planet would -appear as a short curved line rather than a well-defined -dot like a star. Thus the presence of -such a line-image infallibly denotes an asteroid.</p> - -<p>Subsequent calculations are necessary to ascertain -whether the object is a planet already known -or a genuine new discovery. Wolf, and others -using his method in recent years, have made immense -additions to our catalogue of asteroids. -Indeed, the matter was beginning to lose interest -on account of the frequency and sameness -of these discoveries, when the astronomical world -was startled by the finding of the Planet of 1898. -(<a href="#Page_58">Page 58</a>.)</p> - -<p>On August 27, 1898, Witt, of Berlin, discovered -the small body that bears the number -"433" in the list of minor planets, and has received -the name Eros. Its important peculiarity -consists in the exceptional position of the -orbit. While all the other asteroids are farther<span class="pagenum"><a name="Page_106" id="Page_106">[106]</a></span> -from the sun than Mars, and less distant than -Jupiter, Eros can pass within the orbit of the -former. At times, therefore, it will approach -our earth more closely than any other permanent -member of the solar system, excepting our own -moon. So it is, in a sense, our nearest neighbor; -and this fact alone makes it the most interesting -of all the minor planets. The nineteenth -century was opened by Piazzi's well-known discovery -of the first of these bodies (<a href="#Page_59">page 59</a>); it -is, therefore, fitting that we should find the most -important one at its close. We are almost certain -that it will be possible to make use of Eros -to solve with unprecedented accuracy the most -important problem in all astronomy. This is the -determination of our earth's distance from the sun. -When considering stellar parallax, we have seen -how our observations enable us to measure some -of the stars' distances in terms of the distance -"earth to sun" as a unit. It is, indeed, the fundamental -unit for all astronomical measures, and -its exact evaluation has always been considered -the basal problem of astronomy. Astronomers -know it as the problem of Solar Parallax.</p> - -<p><span class="pagenum"><a name="Page_107" id="Page_107">[107]</a></span></p> - -<p>We shall not here enter into the somewhat -intricate details of this subject, however interesting -they may be. The problem offers difficulties -somewhat analogous to those confronting -a surveyor who has to determine the distance of -some inaccessible terrestrial point. To do this, -it is necessary first to measure a "base-line," as -we call it. Then the measurement of angles -with a theodolite will make it possible to deduce -the required distance of the inaccessible point by -a process of calculation. To insure accuracy, -however, as every surveyor knows, the base-line -must be made long enough; and this is precisely -what is impossible in the case of the solar -parallax.</p> - -<p>For we are necessarily limited to marking -out our base-line on the earth; and the entire -planet is too small to furnish one of really sufficient -size. The best we can do is to use the distance -between two observatories situated, as near -as may be, on opposite sides of the earth. But -even this base is wofully small. However, the -smallness loses some of its harmful effect if we -operate upon a planet that is comparatively near<span class="pagenum"><a name="Page_108" id="Page_108">[108]</a></span> -us. We can measure such a planet's distance -more accurately than any other; and this being -known, the solar distance can be computed by -the aid of mathematical considerations based -upon Newton's law of gravitation and observational -determinations of the planetary orbital -elements.</p> - -<p>Photography is by no means limited to investigations -in the older departments of astronomical -observation. Its powerful arm has been -stretched out to grasp as well the newer instruments -of spectroscopic study. Here the sensitive -plate has been substituted for the human -eye with even greater relative advantage. The -accurate microscopic measurement of difficult -lines in stellar spectra was indeed possible by -older methods; but photography has made it -comparatively easy; and, above all, has rendered -practicable series of observations extensive -enough in numbers to furnish statistical information -of real value. Only in this way have we -been able to determine whether the stars, in their -varied and unknown orbits, are approaching us -or moving farther away. Even the speed of this -<span class="pagenum"><a name="Page_109" id="Page_109">[109]</a></span>approach or recession has become measurable, -and has been evaluated in the case of many individual -stars. (See <a href="#Page_21">page 21</a>.)</p> - -<div class="figcenter"> -<a name="P_108" id="P_108"></a> -<br /> -<img src="images/i_108fp.jpg" width="550" alt="" /> -<div class="caption"> -Solar Corona. Total Eclipse.<br /> -<span class="fs80">Photographed by Campbell, January 22, 1898; Jeur, India.</span></div> -</div> - -<p>The subject of solar physics has become a veritable -department of astronomy in the hands of -photographic investigators. Ingenious spectro-photographic -methods have been devised, whereby -we have secured pictures of the sun from -which we have learned much that must have -remained forever unknown to older methods.</p> - -<p>Especially useful has photography proved itself -in the observation of total solar eclipses. It is -only when the sun's bright disk is completely -obscured by the interposed moon that we can see -the faintly luminous structure of the solar corona, -that great appendage of our sun, whose -exact nature is still unexplained. Only during -the few minutes of total eclipse in each century -can we look upon it; and keen is the interest of -astronomers when those few minutes occur. But -it is found that eye observations made in hurried -excitement have comparatively little value. -Half a dozen persons might make drawings of -the corona during the same eclipse, yet they<span class="pagenum"><a name="Page_110" id="Page_110">[110]</a></span> -would differ so much from one another as to -leave the true outline very much in doubt. But -with photography we can obtain a really correct -picture whose details can be studied and discussed -subsequently at leisure.</p> - -<p>If we were asked to sum up in one word what -photography has accomplished, we should say -that observational astronomy has been revolutionized. -There is to-day scarcely an instrument -of precision in which the sensitive plate -has not been substituted for the human eye; -scarcely an inquiry possible to the older method -which cannot now be undertaken upon a grander -scale. Novel investigations formerly not even -possible are now entirely practicable by photography; -and the end is not yet. Valuable as are -the achievements already consummated, photography -is richest in its promise for the future. -Astronomy has been called the "perfect science"; -it is safe to predict that the next generation -will wonder that the knowledge we have -to-day should ever have received so proud a -title.</p> - - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_111" id="Page_111">[111]</a></span></p> - -<h2><a name="TIME_STANDARDS_OF_THE" id="TIME_STANDARDS_OF_THE">TIME STANDARDS OF THE -WORLD</a></h2> - - -<p>The question is often asked, "What is the -practical use of astronomy?" We know, of -course, that men would profit greatly from a study -of that science, even if it could not be turned to -any immediate bread-and-butter use; for astronomy -is essentially the science of big things, and it -makes men bigger to fix their minds on problems -that deal with vast distances and seemingly endless -periods of time. No one can look upon the -quietly shining stars without being impressed by -the thought of how they burned—then as now—before -he himself was born, and so shall continue -after he has passed away—aye, even after his latest -descendants shall have vanished from the -earth. Of all the sciences, astronomy is at once -the most beautiful poetically, and yet the one -offering the grandest and most difficult problems -to the intellect. A study of these problems has<span class="pagenum"><a name="Page_112" id="Page_112">[112]</a></span> -ever been a labor of love to the greatest minds; -their solution has been counted justly among -man's loftiest achievements.</p> - -<p>And yet of all the difficult and abstruse sciences, -astronomy is, perhaps, the one that comes into the -ordinary practical daily life of the people more -definitely and frequently than any other. There -exist at least three things we owe to astronomy -that must be regarded as quite indispensable, -from a purely practical point of view. In the first -place, let us consider the maps in a work on geography. -How many people ever think to ask -how these maps are made? It is true that the -ordinary processes of the surveyor would enable -us to draw a map showing the outlines of a part of -the earth's surface. Even the locations of towns -and rivers might be marked in this way. But one -of the most important things of all could not be -added without the aid of astronomical observations. -The latitude and longitude lines, which are -essential to show the relation of the map to the -rest of the earth, we owe to astronomy. The longitude -lines, particularly, as we shall see farther on, -play a most important part in the subject of time.</p> - -<p><span class="pagenum"><a name="Page_113" id="Page_113">[113]</a></span></p> - -<p>The second indispensable application of astronomy -to ordinary business affairs relates to the -subject of navigation. How do ships find their -way across the ocean? There are no permanent -marks on the sea, as there are on the land, by -which the navigator can guide his course. Nevertheless, -seamen know their path over the trackless -ocean with a certainty as unerring as would -be possible on shore; and it is all done by the -help of astronomy. The navigator's observations -of the sun are astronomical observations; -the tables he uses in calculating his observations—the -tables that tell him just where he is and in what -direction he must go—are astronomical tables. -Indeed, it is not too much to say that without -astronomy there could be no safe ocean navigation.</p> - -<p>But the third application of astronomy is of -still greater importance in our daily life—the furnishing -of correct time standards for all sorts of -purposes. It is to this practical use of astronomical -science that we would direct particular attention. -Few persons ever think of the complicated -machinery that must be put in motion in -order to set a clock. A man forgets some even<span class="pagenum"><a name="Page_114" id="Page_114">[114]</a></span>ing -to wind his watch at the accustomed hour. -The next morning he finds it run down. It -must be re-set. Most people simply go to the -nearest clock, or ask some friend for the time, so -as to start the watch correctly. More careful persons, -perhaps, visit the jeweller's and take the -time from his "regulator." But the regulator -itself needs to be regulated. After all, it is nothing -more than any other clock, except that greater -care has been taken in the mechanical construction -and arrangement of its various parts. Yet it is -but a machine built by human hands, and, like -all human works, it is necessarily imperfect. No -matter how well it has been constructed, it will -not run with perfectly rigid accuracy. Every day -there will be a variation from the true time by a -small amount, and in the course of days or weeks -the accumulation of these successive small amounts -will lead to a total of quite appreciable size.</p> - -<p>Just as the ordinary citizen looks to the jeweller's -regulator to correct his watch, so the jeweller -applies to the astronomer for the correction of his -regulator. Ever since the dawn of astronomy, in -the earliest ages of which we have any record, the<span class="pagenum"><a name="Page_115" id="Page_115">[115]</a></span> -principal duty of the astronomer has been the furnishing -of accurate time to the people. We shall -not here enter into a detailed account, however interesting -it would be, of the gradual development -by which the very perfect system at present in -use has been reached; but shall content ourselves -with a description of the methods now employed -in nearly all the civilized countries of the world.</p> - -<p>In the first place, every observatory is, of course, -provided with what is known as an astronomical -clock. This instrument, from the astronomer's -point of view, is something very different from -the ordinary popular idea. To the average person -an astronomical clock is a complicated and -elaborate affair, giving the date, day of the week, -phases of the moon, and other miscellaneous information. -But in reality the astronomer wants -none of these things. His one and only requirement -is that the clock shall keep as near uniform -time as may be possible to a machine constructed -by human hands. No expense is spared in making -the standard clock for an observatory. Real -artists in mechanical construction—men who have -attained a world-wide celebrity for delicate skill<span class="pagenum"><a name="Page_116" id="Page_116">[116]</a></span> -in fashioning the parts of a clock—such are the -astronomer's clock-makers.</p> - -<p>To increase precision of motion in the train of -wheels, it is necessary that the mechanism be as -simple as possible. For this reason all complications -of date, etc., are left out. We have even -abandoned the usual convenient plan of having -the hour and minute hands mounted at the same -centre; for this kind of mounting makes necessary -a slightly more intricate form of wheelwork. -The astronomer's clock usually has the centres -of the second hand, minute hand, and hour hand -in a straight line, and equally distant from each -other. Each hand has its own dial; all drawn, -of course, upon the same clock-face.</p> - -<p>Even after such a clock has been made as accurately -as possible, it will, nevertheless, not give -the very best performance unless it is taken care -of properly. It is necessary to mount it very -firmly indeed. It should not be fastened to an -ordinary wall, but a strong pier of masonry or -brick must be built for it on a very solid foundation. -Moreover, this pier is best placed underground -in a cellar, so that the temperature of the<span class="pagenum"><a name="Page_117" id="Page_117">[117]</a></span> -clock can be kept nearly uniform all the year -round; for we find that clocks do not run quite -the same in hot weather as they do in cold. -Makers have, indeed, tried to guard against this -effect of temperature, by ingenious mechanical -contrivances. But these are never quite perfect -in their action, and it is best not to test them too -severely by exposing the clock to sharp changes -of heat and cold.</p> - -<p>Another thing affecting the going of fine -clocks, strange as it may seem, is the variation of -barometric pressure. There is a slight but noticeable -difference in their running when the -barometer is high and when it is low. To prevent -this, some of our best clocks have been enclosed -in air-tight cases, so that outside barometric -changes may not be felt in the least by the clock -itself.</p> - -<p>But even after all this has been accomplished, -and the astronomer is in possession of a clock that -may be called a masterpiece of mechanical construction, -he is not any better off than was the -jeweller with his regulator. After all, even the -astronomical clock needs to be set, and its error<span class="pagenum"><a name="Page_118" id="Page_118">[118]</a></span> -must be determined from time to time. A -final appeal must then be had to astronomical -observations. The clock must be set by the -stars and sun. For this purpose the astronomer -uses an instrument called a "transit." This is -simply a telescope of moderate size, possibly five -or six feet long, and firmly attached to an axis at -right angles to the tube of the telescope.</p> - -<p>This axis is supported horizontally in such a -way that it points as nearly as may be exactly -east and west. The telescope itself being square -with the axis, always points in a north-and-south -direction. It is possible to rotate the telescope -about its axis so as to reach all parts of the sky -that are directly north or south of the observatory. -In the field of view of the telescope certain -very fine threads are mounted so as to form -a little cross. As the telescope is rotated this -cross traces out, as it were, a great circle on the -sky; and this great circle is called the astronomical -meridian.</p> - -<p>Now we are in possession of certain star-tables, -computed from the combined observations of -astronomers in the last 150 years. These tables<span class="pagenum"><a name="Page_119" id="Page_119">[119]</a></span> -tell us the exact moment of time when any star -is on the meridian. To discover, therefore, -whether our clock is right on any given night, it -is merely necessary to watch a star with the telescope, -and note the exact instant by the clock -when it reaches the little cross in the field of -view. Knowing from the astronomical tables the -time when the star ought to have been on the -meridian, and having observed the clock time -when it is actually there, the difference is, of -course, the error of the clock. The result can -be checked by observations of other stars, and -the slight personal errors of observation can be -rendered harmless by taking the mean from several -stars. By an hour's work on a fine night it -is possible to fix the clock error quite easily -within the one-twentieth part of a second.</p> - -<p>We have not space to enter into the interesting -details of the methods by which the astronomical -transit is accurately set in the right -position, and how any slight residual error in -its setting can be eliminated from our results by -certain processes of computation. It must suffice -to say that practically all time determinations in<span class="pagenum"><a name="Page_120" id="Page_120">[120]</a></span> -the observatory depend substantially upon the -procedure outlined above.</p> - -<p>The observatory clock having been once set -right by observations of the sky, its error can be -re-determined every few days quite easily. Thus -even the small irregularities of its nearly perfect -mechanism can be prevented from accumulating -until they might reach a harmful magnitude. -But we obtain in this way only a correct standard -of time within the observatory itself. How can -this be made available for the general public? -The problem is quite simple with the aid of the -electric telegraph. We shall give a brief account -of the methods now in use in New York City, -and these may be taken as essentially representative -of those employed elsewhere.</p> - -<p>Every day, at noon precisely, an electric signal -is sent out by the United States Naval Observatory -in Washington. The signal is regulated by -the standard clock of the observatory, of course -taking account of star observations made on the -next preceding fine night. This signal is received -in the central New York office of the telegraph -company, where it is used to keep correct<span class="pagenum"><a name="Page_121" id="Page_121">[121]</a></span> -a very fine clock, which may be called the time -standard of the telegraph company. This clock, -in turn, has automatic electric connections, by -means of which it is made to send out signals -over what are called "time wires" that go all -over the city. Jewellers, and others who desire -correct time, can arrange to have a small electric -sounder in their offices connected with the time -wires. Thus the ticks of the telegraph company's -standard clock are repeated automatically -in the jeweller's shop, and used for controlling -the exactness of his regulator. This, in brief, is -the method by which the astronomer's careful -determination of correct time is transferred and -distributed to the people at large.</p> - -<p>Having thus outlined the manner of obtaining -and distributing correct time, we shall now consider -the question of time differences between different -places on the earth. This is a matter which many -persons find most perplexing, and yet it is essentially -quite simple in principle. Travellers, of course, -are well acquainted with the fact that their watches -often need to be reset when they arrive at their -destination. Yet few ever stop to ask the cause.</p> - -<p><span class="pagenum"><a name="Page_122" id="Page_122">[122]</a></span></p> - -<p>Let us consider for a moment our method of -measuring time. We go by the sun. If we leave -out of account some small irregularities of the sun's -motion that are of no consequence for our present -purpose, we may lay down this fundamental principle: -When the sun reaches its highest position -in the sky it is twelve o'clock or noon.</p> - -<p>The sun, as everyone knows, rises each morning -in the east, slowly goes up higher and higher -in the sky, and at last begins to descend again -toward the west. But it is clear that as the sun -travels from east to west, it must pass over the -eastern one of any two cities sooner than the -western one. When it reaches its greatest height -over a western city it has, therefore, already passed -its greatest height over an eastern one. In other -words, when it is noon, or twelve o'clock, in the -western city, it is already after noon in the eastern -city. This is the simple and evident cause of -time differences in different parts of the country. -Of any two places the eastern one always has later -time than the western. When we consider the -matter in this way there is not the slightest difficulty -in understanding how time differences arise.<span class="pagenum"><a name="Page_123" id="Page_123">[123]</a></span> -They will, of course, be greatest for places that -are very far apart in an east-and-west direction. -And this brings us again to the subject of longitude, -which, as we have already said, plays an -important part in all questions relating to time; -for longitude is used to measure the distance in -an east-and-west direction between different parts -of the earth.</p> - -<p>If we consider the earth as a large ball we can -imagine a series of great circles drawn on its surface -and passing directly from the North Pole to the -South Pole. Such a circle could be drawn through -any point on the earth. If we imagine a pair of -them drawn through two cities, such as New York -and London, the longitude difference of these two -cities is defined as the angle at the North Pole between -the two great circles in question. The size -of this angle can be expressed in degrees. If we -then wish to know the difference in time between -New York and London in hours, we need only -divide their longitude difference in degrees by the -number 15. In this simple way we can get the -time difference of any two places. We merely -measure the longitude difference on a map, and<span class="pagenum"><a name="Page_124" id="Page_124">[124]</a></span> -then divide by 15 to get the time difference. -These time differences can sometimes become -quite large. Indeed, for two places differing 180 -degrees in longitude, the time difference will evidently -be no less than twelve hours.</p> - -<p>Most civilized nations have agreed informally -to adopt some one city as the fundamental point -from which all longitudes are to be counted. Up -to the present we have considered only longitude -differences; but when we speak of the longitude -of a city we mean its longitude difference from -the place chosen by common consent as the origin -for measuring longitudes. The town almost -universally used for this purpose is Greenwich, -near London, England. Here is situated the -British Royal Observatory, one of the oldest and -most important institutions of its kind in the -world. The great longitude circle passing through -the centre of the astronomical transit at the Greenwich -observatory is the fundamental longitude -circle of the earth. The longitude of any other -town is then simply the angle at the pole between -the longitude circle through that town and the -fundamental Greenwich one here described.</p> - -<p><span class="pagenum"><a name="Page_125" id="Page_125">[125]</a></span></p> - -<p>Longitudes are counted both eastward and -westward from Greenwich. Thus New York is -in 74 degrees west longitude, while Berlin is in -14 degrees east longitude. This has led to a -rather curious state of affairs in those parts of the -earth the longitudes of which are nearly 180 -degrees east or west. There are a number of -islands in that part of the world, and if we imagine -for a moment one whose longitude is just 180 degrees, -we shall have the following remarkable result -as to its time difference from Greenwich.</p> - -<p>We have seen that of any two places the eastern -always has the later time. Now, since our imaginary -island is exactly 180 degrees from Greenwich, -we can consider it as being either 180 degrees -east or 180 degrees west. But if we call it -180 degrees east, its time will be twelve hours -later than Greenwich, and if we call it 180 degrees -west, its time will be twelve hours earlier than -Greenwich. Evidently there will be a difference -of just twenty-four hours, or one whole day, between -these two possible ways of reckoning its -time. This circumstance has actually led to considerable -confusion in some of the islands of the<span class="pagenum"><a name="Page_126" id="Page_126">[126]</a></span> -Pacific Ocean. The navigators who discovered -the various islands naturally gave them the date -which they brought from Europe. And as some -of these navigators sailed eastward, around the -Cape of Good Hope, and others westward, around -Cape Horn, the dates they gave to the several -islands differed by just one day.</p> - -<p>The state of affairs at the present time has -been adjusted by a sort of informal agreement. -An arbitrary line has been drawn on the map -near the 180th longitude circle, and it has been -decided that the islands on the east side of this -line shall count their longitudes west from Greenwich, -and those west of the line shall count longitude -east from Greenwich. Thus Samoa is -nearly 180 degrees west of Greenwich, while the -Fiji Islands are nearly 180 degrees east. Yet the -islands are very near each other, though the arbitrary -line passes between them. As a result, -when it is Sunday in Samoa it is Monday in the -Fiji Islands. The arbitrary line described here -is sometimes called the International Date-Line.</p> - -<p>It does not pass very near the Philippine -Islands, which are situated in about 120 degrees<span class="pagenum"><a name="Page_127" id="Page_127">[127]</a></span> -east longitude, and, therefore, use a time about -eight hours later than Greenwich. New York, -being about 74 degrees west of Greenwich, is -about five hours earlier in time. Consequently, -as we may remark in passing, Philippine time is -about thirteen hours later than New York time. -Thus, five o'clock, Sunday morning, May 1st, -in Manila, would correspond to four o'clock, -Saturday afternoon, April 30th, in New York.</p> - -<p>There is another kind of time which we shall explain -briefly—the so-called "standard," or railroad -time, which came into general use in the United -States some few years ago, and has since been -generally adopted throughout the world. It requires -but a few moments' consideration to see -that the accidental situation of the different large -cities in any country will cause their local times -to differ by odd numbers of hours, minutes, and -seconds. Thus a great deal of inconvenience -has been caused in the past. For instance, a -train might leave New York at a certain hour by -New York time. It would then arrive in Buffalo -some hours later by New York time. But -it would leave Buffalo by Buffalo time, which is<span class="pagenum"><a name="Page_128" id="Page_128">[128]</a></span> -quite different. Thus there would be a sort of -jump in the time-table at Buffalo, and it would -be a jump of an odd number of minutes.</p> - -<p>It would be different in different cities, and very -hard to remember. Indeed, as each railway usually -ran its trains by the time used in the principal -city along its line, it might happen that three -or four different railroad times would be used in -a single city where several roads met. This has -all been avoided by introducing the standard -time system. According to this the whole country -is divided into a series of time zones, fifteen -degrees wide, and so arranged that the middle -line of each zone falls at a point whose longitude -from Greenwich is 60, 75, 90, 105, or 120 degrees. -The times at these middle lines are, -therefore, earlier than Greenwich time by an -even number of hours. Thus, for instance, the -75-degree line is just five even hours earlier than -Greenwich time. All cities simply use the time -of the nearest one of these special lines.</p> - -<p>This does not result in doing away with time -differences altogether—that would, of course, be -impossible in the nature of things—but for the<span class="pagenum"><a name="Page_129" id="Page_129">[129]</a></span> -complicated odd differences in hours and minutes, -we have substituted the infinitely simpler series -of differences in even hours. The traveller from -Chicago to New York can reset his watch by -putting it just one hour later on his arrival—the -minute hand is kept unchanged, and no New -York timepiece need be consulted to set the -watch right on arriving. There can be no doubt -that this standard-time system must be considered -one of the most important contributions of -astronomical science to the convenience of man.</p> - -<p>Its value has received the widest recognition, -and its use has now extended to almost all civilized -countries—France is the only nation of importance -still remaining outside the time-zone -system. In the following table we give the -standard time of the various parts of the earth as -compared with Greenwich, together with the date -of adopting the new time system. It will be -noticed that in certain cases even half-hours have -been employed to separate the time-zones, instead -of even hours as used in the United States.</p> - -<p><span class="pagenum"><a name="Page_130" id="Page_130">[130]</a></span></p> - - -<p class="p2 pfs100">TABLE OF THE WORLD'S TIME STANDARDS</p> - -<div class="center"> -<br /> -<table border="0" cellpadding="4" cellspacing="0" width="95%" summary=""> -<tr><td class="tdlbt" colspan="4"></td></tr> -<tr class="fs80"><td class="tdc">When it is Noon<br />at Greenwich<br />it is<br /> </td><td class="tdlbl pad6"><br />In</td> - <td class="tdlbl tdlbr wd1"></td><td class="tdc">Date of Adopting<br />Standard Time<br />System.<br /> </td></tr> -<tr><td class="tdlbt" colspan="2"></td><td class="tdlbl tdlbr"></td><td class="tdlbt"></td></tr> -<tr><td class="tdl pad3">Noon</td><td class="tdlbl pad4">Great Britain.</td><td class="tdlbl tdlbr"></td></tr> -<tr><td class="tdl"></td><td class="tdlbl pad4">Belgium.</td><td class="tdlbl tdlbr"></td><td class="tdl">May, 1892.</td></tr> -<tr><td class="tdl"></td><td class="tdlbl pad4">Holland.</td><td class="tdlbl tdlbr"></td><td class="tdl">May, 1892.</td></tr> -<tr><td class="tdl"></td><td class="tdlbl pad4">Spain.</td><td class="tdlbl tdlbr"></td><td class="tdl">January, 1901.</td></tr> -<tr><td class="tdl"> 1 <span class="fs70">P.M.</span></td><td class="tdlbl pad4">Germany.</td><td class="tdlbl tdlbr"></td><td class="tdl">April, 1893.</td></tr> -<tr><td class="tdl"></td><td class="tdlbl pad4">Italy.</td><td class="tdlbl tdlbr"></td><td class="tdl">November, 1893.</td></tr> -<tr><td class="tdl"></td><td class="tdlbl pad4">Denmark.</td><td class="tdlbl tdlbr"></td><td class="tdl">January, 1894.</td></tr> -<tr><td class="tdl"></td><td class="tdlbl pad4">Switzerland.</td><td class="tdlbl tdlbr"></td><td class="tdl">June, 1894.</td></tr> -<tr><td class="tdl"></td><td class="tdlbl pad4">Norway.</td><td class="tdlbl tdlbr"></td><td class="tdl">January, 1895.</td></tr> -<tr><td class="tdl"></td><td class="tdlbl pad4">Austria (railways).</td><td class="tdlbl tdlbr"></td></tr> -<tr><td class="tdl"> 1.30 <span class="fs70">P.M.</span></td><td class="tdlbl pad4">Cape Colony.</td><td class="tdlbl tdlbr"></td><td class="tdl">1892.</td></tr> -<tr><td class="tdl"></td><td class="tdlbl pad4">Orange River Colony.</td><td class="tdlbl tdlbr"></td><td class="tdl">1892.</td></tr> -<tr><td class="tdl"></td><td class="tdlbl pad4">Transvaal.</td><td class="tdlbl tdlbr"></td><td class="tdl">1892.</td></tr> -<tr><td class="tdl"> 2 <span class="fs70">P.M.</span></td><td class="tdlbl pad4">Natal.</td><td class="tdlbl tdlbr"></td><td class="tdl">September, 1895.</td></tr> -<tr><td class="tdl"></td><td class="tdlbl pad4">Turkey (railways).</td><td class="tdlbl tdlbr"></td></tr> -<tr><td class="tdl"></td><td class="tdlbl pad4">Egypt.</td><td class="tdlbl tdlbr"></td><td class="tdl">October, 1900.</td></tr> -<tr><td class="tdl"> 8 <span class="fs70">P.M.</span></td><td class="tdlbl pad4">West Australia.</td><td class="tdlbl tdlbr"></td><td class="tdl">February, 1895.</td></tr> -<tr><td class="tdl"> 9 <span class="fs70">P.M.</span></td><td class="tdlbl pad4">Japan.</td><td class="tdlbl tdlbr"></td><td class="tdl">1896.</td></tr> -<tr><td class="tdl"> 9.30 <span class="fs70">P.M.</span></td><td class="tdlbl pad4">South Australia.</td><td class="tdlbl tdlbr"></td><td class="tdl">May, 1899.</td></tr> -<tr><td class="tdl">10 <span class="fs70">P.M.</span></td><td class="tdlbl pad4">Victoria.</td><td class="tdlbl tdlbr"></td><td class="tdl">February, 1895.</td></tr> -<tr><td class="tdl"></td><td class="tdlbl pad4">New South Wales.</td><td class="tdlbl tdlbr"></td><td class="tdl">February, 1895.</td></tr> -<tr><td class="tdl"></td><td class="tdlbl pad4">Queensland.</td><td class="tdlbl tdlbr"></td><td class="tdl">February, 1895.</td></tr> -<tr><td class="tdl">11 <span class="fs70">P.M.</span></td><td class="tdlbl pad4">New Zealand.</td><td class="tdlbl tdlbr"></td></tr> -<tr><td class="tdl"> </td><td class="tdlbl pad4"></td><td class="tdlbl tdlbr"></td></tr> -<tr><td class="tdlbt" colspan="4"></td></tr> -</table><br /></div> - -<div class="center"> -<table border="0" cellpadding="4" cellspacing="0" width="80%" summary=""> -<tr><td class="tdl" colspan="7">In the United States and Canada it is</td></tr> -<tr><td class="tdl pad3">4 <span class="fs70">A.M.</span> by</td><td class="tdl" colspan="2">Pacific Time</td><td class="tdc">when</td><td class="tdc">it is</td><td class="tdc">Noon</td><td class="tdc">at</td><td class="tdc">Greenwich.</td></tr> -<tr><td class="tdl pad3">5 <span class="fs70">A.M.</span> "</td><td class="tdl">Mountain</td><td class="tdc">"</td><td class="tdc">"</td><td class="tdc">"</td><td class="tdc">"</td><td class="tdc">"</td><td class="tdc">"</td></tr> -<tr><td class="tdl pad3">6 <span class="fs70">A.M.</span> "</td><td class="tdl">Central</td><td class="tdc">"</td><td class="tdc">"</td><td class="tdc">"</td><td class="tdc">"</td><td class="tdc">"</td><td class="tdc">"</td></tr> -<tr><td class="tdl pad3">7 <span class="fs70">A.M.</span> "</td><td class="tdl">Eastern</td><td class="tdc">"</td><td class="tdc">"</td><td class="tdc">"</td><td class="tdc">"</td><td class="tdc">"</td><td class="tdc">"</td></tr> -<tr><td class="tdl pad3">8 <span class="fs70">A.M.</span> "</td><td class="tdl">Colonial</td><td class="tdc">"</td><td class="tdc">"</td><td class="tdc">"</td><td class="tdc">"</td><td class="tdc">"</td><td class="tdc">"</td></tr> -</table><br /></div> - - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_131" id="Page_131">[131]</a></span></p> - -<h2><a name="MOTIONS_OF_THE_EARTHS_POLE" id="MOTIONS_OF_THE_EARTHS_POLE"></a><a href="#CONTENTS">MOTIONS OF THE EARTH'S POLE</a></h2> - - -<p>Students of geology have been puzzled for -many years by traces remaining from the period -when a large part of the earth was covered with a -heavy cap of ice. These shreds of evidence all -seem to point to the conclusion that the centre of -the ice-covered region was quite far away from -the present position of the north pole of the earth. -If we are to regard the pole as very near the -point of greatest cold, it becomes a matter of -much interest to examine whether the pole has -always occupied its present position, or whether -it has been subject to slow changes of place upon -the earth's surface. Therefore, the geologists -have appealed to astronomers to discover whether -they are in possession of any observational evidence -tending to show that the pole is in motion.</p> - -<p>Now we may say at once that astronomical research -has not as yet revealed the evidence thus -expected. Astronomy has been unable to come<span class="pagenum"><a name="Page_132" id="Page_132">[132]</a></span> -to the rescue of geological theory. From about -the year 1750, which saw the beginning of precise -observation in the modern sense, down to very -recent times, astronomers were compelled to deny -the possibility of any appreciable motion of the -pole. Observational processes, it is true, furnished -slightly divergent pole positions from time -to time. Yet these discrepancies were always so -minute as to be indistinguishable from those slight -personal errors that are ever inseparable from results -obtained by the fallible human eye.</p> - -<p>But in the last few years improved methods of -observation, coupled with extreme diligence in -their application by astronomers generally, have -brought to light a certain small motion of the -pole which had never before been demonstrated -in a reliable way. This motion, it is true, is not -of the character demanded by geological theory, -for the geologists had been led to expect a motion -which would be continuous in the same direction, -no matter how slow might be its annual amount; -for the vast extent of geologic time would give -even the slowest of motions an opportunity to -produce large effects, provided its results could<span class="pagenum"><a name="Page_133" id="Page_133">[133]</a></span> -be continuously cumulative. Given time enough, -and the pole might move anywhere on the earth, -no matter how slow might be its tortoise speed.</p> - -<p>But the small motion we have discovered is -neither cumulative nor continuous in one direction. -It is what we call a periodic motion, the -pole swinging now to one side, and now to the -other, of its mean or average position. Thus -this new discovery cannot be said to unravel the -mysterious puzzle of the geologists. Yet it is -not without the keenest interest, even from their -point of view; for the proof of any form of -motion in a pole previously supposed to be absolutely -at rest may mean everything. No man -can say what results will be revealed by the further -observations now being continued with great -diligence.</p> - -<p>In the first place, it is important to explain that -any such motions as we have under consideration -will show themselves to ordinary observational -processes principally in the form of changes -of terrestrial latitudes. Let us imagine a pair of -straight lines passing through the centre of the -earth and terminating, one at the observer's sta<span class="pagenum"><a name="Page_134" id="Page_134">[134]</a></span>tion -on the earth's surface, and the other at that -point of the equator which is nearest the observer. -Then, according to the ordinary definition of latitude, -the angle between these two imaginary lines -is called the latitude of the point of observation. -Now we know, of course, that the equator is -everywhere just 90 degrees from the pole. Consequently, -if the pole is subject to any motion at -all, the equator must also partake of the motion.</p> - -<p>Thus the angle between our two imaginary lines -will be affected directly by polar movement, and -the latitude obtained by astronomical observation -will be subject to quite similar changes. To clear -up the whole question, so far as this can be done -by the gathering of observational evidence, it is -only necessary to keep up a continual series of -latitude determinations at several observatories. -These determinations should show small variations -similar in magnitude to the wabblings of the -pole.</p> - -<p>Let us now consider for a moment what is -meant by the axis of the earth. It has long been -known that the planet has in general the shape of -a ball or sphere. That this is so can be seen at<span class="pagenum"><a name="Page_135" id="Page_135">[135]</a></span> -once from the way ships at sea disappear at the -horizon. As they go farther and farther from us, -we first lose sight of the hull, and then slowly -and gradually the spars and sails seem to sink -down into the ocean. This proves that the -earth's surface is curved. That it is more or less -like a sphere is evident from the fact that it always -casts a round shadow in eclipses. Sometimes the -earth passes between the sun and eclipsed moon. -Then we see the earth's black shadow projected -on the moon, which would otherwise be quite -bright. This shadow has been observed in a very -large number of such eclipses, and it has always -been found to have a circular edge.</p> - -<p>While, therefore, the earth is nearly a round -ball, it must not be supposed that it is exactly -spherical in form. We may disregard the small -irregularities of its surface, for even the greatest -mountains are insignificant in height when compared -with the entire diameter of the earth itself. -But even leaving these out of account, the earth is -not perfectly spherical. We can describe it best -as a flattened sphere. It is as though one were -to press a round rubber ball between two smooth<span class="pagenum"><a name="Page_136" id="Page_136">[136]</a></span> -boards. It would be flattened at the top and -bottom and bulged out in the middle. This is -the shape of the earth. It is flattened at the -poles and bulges out near the equator. The -shortest straight line that can be drawn through -the earth's centre and terminated by the flattened -parts of its surface may be called the earth's axis -of figure; and the two points where this axis -meets the surface are called the poles of figure.</p> - -<p>But the earth has another axis, called the axis -of rotation. This is the one about which the -planet turns once in a day, giving rise to the well-known -phenomena called the rising and setting of -sun, moon, and stars. For these motions of the -heavenly bodies are really only apparent ones, -caused by an actual motion of the observer on -the earth. The observer turns with the earth on -its axis, and is thus carried past the sun and stars.</p> - -<p>This daily turning of the earth, then, takes -place about the axis of rotation. Now, it so happens -that all kinds of astronomical observations -for the determination of latitude lead to values -based on the rotation axis of the earth, and not -on its axis of figure. We have seen how the<span class="pagenum"><a name="Page_137" id="Page_137">[137]</a></span> -earth's equator, from which we count our latitudes, -is everywhere 90 degrees distant from the -pole. But this pole is the pole of rotation, or -the point at which the rotation axis pierces the -earth's surface. It is not the pole of figure.</p> - -<p>It is clear that the latitude of any observatory -will remain constant only if the pole of -figure and the rotation pole maintain absolutely -the same positions relatively one to the other. -These two poles are actually very near together; -indeed, it was supposed for a very long time that -they were absolutely coincident, so that there could -not be any variations of latitude. But it now -appears that they are separated slightly.</p> - -<p>Strange to say, one of them is revolving -about the other in a little curve. The pole of -figure is travelling around the pole of rotation. -The distance between them varies a little, never -becoming greater than about fifty feet, and it -takes about fourteen months to complete a revolution. -There are some slight irregularities in -the motion, but, in the main, it takes place in the -manner here stated. In consequence of this rotation -of the one pole about the other, the pole<span class="pagenum"><a name="Page_138" id="Page_138">[138]</a></span> -of figure is now on one side of the rotation pole -and now on the opposite side, but it never travels -continuously in one direction. Thus, as we have -already seen, the sort of continuous motion required -to explain the observed geological phenomena -has not yet been found by astronomers.</p> - -<p>Observations for the study of latitude variations -have been made very extensively within -recent years both in Europe and the United -States. It has been found practically most advantageous -to carry out simultaneous series of -observations at two observatories situated in -widely different parts of the earth, but having -very nearly the same latitude. It is then possible -to employ the same stars for observation in -both places, whereas it would be necessary to -use different sets of stars if there were much -difference in the latitudes.</p> - -<p>There is a special advantage in using the same -stars in both places. We can then determine -the small difference in latitude between the two -participating observatories in a manner which will -make it quite free from any uncertainty in our -knowledge of the positions on the sky of the<span class="pagenum"><a name="Page_139" id="Page_139">[139]</a></span> -stars observed; for, strange as it may seem, our -star-catalogues do not contain absolutely accurate -numbers. Like all other data depending on -fallible human observation, they are affected with -small errors. But if we can determine simply -the difference in latitude of the two observatories, -we can discover from its variation the path in -which the pole is moving. If, for instance, the -observatories are separated by one-quarter the -circumference of the globe, the pole will be moving -directly toward one of them, when it is not -changing its distance from the other one at all.</p> - -<p>This method was used for seven years with good -effect at the observatories of Columbia University -in New York, and the Royal Observatory at -Naples, Italy. For obtaining its most complete -advantages it is, of course, better to establish several -observing stations on about the same parallel -of latitude. This was done in 1899 by the International -Geodetic Association. Two stations are in -the United States, one in Japan, and one in Sicily. -We can, therefore, hope confidently that our knowledge -as to the puzzling problem of polar motion -will soon receive very material advancement.</p> - - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_140" id="Page_140">[140]</a></span></p> - -<h2><a name="SATURNS_RINGS" id="SATURNS_RINGS"></a><a href="#CONTENTS">SATURN'S RINGS</a></h2> - - -<p>The death of James E. Keeler, Director of the -Lick Observatory, in California (<a href="#Page_32">p. 32</a>), recalls -to mind one of the most interesting and significant -of later advances in astronomical science. -Only seven years have elapsed since Keeler made -the remarkable spectroscopic observations which -gave for the first time an ocular demonstration of -the true character of those mysterious luminous -rings surrounding the brilliant planet Saturn. -His results have not yet been made sufficiently -accessible to the public at large, nor have they -been generally valued at their true worth. We -consider this work of Keeler's interesting, because -the problem of the rings has been a classic -one for many generations; and we have been -particular, also, to call it significant, because it is -pregnant with the possibilities of newer methods -of spectroscopic research, applied in the older -departments of observational astronomy.</p> - -<p><span class="pagenum"><a name="Page_141" id="Page_141">[141]</a></span></p> - -<p>The troubles of astronomers with the rings -began with the invention of the telescope itself. -They date back to 1610, when Galileo first -turned his new instrument to the heavens (<a href="#Page_49">p. 49</a>). -It may be imagined easily that the bright planet -Saturn was among the very first objects scrutinized -by him. His "powerful" instrument -magnified only about thirty times, and was, -doubtless, much inferior to our pocket telescopes -of to-day. But it showed, at all events, that -something was wrong with Saturn. Galileo put -it, "<i lang="la" xml:lang="la">Ultimam planet am tergeminam observavi</i>" -("I have observed the furthest planet to be -triple").</p> - -<p>It is easy to understand now how Galileo's -eyes deceived him. For a round luminous ball -like Saturn, surrounded by a thin flat ring seen -nearly edgewise, really looks as if it had two little -attached appendages. Strange, indeed, it is -to-day to read a scientific book so old that the -planet Saturn could be called the "furthest" -planet. But it was the outermost known in -Galileo's day, and for nearly two centuries afterward. -Not until 1781 did William Herschel<span class="pagenum"><a name="Page_142" id="Page_142">[142]</a></span> -discover Uranus (<a href="#Page_59">p. 59</a>); and Neptune was not -disclosed by the marvellous mathematical perception -of Le Verrier until 1846 (<a href="#Page_61">p. 61</a>).</p> - -<p>Galileo's further observations of Saturn bothered -him more and more. The planet's behavior -became much worse as time went on. "Has -Saturn devoured his children, according to the -old legend?" he inquired soon afterward; for -the changed positions of earth and planet in the -course of their motions around the sun in their -respective orbits had become such that the ring -was seen quite edgewise, and was, therefore, perfectly -invisible to Galileo's "optic tube." The -puzzle remained unsolved by Galileo; it was left -for another great man to find the true answer. -Huygens, in 1656, first announced that the ring -<em>is</em> a ring.</p> - -<p>The manner in which this announcement was -made is characteristic of the time; to-day it -seems almost ludicrous. Huygens published a -little pamphlet in 1656 called "<cite lang="la" xml:lang="la">De Saturni Luna -Observatio Nova</cite>" or, "A New Observation of -Saturn's Moon." He gave the explanation of -what had been observed by himself and preced<span class="pagenum"><a name="Page_143" id="Page_143">[143]</a></span>ing -astronomers in the form of a puzzle, or -"logogriph." Here is what he had to say of -the phenomenon in question:</p> - -<p>"aaaaaaa ccccc d eeeee g h iiiiiii llll mm -nnnnnnnnn oooo pp q rr s ttttt uuuuu."</p> - -<p>It was not until 1659, three years later, in a -book entitled "<cite lang="la" xml:lang="la">Systema Saturnium</cite>," that Huygens -rearranged the above letters in their proper -order, giving the Latin sentence:</p> - -<p>"<i lang="la" xml:lang="la">Annulo cingitur, tenui plano, nusquam cohaerente, -ad eclipticam inclinato.</i>" Translated -into English, this sentence informs us that the -planet "is girdled with a thin, flat ring, nowhere -touching Saturn, and inclined to the ecliptic"!</p> - -<p>This was a perfectly correct and wonderfully -sagacious explanation of those complex and exasperatingly -puzzling phenomena that had been -too difficult for no less a person than Galileo -himself. It was an explanation that <em>explained</em>. -The reason for its preliminary announcement in -the above manner must have been the following: -Huygens was probably not quite sure of his -ground in 1656, while three years afterward he -had become quite certain. By the publication of<span class="pagenum"><a name="Page_144" id="Page_144">[144]</a></span> -the logogriph of 1656 he secured for himself the -credit of what he had done. If any other astronomer -had published the true explanation -after 1656, Huygens could have proved his -claim to priority by rearranging the letters of his -puzzle. On the other hand, if further researches -showed him that he was wrong, he would never -have made known the true meaning of his logogriph, -and would thus have escaped the ignominy -of making an erroneous explanation. Thus, -the method of announcement was comparable -in ingenuity with the Huygenian explanation -itself.</p> - -<p>We are compelled to pass over briefly the entertaining -history of subsequent observations of -the ring, in order to explain the new work of -Keeler and others. Cassini, about 1675, -been able to show that the ring was double; that -there are really two independent rings, with a -distinct dark space between them. It was a case -of wheels within wheels. To our own eminent -countryman, W. C. Bond, of Cambridge, Mass., -we owe the further discovery (Harvard College -Observatory, November, 1850) of the third<span class="pagenum"><a name="Page_145" id="Page_145">[145]</a></span> -ring. This is also concentric with the other two, -and interior to them, but difficult to observe, -because of its much smaller luminosity.</p> - -<p>It is almost transparent, and the brilliant light -of the planet's central ball is capable of shining -directly through it. For this reason the inner -ring is called the "gauze" or "crape" ring. If -we add to the above details the fact that our -modern large telescopes show slight irregularities -in the surface of the rings, especially when seen -edgewise, we have a brief statement of all that -the telescope has been able to reveal to us since -Galileo's time.</p> - -<p>But of far greater interest than the mere fact -of their existence is the important cosmic question -as to the constitution, structure, and, above -all, durability of the ring system. Astronomers -often use the term "stability" with regard to -celestial systems like the ring system of Saturn. -By this they mean permanent durability. A -system is stable if its various parts can continue -in their present relationship to one another, without -violating any of the known laws of astronomy. -Whenever we study any collection of<span class="pagenum"><a name="Page_146" id="Page_146">[146]</a></span> -celestial objects, and endeavor to explain their -motions and peculiarities, we always seek some -explanation not inconsistent with the continued -existence of the phenomena in question. For -this there is, perhaps, no sufficient philosophical -basis. Probably much of the great celestial procession -is but a passing show, to be but for a -moment in the endless vista of cosmic time.</p> - -<p>However this may be, we are bound to assume -as a working theory that Saturn has always -had these rings, and will always have them; and -it is for us to find out how this is possible. The -problem has been attacked mathematically by -various astronomers, including Laplace; but no -conclusive mathematical treatment was obtained -until 1857, when <ins class="corr" title="Transcriber's Note—Original text: 'James Clark-Maxwell'">James Clerk Maxwell</ins> proved -in a masterly manner that the rings could be -neither solid nor liquid. He showed, indeed, -that they would not last if they were continuous -bodies like the planets. A big solid wheel -would inevitably be torn asunder by any slight -disturbance, and then precipitated upon the planet's -surface. Therefore, the rings must be composed -of an immense number of small detached<span class="pagenum"><a name="Page_147" id="Page_147">[147]</a></span> -particles, revolving around Saturn in separate -orbits, like so many tiny satellites.</p> - -<p>This mathematical theory of the ring system -being once established, astronomers were more -eager than ever to obtain a visual confirmation -of it. We had, indeed, a sort of analogy in -the assemblage of so-called "minor planets" -(<a href="#Page_64">p. 64</a>), which are known to be revolving around -our sun in orbits situated between Mars and -Jupiter. Some hundreds of these are known -to exist, and probably there are countless others -too small for us to see. Such a swarm of tiny -particles of luminous matter would certainly -give the impression of a continuous solid body, -if seen from a distance comparable to that separating -us from Saturn. But arguments founded -on analogy are of comparatively little value.</p> - -<p>Astronomers need direct and conclusive telescopic -evidence, and this was lacking until Keeler -made his remarkable spectroscopic observation in -1895. The spectroscope is a peculiar instrument, -different in principle from any other used -in astronomy; we study distant objects with it -by analyzing the light they send us, rather than<span class="pagenum"><a name="Page_148" id="Page_148">[148]</a></span> -by examining and measuring the details of their -visible surfaces. The reader will recall that according -to the modern undulatory theory, light -consists simply of a series of waves. Now, the -nature of waves is very far from being understood -in the popular mind. Most people, for -instance, think that the waves of ocean consist of -great masses of water rolling along the surface.</p> - -<p>This notion doubtless arises from the behavior -of waves when they break upon the shore, forming -what we call surf. When a wave meets -with an immovable body like a sand beach, the -wave is broken, and the water really does roll -upon the beach. But this is an exceptional case. -Farther away from the shore, where the waves -are unimpeded, they consist simply of particles -of water moving straight up and down. None -of the water is carried by mere wave-action away -from the point over which it was situated at first.</p> - -<p>Tides or other causes may move the water, but -not simple wave-motion alone. That this is so -can be proved easily. If a chip of wood be -thrown overboard from a ship at sea it will be -seen to rise and fall a long time on the waves,<span class="pagenum"><a name="Page_149" id="Page_149">[149]</a></span> -but it will not move. Similarly, wind-waves are -often quite conspicuous on a field of grain; but -they are caused by the individual grain particles -moving up and down. The grain certainly cannot -travel over the ground, since each particle is -fast to its own stalk.</p> - -<p>But while the particles do not travel, the wave-disturbance -does. At times it is transmitted to -a considerable distance from the point where it -was first set in motion. Thus, when a stone is -dropped into still water, the disturbance (though -not the water) travels in ever-widening circles, -until at last it becomes too feeble for us to perceive. -Light is just such a travelling wave-disturbance. -Beginning, perhaps, in some distant -star, it travels through space, and finally the -wave impinges on our eyes like the ocean-wave -breaking on a sand beach. Such a light-wave -affects the eye in some mysterious way. We -call it "seeing."</p> - -<p>The spectroscope (<a href="#Page_21">p. 21</a>) enables us to measure -and count the waves reaching us each second -from any source of light. No matter how far -away the origin of stellar light may be, the spec<span class="pagenum"><a name="Page_150" id="Page_150">[150]</a></span>troscope -examines the character of that light, and -tells us the number of waves set up every second. -It is this characteristic of the instrument -that has enabled us to make some of the most -remarkable observations of modern times. If -the distant star is approaching us in space, more -light-waves per second will reach us than we -should receive from the same star at rest. Thus -if we find from the spectroscope that there are -too many waves, we know that the star is coming -nearer; and if there are too few, we can -conclude with equal certainty that the star is -receding.</p> - -<p>Keeler was able to apply the spectroscope in -this way to the planet Saturn and to the ring -system. The observations required dexterity -and observational manipulative skill in a superlative -degree. These Keeler had; and this work -of his will always rank as a classic observation. -He found by examining the light-waves from -opposite sides of the planet that the luminous -ball rotated; for one side was approaching us -and the other receding. This observation was, -of course, in accord with the known fact of Sat<span class="pagenum"><a name="Page_151" id="Page_151">[151]</a></span>urn's -rotation on his axis. With regard to the -rings, Keeler showed in the same way the existence -of an axial rotation, which appears not -to have been satisfactorily proved before, strange -as it may seem. But the crucial point established -by his spectroscope was that the interior -part of the rings rotates <em>faster</em> than the exterior.</p> - -<p>The velocity of rotation diminishes gradually -from the inside to the outside. This fact is absolutely -inconsistent with the motion of a solid -ring; but it fits in admirably with the theory of -a ring comprised of a vast assemblage of small -separate particles. Thus, for the first time, astronomy -comes into possession of an observational -determination of the nature of Saturn's -rings, and Galileo's puzzle is forever solved.</p> - - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_152" id="Page_152">[152]</a></span></p> - -<h2><a name="THE_HELIOMETER" id="THE_HELIOMETER"></a><a href="#CONTENTS">THE HELIOMETER</a></h2> - - -<p>Astronomical discoveries are always received -by the public with keen interest. Every new -fact read in the great open book of nature is written -eagerly into the books of men. For there -exists a strong curiosity to ascertain just how the -greater world is built and governed; and it must -be admitted that astronomers have been able to -satisfy that curiosity with no small measure of -success. But it is seldom that we hear of the -means by which the latest and most refined -astronomical observations are effected. Popular -imagination pictures the astronomer, as he doubtless -once was, an aged gentleman, usually having -a long white beard, and spending entire nights -staring at the sky through a telescope.</p> - -<p>But the facts to-day are very different. The -working astronomer is an active man in the -prime of life, often a young man. He wastes no -time in star-gazing. His observations consist of<span class="pagenum"><a name="Page_153" id="Page_153">[153]</a></span> -exact measurements made in a precise, systematic, -and almost business-like manner. A night's -"watch" at the telescope is seldom allowed to -exceed about three hours, since it is found that -more continued exertions fatigue the eye and lead -to less accurate results. To this, of course, there -have been many notable exceptions, for endurance -of sight, like any form of physical strength, differs -greatly in different individuals. Astronomical research -does not include "picking out" the constellations, -and learning the Arabic names of individual -stars. These things are not without -interest; but they belong to astronomy's ancient -history, and are of little value except to afford -amusement and instruction to successive generations -of amateurs.</p> - -<p>Among the instruments for carefully planned -measurements of precision the heliometer probably -takes first rank. It is at once the most -exquisitely accurate in its results, and the most -fatiguing to the observer, of all the varied apparatus -employed by the astronomer. The principle -upon which its construction depends is very -peculiar, and applies to all telescopes, even or<span class="pagenum"><a name="Page_154" id="Page_154">[154]</a></span>dinary -ones for terrestrial purposes. If part of a -telescope lens be covered up with the hand, it -will still be possible to see through the instrument. -The glass lens at the end of the tube -farthest from the observer's eye helps to magnify -distant objects and make them seem nearer by -gathering to a single point, or focus, a greater -amount of their light than could be brought -together by the far smaller lens in the unaided -eye.</p> - -<p>The telescope might very properly be likened -to an enlarged eye, which can see more than we -can, simply because it is bigger. If a telescope -lens has a surface one hundred times as large as -that of the lens in our eye, it will gather and bring -to a focus one hundred times as much light from a -distant object. Now, if any part of this telescope -be covered, the remaining part will, nevertheless, -gather and focus light just as though the whole -lens were in action; only, there will be less light -collected at the focus within the tube. The -small lens at the telescope's eye-end is simply a -magnifier to help our eye examine the image of -any distant object formed at the focus by the<span class="pagenum"><a name="Page_155" id="Page_155">[155]</a></span> -large lens at the farther end of the instrument. -For of this simple character is the operation of -any telescope: the large glass lens at one end -collects a distant planet's light, and brings it to a -focus near the other end of the tube, where it -forms a tiny picture of the planet, which, in -turn, is examined with the little magnifier at -the eye-end.</p> - -<p>Having arrived at the fundamental principle -that part of a lens will act in a manner similar to -a whole one, it is easy to explain the construction -of a heliometer. An ordinary telescope lens is -sawed in half by means of a thin round metal -disk revolved rapidly by machinery, and fed continually -with emery and water at its edge. The -cutting effect of emery is sufficient to make such -a disk enter glass much as an ordinary saw penetrates -wood. The two "semi-lenses," as they -are called, are then mounted separately in metal -holders. These are attached to one end of the -heliometer, called the "head," in such a way that -the two semi-lenses can slide side by side upon -metal guides. This head is then fastened to one -end of a telescope tube mounted in the usual<span class="pagenum"><a name="Page_156" id="Page_156">[156]</a></span> -way. The "head" end of the instrument is -turned toward the sky in observing, and at the -eye-end is placed the usual little magnifier we -have already described.</p> - -<p>The observer at the eye-end has control of -certain rods by means of which he can push the -semi-lenses on their slides in the head at the -other end of the tube. Now, if he moves the -semi-lenses so as to bring them side by side exactly, -the whole arrangement will act like an ordinary -telescope. For the semi-lenses will then -fit together just as if the original glass had never -been cut. But if the half-lenses are separated a -little on their slides, each will act by itself. Being -slightly separated, each will cover a different -part of the sky. The whole affair acts as if the -observer at the eye-end were looking through -two telescopes at once. For each semi-lens acts -independently, just as if it were a complete glass -of only half the size.</p> - -<p>Now, suppose there were a couple of stars in -the sky, one in the part covered by the first -semi-lens, and one in the part covered by the -second. The observer would, of course, see<span class="pagenum"><a name="Page_157" id="Page_157">[157]</a></span> -both stars at once upon looking into the little -magnifier at the eye-end of the heliometer.</p> - -<p>We must remember that these stars will -appear in the field of view simply as two tiny -points of light. The astronomer, as we have -said, is provided with a simple system of long -rods, by means of which he can manipulate the -semi-lenses while the observation is being made. -If he slides them very slowly one way or the -other, the two star-points in the field of view will -be seen to approach each other. In this way -they can at last be brought so near together that -they will form but a single dot of light. Observation -with the heliometer consists in thus bringing -two star-images together, until at last they -are superimposed one upon the other, and we see -one image only. Means are provided by which -it is then possible to measure the amount of -separation of the two half-lenses. Evidently the -farther asunder on the sky are the two stars -under observation, the greater will be the separation -of the semi-lenses necessary to make a single -image of their light. Thus, measurement of the -lenses' separation becomes a means of determin<span class="pagenum"><a name="Page_158" id="Page_158">[158]</a></span>ing -the separation of the stars themselves upon -the sky.</p> - -<p>The two slides in the heliometer head are supplied -with a pair of very delicate measures or -"scales" usually made of silver. These can be -examined from the eye-end of the instrument by -looking through a long microscope provided for -this special purpose. Thus an extremely precise -value is obtained both of the separation of the -sliders and of the distance on the sky between -the stars under examination. Measures made in -this way with the heliometer are counted the -most precise of astronomical observations.</p> - -<p>Having thus described briefly the kind of observations -obtained with the heliometer, we shall -now touch upon their further utilization. We -shall take as an example but one of their many -uses—that one which astronomers consider the -most important—the measurement of stellar distances. -(See also <a href="#Page_94">p. 94.</a>)</p> - -<p>Even the nearest fixed star is almost inconceivably -remote from us. And astronomers -are imprisoned on this little earth; we cannot -bridge the profound distance separating us from<span class="pagenum"><a name="Page_159" id="Page_159">[159]</a></span> -the stars, so as to use direct measurement with -tape-line or surveyor's chain. We are forced to -have recourse to some indirect method. Suppose -a certain star is suspected, on account of its brightness, -or for some other reason, of being near us -in space, and so giving a favorable opportunity -for a determination of distance. A couple of -very faint stars are selected close by. These, on -account of their faintness, the astronomer may -regard as quite immeasurably far away. He then -determines with his heliometer the exact position -on the sky of the bright star with respect to the -pair of faint ones. Half a year is then allowed -to pass. During that time the earth has been -swinging along in its annual path or orbit around -the sun. Half a year will have sufficed to carry -the observer on the earth to the other side of -that path, and he is then 185,000,000 miles away -from his position at the first observation.</p> - -<p>Another determination is made of the bright -star's position as referred to the two faint ones. -Now, if all these stars were equally distant, their -relative positions at the second observation would -be just the same as at the former one. But if, as<span class="pagenum"><a name="Page_160" id="Page_160">[160]</a></span> -is very probable, the bright star is very much -nearer us than are the two faint ones, we shall -obtain a different position from our second observation. -For the change of 185,000,000 miles -in the observer's location will, of course, affect -the direction in which we see the near star, while -it will leave the distant ones practically unchanged. -Without entering into technical details, we may -say that from a large number of observations of -this kind, we can obtain the distance of the bright -star by a process of calculation. The only essential -is to have an instrument that can make -the actual observations of position accurately -enough; and in this respect the heliometer is still -unexcelled.</p> - - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_161" id="Page_161">[161]</a></span></p> - -<h2><a name="OCCULTATIONS" id="OCCULTATIONS"></a><a href="#CONTENTS">OCCULTATIONS</a></h2> - - -<p>Scarcely anyone can have watched the sky -without noticing how different is the behavior of -our moon from that of any other object we can -see. Of course, it has this in common with the -sun and stars and planets, that it rises in the -eastern horizon, slowly climbs the dome of the -sky, and again goes down and sets in the west. -This motion of the heavenly bodies is known to -be an apparent one merely, and caused by the -turning of our own earth upon its axis. A man -standing upon the earth's surface can look up -and see above him one-half the great celestial -vault, gemmed with twinkling stars, and bearing, -perhaps, within its ample curve one or two serenely -shining planets and the lustrous moon. -But at any given moment the observer can see -nothing of the other half of the heavenly sphere. -It is beneath his feet, and concealed by the solid -bulk of the earth.</p> - -<p><span class="pagenum"><a name="Page_162" id="Page_162">[162]</a></span></p> - -<p>The earth, however, is turning on an axis, -carrying the observer with it. And so it is continually -presenting him to a new part of the sky. -At any moment he sees but a single half-sphere; -yet the very next instant it is no longer the same; -a small portion has passed out of sight on one -side by going down behind the turning earth, -while a corresponding new section has come into -view on the opposite side. It is this coming into -view that we call the rising of a star; and the -corresponding disappearance on the other side is -called setting. Thus rising and setting are, of -course, due entirely to a turning of the earth, and -not at all to actual motions of the stars; and for -this reason, all objects in the sky, without exception, -must rise and set again. But the moon -really has a motion of its own in addition to this -apparent one caused by the earth's rotation.</p> - -<p>Somewhere in the dawn of time early watchers -of the stars thought out those fancied constellations -that survive even down to our own day. -They placed the mighty lion, king of beasts, -upon the face of night, and the great hunter, too, -armed with club and dagger, to pursue him.<span class="pagenum"><a name="Page_163" id="Page_163">[163]</a></span> -Among these constellations the moon threads her -destined way, night after night, so rapidly that -the unaided eye can see that she is moving. It -needs but little power of fancy's magic to recall -from the dim past a picture of some aged astronomer -graving upon his tablets the Records of -the Moon. "To-night she is near the bright -star in the eye of the Bull." And again: "To-night -she rides full, and near to the heart of the -Virgin."</p> - -<p>And why does the moon ride thus through the -stars of night? Modern science has succeeded -in disentangling the intricacies of her motion, -until to-day only one or two of the very minutest -details of that motion remain unexplained. But -it has been a hard problem. Someone has well -said that lunar theory should be likened to a -lofty cliff upon whose side the intellectual giants -among men can mark off their mental stature, -but whose height no one of them may ever hope -to scale.</p> - -<p>But for our present purpose it is unnecessary -to pursue the subject of lunar motion into its -abstruser details. To understand why the moon<span class="pagenum"><a name="Page_164" id="Page_164">[164]</a></span> -moves rapidly among the stars, it is sufficient to -remember that she is whirling quickly round the -earth, so as to complete her circuit in a little less -than a month. We see her at all times projected -upon the distant background of the sky on which -are set the stellar points of light, as though intended -for beacons to mark the course pursued -by moon and planets. The stars themselves have -no such motions as the moon; situated at a distance -almost inconceivably great, they may, indeed, -be travellers through empty space, yet their -journeys shrink into insignificance as seen from -distant earth. It requires the most delicate instruments -of the astronomer to so magnify the -tiny displacements of the stars on the celestial -vault that they may be measured by human eyes.</p> - -<p>Let us again recur to our supposed observer -watching the moon night after night, so as to -record the stars closely approached by her. Why -should he not some time or other be surprised -by an approach so close as to amount apparently -to actual contact? The moon covers quite -a large surface on the sky, and when we remember -the nearly countless numbers of the stars, it<span class="pagenum"><a name="Page_165" id="Page_165">[165]</a></span> -would, indeed, be strange if there were not some -behind the moon as well as all around her.</p> - -<p>A moment's consideration shows that this must -often be the case; and in fact, if the moon's advancing -edge be scrutinized carefully through a -telescope, small stars can be seen frequently to -disappear behind it. This is a most interesting -observation. At first we see the moon and star -near each other in the telescope's field of view. -But the distance between them lessens perceptibly, -even quickly, until at last, with a startling -suddenness, the star goes out of sight behind the -moon. After a time, ranging from a few moments -to, perhaps, more than an hour, the moon -will pass, and we can see the star suddenly reappear -from behind the other edge.</p> - -<p>These interesting observations, while not at all -uncommon, can be made only very rarely by -naked-eye astronomers. The reason is simple. -The moon's light is so brilliant that it fairly overcomes -the stars whenever they are at all near, except -in the case of very bright ones. Small -stars that can be followed quite easily up to the -moon's edge in a good telescope, disappear from<span class="pagenum"><a name="Page_166" id="Page_166">[166]</a></span> -a naked-eye view while the moon is still a long -distance away. But the number of very bright -stars is comparatively small, so that it is quite unusual -to find anyone not a professional astronomer -who has actually seen one of these so-called -"occultations." Moreover, most people are not -informed in advance of the occurrence of an opportunity -to make such observations, although -they can be predicted quite easily by the aid of -astronomical calculations. Sometimes we have -occultations of planets, and these are the most interesting -of all. When the moon passes between -us and one of the larger planets, it is worth -while to observe the phenomenon even without a -telescope.</p> - -<p>Up to this point we have considered occultations -chiefly as being of interest to the naked-eye -astronomer. Yet occultations have a real scientific -value. It is by their means that we can, -perhaps, best measure the moon's size. By noting -with a telescope the time of disappearance -and reappearance of known stars, astronomers -can bring the direct measurement of the moon's -diameter within the range of their numerical cal<span class="pagenum"><a name="Page_167" id="Page_167">[167]</a></span>culations. -Sometimes the moon passes over a -condensed cluster of stars like the Pleiades. -The results obtainable on these occasions are -valuable in a very high degree, and contribute -largely to making precise our knowledge of the -lunar diameter.</p> - -<p>There is another thing of scientific interest -about occultations, though it has lost some of -its importance in recent years. When such an -event has been observed, the agreement of the -predicted time with that actually recorded by the -astronomer offers a most searching test of the -correctness of our theory of lunar motion. We -have already called attention to the great inherent -difficulty of this theory. It is easy to see -that by noting the exact instant of disappearance -of a star at a place on the earth the latitude and -longitude of which are known, we can both -check our calculations and gather material for -improving our theory. The same principle can -be used also in the converse direction. Within -the limits of precision imposed by the state of -our knowledge of lunar theory, we can utilize -occultations to help determine the position on the<span class="pagenum"><a name="Page_168" id="Page_168">[168]</a></span> -earth of places whose longitude is unknown. It -is a very interesting bit of history that the first -determination of the longitude of Washington -was made by means of occultations, and that this -early determination led to the founding of the -United States Naval Observatory.</p> - -<p>On March 28, 1810, Mr. Pitkin, of Connecticut, -reported to the House of Representatives -on "laying a foundation for the establishment of -a first meridian for the United States, by which -a further dependence on Great Britain or any -other foreign nation for such meridian may be -entirely removed." This report was the result -of a memorial presented by one William Lambert, -who had deduced the longitude of the Capitol -from an occultation observed October 20, -1804. Various proceedings were had in Congress -and in committee, until at last, in 1821, -Lambert was appointed "to make astronomical -observations by lunar occultations of fixed stars, -solar eclipses, or any approved method adapted -to ascertain the longitude of the Capitol from -Greenwich." Lambert's reports were made in -1822 and 1823, but ten years passed before the<span class="pagenum"><a name="Page_169" id="Page_169">[169]</a></span> -establishment of a formal Naval Observatory under -Goldsborough, Wilkes, and Gilliss. But to -Lambert belongs the honor of having marked -out the first fundamental official meridian of -longitude in the United States.</p> - - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_170" id="Page_170">[170]</a></span></p> - -<h2><a name="MOUNTING_GREAT_TELESCOPES" id="MOUNTING_GREAT_TELESCOPES"></a><a href="#CONTENTS">MOUNTING GREAT TELESCOPES</a></h2> - - -<p>There are many interesting practical things -about an astronomical observatory with which -the public seldom has an opportunity to become -acquainted. Among these, perhaps, the details -connected with setting up a great telescope take -first rank. The writer happened to be present -at the Cape of Good Hope Observatory when -the photographic equatorial telescope was being -mounted, and the operation of putting it in position -may be taken as typical of similar processes -elsewhere. (See also <a href="#Page_86">p. 86.</a>)</p> - -<div class="figcenter"> -<a name="P_170" id="P_170"></a> -<br /> -<img src="images/i_170fp.jpg" width="500" alt="" /> -<div class="caption"> -Forty-Inch Telescope, Yerkes Observatory,<br /> -University of Chicago.</div> -</div> - -<p>In the first place, it is necessary to explain -what is meant by an "equatorial" telescope. -One of the chief difficulties in making ordinary -observations arises from the rising and setting of -the stars. They are all apparently moving across -the face of the sky, usually climbing up from the -eastern horizon, only to go down again and set -in the west. If, therefore, we wish to scrutinize -<span class="pagenum"><a name="Page_171" id="Page_171">[171]</a></span>any given object for a considerable time, we must -move the telescope continuously so as to keep -pace with the motion of the heavens. For this -purpose, the tube must be attached to axles, so -that it can be turned easily in any direction. -The equatorial mounting is a device that permits -the telescope to be thus aimed at any part of the -sky, and at the same time facilitates greatly the -operation of keeping it pointed correctly after a -star has once been brought into the field of view.</p> - -<p>To understand the equatorial mounting it is -necessary to remember that the rising and setting -motions of the heavenly bodies are apparent ones -only, and due in reality to the turning of the -earth on its own axis. As the earth goes around, -it carries observer, telescope, and observatory past -the stars fixed upon the distant sky. Consequently, -to keep a telescope pointed continuously -at a given star, it is merely necessary to rotate it -steadily backward upon a suitable axis just fast -enough to neutralize exactly the turning of our -earth.</p> - -<p>By a suitable axis for this purpose we mean -one so mounted as to be exactly parallel to the<span class="pagenum"><a name="Page_172" id="Page_172">[172]</a></span> -earth's own axis of rotation. A little reflection -shows how simply such an arrangement will -work. All the heavenly bodies may be regarded, -for practical purposes, as excessively remote -in comparison with the dimensions of our -earth. The entire planet shrinks into absolute -insignificance when compared with the distances -of the nearest objects brought under observation -by astronomers. It follows that if we have our -telescope attached to such a rotation-axis as we -have described, it will be just the same for purposes -of observation as though the telescope's -axis were not only parallel to the earth's axis, but -actually coincident with it. The two axes may -be separated by a distance equal to that between -the earth's surface and its centre; but, as we -have said, this distance is insignificant so far as -our present object is concerned.</p> - -<p>There is another way to arrive at the same -result. We know that the stars in rising and -setting all seem to revolve about the pole star, -which itself seems to remain immovable. Consequently, -if we mount our telescope so that it -can turn about an axis pointing at the pole, we<span class="pagenum"><a name="Page_173" id="Page_173">[173]</a></span> -shall be able to neutralize the rotation of the stars -by simply turning the telescope about the axis at -the proper speed and in the right direction. Astronomical -considerations teach us that an axis -thus pointing at the pole will be parallel to the -earth's own axis. Thus we arrive at the same -fundamental principle for mounting an astronomical -telescope from whichever point of view we -consider the subject.</p> - -<p>Every large telescope is provided with such an -axis of rotation; and for the reason stated it is -called the "polar axis." The telescope itself is -then called an "equatorial." The advantage of -this method of mounting is very evident. Since -we can follow the stars' motions by turning the -telescope about one axis only, it becomes a very -simple matter to accomplish this turning automatically -by means of clock-work.</p> - -<p>The "following" of a star being thus provided -for by the device of a polar axis, it is, of course, -also necessary to supply some other motion so as -to enable us to aim the tube at any point in the -heavens. For it is obvious that if it were rigidly -attached to the polar axis, we could, indeed, follow<span class="pagenum"><a name="Page_174" id="Page_174">[174]</a></span> -any star that happened to be in the field of view, -but we could not change this field of view at will -so as to observe other stars or planets. To accomplish -this, the telescope is attached to the -polar axis by means of a pivot. By turning the -telescope around its polar axis, and also on this -pivot, we can find any object in the heavens; and -once found, we can leave to the polar axis and its -automatic clock-work the task of keeping that -object before the observer's eye.</p> - -<p>In setting up the Cape of Good Hope instrument -the astronomers were obliged to do a large -part of the work of adjustment personally. Far -away from European instrument-makers, the parts -of the mounting and telescope had to be "assembled," -or put together, by the astronomers of the -Cape Observatory. A heavy pier of brick and -masonry had been prepared in advance. Upon -this was placed a massive iron base, intended to -support the superstructure of polar axis and telescope. -This base rested on three points, one of -which could be screwed in and out, so as to tilt -the whole affair a little forward or backward. -By means of this screw we effected the final ad<span class="pagenum"><a name="Page_175" id="Page_175">[175]</a></span>justment -of the polar axis to exact parallelism -with that of the earth. Other screws were provided -with which the base could be twisted a little -horizontally either to the right or left. Once set -up in a position almost correct, it was easy to perfect -the adjustment by the aid of these screws.</p> - -<p>Afterward the tube and lenses were put in -place, and the clock properly attached inside the -big cast-iron base. This clock-work looked more -like a piece of heavy machinery than a delicate -clock mechanism. But it had heavy work to do, -carrying the massive telescope with its weighty -lenses, and needed to be correspondingly strong. -It had a driving-weight of about 2,000 pounds, -and was so powerful that turning the telescope -affected it no more than the hour-hand of an ordinary -clock affects the mechanism within its case.</p> - -<p>The final test of the whole adjustment consisted -in noting whether stars once brought into -the telescopic field of view could be maintained -there automatically by means of the clock. This -object having been attained successfully, the instrument -stood ready to be used in the routine -business of the observatory.</p> - -<p><span class="pagenum"><a name="Page_176" id="Page_176">[176]</a></span></p> - -<p>Before leaving the subject of telescope-mountings, -we must mention the giant instrument set -up at the Paris Exposition of 1900. The project -of having a <i lang="fr" xml:lang="fr">Grande Lunette</i> had been hailed by -newspapers throughout the world and by the -general public in their customary excitable way. -It was tremendously over-advertised; exaggerated -notions of the instrument's powers were -spread abroad and eagerly credited; the moon -was to be dragged down, as it were, from its -customary place in the sky, so near that we -should be able almost to touch its surface. As -to the planets—free license was given to the journalistic -imagination, and there was no effective -limitation to the magnificence of astronomical -discovery practically within our grasp, beyond -the necessity for printed space demanded by sundry -wars, pestilences, and other mundane trifles.</p> - -<div class="figcenter"> -<a name="P_176" id="P_176"></a> -<br /> -<img src="images/i_176fp.jpg" width="650" alt="" /> -<div class="caption"> -Yerkes Observatory, University of Chicago.</div> -</div> - -<p>Now, the present writer is very far from advocating -a lessening of the attention devoted to astronomy. -Rather would he magnify his office -than diminish it. But let journalistic astronomy -be as good an imitation of sober scientific truth -as can be procured at space rates; let editors -<span class="pagenum"><a name="Page_177" id="Page_177">[177]</a></span>encourage the public to study those things in the -science that are ennobling and cultivating to the -mind; let there be an end to the frenzied effort -to fabricate a highly colored account of alleged -discoveries of yesterday, capable of masquerading -to-day under heavy head-lines as News.</p> - -<p>The manner in which the big telescope came -to be built is not without interest, and shows -that enterprise is far from dead, even in the old -countries. A stock company was organized—we -should call it a corporation—under the -name <i lang="fr" xml:lang="fr">Société de l'Optique</i>. It would appear that -shares were regularly put on the market, and -that a prospectus, more or less alluring, was -widely distributed. We may say at once that the -investing public did not respond with obtrusive -alacrity; but at all events, the promoters' efforts -received sufficient encouragement to enable them -to begin active work. From the very first a -vigorous attempt was made to utilize both the -resources of genuine science and the devices of -quasi-charlatanry. It was announced that the -public were to be admitted to look through the -big glass (apparently at so much an eye), and<span class="pagenum"><a name="Page_178" id="Page_178">[178]</a></span> -many, doubtless, expected that the man in the -street would be able to make personal acquaintance -with the man in the moon. A telescopic -image of the sun was to be projected on a big -screen, and exhibited to a concourse of spectators -assembled in rising tiers of seats within a -great amphitheatre. And when clouds or other -circumstances should prevent observing the planets -or scrutinizing the sun, a powerful stereopticon -was to be used. Artificial pictures of the -wonders of heaven were to be projected on the -screen, and the public would never be disappointed. -It was arranged that skilled talkers -should be present to explain all marvels: and, in -short, financial profit was to be combined with -machinery for advancing scientific discovery. -Astronomers the world over were "circularized," -asked to become shareholders, and, in default of -that, to send lantern-slides or photographs of -remarkable celestial objects for exhibition in the -magic-lantern part of the show.</p> - -<p>The project thus brought to the attention of -scientific men three years ago did not have an -attractive air. It savored too much of charlatan<span class="pagenum"><a name="Page_179" id="Page_179">[179]</a></span>ism. -But it soon appeared that effective government -sanction had been given to the enterprise; -and, above all, that men of reputation -were allowing the use of their names in connection -with the affair. More important still, we -learned that the actual construction had been -undertaken by Gautier, of Paris, that finances -were favorable, and that real work on parts of -the instrument was to commence without delay.</p> - -<p>Gautier is a first-class instrument-builder; he -has established his reputation by constructing successfully -several telescopes of very large size, including -the <em>equatorial coudé</em> of the Paris Observatory, -a unique instrument of especial complexity. -The present writer believes that, if sufficient time -and money were available, the <i lang="fr" xml:lang="fr">Grande Lunette</i> -would stand a reasonable chance of success in -the hands of such a man. And by a reasonable -chance, we mean that there is a large enough -probability of genuine scientific discovery to -justify the necessary financial outlay. But the -project should be divorced from its "popular" -features, and every kind of advertising and charlatanism -excluded with rigor.</p> - -<p><span class="pagenum"><a name="Page_180" id="Page_180">[180]</a></span></p> - -<p>As planned originally, and actually constructed, -the <i lang="fr" xml:lang="fr">Grande Lunette</i> presents interesting peculiarities, -distinguishing it from other telescopes. Previous -instruments have been built on the principle -of universal mobility. It is possible to -move them in all directions, and thus bring any -desired star under observation, irrespective of its -position in the sky. But this general mobility -offers great difficulties in the case of large and -ponderous telescopes. Delicacy of adjustment is -almost destroyed when the object to be adjusted -weighs several tons. And the excessive weight -of telescopes is not due to unavoidably heavy -lenses alone. It is essential that the tube be -long; and great length involves appreciable -thickness of material, if stiffness and solidity are -to remain unsacrificed. Length in the tube is -necessitated by certain peculiar optical defects of -all lenses, into the nature of which we shall not -enter at present. The consequences of these -defects can be rendered harmless only if the instrument -is so arranged that the observer's eye is -far from the other end of the tube. The length -of a good telescope should be at least twelve<span class="pagenum"><a name="Page_181" id="Page_181">[181]</a></span> -times the diameter of its large lens. If the relative -length can be still further increased, so -much the better; for then the optical defects -can be further reduced.</p> - -<p>In the case of the Paris instrument a radical -departure consists in making the tube of unprecedented -length, 197 feet, with a lens diameter of -49¼ inches. This great length, while favorable -optically, precludes the possibility of making the -instrument movable in the usual sense. In fact, -the entire tube is attached to a fixed horizontal -base, and no attempt is made to change its position. -Outside the big lens, and disconnected -altogether from the telescope proper, is mounted -a smooth mirror, so arranged that it can be -turned in any direction, and thus various parts of -the sky examined by reflection in the telescope.</p> - -<p>While this method unquestionably has the advantage -of leaving the optician quite free as to -how long he will make his tube, it suffers from -the compensating objection that a new optical -surface is introduced into the combination, viz., -the mirror. Any slight unavoidable imperfection -in the polishing of its surface will infallibly<span class="pagenum"><a name="Page_182" id="Page_182">[182]</a></span> -be reproduced on a magnified scale in the image -of a distant star brought before the observer's -eye.</p> - -<p>But it is not yet possible to pronounce definitely -upon the merit of this form of instrument, -since, as we have said, the maker has not been -given time enough to try the idea to the complete -satisfaction of scientific men. In the early -part of August, 1900, when the informant of the -present writer left Paris, after serving as a member -of the international jury for judging instruments -of precision at the Exposition, the condition -of the <i lang="fr" xml:lang="fr">Grande Lunette</i> was as follows: Two -sets of lenses had been contemplated, one intended -for celestial photography, and the other to be -used for ordinary visual observation. Only the -photographic lenses had been completed, however, -and for this reason the public could not be -permitted to look through the instrument. The -photographic lenses were in place in the tube, but -at that time their condition was such that, though -some photographs had been obtained, it was not -thought advisable to submit them to the jury. -Consequently, the <i lang="fr" xml:lang="fr">Lunette</i> did not receive a<span class="pagenum"><a name="Page_183" id="Page_183">[183]</a></span> -prize. Since that time various newspapers have -reported wonderful results from the telescope; -but, disregarding the fusillade from the sensational -press, we may sum up the present state of -affairs very briefly. Gautier is still experimenting; -and, given sufficient time and money, he -may succeed in producing what astronomers -hope for—an instrument capable of advancing -our knowledge, even if that advance be only a -small one.</p> - - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_184" id="Page_184">[184]</a></span></p> - -<h2><a name="THE_ASTRONOMERS_POLE" id="THE_ASTRONOMERS_POLE"></a><a href="#CONTENTS">THE ASTRONOMER'S POLE</a></h2> - - -<p>The pole of the frozen North is not the only -pole sought with determined effort by more than -one generation of scientific men. Up in the sky -astronomers have another pole which they are -following up just as vigorously as ever Arctic explorer -struggled toward the difficult goal of his terrestrial -journeying. The celestial pole is, indeed, -a fundamentally important thing in astronomical -science, and the determination of its exact position -upon the sky has always engaged the closest -attention of astronomers. Quite recently new -methods of research have been brought to bear, -promising a degree of success not hitherto attained -in the astronomers' pursuit of their pole.</p> - -<p>In the first place, we must explain what is -meant by the celestial pole. We have already -mentioned the poles of the earth (<a href="#Page_136">p. 136</a>). Our -planet turns once daily upon an axis passing -through its centre, and it is this rotation that<span class="pagenum"><a name="Page_185" id="Page_185">[185]</a></span> -causes all the so-called diurnal phenomena of the -heavens. Rising and setting of sun, moon, and -stars are simply results of this turning of the -earth. Heavenly bodies do not really rise; it is -merely the man on the earth who is turned round -on an axis until he is brought into a position from -which he can see them. The terrestrial poles are -those two points on the earth's surface where it -is pierced by the rotation axis of the planet. -Now we can, if we choose, imagine this axis -lengthened out indefinitely, further and further, -until at last it reaches the great round vault of -the sky. Here it will again pierce out two polar -points; and these are the celestial poles.</p> - -<p>The whole thing is thus quite easy to understand. -On the sky the poles are marked by -the prolongation of the earth's axis, just as on -the earth the poles are marked by the axis itself. -And this explains at once why the stars seem -nightly to revolve about the pole. If the observer -is being turned round the earth's axis, of -course it will appear to him as if the stars were -rotating around the same axis in the opposite -direction, just as houses and fields seem to fly<span class="pagenum"><a name="Page_186" id="Page_186">[186]</a></span> -past a person sitting in a railway train, unless he -stops to remember that it is really himself who is -in motion, and not the trees and houses.</p> - -<p>The existence of such a centre of daily motions -among the stars once recognized, it becomes of -interest to ascertain whether the centre itself always -retains precisely the same position in the -sky. It was discovered as early as the time of -Hipparchus (<a href="#Page_39">p. 39</a>) that such is not the case, -and that the celestial pole is subject to a slow -motion among the stars on the sky. If a given -star were to-day situated exactly at the pole, it -would no longer be there after the lapse of a -year's time; for the pole would have moved away -from it.</p> - -<p>This motion of the pole is called precession. -It means that certain forces are continually at -work, compelling the earth's axis to change its -position, so that the prolongation of that axis -must pierce the sky at a point which moves as -time goes on. These forces are produced by the -gravitational attractions of the sun, moon, and -planets upon the matter composing our earth. -If the earth were perfectly spherical in shape,<span class="pagenum"><a name="Page_187" id="Page_187">[187]</a></span> -the attractions of the other heavenly bodies -would not affect the direction of the earth's rotation-axis -in the least. But the earth is not quite -globular in form; it is flattened a little at the -poles and bulges out somewhat at the equator. -(See <a href="#Page_135">p. 135.</a>)</p> - -<p>This protuberant matter near the equator gives -the other bodies in the solar system an opportunity -to disturb the earth's rotation. The general -effect of all these attractions is to make the -celestial pole move upon the sky in a circle having -a radius of about 23½ degrees; and it requires -25,800 years to complete a circuit of this -precessional cycle. One of the most striking consequences -of this motion will be the change of the -polar star. Just at present the bright star Polaris -in the constellation of the Little Bear is very -close to the pole. But after the lapse of sufficient -ages the first-magnitude star Vega of the constellation -Lyra will in its turn become Guardian of the -Pole.</p> - -<p>It must not be supposed, however, that the -motion of the pole proceeds quite uniformly, and -in an exact circle; the varying positions of the<span class="pagenum"><a name="Page_188" id="Page_188">[188]</a></span> -heavenly bodies whose attractions cause the phenomena -in question are such as to produce appreciable -divergencies from exact circular motion. -Sometimes the pole deviates a little to one side -of the precessional circle, and sometimes it deviates -on the other side. The final result is a -sort of wavy line, half on one side and half on -the other of an average circular curve. It takes -only nineteen years to complete one of these little -waves of polar motion, so that in the whole precessional -cycle of 25,800 years there are about -1,400 indentations. This disturbance of the polar -motion is called by astronomers nutation.</p> - -<p>The first step in a study of polar motion is to -devise a method of finding just where the pole is -on any given date. If the astronomer can ascertain -by observational processes just where the -pole is among the stars at any moment, and can -repeat his observations year after year and generation -after generation, he will possess in time a -complete chart of a small portion at least of the -celestial pole's vast orbit. From this he can obtain -necessary data for a study of the mathematical -theory of attractions, and thus, perhaps, arrive at<span class="pagenum"><a name="Page_189" id="Page_189">[189]</a></span> -an explanation of the fundamental laws governing -the universe in which we live.</p> - -<p>The instrument which has been used most extensively -for the study of these problems is the -transit (<a href="#Page_118">p. 118</a>) or the "meridian circle." This -latter consists of a telescope firmly attached to a -metallic axis about which it can turn. The axis -itself rests on massive stone supports, and is so -placed that it points as nearly as possible in an -east-and-west direction. Consequently, when the -telescope is turned about its axis, it will trace out -on the sky a great circle (the meridian) which -passes through the north and south points of the -horizon and the point directly overhead. The -instrument has also a metallic circle very firmly -fastened to the telescope and its axis. Let into -the surface of this circle is a silver disk upon -which are engraved a series of lines or graduations -by means of which it is <ins class="corr" title="Transcriber's Note—Original text: 'impossible to measure'">possible to measure</ins> -angles.</p> - -<p>Observers with the meridian circle begin by -noting the exact instant when any given star -passes the centre of the field of view of the telescope. -This centre is marked with a cross made<span class="pagenum"><a name="Page_190" id="Page_190">[190]</a></span> -by fastening into the focus some pieces of ordinary -spider's web, which give a well-marked, delicate -set of lines, even under the magnifying power -of the telescope's eye-piece. In addition to thus -noting the time when the star crosses the field -of the telescope, the astronomer can measure by -means of the circle, how high up it was in the sky -at the instant when it was thus observed.</p> - -<p>If the telescope of the meridian circle be turned -toward the north, and we observe stars close to -the pole, it is possible to make two different observations -of the same star. For the close polar -stars revolve in such small circles around the pole -of the heavens that we can observe them when -they are on the meridian either above the pole or -below it. Double observations of this class enable -us to obtain the elevation of the pole above -the horizon, and to fix its position with respect -to the stars.</p> - -<p>Now, there is one very serious objection to -this method. In order to secure the two necessary -observations of the same star, it is essential -to be stationed at the instrument at two moments -of time separated by exactly twelve hours; and if<span class="pagenum"><a name="Page_191" id="Page_191">[191]</a></span> -one of the observations occurs in the night, the -other corresponding observation will occur in -daylight.</p> - -<p>It is a fact not generally known that the -brighter stars can be seen with a telescope, even -when the sun is quite high above the horizon. -Unfortunately, however, there is only one star -close to the pole which is bright enough to be -thus observed in daylight—the polar star already -mentioned under the name Polaris. The fact -that we are thus limited to observations of a single -star has made it difficult even for generations of -astronomers to accumulate with the meridian circle -a very large quantity of observational material -suitable for the solution of our problem.</p> - -<p>The new method of observation to which we -have referred above consists in an application of -photography to the polar problem. If we aim -at the pole a powerful photographic telescope, -and expose a photographic plate throughout the -entire night, we shall find that all stars coming -within the range of the plate will mark out little -circles or "trails" upon the developed negative. -It is evident that as the stars revolve about the<span class="pagenum"><a name="Page_192" id="Page_192">[192]</a></span> -pole on the sky, tracing out their daily circular -orbits, these same little circles must be reproduced -faithfully upon the photographic plate. -The only condition is that the stars shall be -bright enough to make their light affect the sensitive -gelatine surface.</p> - -<p>But even if observations of this kind are continued -throughout all the hours of darkness, we -do not obtain complete circles, but only those -portions of circles traced out on the sky between -sunset and sunrise. If the night is twelve hours -in length, we get half-circles on the plate; if it is -eighteen hours long, we get circles that lack only -one-quarter of being complete. In other words, -we get a series of circular arcs, one corresponding -to each close polar star. There are no fewer than -sixteen stars near enough to the pole to come -within the range of a photographic plate, and -bright enough to cause measurable impressions -upon the sensitive surface. The fact that the -circular arcs are not complete circles does not in -the least prevent our using them for ascertaining -the position of their common centre; and that -centre is the pole. Moreover, as the arcs are<span class="pagenum"><a name="Page_193" id="Page_193">[193]</a></span> -distributed at all sorts of distances from the pole -and in all directions, corresponding to the accidental -positions of the stars on the sky, we have -a state of affairs extremely favorable to the accurate -determination of the pole's place among -the stars by means of microscopic measurements -of the plate.</p> - -<p>It will be perceived that this method is extremely -simple, and, therefore, likely to be successful; -though its simplicity is slightly impaired -by the phenomenon known to astronomers as -"atmospheric refraction." The rays of light -coming down to our telescopes from a distant -star must pass through the earth's atmosphere -before they reach us; and in passing thus from -the nothingness of outer space into the denser -material of the air, they are bent out of their -straight course. The phenomenon is analogous -to what we see when we push a stick down -through the surface of still water; we notice that -the stick appears to be bent at the point where it -pierces the surface of the water; and in just the -same way the rays of light are bent when they -pierce into the air. Fortunately, the mathemati<span class="pagenum"><a name="Page_194" id="Page_194">[194]</a></span>cal -theory of this atmospheric bending of light is -well understood, so that it is possible to remove -the effects of refraction from our results by a -process of calculation. In other words, we can -transform our photographic measures into what -they would have been if no such thing as atmospheric -refraction existed. This having been -done, all the arcs on the plate should be exactly -circular, and their common centre should be the -position of the pole among the stars on the night -when the photograph was made.</p> - -<p>It is possible to facilitate the removal of refraction -effects very much by placing our photographic -telescope at some point on the earth situated -in a very high latitude. The elevation of -the pole above the horizon is greatest in high -latitudes. Indeed, if Arctic voyagers could ever -reach the pole of the earth they would see the -pole of the heavens directly overhead. Now, -the higher up the pole is in the sky, the less will -be the effects of atmospheric refraction; for the -rays of light will then strike the atmosphere in a -direction nearly perpendicular to its surface, which -is favorable to diminishing the amount of bending.</p> - -<p><span class="pagenum"><a name="Page_195" id="Page_195">[195]</a></span></p> - -<p>There is also another very important advantage -in placing the telescope in a high latitude; -in the middle of winter the nights are very -long there; if we could get within the Arctic. -Circle itself, there would be nights when the -hours of darkness would number twenty-four, -and we could substitute complete circles for our -broken arcs. This would, indeed, be most favorable -from the astronomical point of view; but -the essential condition of convenience for the observer -renders an expedition to the frozen Arctic -regions unadvisable.</p> - -<p>But it is at least possible to place the telescope -as far north as is consistent with retaining -it within the sphere of civilized influences. We -can put it in that one of existing observatories on -the earth which has the highest latitude; and -this is the observatory of Helsingfors, in Finland, -which belongs to a great university, is -manned by competent astronomers, and has a -latitude greater than 60 degrees.</p> - -<p>Dr. Anders Donner, Director of the Helsingfors -Observatory, has at its disposal a fine photographic -telescope, and with this some prelimi<span class="pagenum"><a name="Page_196" id="Page_196">[196]</a></span>nary -experimental "trail" photographs were made -in 1895. These photographs were sent to Columbia -University, New York, and were there -measured under the writer's direction. Calculations -based on these measures indicate that the -method is promising in a very high degree; and -it was, therefore, decided to construct a special -photographic telescope better adapted to the particular -needs of the problem in hand.</p> - -<p>The desirability of a new telescope arises from -the fact that we wish the instrument to remain absolutely -unmoved during all the successive hours -of the photographic exposure. It is clear that -if the telescope moves while the stars are tracing -out their little trails on the plate, the circularity -of the curves will be disturbed. Now, ordinary -astronomical telescopes are always mounted upon -very stable foundations, well adapted to making -the telescope stand still; but the polar telescope -which we wish to use in a research fundamental -to the entire science of astronomy ought to possess -immobility and stability of an order higher -than that required for ordinary astronomical purposes.</p> - -<p><span class="pagenum"><a name="Page_197" id="Page_197">[197]</a></span></p> - -<p>It is a remarkable peculiarity of the instrument -needed for the new trail photographs that -it is never moved at all. Once pointed at the -pole, it is ready for all the observations of successive -generations of astronomers. It should -have no machinery, no pivots, axes, circles, -clocks, or other paraphernalia of the usual equatorial -telescope. All we want is a very heavy -stone pier, with a telescope tube firmly fastened -to it throughout its entire length. The top of -the pier having been cut to the proper angle of -the pole's elevation, and the telescope cemented -down, everything is complete from the instrumental -side; and just such an instrument as -this is now ready for use at Helsingfors.</p> - -<p>The late Miss Catharine Wolfe Bruce, of -New York, was much interested in the writer's -proposed polar investigations, and in October, -1898, she contributed funds for the construction -of the new telescope, and the Russian authorities -have generously undertaken the expense of a -building to hold the instrument and the granite -foundation upon which it rests. Photographs -are now being secured with the new instrument,<span class="pagenum"><a name="Page_198" id="Page_198">[198]</a></span> -and they will be sent to Columbia University, -New York, for measurement and discussion. It -is hoped that they will carry out the promise of -the preliminary photographs made in 1895 with -a less suitable telescope of the ordinary form.</p> - - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_199" id="Page_199">[199]</a></span></p> - -<h2><a name="THE_MOON_HOAX" id="THE_MOON_HOAX"></a><a href="#CONTENTS">THE MOON HOAX</a></h2> - - -<p>The public attitude toward matters scientific -is one of the mysteries of our time. It can be -described best by the single word, Credulity; -simple, absolute credulity. Perfect confidence is -the most remarkable characteristic of this unbelieving -age. No charlatan, necromancer, or astrologer -of three centuries ago commanded more -respectful attention than does his successor of -to-day.</p> - -<p>Any person can be a scientific authority; he -has but to call himself by that title, and everyone -will give him respectful attention. Numerous -instances can be adduced from the experience -of very recent years to show how true are these -remarks. We have had the Keeley motor and -the liquid-air power schemes for making something -out of nothing. Extracting gold from sea-water -has been duly heralded on scientific authority -as an easy source of fabulous wealth for the<span class="pagenum"><a name="Page_200" id="Page_200">[200]</a></span> -million. Hard-headed business men not only -believe in such things, but actually invest in them -their most valued possession, capital. Venders -of nostrums and proprietary medicines acquire -wealth as if by magic, though it needs but a moment's -reflection to realize that these persons cannot -possibly be in possession of any drugs, or -secret methods of compounding drugs, that are -unknown to scientific chemists.</p> - -<p>If the world, then, will persistently intrust its -health and wealth into the safe-keeping of charlatans, -what can we expect when things supposedly -of far less value are at stake? The famous Moon -Hoax, as we now call it, is truly a classic piece of -lying. Though it dates from as long ago as -1835, it has never had an equal as a piece of -"modern" journalism. Nothing could be more -useful than to recall it to public attention at least -once every decade; for it teaches an important -lesson that needs to be iterated again and again.</p> - -<p>On November 13, 1833, Sir John Herschel -embarked on the Mountstuart Elphinstone, bound -for the Cape of Good Hope. He took with -him a collection of astronomical instruments,<span class="pagenum"><a name="Page_201" id="Page_201">[201]</a></span> -with which he intended to study the heavens -of the southern hemisphere, and thus extend -his father's great work to the south polar stars. -An earnest student of astronomy, he asked no -better than to be left in peace to seek the truth -in his own fashion. Little did he think that his -expedition would be made the basis for a fabrication -of alleged astronomical discoveries destined -to startle a hemisphere. Yet that is precisely -what happened. Some time about the middle of -the year 1835 the New York <cite>Sun</cite> began the publication -of certain articles, purporting to give an -account of "Great Astronomical Discoveries, -lately made by Sir John Herschel at the Cape of -Good Hope." It was alleged that these articles -were taken from a supplement to the Edinburgh -<cite>Journal of Science</cite>; yet there is no doubt that -they were manufactured entirely in the United -States, and probably in New York.</p> - -<p>The hoax begins at once in a grandiloquent -style, calculated to attract popular attention, and -well fitted to the marvels about to be related. -Here is an introductory remark, as a specimen: -"It has been poetically said that the stars of<span class="pagenum"><a name="Page_202" id="Page_202">[202]</a></span> -heaven are the hereditary regalia of man as the -intellectual sovereign of the animal creation. He -may now fold the zodiac around him with a -loftier consciousness of his mental supremacy." -Then follows a circumstantial and highly plausible -account of the manner in which early and exclusive -information was obtained from the Cape. -This was, of course, important in order to make -people believe in the genuineness of the whole; -but we pass at once to the more interesting account -of Herschel's supposed instrument.</p> - -<p>Nothing could be more skilful than the way in -which an air of truth is cast over the coming account -of marvellous discoveries by explaining in -detail the construction of the imaginary Herschelian -instrument. Sir John is supposed to -have had an interesting conversation in England -"with Sir David Brewster, upon the merits of -some ingenious suggestion by the latter, in his article -on optics in the Edinburgh Encyclopædia -(p. 644), for improvements in the Newtonian -reflectors." The exact reference to a particular -page is here quite delightful. After some further -talk, "the conversation became directed to that<span class="pagenum"><a name="Page_203" id="Page_203">[203]</a></span> -all-invincible enemy, the paucity of light in powerful -magnifiers. After a few moments' silent -thought, Sir John diffidently inquired whether it -would not be possible to effect a <em>transfusion of -artificial light through the focal object of vision</em>! -Sir David, somewhat startled at the originality of -the idea, paused awhile, and then hesitatingly referred -to the refrangibility of rays, and the angle -of incidence.... Sir John continued, -'Why cannot the illuminated microscope, say -the hydro-oxygen, be applied to render distinct, -and, if necessary, even to magnify the focal object?' -Sir David sprang from his chair in an -ecstasy of conviction, and leaping half-way to the -ceiling, exclaimed, 'Thou art the man.' "This -absurd imaginary conversation contains nothing -but an assemblage of optical jargon, put together -without the slightest intention of conveying any -intelligible meaning to scientific people. Yet it -was well adapted to deceive the public; and we -should not be surprised if it would be credited -by many newspaper readers to-day.</p> - -<p>The authors go on to explain how money was -raised to build the new instrument, and then de<span class="pagenum"><a name="Page_204" id="Page_204">[204]</a></span>scribe -Herschers embarkation and the difficulties -connected with transporting his gigantic machines -to the place selected for the observing -station. "Sir John accomplished the ascent to -the plains by means of two relief teams of oxen, -of eighteen each, in about four days, and, aided -by several companies of Dutch boors [<em>sic</em>], proceeded -at once to the erecting of his gigantic -fabric." The place really selected by Herschel -cannot be described better than in his own -words, contained in a genuine letter dated January -21, 1835: "A perfect paradise in rich and -magnificent mountain scenery, sheltered from all -winds.... I must reserve for my next all -description of the gorgeous display of flowers -which adorn this splendid country, as well as -the astonishing brilliancy of the constellations." -The author of the hoax could have had no -knowledge of Herschers real location, as described -in this letter.</p> - -<p>The present writer can bear witness to the -correctness of Herschel's words. Feldhausen -is truly an ideal secluded spot for astronomical -study. A small obelisk under the sheer cliff of<span class="pagenum"><a name="Page_205" id="Page_205">[205]</a></span> -far-famed Table Mountain now marks the site -of the great reflecting telescope. Here Herschel -carried on his scrutiny of the Southern skies. -He observed 1,202 double stars and 1,708 -nebulæ and clusters, of which only 439 were already -known. He studied the famous Magellanic -clouds, and made the first careful drawings -of the "keyhole" nebula in the constellation -Argo.</p> - -<p>Very recent researches of the present royal -astronomer at the Cape have shown that changes -of import have certainly taken place in this -nebula since Herschel's time, when a sudden -blazing up of the wonderful star Eta Argus -was seen within the nebula. This object has, -perhaps, undergone more remarkable changes of -light than any other star in the heavens. It is -as though there were some vast conflagration at -work, now blazing into incandescence, and again -sinking almost into invisibility. In 1843 Maclear -estimated the brilliancy of Eta to be about -equal to that of Sirius, the brightest star in the -whole sky. Later it diminished in light, and -cannot be seen to-day with the naked eye, though<span class="pagenum"><a name="Page_206" id="Page_206">[206]</a></span> -the latest telescopic observations indicate that it -is again beginning to brighten.</p> - -<p>Such was Herschel's quiet study of his beloved -science, in glaring contrast to the supposed discoveries -of the "Hoax." Here are a few things -alleged to have been seen on the moon. The -first time the instrument was turned upon our satellite -"the field of view was covered throughout -its entire area with a beautifully distinct and even -vivid representation of basaltic rock." There -were forests, too, and water, "fairer shores never -angels coasted on a tour of pleasure. A beach of -brilliant white sand, girt with wild castellated -rocks, apparently of green marble."</p> - -<p>There was animal life as well; "we beheld -continuous herds of brown quadrupeds, having -all the external characteristics of the bison, -but more diminutive than any species of the -bos genus in our natural history." There was -a kind of beaver, that "carries its young in -its arms like a human being," and lives in huts. -"From the appearance of smoke in nearly all of -them, there is no doubt of its (the beaver's) being -acquainted with the use of fire." Finally, as<span class="pagenum"><a name="Page_207" id="Page_207">[207]</a></span> -was, of course, unavoidable, human creatures -were discovered. "Whilst gazing in a perspective -of about half a mile, we were thrilled with -astonishment to perceive four successive flocks of -large-winged creatures, wholly unlike any kind -of birds, descend with a slow, even motion from -the cliffs on the western side, and alight upon -the plain.... Certainly they were like human -beings, and their attitude in walking was -both erect and dignified."</p> - -<p>We have not space to give more extended extracts -from the hoax, but we think the above -specimens will show how deceptive the whole -thing was. The rare reprint from which we have -extracted our quotations contains also some interesting -"Opinions of the American Press Respecting -the Foregoing Discovery." The <cite>Daily Advertiser</cite> -said: "No article, we believe, has appeared -for years, that will command so general a -perusal and publication. Sir John has added a -stock of knowledge to the present age that will -immortalize his name and place it high on the -page of science." The <cite>Mercantile Advertiser</cite> -said: "Discoveries in the Moon.—We com<span class="pagenum"><a name="Page_208" id="Page_208">[208]</a></span>mence -to-day the publication of an interesting -article which is stated to have been copied from -the Edinburgh <cite>Journal of Science</cite>, and which made -its first appearance here in a contemporary journal -of this city. It appears to carry intrinsic evidence -of being an authentic document." Many other -similar extracts are given. The New York <cite>Evening -Post</cite> did not fall into the trap. The <cite>Evening -Post's</cite> remarks were as follows: "It is quite -proper that the <em>Sun</em> should be the means of shedding -so much light on the <em>Moon</em>. That there -should be winged people in the moon does not -strike us as more wonderful than the existence of -such a race of beings on the earth; and that there -does or did exist such a race rests on the evidence -of that most veracious of voyagers and circumstantial -of chroniclers, Peter Wilkins, whose celebrated -work not only gives an account of the -general appearance and habits of a most interesting -tribe of flying Indians, but also of all those -more delicate and engaging traits which the author -was enabled to discover by reason of the conjugal -relations he entered into with one of the females -of the winged tribe."</p> - -<p><span class="pagenum"><a name="Page_209" id="Page_209">[209]</a></span></p> - -<p>We shall limit our extracts from the contemporary -press to the few quotations here given, -hoping that enough has been said to direct attention -once more to that important subject, the -Possibility of Being Deceived.</p> - - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_210" id="Page_210">[210]</a></span></p> - -<h2><a name="THE_SUNS_DESTINATION" id="THE_SUNS_DESTINATION"></a><a href="#CONTENTS">THE SUN'S DESTINATION</a></h2> - - -<p>Three generations of men have come and -gone since the Marquis de Laplace stood before -the Academy of France and gave his demonstration -of the permanent stability of our solar system. -There was one significant fault in Newton's -superbly simple conception of an eternal -law governing the world in which we live. The -labors of mathematicians following him had -shown that the planets must trace out paths in -space whose form could be determined in advance -with unerring certainty by the aid of -Newton's law of gravitation. But they proved -just as conclusively that these planetary orbits, -as they are called, could not maintain indefinitely -the same shapes or positions. Slow indeed might -be the changes they were destined to undergo; -slow, but sure, with that sureness belonging to -celestial science alone. And so men asked: -Has this magnificent solar system been built<span class="pagenum"><a name="Page_211" id="Page_211">[211]</a></span> -upon a scale so grand, been put in operation subject -to a law sublime in its very simplicity, only -to change and change until at length it shall lose -every semblance of its former self, and end, perhaps, -in chaos or extinction?</p> - -<p>Laplace was able to answer confidently, "No." -Nor was his answer couched in the enthusiastic -language of unbalanced theorists who work by -the aid of imagination alone. Based upon the -irrefragable logic of correct mathematical reasoning, -and clad in the sober garb of mathematical -formulæ, his results carried conviction to men of -science the world over. So was it demonstrated -that changes in our solar system are surely at -work, and shall continue for nearly countless -ages; yet just as surely will they be reversed at -last, and the system will tend to return again to -its original form and condition. The objection -that the Newtonian law meant ultimate dissolution -of the world was thus destroyed by Laplace. -From that day forward the law of gravitation -has been accepted as holding sway over all phenomena -visible within our planetary world.</p> - -<p>The intricacies of our own solar system being<span class="pagenum"><a name="Page_212" id="Page_212">[212]</a></span> -thus illumined, the restless activity of the human -intellect was stimulated to search beyond for new -problems and new mysteries. Even more fascinating -than the movements of our sun and -planets are all those questions that relate to the -clustered stellar congeries hanging suspended -within the deep vault of night. Does the same -law of gravitation cast its magic spell over that -hazy cloud of Pleiades, binding them, like ourselves, -with bonds indissoluble? Who shall answer, -yes or no? We can only say that astronomers -have as yet but stepped upon the threshold -of the universe, and fixed the telescope's great -eye upon that which is within.</p> - -<p>Let us then begin by reminding the reader -what is meant by the Newtonian law of gravitation. -It appears all things possess the remarkable -property of attracting or pulling each other. -Newton declared that all substances, solid, liquid, -or even gaseous—from the massive cliff of -rock down to the invisible air—all matter can -no more help pulling than it can help existing. -His law further formulates certain conditions -governing the manner in which this gravitational<span class="pagenum"><a name="Page_213" id="Page_213">[213]</a></span> -attraction is exerted; but these are mere matters -of detail; interest centres about the mysterious -fact of attraction itself. How can one thing pull -another with no connecting link through which -the pull can act? Just here we touch the point -that has never yet been explained. Nature withholds -from science her ultimate secrets. They -that have pondered longest, that have descended -farthest of all men into the clear well of knowledge, -have done so but to sound the depths beyond, -never touching bottom.</p> - -<p>This inability of ours, to give a good physical -explanation of gravitation, has led certain makers -of paradoxes to doubt or even deny that there is -any such thing. But, fortunately, we have a simple -laboratory experiment that helps us. Unexplained -it may ever remain, but that there -can be attraction between physical objects connected -by no visible link is proved by the behavior -of an ordinary magnet. Place a small -piece of steel or iron near a magnetized bar, and -it will at once be so strongly attracted that it -will actually fly to the magnet. Anyone who -has seen this simple experiment can never again<span class="pagenum"><a name="Page_214" id="Page_214">[214]</a></span> -deny the possibility, at least, of the law of attraction -as stated by Newton. Its possibility once -admitted, the fact that it can predict the motions -of all the planets, even down to their minutest -details, transforms the possibility of its truth into -a certainty as strong as any human certainty can -ever be.</p> - -<p>But this demonstration of Newton's law is -limited strictly to the solar system itself. We -may, indeed, reason by analogy, and take for -granted that a law which holds within our immediate -neighborhood is extremely likely to be true -also of the entire visible universe. But men of -science are loath to reason thus; and hence the -fascination of researches in cosmic astronomy. -Analogy points out the path. The astronomer -is not slow to follow; but he seeks ever to -establish upon incontrovertible evidence those -truths which at first only his daring imagination -had led him to half suspect.</p> - -<p>If we are to extend the law of gravitation to -the utmost, we must be careful to consider the -law itself in its most complete form. A heavenly -body like the sun is often said to govern the<span class="pagenum"><a name="Page_215" id="Page_215">[215]</a></span> -motions of its family of planets; but such a statement -is not strictly accurate. The governing -body is no despot; 'tis an abject slave of law and -order, as much as the tiniest of attendant planets. -The action of gravitation is mutual, and no -cosmic body can attract another without being -itself in turn subject to that other's gravitational -action.</p> - -<p>If there were in our solar system but two bodies, -sun and planet, we should find each one pursuing -a path in space under the influence of the -other's attraction. These two paths or orbits -would be oval, and if the sun and planet were -equally massive, the orbits would be exactly -alike, both in shape and size. But if the sun -were far larger than the planet, the orbits would -still be similar in form, but the one traversed by -the larger body would be small. For it is not -reasonable to expect a little planet to keep the -big sun moving with a velocity as great as that -derived by itself from the attraction of the -larger orb.</p> - -<p>Whenever the preponderance of the larger -body is extremely great, its orbit will be corre<span class="pagenum"><a name="Page_216" id="Page_216">[216]</a></span>spondingly -insignificant in size. This is in fact -the case with our own sun. So massive is it in -comparison with the planets that the orbit is too -small to reveal its actual existence without the -aid of our most refined instruments. The path -traced out by the sun's centre would not fill a -space as large as the sun's own bulk. Nevertheless, -true orbital motion is there.</p> - -<p>So we may conclude that as a necessary consequence -of the law of gravitation every object -within the solar system is in motion. To say -that planets revolve about the sun is to neglect -as unimportant the small orbit of the sun itself. -This may be sufficiently accurate for ordinary -purposes; but it is unquestionably necessary to -neglect no factor, however small, if we propose -to extend our reasoning to a consideration of the -stellar universe. For we shall then have to deal -with systems in which the planets are of a size -comparable with the sun; and in such systems -all the orbits will also be of comparatively equal -importance.</p> - -<p>Mathematical analysis has derived another fact -from discussion of the law of gravitation which,<span class="pagenum"><a name="Page_217" id="Page_217">[217]</a></span> -perhaps, transcends in simple grandeur everything -we have as yet mentioned. It matters not -how great may be the number of massive orbs -threading their countless interlacing curved paths -in space, there yet must be in every cosmic system -one single point immovable. This point is -called the Centre of Gravity. If it should so -happen that in the beginning of things, some -particle of matter were situated at this centre, -then would that atom ever remain unmoved and -imperturbable throughout all the successive vicissitudes -of cosmic evolution. It is doubtful -whether the mind of man can form a conception -of anything grander than such an immovable -atom within the mysterious intricacies of cosmic -motion.</p> - -<p>But in general, we cannot suppose that the -centres of gravity in the various stellar systems -are really occupied by actual physical bodies. -The centre may be a mere mathematical point in -space, situated among the several bodies composing -the system, but, nevertheless, endowed, in a -certain sense, with the same remarkable property -of relative immobility.</p> - -<p><span class="pagenum"><a name="Page_218" id="Page_218">[218]</a></span></p> - -<p>Having thus defined the centre of gravity in -its relation to the constituent parts of any cosmic -system, we can pass easily to its characteristic -properties in connection with the inter-relation -of stellar systems with one another. It can be -proved mathematically that our solar system will -pull upon distant stars just as though the sun -and all the planets were concentrated into one -vast sphere having its centre in the centre of -gravity of the whole. It is this property of the -centre of gravity which makes it pre-eminently -important in cosmic researches. For, while we -know that centre to be at rest relatively to all -the planets in the system, it may, nevertheless, -in its quality as a sort of concentrated essence of -them all, be moving swiftly through space under -the pull of distant stars. In that case, the attendant -bodies will go with it—but they will -pursue their evolutions within the system, all unconscious -that the centre of gravity is carrying -them on a far wider circuit.</p> - -<p>What is the nature of that circuit? This -question has been for many years the subject of -earnest study by the clearest minds among as<span class="pagenum"><a name="Page_219" id="Page_219">[219]</a></span>tronomers. -The greatest difficulty in the way is -the comparatively brief period during which men -have been able to make astronomical observations -of precision. Space and time are two conceptions -that transcend the powers of definition -possessed by any man. But we can at least -form a notion of how vast is the extent of time, -if we remember that the period covered by man's -written records is registered but as a single moment -upon the great revolving dial of heaven's -dome. One hundred and fifty years have -elapsed since James Bradley built the foundations -of modern sidereal astronomy upon his masterly -series of observations at the Royal Observatory -of Greenwich, in England. Yet so slowly -do the movements of the stars unroll themselves -upon the firmament, that even to this day no -one of them has been seen by men to trace out -more than an infinitesimal fraction of its destined -path through the voids of space.</p> - -<p>Travellers upon a railroad cannot tell at any -given moment whether they are moving in a -straight line, or whether the train is turning -upon some curve of huge size. The St. Goth<span class="pagenum"><a name="Page_220" id="Page_220">[220]</a></span>ard -railway has several so-called "corkscrew" -tunnels, within which the rails make a complete -turn in a spiral, the train finally emerging from -the tunnel at a point almost vertically over the -entrance. In this way the train is lifted to a -higher level. Passengers are wont to amuse -themselves while in these tunnels by watching -the needle of an ordinary pocket-compass. This -needle, of course, always points to the north; -and as the train turns upon its curve, the needle -will make a complete revolution. But the passenger -could not know without the compass -that the train was not moving in a perfectly -straight line. Just so we passengers on the -earth are unaware of the kind of path we are -traversing, until, like the compass, the astronomer's -instruments shall reveal to us the -truth.</p> - -<p>But as we have seen, astronomical observations -of precision have not as yet extended through a -period of time corresponding to the few minutes -during which the St. Gothard traveller watches -the compass. We are still in the dark, and do -not know as yet whether mankind shall last long<span class="pagenum"><a name="Page_221" id="Page_221">[221]</a></span> -enough upon the earth to see the compass needle -make its revolution. We are compelled to believe -that the motion in space of our sun is progressing -upon a curved path; but so far as precise -observations allow us to speak, we can but -say that we have as yet moved through an infinitesimal -element only of that mighty curve. However, -we know the point upon the sky toward -which this tiny element of our path is directed, -and we have an approximate knowledge of the -speed at which we move.</p> - -<p>More than a century ago Sir William Herschel -was able to fix roughly what we call the apex of -the sun's way in space, or the point among the -stars toward which that way is for the moment -directed. We say for the moment, but we mean -that moment of which Bradley saw the beginning -in 1750, and upon whose end no man of those -now living shall ever look. Herschel found that -a comparison of old stellar observations seemed -to indicate that the stars in a certain part of the -sky were opening out, as it were, and that the -constellations in the opposite part of the heavens -seemed to be drawing in, or becoming smaller.<span class="pagenum"><a name="Page_222" id="Page_222">[222]</a></span> -There can be but one reasonable explanation of -this. We must be moving toward that part of -the sky where the stars are separating. Just so a -man watching a regiment of soldiers approaching, -will see at first only a confused body of men; -but as they come nearer, the individual soldiers -will seem to separate, until at length each one is -seen distinct from all the others.</p> - -<p>Herschel fixed the position of the apex at a -point in the constellation Hercules. The most -recent investigations of Newcomb and others -have, on the whole, verified Herschel's conclusions. -With the intuitive power of rare genius, -Herschel had been able to sift truth out of error. -The observational data at his disposal would now -be called rude, but they disclosed to the scrutiny -of his acute understanding the germ of truth that -was in them. Later investigators have increased -the precision of our knowledge, until we can now -say that the present direction of the solar motion -is known within very narrow limits. A tiny circle -might be drawn on the sky, to which an astronomer -might point his hand and say: "Yonder little -circle contains the goal toward which the sun and<span class="pagenum"><a name="Page_223" id="Page_223">[223]</a></span> -planets are hastening to-day." Even the speed -of this motion has been subjected to measurement, -and found to be about ten miles per -second.</p> - -<p>The objective point and the rate of motion -thus stated, exact science holds her peace. Here -genuine knowledge stops; and we can proceed -further only by the aid of that imagination which -men of science need to curb at every moment. -But let no one think that the sun will ever -reach the so-called apex. To do so would mean -cosmic motion upon a straight line, while every -consideration of celestial mechanics points to motion -upon a curve. When shall we turn sufficiently -upon that curve to detect its bending? -'Tis a problem we must leave as a rich heritage -to later generations that are to follow us. The -visionary theorist's notion of a great central sun, -controlling our own sun's way in space, must be -dismissed as far too daring. But for such a central -sun we may substitute a central centre of -gravity belonging to a great system of which our -sun is but an insignificant member. Then we -reach a conception that has lost nothing in the<span class="pagenum"><a name="Page_224" id="Page_224">[224]</a></span> -grandeur of its simplicity, and is yet in accord -with the probabilities of sober mechanical science. -We cease to be a lonely world, and stretch out -the bonds of a common relationship to yonder -stars within the firmament.</p> - - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_225" id="Page_225">[225]</a></span></p> - -<h2><a name="INDEX" id="INDEX"></a><a href="#CONTENTS">INDEX</a></h2> - - -<div class="center fs90"> -<table border="0" cellpadding="4" cellspacing="0" width="95%" summary=""> -<tr><td class="tdl"></td><td class="tdr xs">PAGE</td></tr> -<tr><td class="tdl">Airy, Astronomer Royal,</td><td class="tdr"><a href="#Page_1">1</a></td></tr> -<tr><td class="tdl">Allis, photographs comet,</td><td class="tdr"><a href="#Page_101">101</a></td></tr> -<tr><td class="tdl">Andromeda nebula,</td><td class="tdr"><a href="#Page_28">28</a></td></tr> -<tr><td class="tdl pad4">temporary star,</td><td class="tdr"><a href="#Page_28">28</a>, <a href="#Page_29">29</a>, <a href="#Page_45">45</a></td></tr> -<tr><td class="tdl">Apex, of solar motion, explained,</td><td class="tdr"><a href="#Page_221">221</a></td></tr> -<tr><td class="tdl">Aquila, constellation, temporary star in,</td><td class="tdr"><a href="#Page_40">40</a></td></tr> -<tr><td class="tdl">Arctic regions, position of pole in,</td><td class="tdr"><a href="#Page_194">194</a></td></tr> -<tr><td class="tdl">Argo, constellation, variable star in,</td><td class="tdr"><a href="#Page_205">205</a></td></tr> -<tr><td class="tdl">Association, international geodetic,</td><td class="tdr"><a href="#Page_139">139</a></td></tr> -<tr><td class="tdl"><a name="AST" id="AST"></a>Asteroids, first discovery by Piazzi,</td><td class="tdr"><a href="#Page_59">59</a>, <a href="#Page_106">106</a></td></tr> -<tr><td class="tdl pad4">discovery by photography,</td><td class="tdr"><a href="#Page_64">64</a></td></tr> -<tr><td class="tdl pad4">group of,</td><td class="tdr"><a href="#Page_63">63</a></td></tr> -<tr><td class="tdl pad4">photography of, invented by Wolf,</td><td class="tdr"><a href="#Page_104">104</a></td></tr> -<tr><td class="tdl">Astronomer, royal,</td><td class="tdr"><a href="#Page_1">1</a></td></tr> -<tr><td class="tdl pad4">working, description of,</td><td class="tdr"><a href="#Page_152">152</a></td></tr> -<tr><td class="tdl smcap">Astronomer's Pole, the,</td><td class="tdr"><a href="#Page_184">184</a></td></tr> -<tr><td class="tdl">Astronomy, journalistic,</td><td class="tdr"><a href="#Page_176">176</a></td></tr> -<tr><td class="tdl pad4">practical uses of,</td><td class="tdr"><a href="#Page_112">112</a></td></tr> -<tr><td class="tdl">Atmospheric refraction, explained,</td><td class="tdr"><a href="#Page_193">193</a></td></tr> -<tr><td class="tdl">Axis, of figure of the earth,</td><td class="tdr"><a href="#Page_136">136</a></td></tr> -<tr><td class="tdl pad4">of rotation of the earth,</td><td class="tdr"><a href="#Page_136">136</a></td></tr> -<tr><td class="tdl pad4">polar, of telescope,</td><td class="tdr"><a href="#Page_173">173</a></td></tr> - -<tr><td class="tdl"> </td></tr> -<tr><td class="tdl">Barnard, discovers satellite of Jupiter,</td><td class="tdr"><a href="#Page_51">51</a></td></tr> -<tr><td class="tdl">Bessel, measures Pleiades,</td><td class="tdr"><a href="#Page_15">15</a></td></tr> -<tr><td class="tdl">Bond, discovers crape ring of Saturn,</td><td class="tdr"><a href="#Page_144">144</a></td></tr> -<tr><td class="tdl">Bradley, observes at Greenwich,</td><td class="tdr"><a href="#Page_219">219</a></td></tr> -<tr><td class="tdl">Brahe, Tycho, his temporary star,</td><td class="tdr"><a href="#Page_40">40</a></td></tr> -<tr><td class="tdl">Bruce, endows polar photography,</td><td class="tdr"><a href="#Page_197">197</a></td></tr> - -<tr><td class="tdl"> </td></tr> -<tr><td class="tdl"><span class="pagenum"><a name="Page_226" id="Page_226">[226]</a></span> - Campbell, observes Pole-star,</td><td class="tdr"><a href="#Page_18">18</a></td></tr> -<tr><td class="tdl">Cape of Good Hope, observatory, photography at,</td><td class="tdr"><a href="#Page_101">101</a></td></tr> -<tr><td class="tdl pad4">telescope,</td><td class="tdr"><a href="#Page_170">170</a>, <a href="#Page_174">174</a></td></tr> -<tr><td class="tdl"><i lang="it" xml:lang="it">Capriccio</i>, Galileo's,</td><td class="tdr"><a href="#Page_55">55</a></td></tr> -<tr><td class="tdl">Cassini, shows Saturn's rings to be double,</td><td class="tdr"><a href="#Page_144">144</a></td></tr> -<tr><td class="tdl">Cassiopeia, temporary star in,</td><td class="tdr"><a href="#Page_40">40</a></td></tr> -<tr><td class="tdl">Celestial pole,</td><td class="tdr"><a href="#Page_184">184</a></td></tr> -<tr><td class="tdl">Central sun theory,</td><td class="tdr"><a href="#Page_223">223</a></td></tr> -<tr><td class="tdl">Centre of gravity,</td><td class="tdr"><a href="#Page_217">217</a></td></tr> -<tr><td class="tdl">Chart-room, on ship-board,</td><td class="tdr"><a href="#Page_5">5</a></td></tr> -<tr><td class="tdl">Chronometer, invention of,</td><td class="tdr"><a href="#Page_8">8</a></td></tr> -<tr><td class="tdl">Circle, meridian, explained,</td><td class="tdr"><a href="#Page_189">189</a></td></tr> -<tr><td class="tdl">Clerk Maxwell, discusses Saturn's rings,</td><td class="tdr"><a href="#Page_146">146</a></td></tr> -<tr><td class="tdl">Clock, affected by temperature,</td><td class="tdr"><a href="#Page_117">117</a></td></tr> -<tr><td class="tdl pad4">affected by barometric pressure,</td><td class="tdr"><a href="#Page_117">117</a></td></tr> -<tr><td class="tdl pad4">astronomical,</td><td class="tdr"><a href="#Page_115">115</a></td></tr> -<tr><td class="tdl pad4">astronomical, how mounted,</td><td class="tdr"><a href="#Page_116">116</a></td></tr> -<tr><td class="tdl pad4">astronomical, its dial,</td><td class="tdr"><a href="#Page_116">116</a></td></tr> -<tr><td class="tdl pad4">error of, determined with transit,</td><td class="tdr"><a href="#Page_118">118</a></td></tr> -<tr><td class="tdl pad4">jeweller's regulator,</td><td class="tdr"><a href="#Page_114">114</a></td></tr> -<tr><td class="tdl pad4">of telescope,</td><td class="tdr"><a href="#Page_175">175</a></td></tr> -<tr><td class="tdl">Clusters of stars, photography of,</td><td class="tdr"><a href="#Page_98">98</a></td></tr> -<tr><td class="tdl">Columbia University Observatory, latitude observations,</td><td class="tdr"><a href="#Page_139">139</a></td></tr> -<tr><td class="tdl pad4">polar photography,</td><td class="tdr"><a href="#Page_196">196</a></td></tr> -<tr><td class="tdl">Common, his reflecting telescope,</td><td class="tdr"><a href="#Page_32">32</a></td></tr> -<tr><td class="tdl">Confusion of dates, in Pacific Ocean,</td><td class="tdr"><a href="#Page_125">125</a></td></tr> -<tr><td class="tdl">Congress of Astronomers, Paris, 1887,</td><td class="tdr"><a href="#Page_102">102</a></td></tr> -<tr><td class="tdl">Constellations,</td><td class="tdr"><a href="#Page_162">162</a></td></tr> -<tr><td class="tdl">Control, "mouse," for photography,</td><td class="tdr"><a href="#Page_88">88</a></td></tr> -<tr><td class="tdl">Copernican theory of universe,</td><td class="tdr"><a href="#Page_53">53</a>, <a href="#Page_56">56</a></td></tr> -<tr><td class="tdl pad4">demonstration,</td><td class="tdr"><a href="#Page_94">94</a></td></tr> -<tr><td class="tdl">Corkscrew tunnels,</td><td class="tdr"><a href="#Page_220">220</a></td></tr> -<tr><td class="tdl">Crape ring of Saturn,</td><td class="tdr"><a href="#Page_144">144</a></td></tr> -<tr><td class="tdl">Cumulative effect, in photography,</td><td class="tdr"><a href="#Page_84">84</a></td></tr> - -<tr><td class="tdl"> </td></tr> -<tr><td class="tdl">Date, confusion of, in Pacific Ocean,</td><td class="tdr"><a href="#Page_125">125</a></td></tr> -<tr><td class="tdl">Date-line, international, explained,</td><td class="tdr"><a href="#Page_126">126</a></td></tr> -<tr><td class="tdl">Development of photograph,</td><td class="tdr"><a href="#Page_81">81</a></td></tr> -<tr><td class="tdl"><span class="pagenum"><a name="Page_227" id="Page_227">[227]</a></span> - Dial, of astronomical clock,</td><td class="tdr"><a href="#Page_116">116</a></td></tr> -<tr><td class="tdl">"Dialogue" of Galileo,</td><td class="tdr"><a href="#Page_53">53</a></td></tr> -<tr><td class="tdl">Differences of time, explained,</td><td class="tdr"><a href="#Page_121">121</a></td></tr> -<tr><td class="tdl">Directions, telescopic measurement of,</td><td class="tdr"><a href="#Page_21">21</a></td></tr> -<tr><td class="tdl">Directory of the heavens,</td><td class="tdr"><a href="#Page_103">103</a></td></tr> -<tr><td class="tdl">Distance, of light-source in photography,</td><td class="tdr"><a href="#Page_83">83</a></td></tr> -<tr><td class="tdl pad4">of stars,</td><td class="tdr"><a href="#Page_94">94</a>, <a href="#Page_106">106</a>, <a href="#Page_158">158</a></td></tr> -<tr><td class="tdl pad4">of Sun,</td><td class="tdr"><a href="#Page_67">67</a>, <a href="#Page_97">97</a>, <a href="#Page_106">106</a></td></tr> -<tr><td class="tdl">Donner, polar photography,</td><td class="tdr"><a href="#Page_195">195</a></td></tr> -<tr><td class="tdl">Double telescopes, for photography,</td><td class="tdr"><a href="#Page_86">86</a></td></tr> - -<tr><td class="tdl"> </td></tr> -<tr><td class="tdl">Earth, motions of its pole,</td><td class="tdr"><a href="#Page_131">131</a></td></tr> -<tr><td class="tdl pad4">rotation of,</td><td class="tdr"><a href="#Page_136">136</a>, <a href="#Page_162">162</a>, <a href="#Page_171">171</a>, <a href="#Page_184">184</a></td></tr> -<tr><td class="tdl pad4">shape of,</td><td class="tdr"><a href="#Page_135">135</a></td></tr> -<tr><td class="tdl">Eclipses, photography of,</td><td class="tdr"><a href="#Page_109">109</a></td></tr> -<tr><td class="tdl">Elkin, measures Pleiades,</td><td class="tdr"><a href="#Page_15">15</a></td></tr> -<tr><td class="tdl">Equatorial telescope, explained,</td><td class="tdr"><a href="#Page_170">170</a></td></tr> -<tr><td class="tdl">Eros, discovered by Witt,</td><td class="tdr"><a href="#Page_66">66</a>, <a href="#Page_105">105</a></td></tr> -<tr><td class="tdl pad4">its importance,</td><td class="tdr"><a href="#Page_67">67</a></td></tr> -<tr><td class="tdl">Error of clock, determined by transit,</td><td class="tdr"><a href="#Page_118">118</a></td></tr> -<tr><td class="tdl">Exposure, length of, in photography,</td><td class="tdr"><a href="#Page_84">84</a></td></tr> - -<tr><td class="tdl"> </td></tr> -<tr><td class="tdl">Feldhausen, Herschel's observatory near Capetown,</td><td class="tdr"><a href="#Page_204">204</a></td></tr> -<tr><td class="tdl">Fiji Islands, their date,</td><td class="tdr"><a href="#Page_126">126</a></td></tr> -<tr><td class="tdl">Fixed polar telescope,</td><td class="tdr"><a href="#Page_197">197</a></td></tr> -<tr><td class="tdl">"Following" the stars,</td><td class="tdr"><a href="#Page_88">88</a>, <a href="#Page_173">173</a></td></tr> -<tr><td class="tdl">Four-day cycle of pole-star,</td><td class="tdr"><a href="#Page_24">24</a></td></tr> -<tr><td class="tdl">France, outside time-zone system,</td><td class="tdr"><a href="#Page_129">129</a></td></tr> -<tr><td class="tdl">Fundamental longitude meridian,</td><td class="tdr"><a href="#Page_124">124</a></td></tr> - -<tr><td class="tdl"> </td></tr> -<tr><td class="tdl smcap">Galileo,</td><td class="tdr"><a href="#Page_47">47</a></td></tr> -<tr><td class="tdl pad4">and the Church,</td><td class="tdr"><a href="#Page_48">48</a></td></tr> -<tr><td class="tdl pad4">discoveries of,</td><td class="tdr"><a href="#Page_49">49</a></td></tr> -<tr><td class="tdl pad4">observes Saturn,</td><td class="tdr"><a href="#Page_141">141</a></td></tr> -<tr><td class="tdl">Galle, discovers Neptune,</td><td class="tdr"><a href="#Page_61">61</a></td></tr> -<tr><td class="tdl">Gauss, computes first asteroid orbit,</td><td class="tdr"><a href="#Page_60">60</a></td></tr> -<tr><td class="tdl">Gautier, Paris, constructs big telescope,</td><td class="tdr"><a href="#Page_179">179</a></td></tr> -<tr><td class="tdl">Geodetic Association, international,</td><td class="tdr"><a href="#Page_139">139</a></td></tr> -<tr><td class="tdl"><span class="pagenum"><a name="Page_228" id="Page_228">[228]</a></span> - Geography, maps, astronomical side of,</td><td class="tdr"><a href="#Page_112">112</a></td></tr> -<tr><td class="tdl">Geology, polar motion in,</td><td class="tdr"><a href="#Page_131">131</a></td></tr> -<tr><td class="tdl">Gill, photographs comet,</td><td class="tdr"><a href="#Page_100">100</a></td></tr> -<tr><td class="tdl">Gilliss, at Naval Observatory, Washington,</td><td class="tdr"><a href="#Page_169">169</a></td></tr> -<tr><td class="tdl">Goldsborough, at Naval Observatory, Washington,</td><td class="tdr"><a href="#Page_169">169</a></td></tr> -<tr><td class="tdl"><i lang="fr" xml:lang="fr">Grande Lunette</i>, Paris, 1900,</td><td class="tdr"><a href="#Page_176">176</a>, <a href="#Page_180">180</a></td></tr> -<tr><td class="tdl">Gravitation,</td><td class="tdr"><a href="#Page_13">13</a></td></tr> -<tr><td class="tdl pad4">in Pleiades,</td><td class="tdr"><a href="#Page_14">14</a>, <a href="#Page_212">212</a></td></tr> -<tr><td class="tdl pad4">law of, Newton's,</td><td class="tdr"><a href="#Page_212">212</a></td></tr> -<tr><td class="tdl">Gravity, centre of,</td><td class="tdr"><a href="#Page_217">217</a></td></tr> -<tr><td class="tdl">Greenwich, origin of longitudes,</td><td class="tdr"><a href="#Page_7">7</a>, <a href="#Page_124">124</a></td></tr> -<tr><td class="tdl pad4">time,</td><td class="tdr"><a href="#Page_7">7</a></td></tr> -<tr><td class="tdl">Groombridge, English astronomer,</td><td class="tdr"><a href="#Page_1">1</a></td></tr> - -<tr><td class="tdl"> </td></tr> -<tr><td class="tdl">Harrison, inventor of chronometer,</td><td class="tdr"><a href="#Page_8">8</a></td></tr> -<tr><td class="tdl">Head, of heliometer,</td><td class="tdr"><a href="#Page_156">156</a></td></tr> -<tr><td class="tdl">Heidelberg, photography at,</td><td class="tdr"><a href="#Page_104">104</a></td></tr> -<tr><td class="tdl smcap">Heliometer,</td><td class="tdr"><a href="#Page_152">152</a></td></tr> -<tr><td class="tdl pad4">head of,</td><td class="tdr"><a href="#Page_156">156</a></td></tr> -<tr><td class="tdl pad4">how used,</td><td class="tdr"><a href="#Page_157">157</a></td></tr> -<tr><td class="tdl pad4">principle of,</td><td class="tdr"><a href="#Page_154">154</a></td></tr> -<tr><td class="tdl pad4">scales of,</td><td class="tdr"><a href="#Page_158">158</a></td></tr> -<tr><td class="tdl pad4">semi-lenses of,</td><td class="tdr"><a href="#Page_155">155</a></td></tr> -<tr><td class="tdl">Helsingfors observatory, polar photography at,</td><td class="tdr"><a href="#Page_195">195</a></td></tr> -<tr><td class="tdl">Henry, measures Pleiades,</td><td class="tdr"><a href="#Page_11">11</a>, <a href="#Page_17">17</a></td></tr> -<tr><td class="tdl">Hercules, constellation, solar motion toward,</td><td class="tdr"><a href="#Page_222">222</a></td></tr> -<tr><td class="tdl">Herschel, discovers apex of solar motion,</td><td class="tdr"><a href="#Page_221">221</a></td></tr> -<tr><td class="tdl pad4">discovers Uranus,</td><td class="tdr"><a href="#Page_59">59</a>, <a href="#Page_141">141</a></td></tr> -<tr><td class="tdl pad4">John, the moon hoax,</td><td class="tdr"><a href="#Page_200">200</a></td></tr> -<tr><td class="tdl">Hipparchus, discovers precession,</td><td class="tdr"><a href="#Page_186">186</a></td></tr> -<tr><td class="tdl pad4">early star-catalogue,</td><td class="tdr"><a href="#Page_21">21</a>, <a href="#Page_39">39</a></td></tr> -<tr><td class="tdl pad4">invents star magnitudes,</td><td class="tdr"><a href="#Page_91">91</a></td></tr> -<tr><td class="tdl">Huygens, announces rings of Saturn,</td><td class="tdr"><a href="#Page_142">142</a></td></tr> -<tr><td class="tdl pad4">his logogriph,</td><td class="tdr"><a href="#Page_143">143</a></td></tr> - -<tr><td class="tdl"> </td></tr> -<tr><td class="tdl">Ice-cap, of Earth,</td><td class="tdr"><a href="#Page_131">131</a></td></tr> -<tr><td class="tdl"><i lang="la" xml:lang="la">Index Librorum Prohibitorum</i>,</td><td class="tdr"><a href="#Page_53">53</a></td></tr> -<tr><td class="tdl">International, date-line, explained,</td><td class="tdr"><a href="#Page_126">126</a></td></tr> -<tr><td class="tdl pad4">geodetic association,</td><td class="tdr"><a href="#Page_139">139</a></td></tr> -<tr><td class="tdl"><span class="pagenum"><a name="Page_229" id="Page_229">[229]</a></span> - Inter-stellar motion, in clusters,</td><td class="tdr"><a href="#Page_98">98</a></td></tr> -<tr><td class="tdl pad4">in Pleiades,</td><td class="tdr"><a href="#Page_14">14</a></td></tr> -<tr><td class="tdl">Islands of Pacific, their longitude and time,</td><td class="tdr"><a href="#Page_125">125</a></td></tr> - -<tr><td class="tdl"> </td></tr> -<tr><td class="tdl">Japan, latitude station in,</td><td class="tdr"><a href="#Page_139">139</a></td></tr> -<tr><td class="tdl">Jewellers' correct time,</td><td class="tdr"><a href="#Page_121">121</a></td></tr> -<tr><td class="tdl">Journalistic astronomy,</td><td class="tdr"><a href="#Page_176">176</a></td></tr> -<tr><td class="tdl">Jupiter's satellites, discovered by Galileo,</td><td class="tdr"><a href="#Page_50">50</a></td></tr> -<tr><td class="tdl pad4">discovered by Barnard,</td><td class="tdr"><a href="#Page_51">51</a></td></tr> - -<tr><td class="tdl"> </td></tr> -<tr><td class="tdl">Keeler, observes Saturn's rings,</td><td class="tdr"><a href="#Page_140">140</a>, <a href="#Page_147">147</a>, <a href="#Page_150">150</a></td></tr> -<tr><td class="tdl pad4">photographs nebulæ,</td><td class="tdr"><a href="#Page_32">32</a></td></tr> -<tr><td class="tdl">"Keyhole" nebula,</td><td class="tdr"><a href="#Page_205">205</a></td></tr> - -<tr><td class="tdl"> </td></tr> -<tr><td class="tdl">Lambert, determines longitude of Washington,</td><td class="tdr"><a href="#Page_168">168</a></td></tr> -<tr><td class="tdl">Laplace, discusses Saturn's rings,</td><td class="tdr"><a href="#Page_146">146</a></td></tr> -<tr><td class="tdl pad4">nebular hypothesis,</td><td class="tdr"><a href="#Page_33">33</a></td></tr> -<tr><td class="tdl pad4">stability of solar system,</td><td class="tdr"><a href="#Page_210">210</a></td></tr> -<tr><td class="tdl">Latitude, changes of,</td><td class="tdr"><a href="#Page_133">133</a>, <a href="#Page_138">138</a></td></tr> -<tr><td class="tdl pad4">definition of,</td><td class="tdr"><a href="#Page_134">134</a></td></tr> -<tr><td class="tdl pad4">determining the,</td><td class="tdr"><a href="#Page_6">6</a></td></tr> -<tr><td class="tdl">Leverrier, predicts discovery of Neptune,</td><td class="tdr"><a href="#Page_61">61</a>, <a href="#Page_142">142</a></td></tr> -<tr><td class="tdl">Lick Observatory, Keeler's observations,</td><td class="tdr"><a href="#Page_140">140</a></td></tr> -<tr><td class="tdl">Light, undulatory theory of,</td><td class="tdr"><a href="#Page_19">19</a>, <a href="#Page_148">148</a></td></tr> -<tr><td class="tdl">Light-waves, measuring length of,</td><td class="tdr"><a href="#Page_20">20</a>, <a href="#Page_149">149</a></td></tr> -<tr><td class="tdl">Logogriph, by Huygens,</td><td class="tdr"><a href="#Page_143">143</a></td></tr> -<tr><td class="tdl">Long-exposure photography,</td><td class="tdr"><a href="#Page_85">85</a></td></tr> -<tr><td class="tdl">Longitude, counted East and West,</td><td class="tdr"><a href="#Page_125">125</a></td></tr> -<tr><td class="tdl pad4">determining,</td><td class="tdr"><a href="#Page_6">6</a></td></tr> -<tr><td class="tdl pad4">determining by occultations,</td><td class="tdr"><a href="#Page_167">167</a></td></tr> -<tr><td class="tdl pad4">effect on time differences,</td><td class="tdr"><a href="#Page_123">123</a></td></tr> -<tr><td class="tdl pad4">explained,</td><td class="tdr"><a href="#Page_123">123</a></td></tr> -<tr><td class="tdl pad4">of Washington, first determined,</td><td class="tdr"><a href="#Page_168">168</a></td></tr> - -<tr><td class="tdl"> </td></tr> -<tr><td class="tdl">Maclear, observes Eta Argus,</td><td class="tdr"><a href="#Page_205">205</a></td></tr> -<tr><td class="tdl">Magnitudes, stellar,</td><td class="tdr"><a href="#Page_91">91</a></td></tr> -<tr><td class="tdl">Manila, its time,</td><td class="tdr"><a href="#Page_127">127</a></td></tr> -<tr><td class="tdl">Maps, astronomical side of,</td><td class="tdr"><a href="#Page_112">112</a></td></tr> -<tr><td class="tdl"><span class="pagenum"><a name="Page_230" id="Page_230">[230]</a></span> - Meridian circle, explained,</td><td class="tdr"><a href="#Page_189">189</a></td></tr> -<tr><td class="tdl">Milky-way, poor in nebulæ,</td><td class="tdr"><a href="#Page_33">33</a></td></tr> -<tr><td class="tdl">Minor Planets, see <a href="#AST">Asteroids.</a></td></tr> -<tr><td class="tdl smcap">Moon, Hoax,</td><td class="tdr"><a href="#Page_199">199</a></td></tr> -<tr><td class="tdl pad4">motion among stars,</td><td class="tdr"><a href="#Page_163">163</a></td></tr> -<tr><td class="tdl pad4">mountains discovered by Galileo,</td><td class="tdr"><a href="#Page_49">49</a></td></tr> -<tr><td class="tdl pad4">size of, measured,</td><td class="tdr"><a href="#Page_166">166</a></td></tr> -<tr><td class="tdl">Motion of moon,</td><td class="tdr"><a href="#Page_163">163</a></td></tr> -<tr><td class="tdl"><span class="smcap">Motions</span> of the <span class="smcap">Earth's</span> Pole,</td><td class="tdr"><a href="#Page_131">131</a></td></tr> -<tr><td class="tdl smcap">Mounting Great Telescopes,</td><td class="tdr"><a href="#Page_170">170</a></td></tr> - -<tr><td class="tdl"> </td></tr> -<tr><td class="tdl">Naked-eye nebulæ,</td><td class="tdr"><a href="#Page_28">28</a></td></tr> -<tr><td class="tdl">Naples, Royal Observatory, latitude observations,</td><td class="tdr"><a href="#Page_139">139</a></td></tr> -<tr><td class="tdl">Naval Observatory, Washington, noon signal,</td><td class="tdr"><a href="#Page_120">120</a></td></tr> -<tr><td class="tdl smcap">Navigation,</td><td class="tdr"><a href="#Page_1">1</a></td></tr> -<tr><td class="tdl pad4">before chronometers,</td><td class="tdr"><a href="#Page_3">3</a></td></tr> -<tr><td class="tdl pad4">use of astronomy in,</td><td class="tdr"><a href="#Page_113">113</a></td></tr> -<tr><td class="tdl smcap">Nebulæ,</td><td class="tdr"><a href="#Page_27">27</a></td></tr> -<tr><td class="tdl">Nebula, in Andromeda,</td><td class="tdr"><a href="#Page_28">28</a></td></tr> -<tr><td class="tdl pad4">in Orion,</td><td class="tdr"><a href="#Page_30">30</a></td></tr> -<tr><td class="tdl pad4">"keyhole",</td><td class="tdr"><a href="#Page_205">205</a></td></tr> -<tr><td class="tdl">Nebular, hypothesis,</td><td class="tdr"><a href="#Page_33">33</a></td></tr> -<tr><td class="tdl pad4">structure in Pleiades,</td><td class="tdr"><a href="#Page_17">17</a></td></tr> -<tr><td class="tdl">Nebulous stars,</td><td class="tdr"><a href="#Page_31">31</a></td></tr> -<tr><td class="tdl">Negative, and positive, in photography,</td><td class="tdr"><a href="#Page_82">82</a></td></tr> -<tr><td class="tdl">Neptune, discovery predicted by Leverrier,</td><td class="tdr"><a href="#Page_61">61</a>, <a href="#Page_142">142</a></td></tr> -<tr><td class="tdl pad4">discovery by Galle,</td><td class="tdr"><a href="#Page_61">61</a></td></tr> -<tr><td class="tdl">Newcomb, fixes apex of solar motion,</td><td class="tdr"><a href="#Page_222">222</a></td></tr> -<tr><td class="tdl">Newton, law of gravitation,</td><td class="tdr"><a href="#Page_212">212</a></td></tr> -<tr><td class="tdl pad4">longitude commission,</td><td class="tdr"><a href="#Page_8">8</a></td></tr> -<tr><td class="tdl">New York, its telegraphic time system,</td><td class="tdr"><a href="#Page_120">120</a></td></tr> -<tr><td class="tdl">Noon Signal, Washington,</td><td class="tdr"><a href="#Page_120">120</a></td></tr> -<tr><td class="tdl">Number, of nebulæ,</td><td class="tdr"><a href="#Page_31">31</a>, <a href="#Page_33">33</a></td></tr> -<tr><td class="tdl pad4">of temporary stars,</td><td class="tdr"><a href="#Page_38">38</a></td></tr> -<tr><td class="tdl">Nutation, explained,</td><td class="tdr"><a href="#Page_188">188</a></td></tr> - -<tr><td class="tdl"> </td></tr> -<tr><td class="tdl">Occultations,</td><td class="tdr"><a href="#Page_161">161</a></td></tr> -<tr><td class="tdl pad4">explained,</td><td class="tdr"><a href="#Page_165">165</a></td></tr> -<tr><td class="tdl"><span class="pagenum"><a name="Page_231" id="Page_231">[231]</a></span> - Occultations, use of,</td><td class="tdr"><a href="#Page_166">166</a>, <a href="#Page_167">167</a></td></tr> -<tr><td class="tdl">Orion nebula,</td><td class="tdr"><a href="#Page_30">30</a></td></tr> - -<tr><td class="tdl"> </td></tr> -<tr><td class="tdl">Pacific islands, their longitude and time,</td><td class="tdr"><a href="#Page_125">125</a></td></tr> -<tr><td class="tdl">Parallax, solar,</td><td class="tdr"><a href="#Page_67">67</a>, <a href="#Page_106">106</a></td></tr> -<tr><td class="tdl pad4">stellar,</td><td class="tdr"><a href="#Page_94">94</a>, <a href="#Page_106">106</a></td></tr> -<tr><td class="tdl pad4">measured with heliometer,</td><td class="tdr"><a href="#Page_158">158</a></td></tr> -<tr><td class="tdl">Paris, congress of astronomers, 1887,</td><td class="tdr"><a href="#Page_102">102</a></td></tr> -<tr><td class="tdl pad4">exposition of 1900,</td><td class="tdr"><a href="#Page_176">176</a></td></tr> -<tr><td class="tdl">Periodic motion of earth's pole,</td><td class="tdr"><a href="#Page_133">133</a></td></tr> -<tr><td class="tdl">Perseus, constellation, temporary star in,</td><td class="tdr"><a href="#Page_46">46</a></td></tr> -<tr><td class="tdl">Philippine Islands, their time,</td><td class="tdr"><a href="#Page_127">127</a></td></tr> -<tr><td class="tdl">Photography, asteroid, invented by Wolf,</td><td class="tdr"><a href="#Page_104">104</a></td></tr> -<tr><td class="tdl pad4">congress of astronomical,</td><td class="tdr"><a href="#Page_102">102</a></td></tr> -<tr><td class="tdl pad4">cumulative effect of light,</td><td class="tdr"><a href="#Page_84">84</a></td></tr> -<tr><td class="tdl pad4">distance of light-source,</td><td class="tdr"><a href="#Page_83">83</a></td></tr> -<tr><td class="tdl pad4">double telescopes for,</td><td class="tdr"><a href="#Page_86">86</a></td></tr> -<tr><td class="tdl pad4">general star-catalogue,</td><td class="tdr"><a href="#Page_102">102</a></td></tr> -<tr><td class="tdl pad4 smcap">In Astronomy,</td><td class="tdr"><a href="#Page_81">81</a></td></tr> -<tr><td class="tdl pad4">in discovery of asteroids,</td><td class="tdr"><a href="#Page_64">64</a>, <a href="#Page_104">104</a></td></tr> -<tr><td class="tdl pad4">in solar physics,</td><td class="tdr"><a href="#Page_109">109</a></td></tr> -<tr><td class="tdl pad4">in spectroscopy,</td><td class="tdr"><a href="#Page_108">108</a></td></tr> -<tr><td class="tdl pad4">length of exposure,</td><td class="tdr"><a href="#Page_84">84</a></td></tr> -<tr><td class="tdl pad4">measuring-machine, Rutherfurd,</td><td class="tdr"><a href="#Page_93">93</a></td></tr> -<tr><td class="tdl pad4">motion of telescope for,</td><td class="tdr"><a href="#Page_87">87</a></td></tr> -<tr><td class="tdl pad4">"mouse" control of telescope,</td><td class="tdr"><a href="#Page_88">88</a></td></tr> -<tr><td class="tdl pad4">of eclipses,</td><td class="tdr"><a href="#Page_109">109</a></td></tr> -<tr><td class="tdl pad4">of inter-stellar motion,</td><td class="tdr"><a href="#Page_99">99</a></td></tr> -<tr><td class="tdl pad4">Paris congress, 1877,</td><td class="tdr"><a href="#Page_102">102</a></td></tr> -<tr><td class="tdl pad4">polar,</td><td class="tdr"><a href="#Page_191">191</a></td></tr> -<tr><td class="tdl pad4">Rutherfurd pioneer in,</td><td class="tdr"><a href="#Page_90">90</a></td></tr> -<tr><td class="tdl pad4">star-clusters,</td><td class="tdr"><a href="#Page_98">98</a></td></tr> -<tr><td class="tdl pad4">star-distances measured by,</td><td class="tdr"><a href="#Page_94">94</a></td></tr> -<tr><td class="tdl pad4">summarized,</td><td class="tdr"><a href="#Page_110">110</a></td></tr> -<tr><td class="tdl pad4">wholesale methods in,</td><td class="tdr"><a href="#Page_103">103</a></td></tr> -<tr><td class="tdl">Piazzi, discovers first asteroid,</td><td class="tdr"><a href="#Page_59">59</a>, <a href="#Page_106">106</a></td></tr> -<tr><td class="tdl">Pitkin, report to House of Representatives,</td><td class="tdr"><a href="#Page_168">168</a></td></tr> -<tr><td class="tdl">Planetary nebulæ,</td><td class="tdr"><a href="#Page_31">31</a></td></tr> -<tr><td class="tdl smcap"><span class="pagenum"><a name="Page_232" id="Page_232">[232]</a></span> - Planet of 1898,</td><td class="tdr"><a href="#Page_58">58</a></td></tr> -<tr><td class="tdl">Planetoids, see <a href="#AST">Asteroids.</a></td></tr> -<tr><td class="tdl">Planets known to ancients,</td><td class="tdr"><a href="#Page_58">58</a></td></tr> -<tr><td class="tdl smcap">Pleiades,</td><td class="tdr"><a href="#Page_10">10</a></td></tr> -<tr><td class="tdl pad4">gravitation among,</td><td class="tdr"><a href="#Page_212">212</a></td></tr> -<tr><td class="tdl pad4">motion among,</td><td class="tdr"><a href="#Page_14">14</a>, <a href="#Page_16">16</a>, <a href="#Page_98">98</a></td></tr> -<tr><td class="tdl pad4">nebular structure,</td><td class="tdr"><a href="#Page_17">17</a></td></tr> -<tr><td class="tdl pad4">number visible,</td><td class="tdr"><a href="#Page_11">11</a></td></tr> -<tr><td class="tdl">Polar axis, of telescope,</td><td class="tdr"><a href="#Page_173">173</a></td></tr> -<tr><td class="tdl">Polar photography,</td><td class="tdr"><a href="#Page_191">191</a></td></tr> -<tr><td class="tdl pad4">at Helsingfors,</td><td class="tdr"><a href="#Page_195">195</a></td></tr> -<tr><td class="tdl">Pole, celestial,</td><td class="tdr"><a href="#Page_184">184</a></td></tr> -<tr><td class="tdl pad4">of the earth, motions of,</td><td class="tdr"><a href="#Page_131">131</a></td></tr> -<tr><td class="tdl pad4 smcap">the Astronomer's,</td><td class="tdr"><a href="#Page_184">184</a></td></tr> -<tr><td class="tdl smcap">Pole-Star,</td><td class="tdr"><a href="#Page_18">18</a></td></tr> -<tr><td class="tdl pad4">as a binary,</td><td class="tdr"><a href="#Page_25">25</a></td></tr> -<tr><td class="tdl pad4">as a triple,</td><td class="tdr"><a href="#Page_18">18</a>, <a href="#Page_26">26</a></td></tr> -<tr><td class="tdl pad4">change of,</td><td class="tdr"><a href="#Page_187">187</a></td></tr> -<tr><td class="tdl pad4">its four-day cycle,</td><td class="tdr"><a href="#Page_24">24</a></td></tr> -<tr><td class="tdl pad4">motion toward us,</td><td class="tdr"><a href="#Page_24">24</a></td></tr> -<tr><td class="tdl">Positive, and negative, in photography,</td><td class="tdr"><a href="#Page_82">82</a></td></tr> -<tr><td class="tdl">Potsdam, observatory, photographic star-catalogue,</td><td class="tdr"><a href="#Page_103">103</a></td></tr> -<tr><td class="tdl">Practical uses of astronomy,</td><td class="tdr"><a href="#Page_112">112</a></td></tr> -<tr><td class="tdl">Precession, explained,</td><td class="tdr"><a href="#Page_186">186</a></td></tr> -<tr><td class="tdl">Prize, for invention of chronometer,</td><td class="tdr"><a href="#Page_8">8</a></td></tr> -<tr><td class="tdl">Ptolemaic theory of universe,</td><td class="tdr"><a href="#Page_56">56</a></td></tr> -<tr><td class="tdl">Ptolemy, writes concerning Hipparchus,</td><td class="tdr"><a href="#Page_39">39</a></td></tr> - -<tr><td class="tdl"> </td></tr> -<tr><td class="tdl">Railroad time, explained,</td><td class="tdr"><a href="#Page_127">127</a></td></tr> -<tr><td class="tdl">Refraction, atmospheric, explained,</td><td class="tdr"><a href="#Page_193">193</a></td></tr> -<tr><td class="tdl">"Regulator," the jeweller's clock,</td><td class="tdr"><a href="#Page_114">114</a></td></tr> -<tr><td class="tdl">Ring-nebulæ,</td><td class="tdr"><a href="#Page_31">31</a></td></tr> -<tr><td class="tdl">Rings, of Saturn, see <a href="#SAT">Saturn's rings.</a></td></tr> -<tr><td class="tdl">Roberts, Andromeda nebula,</td><td class="tdr"><a href="#Page_28">28</a></td></tr> -<tr><td class="tdl">Rotation, of Earth,</td><td class="tdr"><a href="#Page_136">136</a>, <a href="#Page_162">162</a>, <a href="#Page_171">171</a>, <a href="#Page_184">184</a></td></tr> -<tr><td class="tdl pad4">of Saturn,</td><td class="tdr"><a href="#Page_150">150</a></td></tr> -<tr><td class="tdl">Royal Astronomer, his duties,</td><td class="tdr"><a href="#Page_2">2</a></td></tr> -<tr><td class="tdl">Royal Observatory, Greenwich,</td><td class="tdr"><a href="#Page_124">124</a></td></tr> -<tr><td class="tdl pad4"><span class="pagenum"><a name="Page_233" id="Page_233">[233]</a></span> - Greenwich, Bradley's observations,</td><td class="tdr"><a href="#Page_219">219</a></td></tr> -<tr><td class="tdl pad4">Naples, latitude observations,</td><td class="tdr"><a href="#Page_139">139</a></td></tr> -<tr><td class="tdl">Rutherfurd, cluster photography,</td><td class="tdr"><a href="#Page_99">99</a></td></tr> -<tr><td class="tdl pad4">invents photographic apparatus,</td><td class="tdr"><a href="#Page_93">93</a></td></tr> -<tr><td class="tdl pad4">pioneer in photography,</td><td class="tdr"><a href="#Page_90">90</a></td></tr> -<tr><td class="tdl pad4">stellar parallax,</td><td class="tdr"><a href="#Page_94">94</a></td></tr> - -<tr><td class="tdl"> </td></tr> -<tr><td class="tdl">Sagredus, character in Galileo's Dialogue,</td><td class="tdr"><a href="#Page_55">55</a></td></tr> -<tr><td class="tdl">Salusbury, Galileo's translator,</td><td class="tdr"><a href="#Page_50">50</a>, <a href="#Page_54">54</a></td></tr> -<tr><td class="tdl">Salviati, character in Galileo's Dialogue,</td><td class="tdr"><a href="#Page_55">55</a></td></tr> -<tr><td class="tdl">Samoa, its date,</td><td class="tdr"><a href="#Page_126">126</a></td></tr> -<tr><td class="tdl smcap"><a name="SAT" id="SAT"></a>Saturn's Rings,</td><td class="tdr"><a href="#Page_140">140</a></td></tr> -<tr><td class="tdl pad4">analogy to planetoids,</td><td class="tdr"><a href="#Page_147">147</a></td></tr> -<tr><td class="tdl pad4">announced by Huygens,</td><td class="tdr"><a href="#Page_142">142</a></td></tr> -<tr><td class="tdl pad4">observed with spectroscope,</td><td class="tdr"><a href="#Page_147">147</a></td></tr> -<tr><td class="tdl pad4">shown to be double by Cassini,</td><td class="tdr"><a href="#Page_144">144</a></td></tr> -<tr><td class="tdl pad4">structure and stability,</td><td class="tdr"><a href="#Page_145">145</a></td></tr> -<tr><td class="tdl">Scales, of heliometer,</td><td class="tdr"><a href="#Page_158">158</a></td></tr> -<tr><td class="tdl">Scorpio, constellation, temporary star in,</td><td class="tdr"><a href="#Page_39">39</a></td></tr> -<tr><td class="tdl">Semi-lenses of heliometer,</td><td class="tdr"><a href="#Page_155">155</a></td></tr> -<tr><td class="tdl">Sextant, how used,</td><td class="tdr"><a href="#Page_4">4</a></td></tr> -<tr><td class="tdl">Sicily, latitude station in,</td><td class="tdr"><a href="#Page_139">139</a></td></tr> -<tr><td class="tdl"><i lang="la" xml:lang="la">Sidereus Nuncius</i>, published by Galileo,</td><td class="tdr"><a href="#Page_52">52</a></td></tr> -<tr><td class="tdl">Simplicio, character in Galileo's Dialogue,</td><td class="tdr"><a href="#Page_55">55</a></td></tr> -<tr><td class="tdl">Sirius, brightest star,</td><td class="tdr"><a href="#Page_205">205</a></td></tr> -<tr><td class="tdl">Size of Moon, measured,</td><td class="tdr"><a href="#Page_166">166</a></td></tr> -<tr><td class="tdl"><i lang="fr" xml:lang="fr">Société de l'Optique</i>,</td><td class="tdr"><a href="#Page_177">177</a></td></tr> -<tr><td class="tdl">Solar parallax, see <a href="#SUN">Sun's distance.</a></td></tr> -<tr><td class="tdl pad4">physics, by photography,</td><td class="tdr"><a href="#Page_109">109</a></td></tr> -<tr><td class="tdl pad4">system, stability of,</td><td class="tdr"><a href="#Page_210">210</a></td></tr> -<tr><td class="tdl">Spectroscope, its use explained,</td><td class="tdr"><a href="#Page_147">147</a></td></tr> -<tr><td class="tdl pad4">used on pole-star,</td><td class="tdr"><a href="#Page_19">19</a></td></tr> -<tr><td class="tdl pad4">to observe Saturn's rings,</td><td class="tdr"><a href="#Page_147">147</a></td></tr> -<tr><td class="tdl">Spiral nebulæ,</td><td class="tdr"><a href="#Page_31">31</a></td></tr> -<tr><td class="tdl">Stability, of Saturn's rings,</td><td class="tdr"><a href="#Page_145">145</a></td></tr> -<tr><td class="tdl pad4">of Solar System,</td><td class="tdr"><a href="#Page_210">210</a></td></tr> -<tr><td class="tdl">Standards, time, of the world,</td><td class="tdr"><a href="#Page_111">111</a></td></tr> -<tr><td class="tdl pad4">table of,</td><td class="tdr"><a href="#Page_130">130</a></td></tr> -<tr><td class="tdl"><span class="pagenum"><a name="Page_234" id="Page_234">[234]</a></span> - "Standard" time, explained,</td><td class="tdr"><a href="#Page_127">127</a></td></tr> -<tr><td class="tdl">Star-catalogue, general photographic,</td><td class="tdr"><a href="#Page_102">102</a></td></tr> -<tr><td class="tdl">Star-clusters, photography of,</td><td class="tdr"><a href="#Page_98">98</a></td></tr> -<tr><td class="tdl">Star-distances</td><td class="tdr"><a href="#Page_94">94</a>, <a href="#Page_106">106</a></td></tr> -<tr><td class="tdl pad4">measured with heliometer,</td><td class="tdr"><a href="#Page_158">158</a></td></tr> -<tr><td class="tdl pad4">Rutherfurd,</td><td class="tdr"><a href="#Page_94">94</a></td></tr> -<tr><td class="tdl">Star magnitudes,</td><td class="tdr"><a href="#Page_91">91</a></td></tr> -<tr><td class="tdl">Star-motion, toward us,</td><td class="tdr"><a href="#Page_21">21</a></td></tr> -<tr><td class="tdl">Star-tables, astronomical,</td><td class="tdr"><a href="#Page_118">118</a></td></tr> -<tr><td class="tdl">Stars, variable,</td><td class="tdr"><a href="#Page_42">42</a></td></tr> -<tr><td class="tdl">St Gothard railway, tunnels,</td><td class="tdr"><a href="#Page_220">220</a></td></tr> -<tr><td class="tdl">Sun, newspaper, the moon hoax,</td><td class="tdr"><a href="#Page_201">201</a></td></tr> -<tr><td class="tdl smcap">Sun-Dial, How to Make a,</td><td class="tdr"><a href="#Page_69">69</a></td></tr> -<tr><td class="tdl smcap"><a name="SUN" id="SUN"></a>Sun's, Destination,</td><td class="tdr"><a href="#Page_210">210</a></td></tr> -<tr><td class="tdl pad4">distance, compared with star distance,</td><td class="tdr"><a href="#Page_97">97</a></td></tr> -<tr><td class="tdl pad4">measured with Eros,</td><td class="tdr"><a href="#Page_67">67</a>, <a href="#Page_106">106</a></td></tr> -<tr><td class="tdl pad4">motion, apex of,</td><td class="tdr"><a href="#Page_221">221</a></td></tr> -<tr><td class="tdl">Sun-spots, discovered by Galileo,</td><td class="tdr"><a href="#Page_49">49</a></td></tr> -<tr><td class="tdl"><i lang="la" xml:lang="la">Systema Saturnium</i>, Huygens,</td><td class="tdr"><a href="#Page_143">143</a></td></tr> - -<tr><td class="tdl"> </td></tr> -<tr><td class="tdl">Telescope, clock,</td><td class="tdr"><a href="#Page_175">175</a></td></tr> -<tr><td class="tdl pad4">at Paris Exposition,</td><td class="tdr"><a href="#Page_176">176</a>, <a href="#Page_180">180</a></td></tr> -<tr><td class="tdl pad4">double, for photography,</td><td class="tdr"><a href="#Page_86">86</a></td></tr> -<tr><td class="tdl pad4">equatorial, explained,</td><td class="tdr"><a href="#Page_170">170</a></td></tr> -<tr><td class="tdl pad4">first used by Galileo,</td><td class="tdr"><a href="#Page_49">49</a></td></tr> -<tr><td class="tdl pad4">motion of,</td><td class="tdr"><a href="#Page_87">87</a></td></tr> -<tr><td class="tdl pad4">mounting great,</td><td class="tdr"><a href="#Page_170">170</a></td></tr> -<tr><td class="tdl pad4">unmoving, for polar photography,</td><td class="tdr"><a href="#Page_197">197</a></td></tr> -<tr><td class="tdl smcap">Temporary Stars,</td><td class="tdr"><a href="#Page_37">37</a></td></tr> -<tr><td class="tdl pad4">in Andromeda nebula,</td><td class="tdr"><a href="#Page_28">28</a>, <a href="#Page_29">29</a>, <a href="#Page_45">45</a></td></tr> -<tr><td class="tdl pad4">in Aquila,</td><td class="tdr"><a href="#Page_40">40</a></td></tr> -<tr><td class="tdl pad4">in Cassiopeia,</td><td class="tdr"><a href="#Page_40">40</a></td></tr> -<tr><td class="tdl pad4">in Perseus,</td><td class="tdr"><a href="#Page_46">46</a></td></tr> -<tr><td class="tdl pad4">in Scorpio,</td><td class="tdr"><a href="#Page_39">39</a></td></tr> -<tr><td class="tdl pad4">their number,</td><td class="tdr"><a href="#Page_38">38</a></td></tr> -<tr><td class="tdl pad4">theory of,</td><td class="tdr"><a href="#Page_42">42</a></td></tr> -<tr><td class="tdl">Time, correct, determined astronomically,</td><td class="tdr"><a href="#Page_113">113</a></td></tr> -<tr><td class="tdl pad4">differences between different places,</td><td class="tdr"><a href="#Page_121">121</a></td></tr> -<tr><td class="tdl smcap"><span class="pagenum"><a name="Page_235" id="Page_235">[235]</a></span> - Time Standards of the World,</td><td class="tdr"><a href="#Page_111">111</a></td></tr> -<tr><td class="tdl pad4">standards of the World, table of,</td><td class="tdr"><a href="#Page_130">130</a></td></tr> -<tr><td class="tdl pad4">system, in New York,</td><td class="tdr"><a href="#Page_120">120</a></td></tr> -<tr><td class="tdl pad4">zones, explained,</td><td class="tdr"><a href="#Page_128">128</a></td></tr> -<tr><td class="tdl">Trails, photographic,</td><td class="tdr"><a href="#Page_191">191</a></td></tr> -<tr><td class="tdl">Transit, for determining clock error,</td><td class="tdr"><a href="#Page_118">118</a></td></tr> -<tr><td class="tdl">Tycho Brahe, his temporary star,</td><td class="tdr"><a href="#Page_40">40</a></td></tr> - -<tr><td class="tdl"> </td></tr> -<tr><td class="tdl">Ulugh Beg, early star-catalogue,</td><td class="tdr"><a href="#Page_21">21</a></td></tr> -<tr><td class="tdl">Undulatory theory, of light,</td><td class="tdr"><a href="#Page_19">19</a>, <a href="#Page_148">148</a></td></tr> -<tr><td class="tdl">Universe, theories of,</td><td class="tdr"><a href="#Page_34">34</a>, <a href="#Page_53">53</a>, <a href="#Page_56">56</a></td></tr> -<tr><td class="tdl">Uranus, discovered by Herschel,</td><td class="tdr"><a href="#Page_59">59</a>, <a href="#Page_142">142</a></td></tr> -<tr><td class="tdl">Use of occultations,</td><td class="tdr"><a href="#Page_166">166</a>, <a href="#Page_167">167</a></td></tr> -<tr><td class="tdl">Uses of astronomy, practical,</td><td class="tdr"><a href="#Page_112">112</a></td></tr> - -<tr><td class="tdl"> </td></tr> -<tr><td class="tdl">Variable stars,</td><td class="tdr"><a href="#Page_42">42</a></td></tr> -<tr><td class="tdl pad4">in Argo,</td><td class="tdr"><a href="#Page_205">205</a></td></tr> -<tr><td class="tdl">Vega, future pole-star,</td><td class="tdr"><a href="#Page_187">187</a></td></tr> -<tr><td class="tdl">Visibility of stars, in day-time,</td><td class="tdr"><a href="#Page_191">191</a></td></tr> -<tr><td class="tdl">Vision, phenomenon of,</td><td class="tdr"><a href="#Page_20">20</a>, <a href="#Page_149">149</a></td></tr> - -<tr><td class="tdl"> </td></tr> -<tr><td class="tdl">Washington, its longitude first determined,</td><td class="tdr"><a href="#Page_168">168</a></td></tr> -<tr><td class="tdl">Waves, explained,</td><td class="tdr"><a href="#Page_148">148</a></td></tr> -<tr><td class="tdl pad4">of light,</td><td class="tdr"><a href="#Page_20">20</a>, <a href="#Page_148">148</a></td></tr> -<tr><td class="tdl">Wilkes, at Naval Observatory, Washington,</td><td class="tdr"><a href="#Page_169">169</a></td></tr> -<tr><td class="tdl">Wilkins, imaginary voyage of,</td><td class="tdr"><a href="#Page_208">208</a></td></tr> -<tr><td class="tdl">Witt, discovers Eros,</td><td class="tdr"><a href="#Page_66">66</a>, <a href="#Page_105">105</a></td></tr> -<tr><td class="tdl">Wolf, M, invents asteroid photography,</td><td class="tdr"><a href="#Page_104">104</a></td></tr> -<tr><td class="tdl pad4">measures Pleiades,</td><td class="tdr"><a href="#Page_11">11</a></td></tr> -<tr><td class="tdl">World's time standards, table of,</td><td class="tdr"><a href="#Page_130">130</a></td></tr> - -<tr><td class="tdl"> </td></tr> -<tr><td class="tdl">Yale College, Pleiades measured at,</td><td class="tdr"><a href="#Page_15">15</a></td></tr> - -<tr><td class="tdl"> </td></tr> -<tr><td class="tdl">Zones, time, explained,</td><td class="tdr"><a href="#Page_128">128</a></td></tr> -</table></div> - - -<div class="transnote"> -<a name="TN" id="TN"></a> - -<p><strong>TRANSCRIBER'S NOTE</strong></p> - -<p>Fractions in the two tables on <a href="#TABLE_1">pg 74</a> and <a href="#TABLE_2">pg 78</a> are in the form "a b-c" -in the original text, for example "2 7-16", and this form is retained -in the etext. A few other basic fractions in the text -such as ½ and ⅖ are displayed in this same form in the etext.</p> - -<p>There is only one Footnote in this book, with its anchor on <a href="#Page_69">pg 69</a>. -It has been placed at the end of the chapter containing the anchor.</p> - -<p>Obvious typographical errors and punctuation errors have been -corrected after careful comparison with other occurrences within -the text and consultation of external sources.</p> - -<p>Except for those changes noted below, all misspellings in the text, -and inconsistent or archaic usage, have been retained. For example, -time zone, time-zone; Le Verrier, Leverrier; light wave, light-wave; -intrust; wabbling; unexcelled; crape; monumented.</p> - -<p> -<a href="#Page_146">Pg 146</a>, 'James Clark-Maxwell' replaced by 'James Clerk Maxwell'.<br /> -<a href="#Page_189">Pg 189</a>, 'impossible to measure' replaced by 'possible to measure'.<br /> -</p> -</div> - - - - - - - - -<pre> - - - - - -End of Project Gutenberg's Practical Talks by an Astronomer, by Harold Jacoby - -*** END OF THIS PROJECT GUTENBERG EBOOK PRACTICAL TALKS BY AN ASTRONOMER *** - -***** This file should be named 53396-h.htm or 53396-h.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/5/3/3/9/53396/ - -Produced by Chris Curnow, John Campbell and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - -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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