454 



NA TURE 



{Sept. 4, 1884 



COMETS 1 



FOR several months past I have anxiously considered how I 

 could best discharge the honourable duty which has been 

 intrusted to me this evening. I have to deliver an astronomical 

 discourse, and to do my very utmost to make that discourse 

 adequate to the subject, adequate to this large and cultivated 

 audience, and adequate to the memorable occasion on which the 

 British Association has first crossed the Atlantic Ocean. 



I propose to address you this evening on the subject of comets, 

 but it will be readily understood that, of a subject so vast and so 

 elaborate, only a slender proportion can be comprised within a 

 single lecture. The first question to be decided was how to 

 select from the vast mass of materials those which would be most 

 suitable for our discussion this evening. To describe the natural 

 history of comets with any approach to completeness would be a 

 very tedious, indeed almost an endless, task. We must rather 

 select those episodes in the history which have especially added 

 to our knowledge, and enabled us to obtain a rational view of 

 the whole subject. Does not Longfellow tell us how impossible 

 it would have been for him to portray the fortunes of Evangeline 

 throughout every detail ? He has only disclosed to us the 

 picturesque and eventful phases of that history. May I be per- 

 mitted to say that I desire to treat my subject in a similar manner, 

 and while concentrating my attention on the really important 

 matters I shall yet follow the wanderers' footsteps, "not through 

 each devious path, each changeful year of existence." 



In pursuance of this scheme I shall at a single blow lop off all 

 the earlier parts of the history. The great primitive discoveries 

 . of the character of comets and of their movements must be 

 entirely omitted. The splendid researches of Sir Isaac Newton, 

 and the classical achievement of Halley, are among this class. 

 They are no doubt familiar to every cultivated mind, for they 

 belong to that wondrous alliance between mathematics and 

 astronomy which imparts a thrill of pleasure to the generous 

 intellect. They are not for our discussion to-night. 



I shall only address you upon the more recent acquisitions to 

 ■ air knowledge of comets, and in order to give definiteness to 

 our programme, I shall select a certain epoch not yet twenty 

 years old, which is to bound our retrospect into time past. There 

 is a special appropriateness in the choice of the year 1S66 as a 

 starting point for the modern history of comets. A very memor- 

 able occurrence in that year attracted universal attention, and 

 threw much and quite unexpected light on the nature of comets. 

 The review of the subject given in this lecture will extend from 

 the year 1866 to the present time. But even in this restricted 

 interval it will not be practicable for me to give anything like an 

 exhaustive account of the different researches that have been 

 made. Every astronomical journal teems with observations of 

 comets. Every year brings us one, or two, or three, or more 

 comets ; organised efforts are made to observe these comets to the 

 utmost, and each season has its own harvest of discoveries. 

 Amid this host of claimants for our attention we must wend our 

 way this evening, glancing at some discoveries, according to 

 others such notice as their importance may merit, but reserving 

 special attention for the three monumental achievements in the 

 modern history of comets. These are, firstly, the determination 

 of the connection between comets and shooting stars ; secondly, 

 the spectroscopic researches on comets : and thirdly, the inves- 

 tigations of the tails of comets. The first of these subjects must 

 be for ever associated with the name of Professor Schiaparelli, 

 the second with the name of Dr. Huggins, the third with the 

 name of Professor Bredichin. 



It was long ago remarked by Kepler, in language of splendid 

 exaggeration, that there were as many comets in the heavens as 

 there were fishes in the ocean. There are comets large and 

 comets small, comets with one tail, comets with two tails, and 

 comets without any tail at all. Comets appear at uncertain and 

 irregular intervals, they are not confined to any special part of 

 the heavens. A comet may be first discovered in one constella- 

 tion, and after a journey across the heavens it may sink to in- 

 visibility in any other constellation. A comet is sometimes only 

 seen for days or for weeks, but sometimes it remains visible for 

 months or even for years. The features of the comet itself are also 

 in a course of incessant transformation during its visit. Its size 

 and its shape are not constant. The interval of a few days, or 

 sometimes of even a few hours, suffices to work wondrous changes 

 in a body almost spiritual in its texture. 



Amid all these elements of confusion where are we to seek for 



1 Lecture by Prof. R. S. Ball, Astronomer-Royal for Ireland, at the 

 Montreal meeting of the British Association. 



the law and the order which really underlie the phenomena ? 

 There is law and there is order. Each one of the myriad comets 

 pursues a definite high-road through space. It is in the province 

 of the mathematician and the astronomer to ascertain by their 

 joint labours what the path is for each comet. The astronomer 

 directs his telescope to the comet, and he reads from the 

 graduated circles attached to his telescope the precise point in 

 the heavens where the comet is located. He repeats this ob- 

 servation a few nights later, he does it a third lime, and his work 

 is done. All the mathematician absolutely requires is to know 

 the place of the comet accurately on three nights. He will no 

 doubt be glad to accept further observations ; they will help to 

 eliminate the errors inseparable from such labours ; they will 

 enable him to obtain three places of the comet purged from all 

 sources of uncertainty. The comet is then within his toils. He 

 can determine the route which the comet is pursuing. He can 

 by his calculations follow the comet in its movements through 

 the profundity of space far beyond the penetration of the 

 telescope. The telescope only watches the comet during a brief 

 portion of its career, but the subtle eye of the mathematician 

 seldom loses sight of a comet once detected. He watches it 

 recede to its greatest distance ; he knows when the comet begins 

 to return ; he sees how it gradually approaches the sun. He 

 assigns the spot on the heavens where the comet is first to appear, 

 and he tells the day and sometimes even the hour when the 

 telescope will welcome the wanderer's return. 



It has long been known that the highway of each comet is one 

 of those graceful curves known to geometers as conic sections. 

 The comets which appear only once sweep through our system 

 in a curve which cannot be distinguished from a perfect parabola. 

 The small but exceedingly interesting class of comets which return 

 periodically revolve in the most beautiful of all curves — the ellipse. 

 The supreme law of gravitation has ordained that the comets must 

 follow a conic section whereof the sun lies at one of the foci. 

 But subject to this imperative restriction the orbit of a comet 

 may have, every degree of variety. A comet may revolve in a 

 path so small that it only requires three years to complete a 

 revolution. Another comet moving in a much longer ellipse will 

 require seventy-five years. There may be every intermediate 

 gradation, and there are some cometary orbits so vast that the 

 mighty journey cannot be accomplished in less than thousands of 

 years, while there are others whose orbits stretch out to a 

 distance so stupendous that we fail to follow them in their 

 wanderings. The ellipses seem to be utterly interminable, and 

 in the language of mathematics we say that the orbit is 

 parabolic. 



In order to enunciate the first of the great modern discoveries 

 which we are to consider to-night, it is necessary to associate with 

 each comet a certain particular elliptic path lying in a particular 

 plane with a particular position in that plane, and with a particular 

 magnitude. The comet is, in fact, to be identified by its path as 

 its only permanent characteristic, for, though the comets may 

 exist in myriads, yet no two comets follow the same course 

 through space : such a contingency is too remote to be worthy 1 >f 

 serious contemplation ; it is, in fact, infinitely improbable. 



There is not, I believe, a greater surprise in the whole of 

 modern astronomy than the discovery of a myriad of small bodies 

 stealthily accompanying a comet in its mighty journey, and the 

 surprise is all the greater when we consider that in another aspect 

 we have been long familiar with these small bodies, and we 

 have called them shooting stars or luminous meteors. It was 

 Schiaparelli who first demonstrated, in 1S66, the wholly unlooked- 

 for connection between the showers of shooting stars and the 

 movements of comets. 



Every one is familiar with the very beautiful spectacle of a 

 shooting star, which is seen to flash into the air and vanish in 

 a streak of splendour. These little bodies were long an enigma 

 in astronomy, but they have gradually been subordinated to law 

 and order. It has been found that the sun which controls the 

 mighty Jupiter does not disdain to guide with equal care the tiny 

 shooting stars, and their movements are now tolerably well 

 known. The received doctrine about the shooting stars has stood 

 the severest test known to science — that is, the test of fulfilled 

 prediction. The first great prediction in this refined branch of 

 astronomy was made about twenty years ago. It was foretold 

 that a splendid shower of shooting stars would occur on the 

 night of November 12th, 1866. All the world knows how 

 triumphantly this prediction was fulfilled. 



If I may be permitted, I would wish to narrate in a few words 

 my own experience of that ever-memorable night. The details 

 of that majestic spectacle have been engraved on my memory. 



