Jan. 3, 18S9] 



NATURE 



^SS 



Ox THE Forces wjiich froducf, the Varfous Forms 

 AND Parts of Comets. 



Before we proceed further with any detailed description, it is 

 necessary to inquire into the causes of the cometary phenomena 

 with which we have so far liecome acquainted— namely, nucleus, 

 jets, envelopes concentric and exceniric, and tails. 



We shall best do this by referring to the various memoirs with 

 which R<iche, of Montpellier, has enriched science. He dealt 

 first with the atmosjiheres of planets ; and, in concluding the third 

 part of a memoir on the fi,'ure of a fluid mais subjected to the 

 attraction of a distant point,' remarked that the inquiry might 

 possibly apply to the theory of comets, if we suppose such an 

 object, fluid and homogeneous, falling in a straight line towards 

 the sun. 



We have seen that a comet when it fir t makes it appearance 

 at Its greatest distance puts on a form resembling a planetary 

 nebula. It is at this point that M. Roche closes with it in order 

 to see what its change of form must be supposing it to be as 

 above stated fluid and homogeneous. 



As it approaches the sun, a tidal action will be set up, as the 

 solar attraction will be greater on the particles nearest ti it : 

 hence there will be an elongation of the swarm, and possibly 

 even one or more separations along a radius v;:ctor. 



Fig. 2 1. 



-Elongation of a cometary swarm. Comet 1882 /', Washingto 

 equatorial. 



If gravitation alone is concerned, the comet will remain 

 s.mmeirical, it will reduce its size as it approaches the sun,'^ and 

 part of its outer portions will be successive'y lost along the 

 radius vector both towards and away from the sm ; there, in 

 fact, will be two outpouring streams — one directed towards the 

 sun, the other away fjom it. There will be the greatest elongation 

 and the greatest loss at perihelion. 



M. Roche makes this out by considering the form of the en- 

 velope in which particles will be equally attracted by the sun 

 and the general ma s of the comet. 



One chief point of the ma'hematical investigations was, in fact, 

 to determine the surface on which the gravity of a small particle 

 wa.s tii/ in consequence of the solar and cometary attractions. 

 This i> calljd the limiting surface. On thi» poi.it 1 quote from 

 M. Faye : — ^ 



" There cxi.sts, for ever}- body placed within the sphera of 

 action of our sun, a surface li nit beyond which its mitter may 

 not pass, under jiain of e caping to that body and falling within 

 the domain of the solar action. This surface limit depends on 

 two things— the ma^s of the body, and its dislarce from the sun. 

 For a planet like the earth, whose mafs is so considerable, this 



' Afemo/res of the Academy of M npellier, vol. ii. p. 23. 

 » Anna/es de I'Obserpatoire de I'uris, vol. v. p. 376. 

 3 " Fonrsof Comets," Natlre, vol. x p. 247. 



surface limit is very distant, and yet, within the still terrestrial 

 region of its satellite, the moon, a child could lift, without much 

 difficulty, a body which would weigh for us 36,000 kilogrammes, 

 so feeble does the attraction of our gloSe become at that di ;tance 

 of to terrestrial radii. A little beyond the lunar orbit, a body 

 would cease to belong to the earth, and would enter the exclusive 

 domain of the sun. But for a comet thi^ surface limit i-; much 

 nearer the nucleus, and, moreover, it draws nearer and nearer 

 in proportion as the c>met approaches the sun . . . The surface 

 which so limits a body in the vicinity of the sun presents two 

 singular points in the direction of the radius vector, setting out 

 from which this surface is widened out into a conical network, 

 in such a ma mer that the dissolution of a body the matter of 

 which reaches or passes beyond these boundaries is effected 

 principally in the vicinity of the points referred to, flying, so to 

 spak, into two pieces, thus obeying at once the attraction of the 

 cjmet and especially, the thenceforth preponderating attraction 

 of the sun. . . . 



Fig 25. —Showing how a cimet approarhiiig the sun, grnvity alone being in 

 question, losfs i scinstituent particles biyond its free .surface, which is 

 cinstsniy di.ninishing, by an outflow in bath direction ; along the radius 

 vector. 



" .\I1 the conditions of instability are fnmd united in comets. 

 Their rrass is extremely small, and, consequently, the surface 

 limit is very near the centre of gravity. Ther distance from the 

 sun diminishes rapidly in the descending branch of their trajec- 

 tory ; consequently this surface limit becomes more and more 

 contracted. Finally, their enormous volume tends unceasingly 

 to dilate, because of the increasing heat of the sun, and to cause 

 the cometary matter to shoot out beyond this surface limit. 



" What becomes of this matter after it is set free by the action 

 of the sun ? Having escaped from that of the comet, it will 

 mne the less preserve the original speed, i e. the speed which 

 the comet itself had at the moment of separation ; this speed 

 will scarcely be altered by the feeble attraction of the cometary 

 nucleus, or by the internal movements of which I have spoken, 

 since these are mea ured by a few metres per second, while the 

 general motion round the sun takes place at the rate of 10, 15, 

 20 leagues and more per second. The molecules, separated and 

 thencefjrward i idep-ndent, then describe isolated orbits around 

 the sun, differing very little from that of the comet. Those 

 which are fjund in advance go a little faster and take the lead ; 

 thoce which are behind remain a little in the rear ; so that the 

 abandoned materials are divided along the trajectory of the 

 comet in front and in rear of the nucleus In time these 

 materials are separated considerably from the body from which 

 they emanate, and are mire and more disseminated ; but, con- 

 sidered at the moment of emission, they will form two visible 

 appendage, two sorts of tails opposed and stratified on thff 

 orbit of the comet." 



So much for the state of things if gravitation alone is in 

 question. 



But is gravitation alone concerned in building up a comet's 

 fo m ? That this is not so was fully recognized long ago, aid it 

 vas suggested by the fact that the t.tils always appeared to 

 I;e driven away from ihe sun ; .Seneca, indeed, was possil)ly 

 acqu.ainted with this fact, as he wrote: "Comae radios solis 

 effngiunt." ' Kepler was ihe first to suggest that the matter of 

 the tails was transported to the regions opposite the sun by 

 the impulsion of the solar rays ; Euler and Laplace accepted 

 this explanation ; ard Newton was the first to give a complete 

 ex 1 ination of the curve of the tail. 



Oibers, whose researches dealt with the phenomena presented 

 by the comet of 181 1, considered that the approach of a comet 

 to the sun might develop electricity in one or the other of these 

 bodies, and to this were ascribed both the repuhive action of the 



'See Pliny, Book 11. c'lap. xx-i. ei seq., for maay references fo mori 

 ancient authorities. 



