Au§. 13, 1874] 



NA TURE 



287 



researches in Chemical Mechanics, and the other on 

 Kc|uilibnum in Gaseous Systems. Prof. Terquem, of 

 Lille, will read various papers in Uptics and Acoustics, 

 and M. G. Tissandier will give a public lecture on Me- 

 teorology and ISalloons. 



On the whole, there will be a fair number of purely 

 scientific papers, though there is an unusually large pro- 

 portion in medical and industrial subjects. 



THE COMETS 



THE following communication appears in the Times 

 from Mr. J. R. Hind, F.R.S., dated Mr. Bishop's 

 Oljservatory, Twickenham, August 10 :— 



" 1 send you positions of the last new comet (Borrelly) 

 for the ensuing ten days ; warnmg the amateur, however, 

 that he must not expect to see it well without a very good 

 telescope. They are deduced from the following orbit, 

 which I have calculated from the first accurate observa- 

 tion at Marseilles on July 26, one at Strasburg on Aug. i, 

 received from Prof. Winnecke, and a third taken at Mr. 

 Bishop's Observatory on the 4th : — 



" Perihelion passage, August, 27'o86i Greenwich time; 

 longitude of perihelion, 344^ 24' 6"; ascending node, 250° 

 59' 50" ; inclination to ecliptic, 41° 39' 52"; distance in 

 perihelion, o'gSogo ; heliocentric motion, direct. 



"The subjoined places are for midnight : — 



" The distances arc expressed, as usual, in jjarts of the 

 earth's mean distance from the sun. 



" It appears that eflorts in various observatories to 

 obtain a daylight view of the late bright comet have been 

 fruitless. I had been most hopeful of it being thus seen 

 witli the powerful telescopes and in the favourable chmate 

 of Marseilles; but 1 learn from M. Coggia that a close 

 search for the comet in fine skies on July 22, and from 

 morning to evening on the 26th, failed to aflbrd a glimpse 

 of it. At Twickenham, under very advantageous circum- 

 stances, about noon on July 23, we could not detect it, 

 when Procyon, the principal star in Canis Minor, at 

 nearly the same angular distance from the sun, was 

 shining brightly in the telescope. It affords additional 

 evidence that proximity to the earth is not so important 

 a condition for visibility of a comet in the daytime as 

 close approach to the sim ; but it was very desirable to 

 have the appearance of Coggia's comet upon record." 



THE FORM OF COMETS* 

 IV. 



WE have seen then that the phenomena of the tails of 

 comets can be explained even including their most 

 complicated appearances. 1 now proceed to deal with 

 other phenomena, for the best proof of the truth of a theory 

 is its capacity to explain a multitude of details which were 

 not at first considered. Examine in the figure (Fig. 8) which 

 I recently showed you of Donati's comet, that singular 

 dark portion which is seen in the axis of the tail to a very 

 considerable distance from the nucleus, and say if that 

 cylindrical space, void of matter, is not the effect of the 

 interposition of a screen — the nucleus, which intercepts 

 the repulsive force, and suppresses in this region all the 

 molecules driven from the head of the comet. This is 



• Continued from p. 270. 



because the repulsixe force, being a surface-action, is spent 

 against that of the nucleus, and is arrested by this simple 

 screen ; it is quite the reverse of attraction, which acts 

 effectually through all matter as if that matter did not exist. 

 This cannot have been the shadow thrown by the nucleus, 

 for two reasons, of which it is enough to mention the 

 first : the black streak, besides being much too long, is 

 not in the exact direction of the luminous ray ; it is in- 

 clined to the radius vector at an angle of several degrees, 

 for which the theory accounts. In short, it widens con- 

 siderably when tails almost straight, composed of the 

 rarest materials, are about to disappear, and we can often 

 follow its trace to the extremity of the tail. 



But I must dwell, in conclusion, upon the curious phe- 

 nomena of the head and upon the luminous sectors which 

 usually appear in the direction of the sun. We find here 

 a new confirmation of the play of the repulsive force. 

 Fig. 1 5 is a drawing on a large scale of the head of the 

 comet of 1S61, made at Rome by Father Secchi. 



Let us not forget, in what follows, that one of the 

 characteristic features of the nebulous layers tvhich sur- 

 round the nucleus, and of which it is perhaps entirely 

 composed, is the transparency which permits us to see 

 small stars through depths much greater than that of our 

 atmosphere. There is reason, then, for believing that the 

 solar rays penetrate across these layers to the central 

 nucleus and heat it, all the more since these same layers 

 are probably not so permeable to dark heat as they 

 are to luminous heat. In the space of three weeks the 

 central heat may thus be raised from the degree of heat 

 of distant space to a temperature sufficiently elevated to 

 volatilise a part of the matter of the nucleus, and perhaps 

 promote chemical reactions arrested till then by the 

 original cold.-* Under this increasing influence the 

 matter is dilated and rapidly separates from the nucleus 

 (19 metres per second for Donati's comet) ; but soon this 

 matter, still too dense to be sensibly repelled, reaches the 

 surface limit beyond which it ceases to belong to the 

 comet. This surface limit presents, as we have seen, two 

 opposite conical points by which the emission takes place. 

 At a later period this matter getting further and further 

 away, and becoming more and more rarefied, falls under 

 the action of this repulsive force, which then makes it 

 turn tail and fly to the rear. This species of conical 

 envelope, turned towards the sun, assumes the appearance 

 of a calyx with inverted edges, -while the opposite envelope 

 with obscure interior contracts under the influence of ttie 

 same action, but without changing its curvature. There 

 will be noticed, in front of this species of calyx, exterior 

 stiata nearer to the sun, to which they present their con- 

 vexity instead of being opened out conformably to the 

 theory which M. Roche had hitherto based solely upon 

 attraction. 1 asked M. Roche to introduce the new force 

 into his investigation of the surfaces assumed by a fluid 

 mass submitted to the double attraction of its own mass 

 and of that of the sun, and we have had the satisfaction 

 of seeing one of the Iw-o singular points of the surface 

 limit disappear. The surfaces are completely enclosed 

 and become curved towards the sun ; there is no room 

 on this side for any loss of matter. But this is to be ex- 

 pected in the exterior layers, which have too little density 

 to obey repulsion ; while in the interior of the head, 

 very near the nucleus, attraction still rules exclusively on 

 account of a density still very great. 



In order to render these somewhat complicated de 

 tails of the theory intelligible, we have only to turn 

 round on its axis Fig. 16 ; it will generate a surface of 

 revolution composed of an e.xterior envelope having a form 

 roughly parabolic, and of two envelopes attached to the 



* Spectrum analysis seoins to prove that this heat may reach the point of 

 giving to the nucleus a light of iis own, presenting, moreover, the character- 

 istics of a light emitted by a gaseous substance, lint up to the present time 

 (1870), indications of this kind are too vague and too doubtful to enable us to 

 derive much help from them. 



