﻿404 Intelligence and Miscellaneous Articles. 



clouds will be formed in the solar atmosphere, thus giving the appear- 

 ance of a tail to the comet. If the shape of the comet be irregular 

 there may be more than one tail. The comet after its perihelion 

 passage recedes from the sun and at the same time gradually cools 

 and contracts ; the tail therefore ought to be greatest and brightest 

 at or shortly after the perihelion, and then slowly diminish and fade 

 away. This is in accordance with observation. 



Your obedient Servant, 



Ernest Carpmael, 

 Scholar of St. John's College. 



Gentlemen, 



In the last Number of the Philosophical Magazine Professor Tyn- 

 dall has propounded the theory that the visible head and tail of a 

 comet is an actinic cloud, resulting from the decomposition of vapour 

 by the solar light. This theory accounts for the extreme tenuity of 

 comets, their polarization, and the motion and development of their 

 tails. 



Now if all our knowledge of comets were derivable from observa- 

 tions with the unassisted eye, this theory would account for the ob- 

 served phenomena ; and, as it is, it is valuable by increasing our 

 knowledge of matter which, if not cometary matter, at all events has 

 striking analogies with it ; but I do not think it affords any explana- 

 tion of cometary phenomena as observed by the telescope. 



In the first place, immediately behind the nucleus, where, accord- 

 ing to this theory, we should expect a very luminous region, we 

 commonly have a dark space. Also the matter which forms the tail 

 usually streams out from the head towards the sun ; this was especi- 

 ally noticed in Halley's comet in 1836. Bond also, speaking of 

 Donati's comet, says, " the material, after being thrown off from the 

 nucleus, instead of at once being driven into the tail, formed a dense 

 cloud of nebulosity into which the luminous matter continued for 

 some time to stream. This cloud extended itself on the sunward side, 

 remaining in its vicinity for several days. When it had acquired a 

 certain stage, the discharge took place mainly from the corners or 

 cusps on either side in two streams, which, coalescing with those 

 issuing from other envelopes, formed the two branches of the tail." 

 Then, too, we have several series of envelopes which rise up towards 

 the sun. In Donati's comet seven were detected ; in the great 

 comet of 1861 no less than eleven were noticed: the force which 

 causes the ascent of these is intermittent and finally dies away. 

 Mr. Webb, in 1861, noticed the descent of the envelopes on the nu- 

 cleus ; also Herschel and Schroeter noticed the same in the comet 

 of 1811. 



From these facts I am disposed to think that although Professor 

 Tyndall's hypothesis accounts for some of the phenomena, yet the 

 true theory is still to be discovered. 



I am, Gentlemen, 



Talfourd Road, Peckham, Your obedient Servant, 



April 18, 1869. W. B. Gibbs, F.R.A.S. 



