192 PROF. O. REYNOLDS ON COMETARY PHENOMENA. 



XXIII. On Comet ary Phenomena. 

 By Professor Osborne Reynolds, M.A. 



Read November 28th, 1871. 



The observation of comets by powerful telescopes has 

 shown them to be in a state of violent internal agitation 

 — a feature which is as much their characteristic as tails 

 or vaporous appearance ; for it is not possessed by any of 

 the planets or fixed stars. Robert Hook seems to have 

 been the first to notice this : when he observed the great 

 comet of 1680 (Newton's comet) through his 14-feet tele- 

 scope, he saw bright streams issuing from some point near 

 the centre of the comet's head and at first taking direction 

 opposite to the tail and towards the sun, then gradually 

 diverging, and finally falling back into the tail. These 

 streams were continually changing in magnitude and di- 

 rection, some of them disappearing and fresh ones appear- 

 ing in their places. Their behaviour was such as to lead 

 the philosopher to the conclusion that they were flame 

 and smoke, or vapour excited by the action of the sun on 

 the constituents of the body of the comet. Robert Hook 

 again noticed this phenomenon in the comet of 1682. In 

 1836 Bessel observed similar appearances in Halley's 

 Comet j and, although he appears to have been in igno- 

 rance of the fact that they had been noticed before, he 

 was led to the same conclusion as Hook as to their origin, 

 viz. that these streams were jets of vapour caused by the 

 action of the sun's heat on the more solid part of the 

 comet. This hypothesis, started by Hook and afterwards 

 by Bessel, seems to have been very generally confirmed by 

 subsequent observation, almost all comets, large and small, 



