PROF. O. REYNOLDS ON COMETARY PHENOMENA. 199 



offers an explanation of, the lagging which is observed in 

 the display of comets. This will be seen from the following- 

 quotation from Herschel : — 



" Their variations in apparent size, during the time they 

 continue visible, are no less remarkable than those of their 

 velocity. Sometimes they make their first appearance as 

 faint and slow-moving- objects with little or no tail; but 

 by degrees accelerate, enlarge, and throw out from them 

 this appendage, which increases in length and brightness 

 till (as always happens in such cases) they approach the sun 

 and are lost in his beams. After a time they again emerge 

 on the other side, receding from the sun with a velocity at 

 first rapid, but gradually decaying. It is for the most part 

 after thus passing the sun that they shine forth in all their 

 splendour, and that their tails acquire their greatest length 

 and development, thus indicating plainly the action of the 

 sun's rays as the exciting cause of that extraordinary 

 emanation." 



The direct heat of the sun would only cause evaporation 

 on that side of the comet to which it was opposite ; and 

 consequently the stream of vapour would be emitted 

 towards the sun, just as appears to be the case from 

 observation; the streams of vapour would first form an 

 atmosphere round the comet, which would increase until 

 the extent was such that its attractive force could no 

 longer prevent the outside being driven away by any 

 force there might be, and so forming a tail or train — such 

 force, for instance, as would be exerted if the sun and 

 vapour were both charged with electricity and acted on 

 each other by induction. 



Again, as the vapour proceeded outwards from the 

 comet it would rapidly expand ; and this expansion would 

 cause clouds to form by condensation, which would travel 

 outwards till they were again dispelled by the sun's 

 rays ; so that on the side towards the sun it is probable 



