On the Formation of the Tails of Comets. 109 



that the matter of the tail is driven off from the head by some 

 force foreign to the comet, and taking effect from the sun out- 

 wards. 5. This force, whatever may be its nature, extends far 

 beyond the earth's orbit. For comets have been seen provided 

 with tails of great length, though their perihelion distances ex- 

 ceeded the radius of the earth's orbit, (e. g. the great comet of 

 181 1.) Nothing can be predicated with certainty with respect to 

 the law of variation of this force, but it is at least probable, that, 

 like all known central forces, it varies inversely as the square of 

 the distance. 



I do not now propose to discuss the question of the nature of 

 this force by which cometic matter is expelled in a direction op- 

 posite to the sun — that is, whether it consists in an impulsive ac- 

 tion of the sun's rays, as Euler imagined, or in a repulsion by the 

 mass of the sun, operating at a distance, as supposed by Olbers 

 and Bessel. I would only remark, that as experiment has failed 

 to discover any impulsive force in the sun's light, the choice must 

 be given to that one of the two views which best explains the 

 phenomena. Whatever may be the nature of the force in ques- 

 tion, I will call it the repulsive force of the sun. Granting its exist- 

 ence, there are two modes in which we may conceive it to ope- 

 rate in forming the tail. We may suppose that it drives off the 

 nebulous matter to greater and greater distances, without destroy- 

 ing the connection of the parts ; so that the tail and the head will 

 always be revolving as one connected mass. Or we may con- 

 ceive that it is continually detaching portions of the nebulosity, 

 and repelling them to an indefinite distance into free space. The 

 first mentioned conception is the theory which has been in vogue 

 hitherto. But it seems to me that there are good and sufficient 

 reasons for rejecting it, and adopting the other in its stead. These 

 it will be my object now to give. 



two such forces adequate to its production. Thus, we might imagine a very rapid 

 flow of the more elevated portions of the nebulous matter from the hemisphere 

 turned towards the sun around to that which is turned away from him, by the mere 

 effect of heat and cold, and that, on reaching this hemisphere, the particles are 

 driven off by the action of a repulsive pole. Something of the same sort might be 

 conceived to be in operation on the earth. Thus we might imagine some subtil 

 fluid flowing from the equator, and accumulating about the poles, and subsequently 

 expelled in detached portions by the repulsion of the magnetic poles, forming the 

 streamers of the aurora borealis and aurora australis ; but to such speculations 

 there is no end. 



