NOMOS. 119 



ment of some of the number. It is torywiHac . 

 difficult to account for the extreme count for the 



movements 



eccentricity of the cometary orbits upon of comets, 



. : . i i i 7 J and explain 



the principles which have been laid their pecuii- 

 down. It is difficult to imagine the exis- 

 tence of differences upon the surface of these airy 

 bodies, which, in their reactions with the Sun, can 

 produce such extremely differing amounts of 

 motion ; nor is it necessary. Now in accounting 

 for the particular movements of comets, we must 

 take into consideration, first of all, the effects of that 

 resistance which these bodies must necessarily en- 

 counter in space, and after this we must weigh the 

 consequences of those remarkable changes of form 

 which they undergo in approaching to and receding 

 from the Sun. It is easy enough to account for the 

 rapid approach to the Sun. The airy comet must en- 

 counter greater resistance in its movements than the 

 ponderous planet (for the resistance must increase in 

 proportion as the density of the moving body ap- 

 proaches to that of the medium in which the body 

 moves), and hence it may be assumed that the sub- 

 tensial impulse which originates in the reactions of the 

 solar and cometary currents may not suffice to send 

 the comet to the distance which is necessary to secure 

 a purely circular orbit. In other words, the excess of 

 resistance to each impulse obliges the comet to stop 

 at a point nearer to the Sun. It may be assumed also 

 that the resistance will act in this way in each suc- 

 ceeding moment, and bring the comet nearer and 

 nearer to the Sun, until the superadded impulse which 

 results from the diminishing distance between the 



I 4 



