150 NOMOS. 



themselves are greatly affected by the attraction of 

 the planets, but the planets appear to be absolutely 

 indifferent to the attraction of the comets. More 

 than once a comet has yielded to the attraction of 

 Jupiter, and passed through the special domain of 

 that planet, without producing the smallest appre- 

 ciable disturbance in the movements of even the 

 smallest of his satellites. Now it is in this extreme 

 rarity of cometary bodies that we may see in part 

 the explanation of those rapid and extensive changes 

 of form which are so distinctive of comets ; for if an 

 ordinary cloud may undergo the great metamorphoses 

 which it does undergo in the comparatively dense 

 region of the atmosphere, a comet, which is infinitely 

 more shadowy than the cloud, may be expected to 

 undergo still more marvellous changes in that atmo- 

 sphere of atmospheres which extends into space. 



The changes which pass over the forms of comets 

 are of a very complex nature. As the comet ap- 

 proaches the sun it diminishes in size, as it recedes 

 it enlarges. As it approaches the sun, also, it often 

 puts out a tail-like process from the side which is 

 most removed from the sun, and it is difficult to know 

 whether this tail is more remarkable for the rapidity 

 with which it is formed, or for the distance to which 

 it is carried. Thus, the tail of the great comet of 

 1680 (the comet observed by Newton), which was 

 60,000,000 miles in length, was emitted in two days ; 

 and the tail of the comet of 1843, which was long 

 enough to have extended from the sun to the orbits 

 of the planetoids, was formed in less than twenty 

 days. The disappearance of the tail is also as rapid 



