120 NOMOS. 



comet and Sun is sufficient to counterbalance the 

 resistance, and carry the comet to the same distance at 

 each succeeding moment, when a movement in a pure 

 circle, or in a polygon of immeasurably small sides 

 which cannot be distinguished from a circle, must 

 commence. It may be assumed also that the increas- 

 ing resistance which the comet may meet with in its 

 passage towards the Sun, may have the effect of 

 neutralising for some time the increment of impulse 

 arising from the diminishing distance between the 

 comet and the Sun, and that the increased resistance 

 may cause the comet to approach nearer to the Sun 

 than it would otherwise have done before it begins 

 to enter upon a uniform and circular path. All this 

 is simple enough, but not so the rest. It is easy to 

 bring the comet nearer and nearer to the Sun, until 

 it finds the circle in which the impulse and the resist- 

 ance are balanced ; but it is not easy to carry the 

 comet out of this circle and away from the Sun. 

 How, then, is this ? Now the answer to this ques- 

 tion appears to be found in the changes in form which 

 the comet has undergone in its passage towards the 

 Sun. In this passage the comet undergoes a remark- 

 able diminution in size. Irregularities in form may 

 accompany this diminution, or they may not, but the 

 diminution is constant. Now there is reason to 

 believe (as we shall have occasion to show more 

 particularly hereafter) that this diminution is owing 

 to the comet having given up, in apparent obedience 

 to the vaporising action of the Sun, one or more of 

 the external layers by which it is invested. In other 

 words, the effect of the change is to expose an inner 



