254 OF N A T U II E. 



projectile or impelled motions, they have no- 

 thing in common, either among themfelves, or 

 with the planets. Each circulates in a different 

 plane; and they defcribe orbits in very different 

 periods of time ; for fome perform their revolu- 

 tions in a few years, and others require feveral 

 centuries. The fun, revolving round his own 

 centre, remains, in other refpects, perfectly at 

 reft in the midft of the whole, and ferves, at 'the 

 fame time, as a torch, a focus, and an axis to all 

 the parts of this vaft machine. 



It is by his magnitude alone that the fun re- 

 mains immoveable, and regulates the motions of 

 the other globes. As the force of attraction is 

 proportioned to the mafs of matter, as the fun 

 is incomparably larger than any of the comets, 

 and contains a thoufand times more matter than 

 the largeft planet, they can neither derange him, 

 nor diminiih his influence, which, by extending 

 to immenfe distances, reftrains the whole, and, 

 at fixed periods, recalls thofe which have ftretch- 

 ed fartheft into the regions of fpace. Some of 

 them, when they return, make fo near an ap- 

 proach to the fun, that, after cooling for ages, 

 they undergo an inconceivable degree of heat. 

 They are fubjected to ftrange viciffitudes from 

 thefe alternate extremes of heat and cold s as 

 well as from the inequalities of their motions, 

 which are fometimes prodigioufly accelerated, and 

 at other times almoft infinitely retarded. When 

 compared with the planets, the comets may be 



confidered 



