sucr. ii.] DISSERTATION SECOND. 49 



Plutarch considers the velocity of the moon's motion as 

 the cause which prevents that body from falling to the 

 earth, just as the motion of a stone in a sling prevents it 

 from falling to the ground. The comparison is, in a cer- 

 tain degree, just, and clearly implies the notion of centri- 

 fugal force ; and gravity may also be considered as pointed 

 at for the cause which gives the moon a tendency to the 

 earth. Here, therefore, a foundation was laid for the true 

 philosophy of the celestial motions ; but it was laid without 

 effect. It was merely the conjecture of an ingenious mind, 

 wandering through the regions of possibility, guided by no 

 evidence, and having no principle which could give stabili- 

 ty to its opinions. Democritus, and the authors of that 

 physical system which Lucretius has so beautifully illus- 

 trated, were still more fortunate in some of their conjec- 

 tures. They taught that the Milky Way is the light of a 

 great number of small stars, very close to one another ; a 

 magnificent conception, which the latest improvements of 

 the telescope have fully verified. Yet, as if to convince 

 us that they derived this knowledge from no pure or cer- 

 tain source, the same philosophers maintained, that the sun 

 and the moon are bodies no larger than they appear to us to 

 be. 



Very just notions concerning comets were entertained by 

 some of the ancients. The Chaldeans considered those bo- 

 dies as belonging to the same order with the planets ; and 

 this was also the opinion of Anaxagoras, Pythagoras, and 

 Democritus. The remark of Seneca on this subject is tru- 

 ly philosophical, and contains a prediction which has been 

 fully accomplished : "Why do we wonder that comets, 

 which are so rare a spectacle in the world, observe laws 

 which to us are yet unknown, and that the beginning and 

 end of motions, so seldom observed, are not yet fully un- 

 derstood 7 " — Veniet tempus, quo isia quae nunc latent, 



