LAWS OF MOTION.- 417 



the motions which they describe may be ascer- 

 tained. It is of importance, however, to ob- 

 serve, that it was not from a previous know- 

 ledge of the quantity and quality of the matter 

 of which the planetary system is composed, 

 that Sir I. NEWTO& discovered, and ascer- 

 tained> the times and the revolutions of its difr 

 ferent parts; but he inferred from the times 

 which the different planets take to describe 

 their revolutions, the quantity, as well as the 

 quality of the matter, belonging to them alL 

 No conclusion was ever made from a more faU 

 lacious, or more unphilosophical assumption. 

 It is not because the indexes of a watch, and of 

 a clock, describe equal areas in equal times, that 

 they are necessarily moved by one and the same 

 cause. A good mathematician, who beholds the 

 regular motion of the index, round the dial plate, 

 is able, by means of proper instruments, to as- 

 certain, in a manner the most precise, that 

 it moves over the space of an inch and a 

 half, in one hour's time ; amounting exactly to 

 eighteen inches in one day. In like manner, a 

 good astronomer, by observation, and the aid 

 of instruments, is able to ascertain, the differ- 

 ent times which the different planets take to 

 perform their revolutions. Although the ma- 

 thematician, and the astronomer, are able by 

 these means, to ascertain the times and the mo- 

 tions of these different bodies, the causes of 



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