PHYSICAL AND LITERARY. 25 



And this power being once eftabliflied, 

 it remains only to be made out, that it is, 

 of a different kind from that which re- 

 fifls a change from reft to motion, and 

 from any degree of motion to a greater. 

 The thing is obvious ; for, as caufes are 

 beft diftinguifhed by their efFecls, the 

 caufe cannot be the fame of two efFedls 

 diametrically oppofite, 'uiz. preferving a 

 body in motion, and oppoiing motion. Ic 

 may ferve to clear this matter, if we at- 

 tend to the difl:in(5lion betwixt impulfe 

 and refiftance. They are fimple ideas,^ 

 and therefore incapable of a definition; 

 but their difference will be obvious, by 

 confidering a man in the ad of puflving 

 againft an obftacle, and barely refuting 

 force applied againft him, without pufh- 

 ing again, or counteracting. The man, 

 no doubt, is adive in refilling, as well as 

 in pufhing; but the aclious are very dif- 

 ferent in their nature, and carefully to 

 be diftinguiflied'in accurate reafoning. 

 In a word, the tendency of impulfe is, to 

 produce motion j the tendency of refiitance 

 is, to prevent it. Thefe povyers, therefore. 



