100 OUTLINES OF NATURAL PHILOSOPHY. 



165. Time is often required for friction to come 

 to its bearing, or to attain its maximum, and in 

 this respect, different substances differ much from 

 one another. 



d. i. COULOMB found, that in wood sliding on wood, 

 without grease, the friction at first increased, but 

 in a minute or two came to a limit, which it did not 

 afterwards exceed. Oak, for example, sliding on 

 oak, though the pressure was varied from 74 Ib. to 

 2474 Ib., had a friction, after a minute, always nearly 

 44 in the hundred. On diminishing the surface as 

 much as possible, the friction was reduced no lower 

 than 41 J per cent. 



b. ii. He found also, that when metal slides on me- 

 tal, the friction is proportional to the pressure; in 

 iron on iron, 28 \ in the hundred ; in iron on brass 

 26, &c. ; and this was the same whether the bodies 

 were just beginning to move from rest, or had ac- 

 quired any velocity whatever; different from the 

 case just mentioned, of wood rubbing on wood. 



. in. When heterogeneous bodies were made to slide 

 on one another, as wood on metal, the friction in- 

 creased slowly with the time, and did not come to 

 its maximum in less than four or five days. Iron 

 against oak, after 10", gave the force of friction 

 = 7J per cent. ; at the end of four days it amounted 

 nearly to one-fifth. In heterogeneous substances, 

 too, the friction increases sensibly with the velocity, 

 and follows nearly an arithmetical, while the velocity 

 follows a geometrical progression. 



d. iv. When 



