MECHANICS. 101 



d. iv. When the surfaces are smeared with unctuous 

 substances, the friction is diminished; but time is 

 necessary for the attainment of the maximum. In 

 a set of experiments, oak rubbing on oak, the sur- 

 faces greased with tallow, and the weights being 

 1650, and 3280 pounds ; in the first, at the end of 

 six days, the friction seemed to become stationary, 

 and was 37 J per cent. ; in the second, the maximum 

 arrived at the end of five days, and the friction was 

 47. 



e. In another set, with brass on iron, and fresh tal- 

 low between them, the friction was four days in 

 coming to its maximum, and it was then between 10 

 and 11 per cent. At the first moment it was 9 per 

 cent., so that here the increase was inconsiderable. 

 These are some of the principal results of COU- 

 LOMB'S experiments, the most accurate and extensive 

 that have been made on this matter : they shew a 

 great difference in the laws of friction, that obtain 

 among different substances. 



Consult PRONY and COULOMB, ubi supra. 



166. Friction is diminished by the unctuous 

 substances mentioned above. Those that are thin- 

 nest and least tenacious are the best ; plumbago 

 also, or Black Lead, as it is called, reduced to 

 powder, and rubbed on the surfaces of wood, 

 metal, stone, &c. serves greatly to diminish fric- 

 tion. 



167. The 



