LUBRICATING SUBSTANCES. 



113 



Fig. 90. 



Grindstone on Friction-wheels. 



ed, for the very 

 plain reason that, 

 in using a grind- 

 stone, nearly all 

 the friction is 

 at the circumfer- 

 ence, or between 

 the stone and the 

 tool, which fric- 

 tion-wheels could 



not, of course, remove. 



SECTION III. 



LUBRICATING SUBSTANCES. 



LuBRiCATiNa substances, as oil, lard, and tallow, ap- 

 phed to rubbing surfaces, greatly lessen the amount of 

 friction, partly by filling the minute cavities, and partly 

 by separating the surfaces. In ordinary cases, or where 

 the machinery is simple, those substances are best for 

 this purpose which keep their places best. Finely- 

 powdered black-lead, mixed with lard, is for this reason 

 better for greasing carriage wheels than some other ap- 

 pUcations. Drying oils, as linseed, soon become stiff 

 by drying, and are of little service. Olive oil, on the 

 contrary, and some animal oils, which scarcely dry at 

 all, are generally preferred. To obtain the full benefit 

 of oil, the application must be frequent. 



According to the experunents made with great care 

 by Morin, at Paris, the friction of wooden surfaces on 

 wooden surfaces is from one quarter to one half the 

 force applied ; and the friction of metals on metals, one 



