LUBEICATIXG SUBSTANCES. 91 



are of little service. Olive oil, on the contrary, and some 

 animal oils, which scarcely dry at all, are generally pre- 

 ferred. To obtain the full benefit of oil, the application 

 must be frequent. 



According to the experiments 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-fifth to one- 

 seventh varying in both cases with the kinds used. 

 Wood on wood was diminished by lard to about one-fifth 

 to one-seventh of what it was before ; and the fiiction of 

 metal on metal was diminished to about half what it was 

 before ; that is, the friction became about the same in both 

 cases after the lard was applied. 



To lessen the friction of wooden surfaces, lard is better 

 than tallow by about one-eiglith or one-seventh ; and tal- 

 low is better thnn dry soap about as two is to one. For 

 iron on wood, tallow is better than dry soap about as five 

 is to two. For cast-iron on cast-iron, polished, the friction 

 with the diflTerent lubricating substances is as follows : 



Water 31 



Soap 20 



Tallow 10 



Lard 7 



Olive oil 6 



Lard and black-lead 5 



When bronze rubs on wrought iron, the fi-iction with 

 lard and black-lead is rather more than with tallow, and 

 about one-fifth more than with olive oil. With steel on 

 bronze, the friction with tallow and with olive oil is about 

 one-seventh less than with lard and black-lead. 



As a general rule, there is least friction with lard when 

 hard wood rubs on hardwood ; with oil, when metal rubs 

 on wood, or metal on metal being about the same in 

 each of all these instances. 



In simple cases, as with carts and wagons, where the 



