114 MECHANICS. 



fifth to one seventh — varying in both cases with the 

 kinds used. Wood on v^ood was diminished by lard 

 to about one fifth to one seventh of what it was before ; 

 and the friction of metal on metal was diminished to 

 about half what it was before ; that is, the friction be- 

 came about the same in both cases after the lard was 

 apphed. 



To lessen the friction of wooden surfaces, lard is bet- 

 ter than tallow by about one eighth or one seventh ; 

 and tallow is better than 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 different lubricating sub- 

 stances is as follows : 



Water 31 



, Soap 20 



Tallow 10 



Lard 1 



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 hard wood ; 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 friction at the axle is but a small portion of the re- 



