116 MECHANICS. 



lision and motion. At present, whenever a "body ac- 

 quires a great velocity, it soon loses it by friction 

 against the surface of the earth. The friction of water 

 against the surfaces it runs over soon reduces the rap- 

 id torrent to a gentle stream ; the fury of the tempest 

 is lessened by the friction of the air on the face of the 

 earth, and the violence of the ocean is subdued by the 

 attrition of its own waters. 



" Its offices in the works of art are equally import- 

 ant. Our garments owe their strength to friction, and 

 the strength of ropes depends on the same cause ; for 

 they are made of short fibres pressed together by twist- 

 ing, causing a sufficient degree of friction to prevent 

 the sliding of the fibres. Without friction, the short 

 fibres of cotton could never have been made into such 

 an infinite variety of forms as they have received from 

 the hands of ingenious workmen."* Deprived of this 

 retaining force, the parts of stone walls, piles of wood 

 and lumber, and the loads of carts and wagons, as well 

 as the wheels themselves, would slide without restraint, 

 as if their surfaces were of the most icy smootliness, 

 and walking without support would be impossible. 



The tractive power of locomotives depends on the 

 friction between the wheels and iron rails, which is 

 equal to about one fifth of the weight of the engine ; 

 that is, a locomotive weighing twenty-five tons will 

 draw with a force of five tons, without producing slip- 

 ping of the wheels. 



* Eneyclopsedia Americana. 



