FEICTION NECESSARY TO EXISTENCE. 93 



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 important. 

 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 twisting, causing 

 a sufficient degree of friction to prevent the sliding of 

 the fibres. Witliout 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 w^heels themselves, 

 would slide without restraint, as if their surfaces were of 

 the most icy smoothness, and walking without support 

 would be impossible. 



The tractive power of locomotives depends on the fric- 

 tion 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 slipping of the 

 wheels. 



CHAPTER YII. 



PRINCIPLES OF DRAUGHT. 



An examination of the nature or laws of friction enables 

 us to ascertain the best line of draught for teams when 

 attached to wagons and carriages. If there were no fric- 

 tion whatever upon the road, the best direction for the 



'Encyclopaedia Americana. 



