FRICTION. 57 



the loins and back are concerned, and likewise the 

 extensors of the legs and thighs ; two of them will 

 walk with 300 lbs. (that is, 150 lbs. each,) at the 

 rate of four miles an hour. 



The last and most effectual way of a man's ex- 

 erting his strength, is in rowing a boat ; he there 

 acts with more muscles than in any other oper- 

 ation ; and the weight of his body also assists him. 



FRICTION. 



By friction is meant the rubbing of bodies 

 against each other. The subject of friction is of 

 great importance in mechanics, as, in consequence 

 of it, the actual performance of machines is much 

 less than what might be expected from calculation. 



However smooth bodies may appear to the eye, 

 yet if they are examined with a microscope, nume- 

 rous inequalities will be observed, and when sur- 

 faces move over each other, the prominences of 

 one fall into the hollows of another, and some, 

 force must be required to lift or drag one over the 

 other, or its prominences must be broken off. 



Friction is greater in moving bodies in propor- 

 tion to their weight or pressure against each other : 

 and also in some degree in proportion to the velo- 

 city of their motion. 



Polished substances, as might be expected, have 

 less friction than rough ones ; and oil, grease, and 

 black lead, by filling up the inequalities of the 

 surfaces of bodies, lessen the friction. 



Metals have more friction when moving on 

 metals of the same kind, than when they move on 

 different metals. Thus, steel and brass are fre- 

 quently employed together to lessen the friction : 



