58 COMMUNICATION OF MOTION IN MACHINES. 



this may be seen in clocks and watches, where the 

 wheels are of brass, and the pinions of steel. 



Hard substances, also, being less subject to wear, 

 are employed to diminish friction ; thus jewels are 

 used in watches for the axles of the wheels to 

 work in. 



Hard wood rubbing against hard wood has less 

 friction than when rubbing against soft wood. 



The wearing of the parts of a machine, by 

 altering the shapes of the parts, and increasing the 

 inequalities, occasions more friction. 



The friction of a single lever is very little. 



The friction of the wheel and axle is in propor- 

 tion to the weight, velocity, and the diameter of 

 the axle ; the smaller the diameter of the axle, the 

 less will be the friction. 



The friction of pulleys is very great, on account 

 of the smallness of their diameters in proportion to 

 that of their axes ; because they very often bear 

 against the blocks, and through the wearing of 

 their wheels and axles. 



In the wedge and screw, there is a great deal of 

 friction. Screws with sharp threads have more 

 friction than those with square threads ; and end- 

 less screws have more than either. 



The most complete method of lessening the 

 friction of wheels is to make their axles turn on 

 the surfaces of smaller wheels, which hence are 

 called friction wheels. 



OF THE COMMUNICATION OF MOTION IN 

 MACHINES. 



Numerous are the methods by which motion 

 may be communicated from one part of a machine 



