452 RECAPITULATION, 



circle. The greatest force is when he pulls the 

 handle upwards from the height of the knee ; and 

 the least where he thrusts from him horizontally. 



Friction. 



1. Friction is owing to the roughness of bodies. 



2. It increases according to the weight and velo- 

 city of moving bodies, and also according to the 

 surface, though in a small degree. 



Communication of Motion in Machines. 



Motion is communicated from one part of a 

 machine to another by various methods ; by levers, 

 cords, wheels, &c. and wheels move each other by 

 the simple contact of their surfaces when pressed 

 together, or by teeth. The proper shape of the 

 teeth is an object of great importance. 



Fly Wheels. 



1. Fly wheels are employed to equalize the 

 motion of a machine. 



2. A fly cannot in any other way add power to 

 the machine. 



Mills. 



1. Water-mills are of three kinds — undershot 

 mills, breast-mills, and over shot-mills. The powers 

 necessary to produce the same effect on each of 

 these, must be as the numbers, 2.4, 1.75, and 1. 



2. But as a fall of water cannot be always had, 

 the wider shot-mills are often used. 



