RECAPITULATION. 451 



Screw. 



1. A screw may be considered as an inclined 

 plane. 



2. It is always used with a lever ; and the power 

 is to the weight, as the distance from one thread, 

 or spiral, to another, is to the circumference of 

 the circle described by the power. 



3. When a screw acts in a wheel, it is called an 

 endless screw. 



Compound Machines. 



1 . In all machines, simple as well as compound, 

 what is gained in power is lost in time. 



2. In any machine, the power is to the weight, 

 when in equilibrio, in a proportion formed by the 

 multiplication of the several proportions which the 

 power bears to the weight, in every simple mecha- 

 nical power used in the machine. 



3. The power of a machine is not at all altered 

 by varying the sizes of the wheels, provided this 

 proportion produced at last by the multiplication 

 of the power of each several part, remain the same. 



Moving poxvers in Machinery. 



1. A horse exerts more strength when drawing 

 horizontally; but the line of draught should not 

 be level with his breast, but incline upwards, 

 making a small angle with the horizontal plane. 



2. When a horse works in a circle, it should not 

 be less than forty feet in diameter. 



3. In turning a winch, a man exerts his strength 

 in different proportions at different parts of the 



g g 2 



