LESSONS IN PHYSICS. 67 



base of the plane. In case a weight is carried up this plane, 

 it rises only through the distance BC, while the power moves 

 through the distance AB, so that power multiplied by AB 

 equals weight multiplied by BC. That is, power is to weight 

 as the height of the plane is to the length of the plane. Casks, 

 etc., are frequently loaded into wagons by an inclined plane. It 

 was by means of the inclined plane that ancients raised the great 

 blocks of stone which we find in the ruins of the pyramids, tem- 

 ples, and other buildings. Common stairs are examples of an 

 inclined plane. 



A wedge consists of two inclined planes joined by their base. 

 It is mostly used for cleaving timber. It is inserted in a crack, 

 and then driven home by blows from a hammer; but as we can- 

 not measure the energy of these blows, we cannot determine the 

 equilibrium of force in the case of a wedge. 



A screw is a cylinder of wood or metal around which runs a 

 spiral ridge called a thread. This cylinder passes through a 

 block in which there is a corresponding groove to fit the ridge or 

 thread of the cylinder. When the cylinder makes one revolution 

 it rises in the nut to a distance equal to the distance between two 

 consecutive sections of the thread. The arm or bar through the 

 head of a screw increases the length of the power arm. The 

 equation of a screw is : power multiplied by the circumference of 

 the circle through which it moves, equals weight multiplied by 

 the distance between two contiguous threads of the screw. The 

 screw is used for raising buildings and for exerting pressure, as 

 in the pressing of hay or cotton; and when the thread is very 

 fine, it may be used to measure short distances, as in the case of 

 the micrometer screw. 



The toggle joint, used commonly in adjusting the tops of car- 

 riages, is a very useful form of the lever, especially in exerting 

 pressure through a short distance. 



In every class of machinery there is loss of power through 

 friction, rigidity of ropes, resistance of air, etc., so that power 

 must be furnished not only to do the work but to run the ma- 

 chine. Illustrations of the simple machines are common, and it 



