22 MECHANICS. 



is but slightly impeded, and a much larger quantity will 

 escape through a channel of any given size. 



When a grindstone is turned rapidly, the water upon 

 its surfiice is thrown off by this tendency to move in 

 straight lines. In the same way, a weight fastened to a 

 cord, whirled by the hand, will keep the cord stretched 

 during the revolution. A cup of water, attached to a 

 cord, may be swung over the head without spilling, the 

 water being held by centrifugal force. The same principle 

 causes a stone, when it leaves a sling, to fly off in a line. 

 This tendency to fly off from a revolving centre is called 

 centrifugal force the word centrifugal meaning flying 

 from the centre. Large grindstones, driven with great 

 velocity by machinery, are sometimes split asunder by 

 centrifugal force. 



The most sublime examples of this force occur in the 

 motion of the earth and planets, which will be more fully 

 explained in a future page. 



CHAPTER in. 



ATTRACTION. 

 GRAVITATION. 



The earth, as is well known, is a mass of matter in the 

 foi-m of a globe, the diameter being upward of 7900 miles. 

 From its enormous size and the small portion seen from 

 one point, the surface aj^pears flat, except where broken 

 into mountains and valleys. 



The tendency which all bodies possess of falling toward 

 the earth is owing to the attractive force which this great 

 mass of matter exerts upon them. This kind of attrac- 



