12 MECHANICS. 



1), are set in motion ali*ke; h is soon brought to rest, 

 while a continues revolving a long time. If now they are 

 placed under the receiver of an air-pump, the air exhaust- 

 ed, and motion given to them alike by the rack-work d^ 

 they wdll both continue in motion during the same period. 



There is no machinery made by man free from the 

 checking influence of friction and the air; and for this 

 reason, no artificial means have ever devised a perpetual 

 motion by mechanical force. But Ave are not without a 

 proof that motion will continue w^ithout ceasing when 

 nothing operates against it. The revolutions of the planets 

 in their orbits furnish a sublime instance ; where removed 

 from all obstructions, these vast globes wheel around in 

 their immense orbits, through successive centuries, and 

 with unerring regularity, preserving undiminished the 

 mighty force given them when first launched into the re- 

 gions of space. 



To set any body in motion, a force is requisite, and the 



heavier the body, the greater must be the force. A small 



stone is more easily thrown by the hand than a cannon 



ball ; speed is more readily given to a skiflf than to a large 



and heavy vessel ; but the same force 



Fig. 2. 1 . , -, 1 . . . 



which sets a body m motion is re- 

 quired to stop it. Thus a wheel or a 

 grindstone, made to revolve rapidly, 

 would need as great an effort of the 

 arm to stop it suddenly as to give it 

 . , , . sudden motion. An unusual exertion 



Inertia Apparatus. 



of the team is necessary in starting a 

 loaded wagon; but when once on its way, it would 

 require the same effort of the horses to stop it as to back 

 it when at rest. 



The force of inertia is finely exhibited by means of a 

 little instrument called the inertia apparatus (fig. 2). A 

 marble or small ball is ])laced on a card, c, resting on a 

 concave stand. A spring snap is then made to strike the 



