44 INERTIA. 



By observing bodies at rest, such as a piece of 

 furniture, a tree, a stone, we have many opportunities 

 of satisfying ourselves that they do not change their 

 state of rest without some external cause, that they do 

 not commence moving of themselves. On the other 

 hand, the observation of moving bodies seems to lead at 

 first to the conclusion, that all moving bodies gradually 

 come to rest. A stone, propelled along the road moves 

 over a short distance, but soon stops again ; if thrown 

 along a sheet of ice, it will pass over a longer space, yet 

 it will come to rest again; it is the same with a cannon- 

 ball projected from a gun, although it may pass over 

 several thousand yards before its motion ceases. But 

 although in all these cases the motion comes to an end, 

 it would be erroneous to conclude that the bodies stop 

 of themselves, for definite causes may be shown to exist 

 which make the motion gradually slower, and finally 

 destroy it altogether. One of these causes is especially 

 the friction which takes place between a moving body 

 and its support. The greater the friction the sooner is 

 the motion of a body stopped ; this is the reason why 

 the stone moves over a greater distance on a smooth 

 sheet of ice than on the rough road. But a moving 

 body has further to overcome the resistance of the air 

 in which it moves. The more the friction and resist- 

 ance of air are diminished by artificial means, the 

 longer the motion of a body lasts, and if it were 

 possible to remove completely every obstacle to motion, 

 it could be proved by actual observation, that a 

 moving body is of itself as incapable of stopping 

 when in motion, as it is incapable of commencing 

 to move of itself when at rest. Friction and resist- 



