MATTER, AND ITS GENERAL PROPERTIES. 17 



motion in a body, requires a power to exist independent 

 of itself. 



It is obvious, from the definition given, that when a body is once put in 

 motion, its inertia will cause it to continue to move until its movement is de- 

 Btroyed, or stopped, bj' some other force. 



A ball fired from a cannon would move on forever, were it not for the re- 

 sistance or friction of the air, and the attraction of the earth. 



21. By Friction, we mean the resistance 

 ^"^uonf"*^" which a moving body meets with from the 



surface on which it moves. 



A marble rolled upon a carpet will move but a short distance, on account 

 of the roughness and unevenness of the surface. Its motion would bo con- 

 tinued much longer on a flat pavement and longer still on fine, smooth ice. 

 If friction, the attraction of the earth, and the resistance of the air, were en- 

 tirely removed, the marble would move on forever. 



Owing to the property of inertia, or the indifference of mat- 

 Whatare Ei- ter to change its state, we find it difficult, in running, to stop 

 ertia ? ^^ "^^ once. The body tends to go on, even after we have ex- 



erted the force of our muscles to stop. We take advantage of 

 this property, by running a short distance when we wish to leap over a ditch 

 or chasm, in order that the tendencj^ to move on, which we acquire by run- 

 ning, may help us in the jump. For the same reason, a running-leap is al- 

 ways longer than a standing one. 



Many of the most frightful railroad accidents which have happened, are due 

 to the laws of inertia. The locomotive, moving rapidly, is suddenly checked 

 by an obstruction, collision, or breakage of machinery ; but the train of cars, 

 in virtue of the velocity previous^ acquired, continue to move, and in conse- 

 quence are driven into, or piled upon each other. 



For the same reasons the wheel of an engine continues to pursue its courso 

 for a time after the driving force has stopped. This property is taken advan- 

 tage of to regulate the motions of machinery. A large, heavy wheel is used 

 in connection vnth the machinery, called a fly-wheel. This hea^y wheel, 

 when once set in motion, revolves with great force, and its inertia causefs it 

 to move after the force which has been imparted to it has ceased to act. A 

 water-wheel or a steam-engine rarely moves perfectly uniformly, but as it is 

 not easy, on the instant, either to check or increase the movement of the 

 heavy wheel, its motion is steady, and causes the machinery to which it is 

 attached to work smoothly and without jerking, even if the action of the driv- 

 ing force be less at one moment than at another. 



22. Attraction is that tendency which all 

 ^actionT^*" ^^^ particles of matter in the universe have to 



approach to each other.* 



• As Attraction, in its various forms and relations to matter, is so comprehensive and 

 Important, it is treated separately in advance. 



