MOTION. 



4. The time employed in going over this space. 



5. The quantity of matter in the moving body. 



6. The force with which it strikes another body 



that is opposed to it. 



In a mechanical sense, every body, by its inertia, 

 resists all change of state. If at rest, it will not 

 begin to move of itself; and if motion is commu- 

 nicated to it by another body, it will continue to 

 move for ever uniformly, except it be stopped by 

 an external agent. It is true, we do not see any 

 instances of bodies continuing to move for ever, 

 after being once put in motion ; but the reason of 

 this is, that all the bodies which we see are acted 

 upon in such a manner, as to have their motions 

 gradually destroyed by friction. For if the friction 

 is diminished by any means, the motion will con- 

 tinue much longer; but as it is impossible to destroy 

 friction entirely, it lessens, and at last destroys, all 

 motions on the surface of the earth. A familiar in- 

 stance may be mentioned to illustrate this tendency 

 of bodies to continue in their present state, whether 

 of rest or motion. If a man be standing in a boat, 

 with his back to the shore when it is pushed off, 

 he is in danger of falling backwards, until he 

 gradually acquires the motion of the boat; and if, 

 after the boat has been in motion some time, it be 

 suddenly stopped, he will fall forwards, because his 

 tendency will then be to continue in a state of 

 motion. 



The velocity of motion is estimated by the time 

 employed in moving over a certain space, or by the 

 space moved over in a certain time. The less the 

 time, and the greater the space moved over, the 

 greater is the velocity. To ascertain the degree 

 of this swiftness or velocity, the space run over 

 must be divided by the time. For example, sup- 



