34 



MATTER AND ITS PHYSICAL PROPERTIES. 



which would cause a body moving at ten miles an hour to increase its rate to 

 eleven miles, would also cause the same body at rest to commence moving at 

 the rate of one mile an hour ; and the same power which would cause a body 

 moving at the rate of ten miles an hour to move at the rate of nine miles in the 

 hour, would cause the same body moving at the rate of one mile an hour to 

 become quiescent. It therefore appears that to increase or diminish the mo- 

 tion of a body is an effect of the same kind as to change the state of rest into 

 that of motion, or vice versa. 



The effects and phenomena which hourly fall under our observation afford 

 unnumbered examples of the inability of lifeless matter to put itself into motion, 

 or to increase any motion which may have been communicated to it. But it 

 does not happen that we have the same direct and frequent evidence of its ina- 

 bility to destroy or diminish any motion which it may have received. And 

 hence it arises, that, while no one will deny to matter the former effect of in- 

 ertia, few will at first acknowledge the latter. Indeed, even so late as the time 

 of Kepler, philosophers themselves held it as a maxim, that " matter is more 

 inclined to rest than to motion ;" we ought not, therefore, to be surprised if, 

 in the present day, those who have not been conversant with physical science 

 are slow to believe that a body once put in motion would continue for ever to 

 move with the same velocity, if it were not stopped by some external cause. 



Reason, assisted by observation, will, however, soon dispel this illusion. 

 Experience shows us in various ways that the same causes which destroy 

 motion in one direction are capable of producing as much motion in the oppo- 

 site direction. Thus, if a wheel, spinning on its axis with a certain velocity, 

 be stopped by a hand seizing one of the spokes, the effort which accomplishes 

 this is exactly the same as, had the wheel been previously at rest, would have 

 put it in motion in the opposite direction with the same velocity. If a carriage 

 drawn by horses be in motion, the same exertion of power in the horses is 

 necessary to stop it, as would be necessary to back it, if it were at rest. Now, 

 if this be admitted as a general principle, it must be evident that a body which 

 can destroy or diminish its own motion must also be capable of putting itself 

 into motion from a state of rest, or of increasing any motion which it has re- 

 ceived. But this latter is contrary to all experience, and therefore we are 

 compelled to admit that a body cannot diminish or destroy any motion which 

 it has received. 



Let us inquire why we are more disposed to admit the inability of matter to 

 produce than to destroy motion in itself. We see most of those motions which 

 take place around us on the surface of the earth subject to gradual decay, and 

 if not rtnewed from time to time, they at length cease. A stone rolled along 

 the grouni. a wheel revolving on its axis, the heaving of the deep after a storm, 

 and all othei motions produced in bodies by external causes, decay, when the 

 exciting cause i s suspended ; and if that cause do not renew its action, they 

 ultimately cease. 



But is there no editing cause, on the other hand, which thus gradually de- 

 prives those bodies oi their motion? — and if that cause were removed, or its 

 intensity diminished, would not the motion continue, or be more slowly re- 

 tarded ? When a stone is rolled along the ground, the inequalities of its shape, 

 as well as those of the ground, are impediments which retard and soon de- 

 stroy its motion. Render the stone round, and the ground level, and the mo- 

 tion will be considerably prolonged. But still small asperities will remain on 

 the stone, and on the surface over which it rolls : substitute for it a ball of 

 highly polished steel, moving on a highly polished steel plane, truly level, and 

 | the motion will continue without sensible diminution for a very long period ; 

 i but even here, and in every instance of motions produced by art, minute asper- 



