36 



MATTER AND ITS PHYSICAL PROPERTIES. 



from a state of rest ; and therefore the power of changing the direction of any 

 motion which it may have received is inconsistent with the quality of inertia. 

 If a body moving from A to B, receive at B a blow in the direction C B E, 



it will immediately change its direction to that of another line B D. The 

 cause which produces this change of direction would have put the body in 

 motion in the direction B E, had it been quiescent at B when it sustained the 

 blow. 



Again, suppose G H to be a hard plane surface ; and let the body be sup- 

 posed to be perfectly inelastic. When it strikes the surface at B, it will com- 

 mence to move along it in the direction B H. This change of direction is 

 produced by the resistance of the surface. If the body, instead of meeting the 

 surface in the direction A B, had moved in the direction E B, perpendicular to 

 it, all motion would have been destroyed, and the body reduced to a state of 

 rest. 



By the former example it appears that the deflecting cause would have put 

 a quiescent body in motion, and by the latter it would have reduced a moving 

 body to a state of rest. Hence the phenomenon of a change of direction is to 

 be referred to the same class as the change from rest to motion, or from motion 

 to rest. The quality of inertia is, therefore, inconsistent with any change in 

 the direction of motion which does not arise from an external cause. 



From all that has been here stated, we may infer generally, that an inani- 

 mate parcel of matter is incapable of changing its state of rest or motion ; that, 

 in whatever state it be, in that state it must for ever persevere, unless disturbed 

 by some external cause ; that if it be in motion, that motion must always be 

 uniform, or must proceed at the same rate, the equal spaces being moved over 

 in the same time ; any increase of its rate must betray some impelling cause, 

 any diminution must proceed from an impeding cause, and neither of these 

 causes can exist in the body itself ; that such motion must not only be con- 

 stantly of the same uniform rate, but also must be always in the same direc- 

 tion, any deflection from its course necessarily arising from some external 

 influence. 



The language sometimes used to explain the property of inertia in popular 

 works, is eminently calculated to mislead the student. The terms resistance 

 and stubbornness to move are faulty in this respect. Inertia implies absolute 

 passiveness, a perfect indifference to rest or motion. It implies as strongly 

 the absence of all resistance to the reception of motion, as it does the absence 

 of all power to move itself. The term vis inertia, or force of inactivity, so fre- 



