The Foundations of Physical Science. 187 



proportioned to the quantity of the body and to the degree of 

 change is equally required, whether to give motion, to take it 

 away, or to bend it." Every one can see that a body at rest 

 might remain so for ever, if no one, and no thing, exerted the 

 force necessary to make it move ; but it is not equally obvious, 

 though equally true, that if once set in motion it would move 

 on for ever, if nothing caused it to stop. Let this truth be 

 felt, and an interesting inquiry must arise concerning what 

 becomes of an arrested force. Science has not yet demon- 

 strated that all forces are correlative ; but we are justified in 

 saying that forces are incessantly at work, and that one force 

 only ceases from doing one kind of work by becoming occupied 

 with another kind of work. We recognize a force by what it 

 is doing, or has done, and we give names which designate 

 distinct actions, although they may become erroneous if sup- 

 posed to designate totally distinct causes. The mechanical 

 force displayed in the swift journey of a cannon-ball to a target, 

 disappears when the object has been struck, and the ball 

 brought to rest ; but it has developed great heat, and altered 

 its own internal state, and also that of the target, in addition 

 to the visible effects of crushing and penetration. 



Nature is full of practical equations. A small body, 

 moving quickly, equals in force and can counterpoise a larger 

 body moving with proportional slowness. There are innu- 

 merable applications of this law; but all are readily compre- 

 hensible, provided the manner in which a lever operates is first 

 understood. Ignorance first, and familiarity afterwards, pre- 

 vents the importance of these simple facts from being per- 

 ceived ; but it is not too much to say, that human existence and 

 civilization would be alike impossible, if small quantities of 

 matter could not be made to balance large quantities of matter, 

 or large quantities made to balance small quantities, by propor- 

 tioning the quantity of motiou imparted to each in a given 

 time. Dr. Arnott puts the question very simply, in explaining 

 that the " apparent paradox of a weight of one pound at the end 

 of a beam being rendered through such medium equal in effect 

 to four pounds placed nearer the centre, is solved by reference 

 to the nature of inertia and motion. The same amount of 

 force which gives any certain velocity to four pounds is just 

 that required to give four times that velocity to one pound ; 

 and owing to the connection of the two weights through the 

 beam, no motion downwards by gravity can occur in the four 

 pounds, without causing a motion upwards just four times as 

 great in the one pound." 



The term inertia has been so long in use, that there is little 

 chance of getting rid of it; but it tends to give a false idea, 

 which sometimes clings to a student's subsequent thoughts. 



