18 FRAGMENTS OF SCIENCE. 



Hence the idea of the conservation of force, as opposed 

 to the destruction of force, which was supposed to occur 

 when inelastic bodies met in collision. 



We now know that the principle of conservation holds 

 equally good with elastic and unelastic bodies. Perfectly 

 elastic bodies develop no heat on collision. They retain 

 their motion afterward, though its direction may be changed ; 

 and it is only when sensible motion is, in whole or in part, 

 destroyed that heat is generated. This always occurs in 

 unelastic collision, the heat developed being the exact 

 equivalent of the motion extinguished. This heat virtually 

 declares that the property of elasticity, denied to the masses, 

 exists among their atoms, and by their recoil and oscillation 

 the principle of conservation is vindicated. 



But ambiguity in the use of the term " force " has been 

 for some time more and more creeping upon us. We called 

 the attraction of gravity a force without any reference to 

 motion. A body resting on a shelf is as much pulled by 

 gravity as when after having been pushed off the shelf it 

 falls toward the earth. We applied the term force also to 

 that molecular attraction which we called chemical affinity. 

 When, however, we spoke of the conservation of force in 

 the case of elastic collision, we meant neither a pull nor a 

 push, which, as just indicated, might be exerted upon inert 

 matter, but we meant the moving force, if I may use the 

 term, of the colliding masses. 



What I have called moving force has a definite me- 

 chanical measure in the amount of work that it can perform. 

 The simplest form of work is the raising of a weight. A 

 man walking up-hill or up-stairs with a pound weight in 

 his hand, to an elevation say of sixteen feet, performs a cer- 

 tain amount of work over and above the lifting of his own 

 body. If he ascend to a height of thirty-two feet, he does 

 twice the work ; if to a height of forty-eight feet, he does 

 throe times the work ; if to sixty-four feet, he does four 



