MOTION. I9 



the initial motion is impeded, the less is the resulting heat. 

 Again, if a body move in a fluid, although some heat is pro- 

 duced, the heat is apparently trifling, because the particles of 

 the fluid themselves move, and continue the motion originally 

 communicated to the moving body : for every portion of 

 motion communicated to them this loses an equivalent, and 

 where both lose, then an equivalent of heat results. 



As the converse of this proposition, it should follow that 

 the more rigid the bodies impinging on each other the greater 

 should be the amount of heat developed by friction, and so 

 we find it. Flint, steel, hard stones, glass, and metals are 

 those bodies which give the greatest amount of heat from 

 friction or percussion ; while water, oil, &c. give little or no 

 heat, and from the ready mobility of their particles lessen its 

 development when they are interposed between rigid moving 

 bodies. Thus, if we oil the axles of wheels, we have more rapid 

 motion of the bodies themselves, but less heat ; if we increase 

 the resistance to motion, as by roughening the points of con- 

 tact, so that each particle strikes against and impedes the 

 motion of others, then we have diminished motion, but in- 

 creased heat ; or if the bodies be smooth, but instead of sliding 

 past each other be pressed closely together and then rubbed, 

 we shall in many cases evolve more heat than by the 

 roughened bodies, as we get a greater number of particles in 

 contact and a greater resistance to the initial motion. I cannot 

 present to my mind any case of heat resulting from friction 

 which is not explicable by this view : friction, according to 

 it, is simply impeded motion. The greater the impediment, 

 the more force is required to overcome it, and the greater is 

 the resulting heat ; this resulting heat being a continuation 

 of indestructible force, capable, as we shall presently see, of 

 reproducing palpable motion, or motion of definite masses. 



Whatever be the nature of the bodies, rough or smooth, 

 solid or liquid, provided there be the same initial force, and 

 the whole motion be ultimately arrested, there should be the 

 same amount of heat developed, though where the motion is 

 carried on through a great number of points of matter we do 

 not so sensibly perceive the resulting heat from its greater 

 dissipation. The friction of fluids produces heat, an effect first 



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