30 COBEELATION OF PHYSICAL FORCES. 



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

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

 developement when interposed between rigid moving bodies. 

 Thus, if we oil the axles of wheels, we have more rapid mo- 

 tion 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 slid- 

 ing 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 can- 

 not present to my mind any case of heat resulting from fric- 

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

 to it, is simply impeded motion. The greater the impedi- 

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

 greater is the resulting heat ; this resulting heat being a con- 

 tinuation of indestructible force, capable, as we shall pres- 

 ently see, of reproducing palpable motion, or motion of defi- 

 nite 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 noticed I believe by Mayer. The total heat produced by 

 the friction of fluids should, therefore, it will be said, be 

 equal to that produced by the friction of solids ; for although 

 each particle produces little heat, the motion being readily 

 taken up by the neighbouring particles, yet by the time the 

 whole mass has attained a state of rest there has been the 

 same impeding of the initial motion as by the friction of sol- 



