2d CORRELATION OF PHYSICAL FORCES. 



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 solids if 

 produced by the same initial force. If the heat be viewed in 

 the aggregate, and allowance be made for the specific thermal 

 capacity of the substances employed, it probably is the same, 

 though apparently less ; the heat in the case of solids being 

 manifested at certain defined points, while in that of fluids it 

 is dissipated, both the time and space during and through 

 which the motion is propagated differ in the two cases, so that 

 the heat in the latter case is more readily carried off by sur- 

 rounding bodies. 



If the body be elastic, and by its reaction the motion im- 

 pressed on it by the initial force be continued, then the heat 

 is proportionately less ; and were a substance perfectly elastic 

 and no resistance opposed to it by the air or other matter, 

 then the movement once impressed would be perpetual, and 

 no heat would result. A ball of caoutchouc bandied about 

 for many minutes between a racket and a wall is not per- 

 ceptibly heated, while a leaden bullet projected by a gun 

 against a wall is rendered so hot as to be intolerable to the 

 touch : in the former case, the motion of the mass is con- 

 tinued by the reaction due to its elasticity ; in the latter the 

 motion of the mass is extinguished and heat ensues. 



A pendulum started in the exhausted receiver of an air- 

 pump continues its oscillation for hours or even days ; the 

 friction at its point of suspension and the resistance of the air 

 is minimised, and the heat is therefore imperceptible, but these 

 trifling resistances in the end arrest the motion of the mass, 

 the one giving it out as heat, the other conveying the force to 

 the receiver, and thence to surrounding bodies. Similar 

 reasoning may be applied to the oscillation of a coiled spring 

 and balance-wheel. 



To wind up a clock a certain amount of force is expended 

 by the muscles ; this force is given back by the descent of 



