MOTION. 31 



ids if produced by the same initial force. If the heat be 

 viewed in the aggregate, and allowance be made for the spe 

 cific 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 car- 

 ried off by surrounding 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 elas- 

 tic, and no resistance opposed to it by the air or other mat- 

 ter, 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 

 perceptibly 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 contin- 

 ued 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 imperceptible, but these tri- 

 fling 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 reason- 

 ing 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 arm ; this force is given back by the descent of the 

 weight, the wheels move, the pendulum is kept oscillating, 

 heat is generated at each point of friction, and the surround- 

 ing air is set in motion, a part of which is made obvir c< *- 



