HEAT. 71 



prevented, we call mechanical effect, or may view as converted 

 into mechanical effect ceasing to be heat ; but, throwing out 

 of the question nervous sensation, this expansion or mechani 

 cal effect is all the evidence we have of heat, for if the air is 

 allowed to expand freely, this expansion becomes the index 

 of the heat ; if the air be confined, the expansion of the 

 matter of the vessel confining it, or of the mercury of a ther 

 mometer in contact with it, &c., are the indices of the heat. 



If, again, the air which has been expanded be, by mechani 

 cal pressure or by other means, restored to its original bulk, 

 it is capable of heating or expanding other substances to a 

 degree to which it would not be equal, if it had remained in 

 its expanded state. To produce continuous motion, or the 

 up and down stroke of a piston, we must heat and (tool, just 

 as with a magnetic machine we must magnetise and demagne 

 tise in order to produce a continuous mechanical effect ; and 

 although, from the impossibility of insulating heat, some heat 

 is apparently lost in the process, the result may be said to be 

 effected by the transfer of heat from the hot to the cold body, 

 from the furnace to the condenser. But we may equally well 

 say that the heat has been converted into mechanical force, 

 and the mechanical force back into heat ; the effects are 

 always correlative, as are the mechanical effects of an air 

 pump, with which, as we dilate the air on one side, we con 

 dense it on the other ; and as we cannot dilate without the 

 reciprocal condensation, so we cannot heat without the recip 

 rocal cooling, or vice versa. 



Hitherto the resistances of the piston or of any superim 

 posed weight have been thrown out of consideration, or, what 

 amounts to the same thing, it has been assumed that the 

 weight raised by the piston has descended with it. The heat 

 has not merely been employed in dilating the air or vapour, 

 but in raising the piston with its weight. If, as the vapour 

 is cooled, the weight be permitted to descend, its mechanical 

 force restores the heat lost by the dilatation ; but in this case 



