1861.] Conservation of Force applied to Organic Nature. 349 



when it hangs straightly down, perpendicular. Now if you let it fall, 

 and it comes to its position of equilibrium, it has gained a certain 

 velocity. Therefore, at first, you had motive power in the form of a 

 raised weight. If the pendulum comes again to the position of equi- 

 librium, you have motive power in the form of vis viva, in the form of 

 velocity, and then the pendulum goes again to the other side, and it 

 ascends again till it loses its velocity ; then again, vis viva or velocity 

 is changed into elevation of the weight : so you see in every pendulum 

 that the power of a raised weight can be changed into velocity, and 

 the velocity into the power of a raised weight. These tyo are 

 equivalent. 



Then take the elasticity of a bent spring. It can do work, it can 

 move machines or watches. The cross-bow contains such springs. 

 These springs of the watch and cross-bow are bent by the force of the 

 human arm, and they become in that way reservoirs of mechanical 

 power. The mechanical power which is communicated to them by the 

 force of the human arm, afterwards is given out by a watch during 

 the next day. It is spent by degrees to overpower the friction of the 

 wheels. By the cross-bow, the power is spent suddenly. If the 

 instrument is shot off, the whole amount of force which is communi- 

 cated to the spring is then again communicated to the shaft, and gives 

 it a great vis viva. 



Now the elasticity of air can be a motive power in the same way 

 as the elasticity of solid bodies ; if air is compressed, it can move 

 other bodies ; let us take the air-gun ; there the case is quite the same 

 as with the cross-bow. The air is compressed by the force of the 

 human arm ; it becomes a reservoir of mechanical power ; and if it is 

 shot off, the power is communicated to the ball in the form of vis viva, 

 and the ball has afterwards the same mechanical power as is com- 

 municated to the ball of a gun loaded with powder. 



The elasticity of compressed gases is also the motive power of 

 the mightiest of our engines, the steam-engine ; but there the case is 

 different. The machinery is moved by the force of the compressed 

 vapours, but the vapours are not compressed by the force of the 

 human arm, as in the case of the compressed air-gun. The compressed 

 vapours are produced immediately in the interior of the boiler by the 

 heat which is communicated to the boiler from the fuel. 



You see, therefore, that in this case the heat comes in the place 

 of the force of the human arm, so that we learn by this example, that 

 heat is also a motive power. This part of the subject, the equivalence 

 of heat as a motive power, with mechanical power, has been that 

 branch of this subject which has excited the greatest interest, and has 

 been the subject of deep research. 



It may be considered as proved at present, that if heat produces 

 mechanical power, that is, mechanical work, a certain amount of heat 

 is always lost. On the other hand, heat can be also produced by 

 mechanical power, namely, by friction and the concussion of unelastic 

 bodies. You can bring a piece of iron into a high temperature, so 



