Prof. Helmholtz on the Interaction of Natural Forces. 495 



our watches by means of the tension of springs. A weight 

 which lies on the ground, an elastic spring which is without ten- 

 sion, can produce no effects : to obtain such we must first raise 

 the weight or impart tension to the spring, which is accom- 

 plished when we wind up our clocks and watches. The man 

 who winds the clock or watch communicates to the weight 

 or to the spring a certain amount of power, and exactly so 

 much as is thus communicated is gradually given out again 

 during the following twenty-four hours, the original force being 

 thus slowly consumed to overcome the friction of the wheels and 

 the resistance which the pendulum encounters from the air. The 

 wheelwork of the clock therefore exhibits no working force which 

 was not previously communicated to it, but simply distributes 

 the force given to it uniformly over a longer time. 



Into the chamber of an air-gun we squeeze, by means of a 

 condensing air-pump, a great quantity of air. When we after- 

 wards open the cock of the gun and admit the compressed air 

 into the barrel, the ball is driven out of the latter with a force 

 similar to that exerted by ignited powder. Now we may deter- 

 mine the work consumed in the pumping-in of the air, and the 

 living force which, upon firing, is communicated to the ball, but 

 we shall never find the latter greater than the former. The com- 

 pressed air has generated no working force, but simply gives to the 

 bullet that which has been previously communicated to it. And 

 while we have pumped for perhaps a quarter of an hour to charge 

 the gun, the force is expended in a few seconds when the bullet 

 is discharged ; but because the action is compressed into so short 

 a time, a much greater velocity is imparted to the ball than would 

 be possible to communicate to it by the unaided effort of the arm 

 in throwing it. 



From these examples you observe, and the mathematical theory 

 has corroborated this for all purely mechanical, that is to say, 

 for moving forces, that all our machinery and apparatus gene- 

 rate no force, but simply yield up the power communicated to 

 them by natural forces, — falling water, moving wind, or by the 

 muscles of men and animals. After this law had been established 

 by the great mathematicians of the last century, a perpetual 

 motion, which should only make use of pure mechanical forces, 

 such as gravity, elasticity, pressure of liquids and gases, could 

 only be sought after by bewildered and ill-instructed people. 

 But there are still other natural forces which are not reckoned 

 among the purely moving forces, — heat, electricity, magnetism, 

 light, chemical forces, all of which nevertheless stand in manifold 

 relation to mechanical processes. There is hardly a natural 

 process to be found which is not accompanied by mechanical 

 actions, or from which mechanical work may not be derived. 



