28o CLASSICS OF MODERN SCIENCE 

 consumed to overcome the friction of the wheels and the resistance 

 which the pendulum encounters from the air. The wheel-work of 

 the clock therefore exhibits no working force which was not previously 

 communicated to it, but simply distributes the force given to it uni- 

 formly over a longer time. 



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

 ing air-pump, a great quantity of air. When we afterwards open the 

 cock of a 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 determine the work consumed in the pumping- 

 in of the air, and the living force which, upon firing, is com- 

 municated to the ball, but we shall never find the latter greater than 

 the former. The compressed air has generated no working force, but 

 simply gives to the bullet that which has been previously communica.ed 

 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 generate 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 make only use of pure 

 mechanical forces, such as gravity, elasticity, pressure of liquids and 

 gases, could only be sought after by bewildered and ill-instructed peo- 

 ple. 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 me- 

 chanical work may not be derived. Here the question of a perpetual 

 motion remained open ; the decision of this question marks the prog- 

 ress of modern physics. 



In the case of the air-gun, the work to be accomplished in the pro- 



