ON THE INTERACTION OF NATURAL FORCES. 145 



when the bullet is discharged ; but because the action is com- 

 pressed 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 use solely of pure me- 

 chanical forces, such as gravity, elasticity, pressure of liquids 

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

 structed 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 never- 

 theless 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. Here the question of a perpetual 

 motion remained open ; the decision of this question marks the 

 progress of modern physics, regarding which I promised to 

 address you. 



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

 the propulsion of the ball was given by the arm of the man 

 who pumped in the air. In ordinary firearms, the condensed 

 mass of air which propels the bullet is obtained in a totally 

 different manner, namely, by the combustion of the powder. 

 Gunpowder is transformed by combustion for the most part 

 into gaseous products, which endeavour to occupy a much 

 greater space than that previously taken up by the volume of 

 the powder. Thus you see that, by the use of gunpowder, the 

 work which the human arm must accomplish in the case of the 

 air-gun is spared. 



In the mightiest of our machines, the steam-engine, it is a 

 strongly compressed aeriform body, water vapour, which, by its 



