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H. Helmholtx on the Interaction of Natural Forces. 1^5 



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

 to that exerted hj^ ignited powder. Now we may determine 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 corn- 

 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 m 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 the- 

 ory 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 only make use of pure mechanical forces, such as 

 gravity, elasticity, pressure of liquids and gnses, could only be 

 souglit 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, chem- 

 ical forces, all of which nevertheless stand in manifold relation 

 to mechnnical 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 

 ^vhich I promised to address you. ^ i . , 



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 

 banner, namelv, by the combustion of the powder. Gunpowder 

 is transformed by combustion for the most part into gaseous pro- 

 ducts, which endeavor to occupy a mucli larger 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 

 stronglv compressed aeriform body, water vapor, which, by its 

 effort to expand, sets the machine in motion. Here al«o vve^ do 

 not condense the steam by meana of an external mechanical 

 force, but by communicating heat to a mass of water in a closea 



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