CHAP, iv.] APPLICATIONS OF COMPRESSED AIR. CO 



CHAPTER IV. 



INDUSTRIAL APPLICATIONS OF COMPRESSED AIR. 



L THE AIR-GUN. 



TTTE have just seen how atmospheric pressure may be utilized as a 

 V V motive power. Forthat purpose it is sufficient to make a vacuum 

 by means of air-pumps in the space through which the vehicle is to be 

 moved ; thus establishing a difference in the pressures exercised on 

 the different sides of the moving body regarded as a piston. This 

 difference of pressure can be obtained in another way; instead of 

 rarefying the air in front, it can be compressed behind. The elastic 

 power with which this air will be pressed against the walls will then 

 be useful in different ways, and give rise to applications, to the 

 most important of which we are now about to refer. 



We noticed in the Forces of Nature the arrangements given to 

 machines used to compress air or other gases. These are pumps 

 which only differ from air-pumps in the working of the valves 

 which are reversed. 



The air-gun is one of the oldest applications of compressed air. 

 The invention dates as far back as 1560, and it even appears that the 

 ancients knew of a similar machine, as, according to Philon, Ctesibius 

 made a tube out of which an arrow 7 was sent by means of compressed 

 air. However that may be, the air arquebus had been for some 

 time in use in the army. In the present day it is only looked upon 

 as a curiosity. The mechanism is as follows. 



The butt-end of the gun is hollow and of metal ; inside this is the 

 reservoir in which the air is compressed by means of a force-pump. 

 Formerly this was placed in the butt-end itself, and the reservoir 



