MECHANICS. 



are subject. If increased elasticity be 

 communicated to air by mechanical 

 means, it must be by compression or 

 condensation. It is evident, that in this 

 case., no power whatever will be gained, 

 in as much as it will require exactly as 

 much power to produce a given degree 

 of condensation in a given quantity of 

 air, as is equal to the increased elasti- 

 city with which that condensed air will 

 be endued. However, in this case, as 

 in that of the ordinary use of atmos- 

 pheric pressure, although no power be 

 gained by mechanical condensation, yet 

 considerable advantage may be derived 

 from this as a method of transmuting 

 one power into another, and as means 

 of accumulating the effects of a small 

 intermitting power, so as to convert it 

 into a severe or continued pressure. 



We have already seen an instance of 

 this in the air-gun. (PNEUMATICS, 

 Art. 52.) If we attempted, by mere 

 manual force, to project a bullet, we 

 should find our efforts attended with but 

 a small effect ; but if it were possible to 

 unite in one impulse the combined force 

 of a vast number of separate impulses, 

 we should produce the desired effect. 

 The air-gun, then, is nothing more than 

 a contrivance, by which a great number 

 of separate exertions of our strength 

 are accumulated and combined, and 

 made to act simultaneously. The pro- 

 cess of condensing the air is conducted 

 by a number of successive muscular 

 exertions ; and the elastic force which 

 the condensed air thus receives, is ex- 

 actly equal to the sum of these seve- 

 ral exertions of human strength, and 

 may, therefore, be considered as a ma- 

 gazine in which these separate exertions 

 are contained in such a manner, that 

 their combined intensity may be, at any 

 moment, applied to the ball or other 

 missile to be projected. 



In this instance, the object to be at- 

 tained is the production of a severe but 

 instantaneous effect. The elastic pro- 

 perty of air is also sometimes used to 

 convert an intermitting or reciprocating 

 action into a continued and uniform 

 one. The fire-engine, described in our 

 treatise on PNEUMATICS, Art. 48, is an 

 instance of this. The force which works 

 the pistons is intermitting or reciproca- 

 ting, while the pressure of the condensed 

 air in the air-vessel, produced by that 

 intermitting force, is continuous in its 

 action. Its total action, however, must 

 be precisely equal to the sum of the 

 forces which depress the pistons. 



The force of condensed air may be 

 applied to produce a severe and conti- 

 nued pressure, on a principle similar to 

 that of Bramah's hydrostatic press, al- 

 ready described. Let B (fig. 26.) be a 



large cylinder, in which a solid piston 

 or plunger moves air-tight. Let D E be 

 a small tube, having a stop-cock at G, 

 and terminated in a screw at E. Let 

 C be a strong metal ball, capable of 

 bearing an intense bursting pressure, 

 having a small tube, terminated by a 

 screw at E, by which it may be con- 

 nected occasionally with the tube 

 D E, or with a condenser, (Pneuma- 

 tics, Art. 38,) and also furnished with a 

 stopcock at F. 



By means ef a condenser screwed 

 upon E, the stopcock F being opened, 

 let air be forced into the ball C, until it 

 presses against the cock F, when closed, 

 with a force of more than one tori. The 

 condenser being then removed from E, 

 the air cannot escape, the cock F being 

 closed. Let the ball and tube CFE 

 be then screwed upon the tube D E, 

 and the cocks F and G both opened. 

 The condensed air will expand through 

 the tube D, artd fill the part of the 

 cylinder below the piston. If, after 

 this expansion, the elastic force of the 

 compressed air is such that it would 

 press on the stopcocks with a force 

 exceeding that of the atmosphere by 

 one ton, there will be an effective pres- 

 sure against the piston A, of as many 

 tons as the number of times that the 

 section of the tube D is contained in 

 that of the piston. Suppose the sec- 

 tion of the tube to be a quarter of 

 an inch in diameter, and the piston to 

 be one foot, the pressure on the piston 

 will then be equal to 2304 tons. 



In this case, like all the former, air 

 is only used as a convenient means of 

 accumulating mechanical force; and 

 ought not, properly speaking, to be 

 looked upon as the prime mover. As in 

 using the weight or pressure of the at- 

 mosphere, we consider that cause, what- 

 ever it be, that produces the vacuum, or 

 the rarefaction, to be properly the prime 



