PNEUMATICS. 



We shall devote the present chapter to 

 explain the construction of this instru- 

 ment, and the principles which govern 

 its action. 



(33.) The air-pump is exhibited under 

 various forms, each of which is attended 

 with particular advantages and disad- 

 vantages, according to the purposes to 

 which it is applied. There are, however, 

 some general principles in which all 

 modifications of this interesting machine 

 agree, and which we shall first explain. 



Let R (Jig. 15.) be the section of a 

 glass vessel closed at the top T, but 

 open at the bottom, and having its 

 lower edge ground smooth, so as to rest 

 in close contact with a smooth brass 

 plate, of which S S is a section. When 

 the receiver R is thus placed upon the 

 plate S S it will, with the assistance of a 

 little unctuous matter previously rubbed 

 on the edge of the glass, be in air-tight 

 contact. In the plate is a small aper- 

 ture A, which communicates, by a tube 

 AB, with a cylinder in which a solid 



fy. 15. 



n 



piston P is moved. The piston-rod C 

 moves in an air-tight collar D, and a 

 valve V is placed in the bottom of the 

 cylinder opening outwards. 



Let the air in the receiver R, the 

 exhausting tube AB, and the barrel 

 S V be first supposed to have the same 

 density as the external air. Upon de- 

 pressing the piston, after ifc has passed 

 the aperture B, th? air in the barrel S V 

 will l^e compressed by the piston. Its 

 density, and therefore i^s elasticity, 

 will be increased, and will become 

 greater than that of the external air. 

 This superior elastic force will open the 

 valve V, through which as the piston 



descends, the air in the barrel will be 

 driven into the atmosphere. When the 

 piston has reached the bottom of the 

 cylinder, the valve V will be closed by 

 a spring or otherwise, and will be 

 pressed into its seat also by the atmos- 

 pheric pressure. 



. When the piston has thus arrived at 

 the bottom of the barrel, the air which 

 before filled the receiver R and the ex- 

 hansting tube A B, will have expanded 

 by its elastic property, and diffused itself 

 also through the barrel above the piston. 

 But upon again raising the piston it will 

 be forced back into its former bounds, 

 until the piston has passed the aperture 

 B. As the piston ascends, it leaves 

 beneath it a vacuum, into which the 

 external air is prevented from entering 

 by the valve V. When, therefore, the 

 piston has been raised beyond the aper- 

 ture B, the air in the receiver R and 

 the exhausting tube AB will expand 

 once more, and also fill the barrel S V. 



Upon a second depression of the pis- 

 ton the air which fills the barrel will be 

 discharged, and similar effects will fol- 

 low its ascent, and so the process may 

 be continued at pleasure. 



It will be perceived that this instru- 

 ment depends for its efficacy entirely on 

 the elastic quality in the air, by which, 

 while there is any portion of air in the 

 .receiver and exhausting tube, that por- 

 tion, however small, will expand and 

 diffuse itself equally through the barrel 

 in addition to the space it before filled. 

 It must be pretty evident, with very 

 little consideration, that by this process 

 a perfect vacuum can never be produced 

 under the receiver. For some air, how- 

 ever small the quantity be, must remain 

 after every depression of the piston. 

 Let us, however, examine how nearly 

 we may approach to a vacuum, or more 

 properly speaking, let us determine what 

 degree 'of rarefaction may be effected, 

 supposing the mechanical construction 

 of the instrument we have described to 

 be perfect, and no obstructions to arise 

 from circumstances merely practical. 



At the commencement of the process 

 the air which fills the receiver, exhaust- 

 ing tube, and barrel, is of the density of 

 the external ah-; let its entire quantity 

 in this state be called one. Let the 

 capacity of the barrel SV bear any 

 proposed proportion to that of the re- 

 ceiver and tube ; suppose that it is one- 

 third of their united magnitudes, and 

 therefore that it contains one-fourth of 

 the air contained within the valve V m 



