THE AIH- PUMP, ETC.] MECHANICAL PHILOSOPHY. PNEUMATICS. 



775 



Fig. 199. 



high above the mouth to enable the persons sitting on 

 them to keep their heads free from the water, which 

 rises in the bel], in virtue of the upward pressure, as it 

 descends into the sea, and to breathe the air which is 

 thus forced up and condensed, above the rising water. 

 By means of a flexible tube, communicating with the 

 upper atmosphere through the top of the bell, fresh air 

 can be pumped in, and the air unfit for respiration 

 let out. 



It is easy to find the space in to which the air, originally 

 in the bell, will be compressed when the chest is sunk 

 to any depth below the surface. 



Let A B (Fig. 199) be the distance between the surface 

 of the water above, and the sur- 

 face of the water in the bell 

 Let also h be the height of a 

 column of the water, the pres- 

 sure of which is equal to that 

 of the atmosphere : then the 

 original air hi the bell sustained 

 a pressure equal to that of a 

 column of water, of the same 

 horizontal section and height h ; 

 but when condensed by being 

 forced up to B, it sustains a 

 pressure equal to the same 

 column h, and the additional 

 column of water that is above 

 B ; that is, the pressure now 

 sustained is equal to that of a column of water of height 

 h + A B. 



. vol . of condensed air _ D B h ( D aze "69) 



vol. of original air ~~D C **h -f-'AB 

 Putting, therefore, x for D B, the depth of the com- 

 pressed air between the roof of the bell and the surface 

 of the water within it, we have 



DC 



Hence, by solving this quadratic equation, which has 

 but one positive root, the depth DB of the compressed 

 air may readily be ascertained when the height D C of 

 the bell, and the depth A D, to which its upper surface 

 is sunk, are given. 



THE COXDKXSER. The condenser consists of a 

 strong vessel AH (Fig. 200), called the receiver, into 

 which atmospheric air is forced and condensed by means 

 of the following apparatus : 



A cylindrical barrel, opening into the receiver, and 

 having a valve opening downwards at C, is furnished 

 with a piston D, having a valve pig. 200. 



also opening downwards. As the 

 piston descends, the valve D closes 

 by the resistance of the air in C D, 

 and the pressure opens the valve 

 C, the compressed air passing into 

 the receiver A B. 



This piston having thus forced 

 all the air originally in the barrel 

 into the receiver, is raised up ; 

 and the superior pressure of the 

 condensed air in A B immediately 

 closes the valve C, while the down- 

 ward pressure of the external at- 

 mosphere opens the valve D ; so 

 that the barrel becomes again filled 

 with common air, and the operation 

 is repeated. 



In some condensers the piston D is made solid, and a 

 small orifice is made at p near the top of the barrel, 

 through which, upon raising the piston above it, tin; 

 barrel is supplied with fresh air from without. 



The density of the compressed air in the receiver, 

 after n descents of the piston, may be ascertained thus : 

 Let R and B be the respective capacities of the receiver 

 and the barrel, and let D be the density of the atmo- 

 spheric air : then, R being the volume of this air in the 

 receiver at first, and R + B the volume of it in the 

 receiver after the first descent of the piston, the entire 



mass of air then in A B will be D . R -(- D . B ; as the 

 additional mass D . B ia forced in at the second descent, 

 A B will then contain the mass D . R -\- 2D . B ; and a 

 fresh mass D . B being thus forced in at every descent, 

 we have for the whole quantity of air, after n descents, 



Quantity of air = D (R + n B) .', its density 



which increases in arithmetical progression. 



THE WINE-TASTER. We may here notice, as 

 among the minor contrivances by which the pressure of 

 the air has been made available for practical purposes, 

 the convenient little instrument called the wine-taster, 

 which is much used in wine and ale cellars to draw out, 

 Fig. 201. through the bung-hole of a cask, a specimen of 

 its contents. The marginal representation (Fig, 

 201) will give a sufficiently clear idea of the form 

 of this contrivance, which is hollow, and has a 

 small perforation at each end. When dipped 

 into any liquid, the upper orifice being kept 

 open to the atmosphere, the fluid rises through 

 the lower orifice, till the level is the same inside 

 and outside ; the thumb is then pressed on the 

 upper orifice, and the vessel withdrawn. The 

 air previously within the common atmospheric 

 air, reaching from the surface of the liquid to 

 the upper hole expands and fills the enlarged 

 space which the descent of the liquid leaves as 

 the vessel is raised. The pressure within is 

 thus diminished, while the external pressure 

 upwards, on the lower orifice, is that of the un- 

 rarefied atmosphere ; so that a portion of the 

 liquid remains suspended in the tube, under 

 which, if a glass be held, and the thumb removed, the 

 sample will run out by its own weight. 



THE AIR-PUMP. The oflice of the air-pump is the 

 opposite of that of the condenser, the purpose of it being 

 to exhaust a receiver of the air contained in it. There 

 are several forms of this machine ; one of the best known 

 is represented in Fig. 202, and is called Hawksbee's 

 air-pump. The receiver, containing the air to be with- 

 drawn, communicates, by means of a pipe, with two 

 Fig. 202. Fig. 20,1. 



barrels, generally of polished brass, in which two closely- 

 fitting pistons move by rack-work, as in the sectional 

 outline exhibited in Fig. 203, where it will be observed 

 that the descent of one piston compels the ascent of the 

 other. The four valves, marked a, b, e, f, all open 

 upwards. As one of the pistons descends, its valve 

 opens, and the air in the barrel passes through, anil 

 rests above the piston when it has arrived at the bottom. 

 During this operation the other piston ascends, with its 

 valve closed, emptying the barrel in which it is fitted, 

 of its air, the vacuum being supplied from the air in 

 the receiver with which the barrel is in communication. 

 The first piston is now raised, and iti barrel emptied in 



