PNEUMATICS. 



17 



cury. As the rarefaction proceeds, the 

 air in the receiver losing a part of its 

 elastic force no longer balances the pres- 

 sure of the atmosphere at F, and conse- 

 quently the mercury rises in the tube 

 E G. The weight of the column of 

 mercury suspended in this tube, toge- 

 ther with the elastic force of the air in 

 the receiver which presses on its surface 

 at H, are the forces which balance the 

 atmospheric pressure at F. They are, 

 therefore, together equal to the atmos- 

 pheric pressure; and hence it follows 

 that the column of mercury in the tube 

 is always equal to the excess of the at- 

 mospheric pressure above the elastic 

 force of the air in the receiver. 



Since the column of mercury sus- 

 pended in the common barometer is 

 always a measure of the atmospheric 

 pressure, it is evident that the difference 

 between this column and that suspended 

 in the gauge E G of the pump will be 

 the exact measure of the pressure of the 

 rarefied air remaining in the receiver. 



(35.) By a gauge of this form (and 

 it is the usual one for standing air- 

 pumps) the elastic force of the rare- 

 fied air can only be known by com- 

 parison with a barometer. A' gauge 

 might, however, be very easily con- 

 structed which would give the amount 

 of the pressure immediately. Let G 

 (Jig. 17.) be a cistern containing mercury, 

 and closed at the top, communicating 



fig. 17. 



with the exhausting tube AB, or in any 

 other convenient manner with the re- 

 ceiver. In the top of this let a barome- 

 ter-tube filled with mercury be inserted, 

 so as to be air-tight at T. The atmos- 

 pheric pressure at F sustains as usual 

 the barometric column. But upon rare- 



fying the air in the receiver by means of 

 the pump, this pressure will be dimi- 

 nished, and the mercury at E will ac- 

 cordingly fall, and the column which will 

 be sustained will measure the elastic 

 force of the rarefied air. 



Such a gauge, although simpler in 

 principle than the common one, would 

 not however be attended with the same 

 practical advantages. 



(36.) Such is the general theory of 

 the Air-pump. The varieties of its 

 construction are very considerable, and 

 it would not be consistent with our 

 plan to enter into details respecting 

 them. We shall, therefore, conclude 

 this chapter with a description of the 

 air-pump which is in most general use. 



A sectional drawing of this apparatus, 

 with some trifling transposition of parts 

 to bring them all into view, is given in 

 the annexed Jig. 18. R is the glass re- 

 ceiver placed on the pump-plate S S, 

 T T T is the exhausting tube communi- 

 cating with two pump barrels B B', and 

 furnished with a cock C by which the 

 communication between the receiver 

 and barrels may be cut off at pleasure. 



V V are parchment valves in the bot- 

 toms of the pump-ban-els, opening up- 

 wards, so that air may pass through 

 them from the tube T T, into the barrel, 

 but cannot return. 



P P' are two pistons, fitting, air-tight, 

 in the ban-els, and furnished with valves 

 similar to V V, which also open up- 

 wards. The piston-rods are furnished 

 with racks E E', which are wrought by 

 a toothed wheel W. This wheel is 

 turned, in the usual way, by a winch D, 

 and by alternately turning ft in opposite 

 directions, the pistons are elevated and 

 depressed. 



G is the barometer gauge, communi- 

 cating with the receiver at H. 



Let us now suppose the piston P 

 ascending, and P descending. Since 

 the valve in P opens upwards, no air 

 can pass from above through it ; as it 

 ascends, therefore, the air in P V, ex- 

 panding into the space deserted by the 

 piston, becomes rarefied, and presses 

 with diminished force on the valve V. 

 The superior elasticity of the air in the 

 receiver and tube will force open the 

 valve V, and will continue to pass 

 through that valve until its elasticity 

 exceeds that of the air in the barrel 

 P V, by a force less than that which is 

 requisite to raise the valve V. 



In the meantime the piston P' has 

 been descending, and the air in P' V is 

 c 



