PNEUMATICS. 



21 



H* 



CHAPTER VI. OfPutnn* and Syphons 

 Air-balloon and Air-gun. 



I. The common Suction-Pump. 



(40.) The suction-pump consists of 

 two hollow cylinders AB, BC, (Jig. 23.) 

 placed one under the other, and com- 

 municating by a valve V Jig. 23. 

 which opens upwards. 

 The cylinder B C is called 

 the suction-tube, and has 

 its lower end C immersed 

 in the well or reservoir 

 from which the water is 

 to be raised. In the upper 

 cylinder, A B, a piston 

 P is moved, having a valve 

 in it which opens up- 

 \vards ; this piston should 

 move air-tight in the cy- 

 linder. At the top of the 

 cylinder A B is a spout S, 

 for the discharge of the 

 water. 



We will first suppose 

 the piston to be at the 

 bottom of the cylinder 

 AB, and in close contact 

 with the valve V. Upon 

 elevating it, the piston 

 valve is kept closed by the 

 atmospheric pressure, and 

 if the valve V were not 

 permitted to rise, a va- 

 cuum would be produced 

 between it and the piston, 

 the elevation of which 

 would then require a force equal to 

 about lolbs., multiplied by as many 

 square inches as are in the section of 

 the piston. But this is not the case. The 

 moment the piston begins to ascend, the 

 elasticity of the air in BC opens the valve 

 V, and the air rushing in through it, ba- 

 lances part of the atmospheric pressure 

 on the piston. Now, if the water at C 

 were not permitted to rise, the air be- 

 tween the piston and the surface C 

 would be rarefied by the ascent of the 

 piston. It would therefore press against 

 the low r er surface of the water with a 

 force less than the atmosphere. But 

 the entire force of the atmosphere 

 presses on the surface of the water in 

 the well. The diminished elasticity of 

 the air in the suction-pipe not being a 

 counterpoise for this, the water is ne- 

 cessarily pressed up into that pipe. Let 

 us consider to what height it will rise. 



If the surface of the water in the 

 suction-pipe rest at H, and rise no 

 higher, there is a compound column of 





air and water pressing on the level C ; 

 viz. the column of water C H, and the 

 elastic force of the air in BH. These 

 two together balance the atmospheric 

 pressure on the external surface of the 

 water in the well. It consequently fol- 

 lows, that the air in B H must be rare- 

 fied, since its elasticity falls short of the 

 atmospheric pressure by the pressure of 

 the column of water C H. As a column 

 of water, about thirty-four feet in height, 

 balances the atmosphere, it follows, that 

 the elastic force of the air in BH is 

 equal to the pressure of a column of 

 water whose height is the excess of 

 thirty-four feet above B H. 



Upon the descent of the piston, the 

 air compressed between it and the valve 

 V, escapes through the piston valve in 

 the same manner as we have described 

 in the air-pump ; and upon the suc- 

 ceeding ascent, the elastic force of the 

 air in B H, raising the valve V, passes 

 into the space in which the piston would 

 otherwise leave a vacuum. The air in 

 BH being thus rarefied, its elastic pres- 

 sure on the surface H of the water in the 

 suction-pipe is diminished ; and, there- 

 fore, when added to the pressure of the 

 column of water C H, is no longer 

 equivalent to the atmospheric pressure 

 on the external surface of the water in 

 the well. This pressure must therefore 

 force more water up in the suction-pipe, 

 and will continue to do so, until the 

 pressure of the increased column C H', 

 added to the elasticity of the air in 

 B H', is an exact balance for the atmos- 

 pheric pressure on the external surface. 

 Upon the principle already explained it 

 follows, that the elastic pressure of the 

 air in B H', is equal to the pressure of a 

 column of water, whose height is equal 

 to the excess of thirty-four feet above 

 the height C H'. 



While the water is rising in the suc- 

 tion-pipe B C, the machine is in fact an 

 air-pump, the suction-pipe itself acting 

 the part of receiver. The air which ori- 

 ginally filled the suction-pipe B C, is 

 gradually pumped out, and its place is 

 in part filled by the water which is 

 forced in by the pressure of the external 

 air. Now, upon the principles already 

 established, respecting the action of the 

 air-pump, it is quite apparent that a 

 perfect exhaustion can never be effected 

 in the suction-pipe B C ; and therefore 

 a column of water can never be raised, 

 whose pressure is equal to that of the 

 atmosphere; and hence we deduce a 

 consequence of the most vital import- 



