INTRODUCTION TO PNEUMATICS. 



the piston, which, opening upwards, admits the water to rise through it, 

 but prevents its returning ; and y a similar valve in the body of the 

 pump. When the pump is in a state of inaction, the two valves are closed 

 by their own weight ; but when, by drawing down the handle of the 

 pump, the piston ascends, it raises a column of air which rested upon it, 

 and produces a vacuum between the piston and the lower valve, Y: the 

 air beneath this valve, which is immediately over the surface of the water, 

 consequently expands, and forces its way through it; the water then, 

 relieved from the pressure of the air, ascends into the pump. A few 

 strokes of the handle totally exclude the air from the body of the pump, 

 and fill it with water, which, having passed through both the valves, 

 flows out at the spout. Thus the air and the water successively rise in 

 the pump on the same principle that the mercury rises in the barometer. 

 Water is said to be drawn up into a pump by suction ; but the power of 

 suction is no other than that of producing a vacuum over one part of the 

 liquid, into which vacuum the liquid is forced by the pressure of the 

 atmosphere on another part. . v The action of sucking through a straw con- 

 sists in drawing in and confining the breath, so as to produce a vacuum, 

 3>r at least to lessen materially the quantity of air, in the mouth : in con- 

 sequence of which, the air within the straw rushes into the mouth, and is 

 followed by the liquid, into which the lower end of the straw is immersed. 

 The principle is the same ; and the only difference consists in the mode of 

 producing a vacuum. In suction, the muscular powers answer the pur- 



Fig. 2. Fig. 3. 



pose of the piston and valves. The distance from the level of the water 

 in the well to the valve in the piston ought not to exceed thirty-two 

 feet, otherwise the water would not be sure to rise through that valve, 

 for the weight of the air is sometimes not sufficient to raise a column of 

 mercury more than twenty-eight inches, or a column of water much more 

 than thirty-two feet ; but when once it has passed that opening, it is no 

 longer the pressure of air on the reservoir which makes it ascend it is 

 raised by lifting it up, as you would raise it in a bucket, of which the 



