PNEUMATICS. 



Fig. 31- 



WATER-PUMPS. 



The effect of atmospheric pressure on water 

 affords a convenient method of raising it above its 

 ordinary level ; this is 

 effected by pumps, which 

 may be termed both 

 hydraulic and pneumatic 

 machines. Fig. 31 repre- 

 sents the outline of a 

 common suction - pump. 

 It consists of a cylinder, 

 furnished with a piston, 

 A, made to fit air-tight. 

 In this piston there is a 

 valve opening upwards, 

 but here indicated as 

 closed. At the bottom 

 of the cylinder or barrel 

 there is another valve, B, 

 opening upwards ; and 

 from the bottom proceeds 

 a feeding-pipe, and dips 

 more or less into the 

 water. When the piston 

 is raised, the air under it 

 is rarefied more and more 

 at each stroke, and the 

 pressure of the air upon the surface of the water 

 on the outside of the pipe causes the water to rise 

 inside where the pressure is lessened. The valve 

 B is at the same time opened upwards, and the 

 water, after several strokes, rushes in above it. 

 When the upward stroke of the piston is complete, 

 it is again depressed the water passes through 

 the valve in the piston, and on the next stroke it 

 is discharged at the spout. 



In this form of pump, the greatest height to 

 which the water can be raised, counting from the 

 level of the water in the well to 

 the bottom valve, B, is, in theory, 

 34 feet. In practice, however, 

 owing to the imperfect vacuum, 

 the limit is usually from 20 to 28 

 feet But with the forcing-pump, 

 fig. 32, there is no such limit. 

 The mode of its action will be 

 understood from the accompany- 

 v r ing outline. A feeding-pipe con- 



nects the water and the barrel, as 

 p v in the suction-pump ; but the 

 piston, P, is solid, or without a 

 valve, and a pipe, called the 

 ascension-pipe, provided with a 

 valve opening from the barrel, 

 enters near the bottom. When 

 the piston is lifted, the valve, S, 

 closes, and the water rises through 



Fig. 32. 



she feeding-pipe and suction-valve, on the same 

 principle as in the suction-pump. The piston 

 being raised to the top of the barrel, and the space 

 below it being filled with water, it is now pressed 

 Uown ; the valve V closes, that at S opens, and 

 i he water is forced up the pipe to the required 

 height. 



The Fire-engine, The fire-engine is an applica- 

 uon of the force-pump. The principle of its 

 action, and its connection with the pumps, &c. will 

 be easily understood by the aid of the annexed 

 diagram (fig. 33), where a represents in section a 

 piston ascending, d the other piston descending, 



J the pipe or hose communicating with the 

 water-supply, g the hose 

 that conveys the issuing 

 stream to the fire, be the 

 level of the water in the 

 air-chamber, e the space 

 above filled with com- 

 pressed air. The rising 

 piston raises the water 

 from/to fill its cylinder ; 

 the descending piston 

 forces the water contained 

 in its cylinder into the 

 bottom of the air-chamber, 

 and thereby compresses 

 the air in e. The pistons 

 rise and descend alter- 

 nately. The compressed 

 air reacts by its elas- 

 ticity, and pressing upon Fig. 33. 

 the surface fa, forces the 



water through the hose g. In the space e, above 

 fa, the whole of the air that formerly filled the 

 chamber is supposed to be compressed. Assuming 

 | this to be one-third of its original bulk, its pressure 

 will be about 45 Ibs. to the square inch, and this 

 pressure will be continuous and nearly steady, if 

 the pumps act with sufficient force and rapidity to 

 keep the water at that level. 



The Syphon. The action of the syphon will be 

 readily understood from the principles already 

 illustrated. It is simply a 

 bent tube, with one end in- 

 serted in a liquid, as shewn 

 in fig. 34. To begin the 

 action, the air is withdrawn 

 from the tube by means of 

 the mouth or a syringe, 

 when the liquid enters and 

 fills it, as in other cases of 

 suction. When the suction 

 is stopped, there are two 

 forces acting on the liquid 

 in the tube the pressure Fig. 34. 



of the atmosphere on the 



liquid in the vessel, forcing it in the direction 

 from C towards B and A, and the pressure 

 at A forcing it in the opposite direction. These 

 two pressures are of themselves equal, and when 

 the lengths of the columns of liquid in the 

 two legs are equal, there is no flow ; but when 

 one column, as AB, is longer than the other, its 

 weight destroys the balance, and produces a flow 

 towards itself. 



Pressure of the Atmosphere on the Human 

 Body. The body of a man has, on an average, 

 a surface of about 2000 square inches. At the 

 rate, then, of fifteen pounds to the square inch, 

 the whole pressure which a person of an ordinary 

 size sustains on the surface of his body is 30,000 

 pounds, or nearly fourteen tons. It gives a wrong 

 notion of this pressure to speak of it as a load. It 

 has, on the contrary, a buoyant effect, and makes 

 a man lighter, or press less on the earth, than he 

 would without it. 



The reason why this compression is not felt, 

 and does not in effect squeeze the body into less 

 bulk, is, that there is an equal pressure outwards 

 at all parts. In fig. i, a force of a pound pressing 

 in the plug a, is felt as an equal force pressing out 

 the plug b. Now, the blood and other liquids in 



