PUMICE 



PUMP 



Though many stories are told of the treachery 

 of the puma, and of its habit of springing on 

 rs-by from trees, in reality it seems to be 

 less ferocious than other wild cats and to be 

 reluctant to attack man. Ranchers regard it as 

 a pest because in attacking a sheepfold it is 

 not content with killing one sheep, but must 

 make way with a hundred or more. 



PUMICE, pum'is, a spongy rock, so light 

 that it floats on water, used for polishing wood, 

 metals, ivory and other articles. It may be 

 gray, white or some shade of brown. It is a 

 sort of lava (which see) and contains more or 

 less glass in fine particles, which fact gives the 

 stone its value for polishing surfaces. 



PUMP, one of the common devices by which 

 man utilizes simple laws of physics and com- 

 pels the forces of nature to do' part of his 

 work for him. Not all pumps operate accord- 



THE LIFT PUMP 



In the figure c is the cylinder ; p, the piston ; 

 i, the intake valve ; o, the outlet valve. 



In the first position the piston is being: raised. 

 The outlet valve is air-tight, and all the air is 

 being forced out through the spout. 



The pressure of the water raises the intake 

 valve, and water takes the place of the air re- 



IMMV.-rl. 



In the second position the piston is being low- 

 ered through the water which has entered. The 

 ure now opens the outlet valve and clones 

 tin- intake valve, preventing the return of the 

 water to the cistern below. 



In the last poult Ion the water is being forced 

 out an the air was In the first instance, and more 

 water is entering. 



ing to the same principle. The suction, or 

 lift, pump, the centrifugal pump and thr tur- 

 l)in pump all depend upon different laws; tin- 

 force pump is a suction pump with an addi- 

 tional feature. 



The Lift Pump. This is the common kit dim 

 or wi II pump, so indispensable in rural com- 

 munities. Before Galileo's time scholars thought 

 that the suction of such a pump was due to 

 nature's refusal to permit a vacuum, but Gali- 

 lat<> in the sixteenth century, notiod tint 

 tin- pump would not raise water much 



than thirty feet, and correctly attributed its 

 workings to the fact that air has weight. If 

 in the pump shown in the illustration the cylin- 

 der and the pipe leading to it from below 

 were emptied of air, there would be no pres- 

 sure on the water under the mouth of the pipe. 

 But air would still be weighing on the water 

 around it, and since pressure on a liquid is 

 transmitted through it in all directions, water 

 would be pressed upward into the pipe. How 

 the pump is constructed so as to take advan- 

 tage of this law is shown in the illustration. 

 The labor which the pumper performs merely 

 lifts the water to the spout after nature has 

 raised it to the intake valve. 



Sometimes a pump refuses to work until it 

 has been primed, by having water poured into 

 it. This is because its valves are not air-tight 

 unless they are wet. 



The Force Pump. The intake valve of a suc- 

 tion pump can 

 never be more 

 than thirty-three 

 feet above the 

 water in the cis- 

 tern (see BAROME- 

 TER), and because 

 of the imperfect 

 valves of most 

 pumps this limit 

 is reduced in prac- 

 tice to about 

 twenty-seven feet. 

 But above the in- 

 takr valve the 

 water is raised by 

 the force exerted 



on the piston so 

 .. A , . 



if the spout is 



turned upward pump emptleg tne cyl _ 



the only limit to inder of air in the same fnsh- 



u u \L i- * on aa the "^ pump, but on 



the height lies m the return stroke the wnt-r. 



tlii* fnrr. instead of being admitted 



w - through a valve in the piston. 



The principle of l forced through an oponlnK 



at the side and past the outlet 



the force pump is valve. 



thorr.fr.ro K Tne alr dome serves to 

 tnereiore the malntaln the now between the 



same as that of pressure strokes. When the 

 ...... . . water Is being forced through 



Utt lilt pump, but the outlet valve it comprewen 



,,n,ifhor I..M.- rtf the air In the dome; as soon 

 another law of R8 the ptston 8tarta to wltn . 



physics is com- draw the outlet valve closes 



and the compressed air ex- 



monly utilized to panda, driving more water 



gain smoothness mto the outlet plpe ' 

 in operation. This is the law that comprecd 

 air always seeks to expand. How this helps to 

 flow is shown in the second 



THE FORCE PUMP 

 In the figure d is the air 

 dome ; o, the outlet valve ; i, 

 the intake valve; a, the air 



a constant 

 ration. 



