DEVICES USING AIR PRESSURE 



plants. Many fire engines are equipped with force 

 pumps that make it possible to fight fires in high 

 buildings. 



How do lift and force pumps work? The experi- 

 ment has taught you that as the piston A of the lift 

 pump in Figure 17 moves upward, the pressure in 

 the cylinder under it will be lowered below that of the 

 atmosphere pressing on the water in the well W . This 

 greater pressure on the surface of the water in the 

 well will push some of the water up into the cylinder 

 C of the pump, forcing the foot valve F open as it 

 enters, On the down stroke of the piston, valve F 



FIG. 18. SIMPLE FORCE PUMP FIG. 19. AIR DOME FORCE PUMP 



will close, preventing the water from running back 

 into the well. The piston valve V will be forced open 

 as it pushes against the water in the cylinder and 

 will allow much of it to pass into the upper part of 

 the cylinder B. On the next up stroke of the piston 

 this water will be raised to where it can run out of 

 spout S while at the same time more water from the 

 well is being pushed past the valve F. The foot valve 

 F, which is made of leather, often dries out, as does 

 the leather part of the piston. This prevents the re- 

 duction of pressure under the piston until the pump 

 is "primed" by pouring water into the top, thus moist- 

 ening the leather parts. 



The simplest kind of force pump differs from the 

 lift pump only by lacking the valve in the piston and 

 in having an opening in the bottom of the cylinder 

 which is closed by valve F as shown in Figure 18. 

 This valve opens outward. When the piston goes up, 

 water is pushed from the well past the foot valve into 

 the cylinder C of the pump. This is exactly the same 

 as the lift pump. When the piston goes down, the 

 water in the cylinder is forced through the valve F 

 and delivered through a pipe to an upper floor. This 

 simple force pump delivers water only on the down 

 stroke of the piston, thus causing the liquid to spurt 

 out of the opening in jerks. This is not desirable; 

 for example, in fighting fires if the water came only 

 in spurts it would prove of little use. It is possible to 



secure an even flow of water from a force pump by 

 adding a very simple part to the one described above. 

 An air dome is placed in the delivery pipe and the 

 water comes in a steady stream. The water gets into 

 the cylinder C (Fig. 19) just as in the lift pump and 

 simple force pump. It is also forced out of the cylinder 

 on the down stroke of the piston P. As it leaves the 

 cylinder C it forces valve F open and enters the air 

 dome or chamber A, The air is compressed as the 

 water enters and acts as a cushion or spring. During 

 this stroke water is also forced out of the opening 

 D. As the piston begins its up stroke the pressure is 

 removed from the air in the dome and it tends to ex- 

 pand as does a spring which has been coiled under 

 force. It pushes against the water in the chamber A ; 

 since the water cannot get past valve F it is pushed 

 out of the spout at D. You will see then that on the 

 down stroke of the piston water is pushed out of the 

 spout by the force of the piston while on the up stroke 

 it is pushed out by the force of the expanding air in 

 the air chamber. This makes an even flow on both 

 strokes of the piston. Have you ever noticed the air 

 dome on a fire engine force pump or on a pump about 



the farm? _ 



~~How doeTan automobile or bicycle 

 pump work? In the early days of the 

 automobile one of the most important 

 tools in the tool box was the tire 

 pump. It was almost as common to 

 see people stranded by the side of the 

 road mending and pumping up a tire 

 as it was to see them motoring along. 

 This has changed, thanks to the im- 

 provements which have been made in 

 the quality of tires. The air pump, 

 nevertheless, is still an important 

 part of automobile equipment and is 

 carried by almost every motorist. 

 While bicycles are not so common as 

 they were at one time, they still must 

 use compressed air in their tires, and 

 one of the simplest ways of getting it is with the 

 hand air pump. 



This pump is interesting in its construction and 

 operation. It has a cylinder enclosing a piston which 

 moves up and down inside. This piston is a flexible 

 leather circle cut a little larger than the cylinder. This 

 allows the air to get past the piston on the up stroke 

 but prevents it from doing so on the down stroke 

 as the leather then is forced tightly against the cyl- 

 inder walls by the air which is below it. This air is 

 compressed as the piston comes down and is forced 

 from the delivery hose past the opening D. The tire 

 valve prevents the air in a tire from escaping back 

 into the pump. 



FIG. 20. CROSS 

 SECTION OF BI- 

 CYCLE PUMP 



