COMPRESSED AIR 



43 



Discharge 



valve. The pressure of the water in the discharge pipe compresses 

 the air in the air chamber. When, now, the piston is raised once 

 more, the discharge valve is closed by the pressure of the water in 

 the discharge pipe; yet the stream of water does not stop flowing, 

 because of the pressure of the air in the air chamber. The steam 

 fire-engine is a good example of a force pump. 



A rotary pump (Fig. 35) consists of a wheel with inclined blades. 

 The wheel is turned rapidly by an engine, and removes the air of a 

 pipe dipping into water. The 

 water is then raised by atmos- 

 pheric pressure into the case 

 in which the wheel revolves. 

 The blades of the wheel force 

 the water out through the dis- 

 charge pipe. 



Since the rotary pump has 

 no valves, it can pump water 

 carrying grit and dirt, mate- 

 rials which would clog a pump 

 with valves. This fact makes 

 it especially valuable for the 

 draining of swamp lands, 

 marshes, etc., and for pumping into canals and ditches the water 

 that is to be used in irrigation, the artificial watering of land. 



43. Compressed Air. While an ordinary pump re- 

 moves air from a given space, a compression pump (cf. 

 71) packs as much air as possible into a given space. A 

 bicycle or automobile tire pump is such a pump. 



When compressed air is released, it expands, and, like 

 other expanding gases, it can do work (cf. 24). Thus, 

 a sand blast is a current of air released from pressure, and 

 carrying sharp sand with great speed. If the sand strikes 

 glass, it chips its surface, forming ground glass. If part 

 of the glass is protected with wax, while the rest is exposed 



FIG. 35. 

 This rotary, or centrifugal, 



belt placed over the pulley. 



riven by a 



