=68. The Air Brake.=--Compressed air is used to do work in many machines, such as pneumatic drills, hammers, and air brakes. The Westinghouse air brake (Fig. 50) uses air at a pressure of about 70 lbs. to the square inch. The essential parts as shown are a reservoir _R_, the brake cylinder _C_ and a triple valve _V_, placed under each car with an air pipe _P_, leading to the engine. This is connected to _R_ by the triple valve _V_. When the pressure in _P_ is reduced by the engineer or by accident, the triple valve operates so as to admit air from _R_ into the cylinder _C_ pushing the piston _H_ to the left. _H_ is connected to the brakes by levers which press the brake shoes strongly against the wheels. When the air pressure in _P_ is restored the triple valve acts so as to permit the air in _C_ to escape while _R_ is filled again from _P_. The hissing sound heard when a train stops is caused by air escaping from cylinder _C_. The spring in _C_ keeps the brakes from the wheels except when the "air is on." [Illustration: FIG. 51.--Cross-section of a gas meter showing its construction and action.] =69. The Gas Meter.=--The gas meter consists of a box divided into two parts by a vertical partition (Fig. 51). Two bellows are attached to this partition, one on each side. The valves that regulate the flow of gas to and from the bellows and the chambers _A_ and _D_ are opened and closed by levers connected with the bellows. These levers also operate the hands upon the dials. When the inlet to the bellows _B_ is opened, the outlet of _A_ is also opened. Gas entering _B_ opens the bellows and forces the gas in _A_ out into the house-pipe _E_. When _B_ is full its inlet valve closes and its outlet valve opens. The inlet of _A_ also opens and its outlet closes. Gas now flows into _A_, compressing the bellows and _B_, and forcing the gas from it into the house-pipe. At each filling of the bellows _B_ there will be displaced from _A_ and forced into the house-pipe as much gas as enters _B_. It is evident that at each emptying of _B_ an equal amount of gas enters _A_. Thus we have _A_ and _B_ alternately filling and emptying as long as the gas burner is open. To have a continuous flow of gas in the house-pipes two pipes and two chambers are necessary, one being filled while the other is being emptied. Fig. 52 represents the dials upon a gas meter showing a reading of 54,600 cu. ft. [Illustration: FIG. 52.--Dials of a gas meter.] =70. Centrifugal Pumps.= Fluids, such as water and air, are often put in motion by devices called _centrifugal pumps_ (see Art. 78). These pumps contain a revolving part, like a wheel without a rim, whose spokes are replaced by thin blades. This revolving part resembles the paddle wheel of some steam boats and is enclosed in a case or cover having one opening at the rim and another opening on one side about the axle. [Illustration: FIG. 53.--A vacuum sweeper. (_Courtesy of the Hoover Suction Sweeper Co._)]