AIR GAUGES 



391 



but not in service, lap, or emergency positions. The other 

 gauge connection T, a part of the right-hand gauge, is piped 

 to the connection R of the brake valve, so that this hand, colored 

 red, indicates main-res- 

 ervoir pressure. 



An inside view of the 

 air gauge is shown in 

 view (&), in which A 

 and B are two bent 

 tubes of elliptic shape, 

 as shown in (d). The 

 tube A is connected to 

 the fitting M, and the 

 tube B to the fitting T. 

 The bottom ends of the 

 tubes are held fast and 

 the top ends are closed 

 and free. The action 

 of the gauge may be 

 thus explained: If a 

 tube of elliptic section 

 is bent, as ' shown in 

 view (&), and then sub- 

 jected to an internal 

 pressure, the force 

 exerted will tend to 

 straighten the tube, be- 

 cause the force exerted 

 within the tube tends 

 to make it assume the 

 circular form shown 

 dotted in view (c). In 

 assuming the circular 

 form, the concave side 

 a of the bent tube tends 

 to lengthen, while the 



convex side 6 tends to shorten. These combined efforts tend 

 to straighten out the tube, and therefore impart a movement 

 to its free end. 



