THE BOILER. 85 



valve, (to prevent the pressure from reaching a dangerous 

 height), and a "blow-off" valve, (for draining the boiler). 

 A boiler is usually fitted also with a whistle for signaling, 

 and a blower for forcing the draft. The water feeders, water 

 glass and gage cocks have been treated under the ".Feed 

 Water" in Chapter II. 



The 'Steam Gage. The steam 

 gage indicates the steam pressure 

 in the boiler in pounds per square 

 inch. The cut shows the interior 

 of the gage used on Case engines. 

 The curved tube or Bourdon 

 spring has an oval cross section, 

 and when exposed to pressure 



FIG. 27. INTERIOR OF GAGE. from the inside? tends to straight- 

 en, as a hose will do when under water pressure. The free 

 end of the Bourdon tube is connected to the pointer by means 

 of a segment lever and pinion so that the pointer, which is on 

 the same shaft as the pinion, revolves, indicating on the dial 

 the pressure on the inside of the tube, which is the same as 

 that in the boiler. In order to prevent the temper of the tube 

 from being injured by hot steam, a siphon, which con- 

 denses the steam and keeps the tube filled with water, 

 is placed between the gage and the boiler. The sectional 

 view of the siphon shows a loose cap over the pipe, 

 which extends into the globe chamber ; this deflects the 



