CKEAM. 287 



creased wear and tear, but more often resulting in an explosion 

 of a more or less serious nature. 



Water-glass ajid Water-gauges. — Every boiler should have 

 three water-gauges in addition to a water-glass ; these are 

 usually attached to a hollow cast-iron cylinder or tube con- 

 nected with the water and steam spaces. 



The water-glass should be blown out daily, and, if clogged, 

 can be safely cleaned with a bent wire. 



In no case should the water glass alone be depended upon to 

 indicate the water level. 



Steam-gauge. — Each boiler should be provided with a steam- 

 gauge, which gauge should be directly connected with it. 



Safety-valve. — Every boiler should be provided with a safety- 

 valve having direct communication with the steam space, and 

 there should, moreover, be an intervening valve. Some of 

 the most disastrous explosions can be traced to faulty ar- 

 rangement in this respect. The valve thoughtlessly left 

 closed after cleaning or repairs prevents the safety-valve 

 from relieving the pressure when it rises above the safe 

 working pressure of the boiler. 



Safety-valves are of two kinds : spring and lever safety- 

 valves. Of the two valves the lever-valve has the most dis- 

 advantages, one of the most important being the ease with 

 which it may be made useless by adding an additional weight 

 to that already provided, in order to keep the valve on its seat, 

 and therefore greatly increasing the pressure at which it will 

 blow off. 



A safety-valve should be raised each day by hand so as to 

 allow steam to escape; this prevents clogging and rusting. 



The dealer will usually set the spring-valve so that it will 

 blow off at the desired pressure. It can be adjusted, however, 

 by loosening or tightening a screw provided for that purpose. 



The lever-valve may be set v,'ith the aid of the following 

 formula: 



_ bPA — Vb — w: 



7f ' 



/— distance from weight to fulcrum; 

 b ^ " *' valve centre to fulcrum; 



c = distance from the centre of gravity of the lever of the ful- 

 crum; 



