324 APPLIED SCIENCE 



and a slow fire, although the former arrangement is the more 

 economical. 



An economical manner of banking fires is to push the live 

 coals back against the bridge wall, leaving the forward part 

 of the grates covered with ashes only, then covering the live 

 coals with a moderately thick layer of fresh coal. Fine coal 

 is preferable as the air does not readily pass through it, es- 

 pecially when the draught is diminished by closing the damper; 

 this should be done just before covering the fire with fresh 

 coal. The damper should be left open a very little to avoid 

 the accumulation of gas in the furnace and the possibility 

 of an explosion. This method of banking fires saves much 

 time when preparing to start again. The grates may be 

 quite thoroughly cleaned without disturbing the low fire at 

 the bridge wall. 



In case the water level becomes dangerously low, the fire 

 should be drawn immediately. The engine should continue 

 to run, and water should not enter the boiler in any quantity. 

 When the furnace has cooled down to about the same tem- 

 perature as the boiler, the water level may be raised very 

 gradually until water appears in the glass. The boiler may 

 then be filled more rapidly and 'the fire started. 



365. Temperatures of Steam. After steam has been once 

 generated, the temperature remains constant, and the latent 

 heat, not observable by the thermometer, is absorbed. The 

 temperature of steam in contact with the water from which 

 it is generated depends upon the pressure. If the vessel is 

 closed, as in boilers, the pressure becomes greater and raises 

 the boiling point of the water. Steam under pressure and 

 confined has considerable energy due to heat, which is 

 measured, as already noted, by the heat unit, B. T. U. 



