THE STEAM-ENGINE. 



497 



Fig. 54. 



The door by which fuel is introduced upon the grate is represented at A, 

 and the door leading to the ash-pit at B. The fire-bars at C slope downward 

 from the front at an angle of about 25, giving a tendency to the fuel to move 

 from the front toward the back of the grate. The ashrpit D is constructed of 

 such a magnitude, form, and depth, as to admit a current of atmospheric air to 

 the grate-bars, sufficient to sustain the combustion. The form of the ash-pit is 

 usually wide below, contracting toward the top. 



The fuel when introduced at the fire-door A, should be laid on that part of 

 the grate nearest to the fire-door, called the dead plates : there it is submitted 

 to the process of coking, by which the gases and volatile matter which it con- 

 tains are expelled, and being carried by a current of air, admitted through small 

 apertures in the fire-door over the burning fuel in the hinder part of the grate, 

 they are burnt. When the fuel in front of the grate has been thus coked, it is 

 pushed back, and a fresh feed introduced in front. The coal thus pushed back 

 soon becomes vividly ignited, and by continuing this process, the fuel spread 

 over the grate is maintained in the most active state of combustion at the 

 hinder part of the grate. By such an arrangement, the smoke produced by 

 the combustion of the fuel may be burnt before it enters the flues. The flame 

 ana heated air proceeding from the burning fuel arising from the grate, and 

 rushing toward the back of the furnace, passes over the fire-bridge , and is 

 carried through the flue F which passes under the boiler. This flue (the 

 cross section of which is shown in fig. 54, by the dark shade put under the 

 boiler) is very nearly equal in width to the bottom of the boiler, the space at 

 the bottom of the boiler, near the corners, being only what is sufficient to give 

 the weight of the boiler support on the masonry forming the sides of the flue. 

 The bottom of the boiler being concave, the flame and heated air as they pass 

 along the flue rise to the upper part by the effects of their high temperature, 

 and lick the bottom of the boiler from the fire-bridge at E to the further end G. 



At G the flue arises to H, and turning to the side of the boiler at I 1, con- 



VOLi. II. 351 



