The advocates of [Fig. 107] claim that the office of the flame is to get into as close contact with the bottom as possible, and this form compels the flame to do so. In burning soft coal this form is found to soot up the bottom of the boiler very badly.
[Fig. 108] is followed more extensively than any other, the variations being the depth of chamber; with depth generally from 36 to 40 inches.
[Fig. 109] has nothing to commend it, except in cases where bridge is too low.
[Fig. 110] is followed a great deal and gives very good satisfaction. This form allows for the theory of combustion, namely, the expansion of the gases after leaving bridge wall.
Space behind the bridge wall should be enlarged, as it will reduce the velocity of fire gases, and thus have them give up more of their heat to the boiler.
The bridge wall should not be less than 18 inches at bottom, but may be tapered off toward top to 9 or 13 inches.
Setting of Water Tube Boilers.
On page 67, [Fig. 26], is exhibited a steam boiler with inclined tubes. The setting in this style of boilers is as follows:
A brick wall is laid for the front with suitable openings for the doors of the furnace and ash pit, and protected on the outside by a front of cast iron, and on the inside by a lining of fire brick.