[Illustration:  Fig. 3]

85. Q.—­This species of boiler has not an internal furnace, but is set in brickwork, in which the furnace is formed?

A.—­Precisely so.  The general arrangement and configuration will be at once understood by a reference to the annexed figure (fig. 3), which is a transverse section of a wagon boiler.  The line b represents the top of the grate or fire bars, which slope downward from the front at an angle of about 25 deg., giving the fuel a tendency to move toward the back of the grate.  The supply of air ascends from the ash pit through the grate bars, and the flame passes over a low wall or bridge, and traverses the bottom of the boiler.  The smoke rises up at the back of the boiler, and proceeds through the flue F along one side to the front, and returns along the other side of the boiler, and then ascends the chimney.  The performance of this course by the smoke is what is termed a wheel draught, as the smoke wheels once round the boiler, and then ascends the chimney.

86. Q.—­Is the performance of this course by the smoke universal in wagon boilers?

A.—­No; such boilers sometimes have what is termed a split draught.  The smoke and flame, when they reach the end of the boiler, pass in this case through an iron flue or tube, reaching from end to end of the boiler; and on arriving at the front of the boiler, the smoke splits or separates—­one half passing through a flue on the one side of the boiler, and the other half passing through a flue on the other side of the boiler—­both of these flues having their debouch in the chimney.

87. Q.—­What are the appliances usually connected with a wagon boiler?

A.—­On the top of the boiler, near the front, is a short cylinder, with a lid secured by bolts.  This is the manhole door, the purpose of which is to enable a man to get into the inside of the boiler when necessary for inspection and repair.  On the top of this door is a small valve opening downward, called the atmospheric valve.  The intention of this valve is to prevent a vacuum from being formed accidentally in the boiler, which might collapse it; for if the pressure in the boiler subsides to a point materially below the pressure of the atmosphere, the valve will open and allow air to get in.  A bent pipe, which rises up from the top of the boiler, immediately behind the position of the manhole, is the steam pipe for conducting the steam to the engine; and a bent pipe which ascends from the top of the boiler, at the back end, is the waste-steam pipe for conducting away the steam, which escapes through the safety valve.  This valve is set in a chest, standing on the top of the boiler, at the foot of the waste-steam pipe, and it is loaded with iron or leaden weights to a point answerable to the intended pressure of the steam.

88. Q.—­How is the proper level of the water in the boiler maintained?