A.—­By means of a balanced buoy or float.  This float is attached to a rod, which in its turn is attached to a lever set on the top of a large upright pipe.  The upper part of the pipe is widened out into a small cistern, through a short pipe in the middle of which a chain passes to the damper; but any water emptied into this small cistern cannot pass into the pipe, except through a small valve fixed to the lever to which the rod is attached.  The water for replenishing the boiler is pumped into the small cistern on the top of the pipe; and it follows from these arrangements that when the buoy falls, the rod opens the small valve and allows the feed water to enter the pipe, which communicates with the water in the boiler; whereas, when the buoy rises, the feed cannot enter the pipe, and it has, therefore, to run to waste through an overflow pipe provided for the purpose.

89. Q.—­How is the strength of the fire regulated?

A.—­The draught through the furnaces of land boilers is regulated by a plate of metal or a damper, as it is called, which slides like a sluice up and down in the flue, and this damper is closed more or less when the intensity of the fire has to be moderated.  In wagon boilers this is generally accomplished by self-acting mechanism.  In the small cistern pipe, which is called a stand pipe, the water rises up to a height proportional to the pressure of the steam, and the surface of the water in this pipe will rise or fall with the fluctuations in the pressure of the steam.  In this pipe a float is placed, which communicates by means of a chain with the damper.  If the pressure of the steam rises, the float will be raised and the damper closed, whereas, if the pressure in the boiler falls, the reverse of this action will take place.

[Illustration:  Fig. 4.]

[Illustration:  Fig. 5.]

90. Q.—­Are all land boilers of the same construction as that which you have just described?

A.—­No; many land boilers are now made of a cylindrical form, with one or two internal flues in which the furnace is placed.  A boiler of this kind is represented in Figs. 4 and 5, and which is the species of boiler principally used in Cornwall.  In this boiler a large internal cylinder or flue runs from end to end.  In the fore part of this cylinder the furnace is placed, and behind the furnace a large tube filled with water extends to the end of the boiler.  This internal tube is connected to the bottom part of the boiler by a copper pipe standing vertically immediately behind the furnace bridge, and to the top part of the boiler by a bent copper pipe which stands in a vertical position near the end of the boiler.  The smoke, after passing through the central flue, circulates round the sides and beneath the bottom of the boiler before its final escape into the chimney.  The boiler is carefully covered over to prevent the dispersion of the heat.

[Illustration:  Fig. 6]