EVAPORATING POWER-FURNACE.
rendered compatible with great power of evaporation by expe-
dients which shall produce, in a small furnace, an extremely
intense combustion, and which shall ensure the transmission to
the water completely, and immediately, of the heat developed in
such combustion.
17. The heat developed in the combustion of fuel in a furnace
is propagated in two ways. A part radiates from the vivid fuel
in the manner, and according to nearly the same laws which
govern the radiation of light. These rays of heat, diverging in
every direction from burning fuel, strike upon all the surfaces
which surround the furnace. Now, as it is essential that they
should be transmitted immediately to the water in the boiler, it
follows that the furnace ought to be surrounded on every side
with a portion of the boiler containing water; in short, a hollow
casing of metal, filled with water, ought to surround the fire-
place. By this expedient, the heat radiating from the fuel, strik-
ing upon the metal which forms the inner surface of such casing,
will enter the water, and become efficient in producing
evaporation.
Whatever then be the particular form given to the engine, the furnace must be surrounded by such a casing. This casing is called the FIRE-BOX. The bottom of it is occupied by a grate, which should consist of bars sufficiently deep to prevent them from being fused by the fuel which rests upon them, having sufficient space between them to allow the air to enter so freely as to sustain the combustion, but not such as to allow the unburned fuel to fall through them.
18. The limited magnitude of locomotive boilers renders the construction of the extensive flues used in stationary boilers impracticable; and accordingly, in the early engines, a great waste of heat was occasioned, owing to the flame and heated air being permitted to issue into the chimney before their tempera- ture was sufficiently reduced by contact with the flues.
At length an admirable expedient was adopted which com- pletely attained the desired end. The boiler was traversed by a considerable number of small tubes of brass or copper, running parallel to each other from end to end, the furnace being at one end of the boiler, and the chimney at the other. The flame and heated air which passed from the furnace had no other issue to the chimney except through these tubes. It was thus driven, in a multitude of threads, through the water. The magnitude and number of the tubes was so regulated, that when the air arrived at the chimney, it had given out as much of its heat as was prac- ticable to the water. The full importance of this expedient was not appreciated until121
