16 Steam Engine, 
formerly stated, together with the form of the furnace, and 
its variations after-mentioned ; the success of the furnace 
depends also upon a considerable degree on what is called 
the draught of the furnace; that is, the chimney and flues 
so constructed, as a sufficient current of air may pass 
tlirough the fire to bring it to a proper degree of heat ; al- 
so, that the current of fresh air may have such force as to 
come pretty much in contact with the burning fuel, and 
to convey the smoke along with it through the hottest of 
the flame ; if this is not the case, the smoke will not 
be so completely consumed in these furnaces. The 
hopper is allowed to be kept as full of coal as possible, 
and either wholly or in part small coal, so as to prevent 
air as much as possible getting in by that passage ; this 
must be attended to when the furnace is in its ordinary 
working state ; yet, sometimes, it is necessary to keep 
this opening of the hopper, either wholly, or in part, open, 
when there is little heat wanted. 
Description and reference to the draughts. 
Fig. 7, is a side view, or section of the furnace, as if ap- 
plied to a boiler, and cut through the middle. Fig. 8, is 
a front view of the furnace in perspective, the hopper hav- 
ing a division in the middle, and two regulating plates for 
the admission of fresh air. Figs. 7 and 8, A, the hopper 
into which the coals are put to supply the furnace, a the 
regulating plate ; above which and below, the higher plate 
^ of the hopper, a stream of fresh air is admitted to this 
plate ; a is made to turn on a pin pivot or centre, so that 
it may be brought nearer to, or recede further from the 
uppermost at the point n, Fig. 7, for regulating the quan- 
tity of air admitted betwixt the plate as occasion requires ; 
this plate is sometimes made fast, or kept so, after adopt- 
ing a space which is judged sufficient for the quantity of 
fresh air but on other occasions, as in steam engines, it is 
