19 
Steam Engine. 
indeed, this may be accomplished many ways in a greater 
or lesser degree, by letting in a definite quantity, directed 
in such a manner as to come in contact with the fuel, 
though perhaps in no way better than the methods explain- 
ed ; to make this furnace more complete, doors are placed 
to the front, witli a sufficient space below them to let air 
into the furnace, these doors are represented by S S, Fig, 
8, as standing open ; they are kept shut wffiile the furnace 
is going, except when coals are put in, or the bars cleans- 
ed, or the like when they are opened for that purpose ; 
the use of these doors is to prevent, in a great measure, 
the radient heat that comes from under the bars and from 
the opening at the foreside from escaping ; this also adds 
something to the draught or well-going of the furnace, 
as this heat when suffered to escape rarifies the air before 
the furnace, which air passes upwards, performing in a small 
degree that wffiich a chimney does, and depriving the fire 
of a portion of air which it otherwise has when the doors 
are shut ; these furnaces are made of various sizes, accord- 
ing to the uses they are applied to ; in some large ones 
there is a division in the middle of the hopper, as in Fig« 
8 ; this size, as measured by the scale, will serve a steam- 
engine boiler of about nine or ten feet diameter ; lesser 
furnaces do not need this division in the middle of the 
hopper. In witness whereof, &c. 
Of Steam and the Steam Engine. 
Before I enter directly upon the subject of the steam 
engine, it will be necessary that the reader should fully 
comprehend the doctrines of fluidity, evaporation, latent 
heat and specific heat ; and also the facts ascertained res-- 
pecting the expansive force of steam. The first of the 
following extracts, which with great brevity and great 
dearness gives a summary of the doctrine of heat or calo- 
