352 
Steam Engine. 
fuel ; and when the vertical sides of the body of the boilei , 
and its upper part are suitably enveloped, in order to pre- 
vent the loss of heat by these surfaces, this apparatus may 
be employed with much advantage in all cases where it is 
required to boil water for procuring steam. 
And as in the case where the boiler is constructed on a 
great seal , the seven tubes that descend from the bottom 
of the boiler into the fire may be made of cast iron, whilst 
the body of the boiler is composed of sheet iron, or sheet 
copper ; it is certain that a boiler of this kind, sufficiently 
large for a steam engine, a dyeing house, or a spirit dis= 
tillery, would cost much less than a boiler of the usual 
form, of equal surface and power. 
But in all cases where it is required to produce a great 
quantity of steam, it will be always preferable to employ 
several boilers of a middling size, placed beside each other, 
and heated each by a separate fire, instead of using one 
large boiler heated by one fire. 
I have shewn, in my sixth essay, on the management 
of fire, and the oeconomy of fuel, that beyond a certain 
limit, there is no advantage derived from augmenting the 
capacity of a boiler. 
It will be perceived, that the boiler which I have the 
honour of presenting to this society, is of a form fit for 
being placed in a portative furnace, and it was actually in« 
tended for that purpose. 
Its furnace, which is made of bricks, with a circular 
iron grate of six inches in diameter, is built in the inside 
of a cylinder of sheet iron, seventeen inches in diameter, 
and three feet high, and can be easily transported from 
place to place, by two men. 
This cylinder of sheet iron, which is divided into two 
parts, in order to facilitate the construction of the masonry 
weighs only forty- six pounds. The masonry weighs 
