10 
THEORY OF HEAT AND ITS PRACTICAL APPLICATION. 
private gentlemen. They resemble two glass argand-lamp burners of different 
breadths, the smaller placed within the larger. Thus, supposing the small inner 
cylinder to be eight inches wide in the clear, and the outer one twelve inches, a 
space (allowing for the thickness of the metal) of two to three inches remains, and 
this space contains the water. The two cylinders are united at top and at bottom 
by rims, either brazed, if of copper, or by process of casting in the mould, if of 
iron. The bottom of the inner cylinder rests upon a case, into which slides a sort 
of gridiron grating, on which the fire rests. An exit, or flow-pipe, passes from the 
outer cylinder, half an inch below its rim, which space is therefore occupied by a 
thin stratum of air, that, by expanding under the force of heat, propels the water 
into the pipes or channels, round which it circulates, till it arrives at the return- 
pipe ; this enters the boiler near its bottom by a rapid and steep inclination, thus 
insuring the flow of the water by the conjoint agency of atmospheric pressure and 
gravitation. 
This simple machinery is excellent in its capabilities, but is susceptible of im- 
provement, as we shall now endeavour to prove ; but, in justice, it must be under- 
stood that there are no et ceteras in the charges ; a distinct calculation can be made 
from the first ; and a man, therefore, knows, to within a few shillings, what 
expenditure he must incur : a very important consideration this, since it would be 
easy to show from evidence that a party who has been induced to purchase a new- 
fashioned boiler of small dimensions, very specious at first sight, at the cost of five 
or six pounds, found himself, in the long run, charged above forty pounds, ere all 
the adjuncts were properly adapted, and in the very first winter an entire crop of 
grapes was lost in consequence of the utter uncertainty of the fire, which, if 
minutely attended to, raged on to the temperature of 100° to 120°, or died out at 
once, if not continually fed, at a loss (during frost) of 80°. 
The vice of our cylinder furnaces is, that they require coke as fuel, and this implies 
undeviating attention. There is, however, a boiler of new construction announced, 
on which the fire acts within and all round the outside of the case of water. At 
a future period we may be enabled to notice the machinery more particularly ; but 
for the present will endeavour to describe a furnace which will work both flue and 
water channels by the aid of the commonest and least expensive fuel, and it is one 
that any gentleman or grower for the market can have constructed under his own 
superintendence, by an iron founder and intelligent labourer who knows how 
to handle a trowel with precision. 
The model was suggested by inspecting two cast iron furnaces, with double 
doors and grating corresponding in appearance with the common brick ovens used 
in forcing houses. Their original cost was 10/. each ; but they had already gone 
through the work of vinery-forcing for twenty years without repair. Now, upon 
this principle, let the boiler be a cast-iron oven, but instead of being single, let it 
be cased, or double on the two sides and top, leaving a space in front for the door, 
and another at the further end for the exit of the smoke. 
