ARTIFICIAL HEAT AND APPARATUS 45 
hermetically sealed glass bottles, the generated heat is 
voustautly passing off through the crevices, more particu- 
larly those in the upper part of the roof, and the moisture 
along with it, The main difference, then, between the 
two is, that one only raises 8 the temperature, while the 
that escape through the joints and the bricks, in quantity 
according to the good or bad workmanship, or the porosity 
of the material. “Smoke has thin shoulders,” is an old 
adage among gardeners, and we often find it verified in 
the use of flues. Notwithstanding these disadvantages, 
good grapes may be forced by flue heat, and in some casez 
this is the only means at command; when 50, it is advisa- 
ble to make the best arrangement we can. If a steady 
continuous heat be required, there ought to be enougk 
room in the furnace to hold a large bulk of fuel, and if it 
is situated somewhat below the level of the flue, the heat 
will flow more evenly, and disseminate itself more equally, 
as it travels along with the draft, by which a greater por- 
tion will be absorbed by the bricks, and given off into the 
house. A small furnace, with a quick draft, is never eco- 
nomical, as a considerable portion of heat passes off by the 
chimney and is lost. A narrow and small flue is also ob- 
jectionable, as the heat acts directly upon the mason-work, 
and the bricks become so hot as to abstract the oxygen of 
the air, rendering it so devoid of moisture as not to be fit 
for the leaves to respire in. To a certain extent, this evil 
may be counteracted, by keeping shallow vessels filled 
with water upon the flue, and damping the floor of the 
house; but most of the moisture produced in this way is — 
in a vaporous state, and not in the exact solution required 
= by the absorbing stomata of plants. The analogy be 
