if you can: if
not, get a few sash, at least. Don't put it off till next year; do it
now!

[Illustration: Fig. 6--The regular even-span type. A indicates a row of
pipe standards; BB, braces from these to the purlins. There is a fitting
made for the junction C.]

In the next chapter we will take up the handling of vegetables and
flowers in the small greenhouse. But don't be content to _read_ about
it. It's the pleasantest kind of _work_--try it yourself!

CHAPTER XXII

METHODS OF HEATING

In the foregoing chapter on homemade greenhouses very brief reference
was made to the various methods of heating. It will be well to
understand a little more in detail how to heat glass structures, as
temperature is, next to moisture, the most important factor of success.
If steam or hot water is used in the dwelling house and a greenhouse of
the lean-to type is used, the problem becomes a very simple one, as
additional pipes can be run through the greenhouse. But as this
advantage is not always ready to hand, we will consider the heating of
an isolated house, and the principles involved may be adapted to
individual needs. There are three systems of heating: flues (hot air),
hot water, and steam--the latter we need not take up as it is economical
only for larger structures than the amateur is likely to have.

[Illustration: Fig. 7--The best arrangement for heating a greenhouse by
hot air, is to run a brick or cement flue from the furnace around under
the benches and into the chimney over the fire. AA--storages space;
B--furnace; C--chimney; DDD--benches; E--furnace door.]

Heating by hot air carried through brick or tile flues is the simplest
and cheapest method for very small houses. The best way of constructing
such a system is illustrated in the diagram adjoining, which shows the
flue returning into the chimney (after traveling the length of the house
and back). This method does away with the greatest trouble with flue
heating--a poor draft; for immediately the fire is started, the air in
the chimney becomes heated, and rising, draws the hot air from the
furnace around through the flue with a forced draft. This forced draft
accomplishes three other good things: it does away with the escape of
noxious gases into the greenhouses, lessens the accumulation of moisture
and dust from wood smoke, and distributes the heat much more evenly
throughout the house. The furnace may be built of solid brick, with
doors and grates a