GENERAL REMARES ON HEATING. 147 
while there is also more risk of an explosion in frosty 
weather than under the low-pressure hot water system. 
Where the exhaust steam from engines of any kind is 
available, this may be very economically employed to heat 
any green- or forcing-houses, with good results, a few 
slight precautions being observed. It should, however, 
be borne in mind that the act of discharging the waste 
steam into such pipes, instead of directly into the 
atmosphere, is almost certain to cause a slight back-pressure 
in the cylinders of the engine. In heating by steam the 
pipes should be fixed with a slight downward inclination 
from the point where the steam enters them to the 
farthest extremity where the exit is, so as to allow the 
condensed water to run away freely. A comparatively 
small outlet pipe, or cock, placed at the farther end, 
leading into a drain, etc., will suffice to carry off the 
condensed water. 
Heating by means of hot air—of which the old-fashioned 
flue is one form—has also been tried and employed to 
some extent, but the chief objection to its use, as to that of 
steam, is that the pipes, or radiating surfaces of whatever 
kind, are apt to become overheated and burn the air. 
It is also a difficult matter to secure a proper and regular 
circulation over considerable areas, as easily accomplished 
with hot water, and to have all, or the greater part of, 
the heat in one place is fatal to good plant culture. 
Stoves of the ordinary close type, on the slow-combustion 
principle, such as the little ‘ Tortoise”’ stoves (without hot 
water), are sometimes used for heating small plant-houses, 
but though useful to a certain extent where frost only 
has to be excluded, they are open to several serious 
L 2 
