44 VEGETABLE FORCING 
able for the heating of greenhouses, but space will not 
permit a discussion of the merits and characteristics of 
each. A few general factors, however,.should be taken 
into account in the selection of a boiler, and they may 
be enumerated as follows: (1) The boiler should be 
amply large. It is uneconomical in every respect to force 
a boiler which is too small for the required radiation. 
(2) The boiler should secure perfect combustion of the 
fuel used. (3) A long fire travel is essential to the great- 
est efficiency. (4) Thin waterways are a decided advan- 
tage. (5) Horizontal fire box surfaces are superior to 
perpendicular tubes or flues. (6) The boiler should be 
easily cleaned. (7) There should be at least two boilers, 
even for small houses, so that in case of accident to one, 
the other may be used. 
Thermostats are used to some extent among green- 
house growers. They are electrical devices for the auto- 
matic regulation and indication of temperatures. An 
electric circuit connects with battery cells and a bell, if 
the thermostat is to be used as an alarm. Sufficient ex- 
pansion or contraction of a substance, such as rubber or 
metal, closes the circuit and causes the bell to ring when 
the temperature has reached a dangerous point. Thermo- 
stats are sometimes used in greenhouses which are not 
large enough to require a fireman throughout the night. 
In such cases they may connect with a bell in the bed- 
room of the fireman, or, if preferred, to a small motor 
which will automatically open or close the dampers of 
the boiler. 
Thermostats in large commercial houses are in keeping 
with the “safety first” policy. Why take chances on 
the damaging or perhaps total loss of a crop, when a 
few dollars will provide a sleepless night watchman that 
will sound a warning the very moment the temperature 
in the greenhouse has dropped to a dangerous point? 
Furthermore, the night fireman and the foreman are 
