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 



