168 GREENHOUSE CONSTRUCTION AND HEATING. 
ascertaining the amount of piping necessary to afford 
temperatures suitable for different purposes. Of these 
some are altogether too complicated, and others totally 
absurd, while even that standard authority, Hood, only 
gives a table showing the amount of piping required to 
heat given quantities of air per minute—a totally different 
thing from merely maintaining a certain temperature in 
a structure of any kind, and consequently quite useless 
for the purpose under consideration. 
Decidedly the most practical and reliable rule—like 
nearly all other good things, a simple one—is as follows: 
Multiply the length of the structure by the width, and 
the result by the average height (this being easily obtained, 
in most cases, by adding the greatest and least heights 
and dividing by two). This gives the actual cubic contents 
of the house, in feet, and this, divided by 30, shows the 
number of feet of 4in. piping necessary to thoroughly 
exclude frost (under ordinary circumstances) and maintain 
the usual greenhouse temperature suitable for plant-houses, 
cool conservatories, late vineries, etc. ; divided by 20, the 
above product gives the required amount of similar piping 
for what are termed intermediate houses, cool stoves, 
early vineries, etc.; by 15 that for ordinary stoves or 
hot-houses; and by 10 that required for forcing and 
propagating-houses, Hast Indian orchid-houses, and other 
structures in which a high temperature (65° to 70°), 
has to be maintained in any weather. Stated in a 
somewhat different way, but amounting to much the 
same thing, 30ft. to 40ft. run of 4in. piping must be allowed 
‘to each 1000ft. of cubic contents for greenhouses, con- 
servatories, and other cool structures ; 50ft. per 1000 for 
