40 VEGETABLE FORCING 
using hot water, and they also claim economy in fuel 
consumption, and better atmospheric conditions for the 
growth of the plants. Some growers have a combination 
of steam and hot water. They use the steam only in 
extremely cold weather and for sterilizing the soil, and 
also for operating the pumps. 
The differences in the various methods of steam and hot 
water heating are so great that the two general systems 
can scarcely be compared. It may be said, however, that 
there is an increased tendency to use the improved 
methods of hot water heating in very large ranges, and 
that they are unquestionably more economical for small 
houses. The greater durability of pipes constantly filled 
with water is a strong point in favor of the hot water 
system. 
Radiation required.—The radiation required to heat a 
house properly depends upon the exposure and the pro- 
tection of the building, the area of glass exposed, the 
temperature requirements of the crops grown, the 
severity of the weather and the system of heating. 
One of the leading manufacturers of greenhouse boilers 
uses the following data for finding the number of square 
feet of pipe surface required to heat the house to various 
temperatures with the gravity hot water system when the 
outside temperature is zero: For 60 degrees to 65 degrees 
divide square feet of glass and equivalent by 2.62; for 55 
degrees to 60 degrees divide by 3; for 50 degrees to 55 
degrees divide by 3.46; for 45 degrees to 50 degrees divide 
by 4; for 40 degrees to 45 degrees divide by 4.67, Six 
square feet of wall area should be figured as the equiva- 
lent of one square foot of glass. The divisors named un- 
doubtedly provide much more liberal radiation than is 
common in most greenhouses which are devoted to vege- 
table forcing, but it is better to have too much radiation 
than not enough. Steam and the pressure systems of hot 
water require less radiation. So many factors enter into 
