HEATING THE RANGE 103 



" For 50 to 55 divide square feet of glass and equiva- 

 lent by 3.46. 



" For 45 to 50 divide square feet of glass and equiva- 

 lent by 4. 



" For 40 to 45 divide square feet of glass and equiva- 

 lent by 4.67. 



" For 35 to 40 divide square feet of glass and equiva- 

 lent by 5.5." 



81. Explanation of the table. " From the use of 

 such a list of divisors it will appear that greenhouse heat- 

 ing, as practiced, is a rule of thumb business. The list 

 of divisors is not, however, entirely a rule of thumb, but 

 it amounts to nearly the same thing, inasmuch as it is a 

 fact that in the derivation of the formula from which 

 the divisors are obtained the premises are not all absolutely 

 correct. Therefore, the conclusion can hardly be true 

 to all cases. 



"The formula is (r ~ r) ^=R, where T equals the 

 (150 -T)J, 



temperature required in a greenhouse, T f equals the 

 coldest outside temperature, G equals glass and equiva- 

 lent exposure, 150 equals average temperature in degrees 

 of water in pipes, 2 equals the B. T. U. 1 given off per 

 square foot of pipe surface per hour per degree of differ- 

 ence between the temperature of the air, and that of the 

 water in the pipes, and R equals the required number 

 of square feet of radiating surface. This formula is con- 

 structed on the theory that every square foot of glass 

 surface loses 1 B. T. U. per hour per degree of difference 



1 A B. T. U. is a British Thermal Unit, the standard of heat- 

 ing measurement. It represents the quantity of heat required 

 to raise the temperature of 1 Ib. of water 1 Fahr. 



