104 THE PRINCIPLES OF FLORICULTURE 



between the inside temperature and the temperature out- 

 side, and that every square foot of pipe surface effuses 

 2 B. T. U. per hour per degree of difference between the 

 temperature of the water in the pipes and the air sur- 

 rounding the pipes. It is evident that if one square foot 

 of ordinary vertical window glass surface loses 1 B. T. U. 

 per hour per degree of difference between the inside and 

 the outside temperatures, one square foot of greenhouse 

 roof surface with its lapped glass, between the lights of 

 which the wind blows at will, and which surface is nearer 

 horizontal than vertical, will lose heat at a considerably 

 greater rate. Also, horizontal pipe coils do radiate heat 

 at a rate of 2.25 B. T. U. and upward per square foot 

 per hour per degree of difference, and the rate may be 

 taken as 2 B. T. U. without appreciable error, and 

 finally, the average temperature of the pipes is usually 

 10 or 15 higher than 150, and on demand it may be 

 brought to an average temperature of 180. So it is 

 evident that the formula is constructed from data which 

 are not absolutely correct. Nevertheless, the results ob- 

 tained are to a degree constant. 



' The quantities of radiation produced by these divi- 

 sors are large, and assurance is felt that the temperatures 

 required will be maintained through eight to ten hours of 

 inattention to the fire during the night, during which 

 time the fires are banked and provided with very little 

 draft. It is owing to this feature, namely, that hot- 

 water heaters generally get no attention at night, that 

 large pipes are preferable to small pipes for greenhouse 

 heating by gravity circulation of hot-water. These large 

 pipe systems, contemptuously called ' water logged' by 

 manufacturers of different devices, which are calculated, 

 or are represented to produce the same or a greater amount 



