PRINCIPLES OF HEATING HOT-HOUSES. 163 



tion the great difference of the external radiation, and the con- 

 duction of heat by the materials of the building. 



The loss of heat by buildings covered with glass is very great. 

 It appears, by experiment, that one square foot of glass will cool 

 down 1-279 cubic feet of air as many degrees per minute as the 

 internal temperature of the house exceeds the temperature of 

 the external air; thus, if the difference between the external 

 temperature and the temperature of the house be 30 degrees, 

 then 1-279 cubic feet of air will be cooled 30 degrees by each 

 square foot of glass ; or, more correctly, as much heat as is equal 

 to this will be given off by each square foot of glass, for, in real- 

 ity, a very much larger quantity of air will be affected by the 

 glass, but it will be cooled to a less extent. The real loss of 

 heat, however, from the house will be what is here stated. 



There are various causes likely to affect these calculations, 

 such as, — 



High winds, which are found to reduce the internal tempera- 

 ture more than actual cold, or even frost ; 



Condensation of moisture on the glass, which prevents the 

 escape of heated air ; and, when a certain temperature is main- 

 tained within, prevents radiation from the glass to a great 

 degree ; 



The extent of wood in the roof of the house, which also pre- 

 vents radiation and conduction, as in the case of metallic roofs. 



These circumstances will be found to affect, in a greater or 

 less degree, the air of the house, though, under general circum- 

 stances, these calculations will be nearly correct. 



In estimating the quantity of glass surface contained in a 

 building, the extent of wood surface must be carefully excluded. 

 This is particularly necessary in all horticultural buildings, 

 where the maximum of heating power is dependent upon the 

 estimate taken. The readiest way of calculating, and suffi- 

 ciently accurate for ordinary purposes, is to take the square sur- 

 ►faces of the sashes, and then deduct one eighth of the amount for 

 wood work. In the generality of horticultural buildings, the 

 wood work fully amounts to this quantity. When the frames 

 and sashes are made of metal, the radiation of heat will be quite 



