166 PRINCIPLES OF HEATING HOT-HOUSES. 



lated, as the specific heat of equal volumes of air and water is 

 as 1 to 2990. The above average will show that each square 

 foot of glass will cool 1*279 cubic feet of air one degree per 

 minute, when the temperature of the glass is one degree above 

 that of the external air. 



But by this we can only find the effect of glass in a still at- 

 mosphere, and, therefore, to find the effect of glass in cooling 

 the volume of a hot-house, especially when exposed to the action 

 of winds, further experiments are necessary, of which we shall 

 treat in a subsequent part of this work, in connection with " pro- 

 tection of hot-house roofs during the night." 



