162 PRINCIPLES OF HEATING HOT-HOUSES. 



heat taken up by different substances. To ascertain, therefore, 

 the effect a certain quantity of water will produce in warming 

 the air of a hot-house, there appears to be no better method 

 than that of computing from the specific heat of gases compared 

 with water. 



Every substance has its peculiar specific heat. Now, one 

 cubic foot of water, by losing one degree of heat, will raise the 

 temperature of 2990 cubic feet of air the extent of one degree ; 

 and, by the same rule, by losing 10 of its heat, it will raise the 

 temperature of 2990 cubic feet of air 10 degrees; and so with 

 similar quantities in similar proportions. 



In order to know the time it will take to heat a certain quan- 

 tity of air any required number of degrees, by means of hot 

 water contained in metal pipes, we must calculate the effect 

 from direct experiment ; and, as the radiating and conducting 

 powers of different substances differ considerably, it is necessary 

 that the experiment be made with the same material as the 

 pipes for which we wish to estimate the effect. 



From data obtained by experiments on the cooling of iron 

 pipes, it appears that the water contained in a pipe 4 inches in 

 diameter loses -851 of a degree of heat per minute, when the 

 excess of its temperature is above 125 degrees above that of the 

 surrounding air. There one foot in length of a pipe 4 inches 

 diameter will heat 222 cubic feet of air one degree per minute, 

 when the difference between the temperature of pipe and the 

 air is 125 degrees. 



To calculate from this data, however, the length of a pipe, of 

 any given size, that will be necessary to warm a house, and to 

 maintain it at any given temperature under a certain external 

 temperature, it will be necessary to estimate the heat lost by 

 the conducting and radiating power of the glass, and of any 

 metallic substance used in the structure. 



Heating horticultural structures is a very different matter 

 from heating solid opaque buildings ; and here many erectors 

 of heating apparatus fall into error. They suppose, because an 

 apparatus of certain power heated a large building, a church 

 or a hall, one of proportionate dimensions should warm a hot- 

 house of proportionate size, without taking into full considera- 



