PRINCIPLES OF HEATING HOT-HOUSES. 



165 



the temperature which corresponds to that of the external air, 

 which may be the medium (or average) of your locality. 

 Then, in the other column, find the temperature required in the 

 house ; then, in this latter column, and on the line which cor- 

 responds with the external temperature, the required number of 

 feet of pipe will be found. 



Supposing, now, that a forcing-house is to be kept at 75 de- 

 grees, and the average of the external thermometer in the 

 coldest weather, taken at 10 (Fah.) ; then, by the foregoing 

 table, we find, under the column 75, and on the line 10, for 

 external temperature, the quantity 292, which is the number of 

 feet of pipe required to heat 1000 cubic feet of air per minute, 

 the proposed number of degrees. Of course, the volume of air 

 in the house must be previously ascertained. Any other differ- 

 ence of temperature may be found in the same way. 



It will thus be perceived, that the amount of heat required 

 for warming a glazed structure is much greater than that re- 

 quired for warming an opaque building of the same size, in 

 consequence of the radiation of heat from its surface ; and the 

 difference is much greater than the allowance made by erectors 

 of heating apparatuses, under general circumstances. 



To ascertain the effect of glass windows in cooling the atmos- 

 phere of a house, the following experiments were made, with a 

 vessel as nearly as possible the same thickness as the glass 

 ordinarily used for glazing. The temperature of the house, in 

 these experiments, was 65 ; the thickness of the glass was .0825 

 of an inch ; the surface of the vessel measured 34-296 square 

 inches, and it contained 9-794 cubic inches of water. The time 

 in which this vessel cooled, when filled with hot water, is shown 

 as follows : 



From the average rate of cooling here given, the effect of 

 glass in cooling the atmosphere of a room may easily be calcu- 



