440 Polmaise Method of Heating Greenhouses, ^c. 



the flue as we too often find it, but the former can be to a 

 very great extent done away with, and the latter prevented 

 in toto. 



We are fully satisfied that flues built well, of hard brick, 

 and covered with slate, when not heated beyond 100*^, radi- 

 ate a heat in every respect as pure as hotwater pipes ; no gas 

 is eliminated, and the absorption of vapors is almost inappre- 

 ciable. 



It is essential to observe here that other causes of aridity 

 exist which are seldom taken into full consideration in judg- 

 ing the specific properties of the different methods of heat- 

 ing hothouses. The greatest of these is the radiation of 

 heat from the roof of the house, and the exit of the air from 

 the interstices of the glass. Before, therefore, a proper esti- 

 mate of the heat generated in a house can be made, this 

 ought to form a paramount consideration. By overlooking 

 these circumstances which are intimately connected with the 

 principles of heating hothouses, many of the plans hitherto 

 brought before the public can be considered as little better 

 than wild speculations, or fanciful whimsicalities, unsupport- 

 ed by any reasonable or practical foundation. 



In a paper published by Mr. Daniell in the Transactions of 

 the London Horticultural Society, as far back as 1824, he 

 says that "the glass of a hothouse at night cannot exceed 

 the mean of the external and internal atmospheres, and tak- 

 ing the mean at 80° and 40°, then 20 degrees of dryness are 

 kept up in the interior of a house, or a degree of saturation 

 not exceeding .528. To this, in a clear night, we may add 

 at least 6 degrees for the eflfects of radiation, to which the 

 glass is particularly exposed,* which will reduce the hygro- 



*The amount of external radiation differs under various circumstances, as in the 

 case of wind, exposure, &c., it increases in rapid ratio at very low temperatures, and in 

 our severe winters here, when the thermometer is below zero, the loss of heat by exter- 

 nal radiation is sometimes even as much as one third of the whole heat generated. Mr. 

 Daniell's allowance is evidently too low under any circumstances. I have come to a 

 tolerable estimate in clear nights by taking one quarter of the difference between the 

 two atmospheres, which in this case would be 10 degrees instead of 6. I have found no 

 accurate rule, however, that can be generally applicable to ascertain the exact amount 

 of heat radiated from hothouse roofs, as the result will differ widely in houses standing 



