Pohnaise Method of Heating Greenhouses, S^c. 441 



metrical point to .424,* and this is a degree of dryness which 

 cannot be otherwise than injurious to vegetable life. It will 

 be allowed that this is not an extreme case, and much more 

 favorable than must occur during the winter season. 



From what has already been said, the advantage of having 

 not only conducting, but also radiating bodies distributed 

 equally over the lower surface of the house, will be suffi- 

 ciently obvious, if we would obtain the advantage of the 

 laws by which heated bodies give off their caloric. To 

 effect this we must use conductors as well as radiators, other- 

 wise an equal distribution of heat cannot be obtained in a 

 hothouse. As a traveller and carrier of heat, hot water is 

 decidedly the best agent that has yet been discovered, and it 

 is matter of surprise that prejudices should still exist against 

 this excellent method. This, however, is only the case, as 

 we know from experience, where its merits and its principles 

 are alike imperfectly understood ; and, therefore, the failures 

 which have occurred in the practical application of this 

 method of heating, are distinctly referable to the want of a 

 proper knowledge in their construction and of the principles 

 on which they work. Its merits, however, will best appear 

 by the plainest statement of facts, and as no method, how- 

 ever perfect in itself, can give satisfaction unless properly 

 constructed and properly applied, and it may be remarked 

 that many apparatus have proved wholly abortive, through 

 the most trifling causes, I shall therefore proceed to describe 

 some of the causes of failure, and how success may be most 

 easily attained. 



Boston, August, 1850. 



( To he continued. ) 



close by each other. The Table of cooling bodies by radiation, given in a preceding 

 part of this article, page 336 of the September number, which is calculated from accu- 

 rate data, will assist the learner in his study of this part of our subject. 



* It may be necessary here to explain what is meant by the point of saturation. The 

 natural scale of the hygrometer is included between the points of perfect dryness and 

 perfect moisture, the latter of course being that state of the atmosphere at which the daw 

 point coincides with that of the air. The intermediate degrees may be ascertained by 

 dividing the elaslicit}- of vapor at the temperature of the dew point by the elasticity at 

 the temperature of the air. The quotient will express the proportion of moisture actuai- 

 VOL. XVI. NO. X. 56 



