Polmaise Method of Heating Hothouses. 151 



an important point in any heating apparatus, and ought if 

 possible to be secured. 



I will just advert to another argument of the promoters of 

 Polmaise, and this is considered by them the most important 

 of all, viz.: its capability of creating a motion in the atmos- 

 phere of the house. Any system of heating by hot air, pos- 

 sesses theoretically some advantages, over other methods, 

 where the surface of radiation is larger. Strictly examining 

 the matter however, we find that this theory, however plau- 

 sible it may appear, has scarcely a practical foundation. As 

 far as regards its influence and utility in a hothouse, we 

 know very well that currents are created by the rarefaction 

 of air, or the expansion of its particles by heat. But, how- 

 ever beneficial a motion in the atmosphere may be, and I 

 will not dispute the fact, we know also, and that too by ex- 

 perience, that the more we increase the unlimited action of 

 this law in the atmosphere of a hothouse, the more injurious 

 it becomes to vegetable and animal life. If it be desirable 

 to create a torrid tornado in a hothouse, the apertures of in- 

 gress must admit a current of air heated to a degree that 

 would quickly contaminate the whole volume, and render 

 it incapable of sustaining either animal or vegetable existence. 



The motion given to the atmosphere of a hothouse, by a 

 current of heated air, depends upon the temperature to which 

 it is heated, and not upon the quantity or volume of air pass- 

 ing over the heating body. The motion created, will, there- 

 fore be greater, in proportion as the aperture is diminished, 

 and the temperature of the current increased. The equaliza- 

 tion of the internal volume, however, will not be in propor- 

 tion to the heat of the current with the latter, but to the 

 internal and external surfaces of radiation, so that a current 

 of highly heated air, entering a house, will not equalize the 

 temperature of the internal atmosphere ; in fact, some parts 

 of it will not be influenced by the current at all ; this I have 

 proved by actual experiment. But, strictly speaking, the 

 atmosphere of a house artificially heated, is always in motion ; 

 it must necessarily be so, by virtue of the law already re- 

 ferred to ; and we find on examination that it is continually 



