EFFECTS OF VENTILATION. 269 



the internal and external temperature, the greater will be the 

 demand for moisture. Thus, if the external air be at the freez- 

 ing point, (32,) and the air in the house heated to 50 degrees, 

 then there is three times more moisture carried away by escap- 

 ing air than is brought in by the returning quantity; and, 

 escaping at 90, it carries away four times as much, and so on, 

 in proportion to the difference of the two atmospheres ; the ex- 

 ternal air, however, increasing in ratio as it decreases in tem- 

 perature. 



According to these calculations, atmospheric air, entering a 

 house at 32, and escaping at 100, carries away nearly six 

 times as much moisture as it brings in. This, in a short time, 

 would render the atmosphere of a house deleterious to either 

 animal or vegetable life ; and in large and lofty houses this is 

 practically the case. We have managed a lofty plant-house, 

 where the plants on the side shelves were nearly frozen, while the 

 thermometer, hung in the angle of the roof, about 45 feet high, 

 stood at 100 degrees. Now this heated air, escaping at the top 

 of the roof, as is generally the case as well as here, carried away 

 more moisture than the small evaporating surface could supply ; 

 the effects were, consequently, ruinous to the plants. However 

 imperfect the above calculations may be, they are within the 

 bounds of truth, and are sufficiently accurate to show the im- 

 portance of this subject to exotic horticulture ; and it will more 

 effectually impress upon our minds the amount of care and con- 

 sideration which the ventilating of hot-houses demands. If air 

 must be admitted, for the purpose of regulating the internal 

 temperature, every precaution should be taken to prevent it 

 from entering in strong currents, and it should be taken in from 

 the warmest side of the house, and, if possible, over a warm 

 surface, as hot-water pipes, or whatever heating apparatus may 

 be employed, so that the internal atmosphere may be gradually 

 reduced ; and, at the same time, the utmost precaution should 

 be used to prevent the escape of heated air, at least as little as 

 possible, by direct ascension ; this is easily accomplished by the 

 improved methods of ventilation now adopted, some of which I 

 shall hereafter endeavor to describe. Thus the cultivator is 

 enabled to modify the two atmospheres, previous to their com- 



