EFFECTS OF VENTILATION. 271 



air will be cooled down the 20 degrees ; thus, 60 minus 40 gives 

 the difference, which is 20. If the house contains 800 square 

 feet of glass, presented to the action of the external atmosphere, 

 1000 cubic feet of air will lose 20 degrees of heat ; consequent- 

 ly, the moisture this air held in invisible solution, in virtue of 

 its 20 degrees of temperature, will be condensed by the external 

 cold, and deposited on the glass ; and it will also be found, that 

 the greater the difference between the external and internal 

 temperatures, the greater will be the amount of condensation. 

 The quantity of moisture abstracted from plants, at high tem- 

 peratures, is enormous. This fact is sufficiently de-monstrated 

 in a hot summer day, when the leaves of the trees are wilted, 

 and the garden vegetables flag and droop their leaves. The 

 earth gives out its moisture, and the atmosphere carries it away. 

 The same thing takes place in hot-houses ; the moisture is ab- 

 stracted by the heated air, and is carried off in the form of 

 invisible vapor, till its upward progress is arrested by the glass, 

 and the cold again reduces it to water. 



If we take, for example, the roof of a hot-house, comprising 

 750 superficial feet of glass, and calculate that every square foot 

 of that glass will cool down 1| cubic feet of heated air 36° 

 per minute, and calculating the internal temperature at 65°, we 

 shall find that 937 cubic feet of air will be cooled down 36 de- 

 grees per minute. Now air, saturated at the temperature of 65 

 degrees, contains about 6-59 grains of water per cubic foot, and 

 at the temperature of 30 degrees, it is saturated with 2-25 

 grains ; this gives 4'34 grains of water lost, in condensation on 

 the glass, per minute ; or further, each square foot of glass con- 

 denses 1| cubic feet, or about 5*42 grains of water, per minute ; 

 and supposing the atmosphere of a house, such as we have de- 

 scribed, to be constantly supplied with moisture, by evaporation, 

 or otherwise, there would be abstracted from it about ^ of a pint 

 of water per minute, which is about 12 quarts per hour, or at 

 the rate of nearly 72 gallons in 24 hours. This enormous 

 amount of water, evaporated into the atmosphere of a hot-house, 

 when reduced to calculation, and displayed in plain figures, 

 seems to startle the imagination, and looks very like exaggera- 

 tion ; although it is much below the mark which, by a more 



