282 On the Means of supplying 



glazed buildings is considerable at all times, even when the lights 

 are closed; and if the glazing be defective, and the laps be 

 not puttied, it is very great indeed. The amount of moisture 

 thus abstracted cannot be very easily estimated, varying ex- 

 ceedingly according to the height and construction of the building 

 heated. 



There exists however another drain of moisture, constantly 

 affecting all hot-houses, however perfectly constructed, and how- 

 ever cautiously ventilated, viz. the condensation on the glass. In 

 this case the expenditure is capable of pretty accurate calcula- 

 tion. It has been ascertained by experiment * that each square 

 foot of glass will cool 1 j cubic foot of air as many degrees per 

 minute as the temperature of inner air exceeds that of outer air : 

 that is to say, if the temperature of outer air be 44°, and 

 of the house 66°, for every square foot of glass, 1 \ cubic foot 

 of air will be cooled 22° per minute ; and the moisture which 

 this air held in solution, in virtue of its 22° of heat, will be 

 deposited on the glass, and will either drain away out of the 

 house or fall in drip. The greater the difference between the 

 temperatures of internal and external air, the greater will be the 

 amount of condensation ; and be it observed, that the capacity 

 of air for moisture does not increase simply in the arithmetical 

 ratio of its temperature, but by a scale considerably more rapid, 

 so that the expenditure of moisture at high temperatures is much 

 greater than at low temperatures, for equal differences between 

 internal and external air. 



To put this matter in a clearer light, and give some idea of 

 the real waste of moisture by this means, we will take the case 

 of a small vinery 25 ft. long, by 13 ft. 6 in. wide in the roof, 

 maintained at 65°, when outer air is 35°, a matter of daily occur- 

 rence in early forcing. 



The area of such a roof is about 350 ft. square including 

 woodwork, and, as the glass of the ends will more than com- 

 pensate for this reckoning, we will take 350 ft. as the area of 

 glass. Every square foot will cool 1 ^ cubic foot of air 30° per 

 minute ; that is to say, 437 cubic feet of air will be cooled 30° 

 per minute. Now, air saturated at 65° contains 6*59 grains of 

 water per foot cube ; at 30° it is saturated by 2*25 ; consequently 

 4*34 grains per minute is the amount of water deposited in con- 

 densation by each cubic foot, or each square foot of glass con- 

 denses per minute 5*42 grains of water : that is to say, there 

 is subtracted from the atmosphere of such a house 1897 grains 

 of water per minute, or very nearly a quarter of a pint ; that is, 

 14 "8 lb., or nearly one gallon and a half per hour. Hence in 

 24 hours, if the temperature be maintained as above, there would 



* See Hood's Treatise on "Warming by Hot Water, art. 145. 



