80 



ATMOSPHERIC 3I0ISTURE, CONSIDERED 



lion of the air could not have exceeded 910." If the h}^grometer were cou-- 

 sulted in hothouses as commonly managed, Mr. Daniell observes, it would 

 be no uncommon thing- to find in them a difference of 20^ between the point 

 of condensation and the air, or a degree of moisture falling short of 500." 

 The causes of the dryness of our artificial climates has been admirably 

 pointed out by Mr. Rogers. 



252. The causes whose constant operation renders our artificial climates 

 unnaturaUii dry are principally^ two : the condensation of moisture on the 

 glass, and the escape of heated and damp air through the crevices of the 

 building, the space which it occupied being constantly supplied by dry ex- 

 ternal air. A third drain of moisture formerly existed in the absorbing surfaces 

 of brick flues, w^hich drank up the moisture of the air in contact with 

 them, and carried it off" wuth the smoke into the outer air. The very general 

 use of hot water in iron pipes has removed this nuisance, and we have now 

 only to contend with the two first mentioned. 



253. Some idea of the drain of moisture by the escape of heated air may 

 be formed from the following considerations. The capacity of air for moisture, 

 that is to say, the quantit}'^ of water which a cubic foot of air will hold in 

 invisible solution, depends upon its temperature, and increases with it in a 

 rapid ratio. It is doubled betw^een 44° and 66°. The consequence is, that 

 every cubic foot of air which escapes at the latter temperature carries 

 off with it twice as much moisture as it brought in. Where the difference 

 of temperature is greater, the drain becomes greater also : air entering at 

 44^, and escaping at 80°, carries off^ three times as much as it brought in ; 

 escaping at 90°, four times. Now^ the escape of air from our best 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 in- 

 deed. The amount of moisture thus abstracted cannot be very easily esti- 

 mated, varying exceedingly according to the height and construction of the 

 building heated. 



254. There exists, however, another drain of moisture^ constantly 

 aff^ecting all hothouses, however perfectly constructed, and however cau- 

 tiously ventilated : viz., the condensation on the glass. In this case the ex- 

 penditure is capable of pretty accurate calculation. It has been ascertained 

 by experiment, that each square foot of glass will cool 1^ cubic foot of air 

 as many degrees per minute as the temperature of mner 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 feet 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 

 temperature 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 diff^erences 

 betw^een internal and external air." (^Gard. Mag. vol. xvi. p. 282.) 



255. This dryness of the atmosphere of hothouses Mr. Daniell has shown 

 to be frequently accompanied by an injurious degree of cold to the roots of 

 plants. " The danger of overwatering most of the plants, especially at par- 

 ticular periods of their growth, is in general ver}' justly appreciated ; and in 



