36 ATMOSPHERIC MOISTURE, CONSIDERED 



degrees between the temperature of the air and the dew-point ; making 

 the degree of saturation about 730, but most frequently 5 degrees, or 

 850 ; and the mean saturation of the air could not have exceeded 910." 

 In our hothouses as commonly managed, he 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 this unnatural dryness in our artificial climates are 

 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 external air. A third drain 

 of moisture formerly existed in the absorbing surfaces of brick flues, 

 which drank up the moisture of the air in contact with them, and 

 carried it off with 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 other two robbers of moisture. 



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 quantity of water which a cubic foot of 

 air will hold in invisible suspension, depends upon its temperature, and 

 increases with it in a rapid ratio. It is doubled between 44 and 60. 

 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. 

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

 mated, as it varies exceedingly according to the height and construction 

 of the building heated, and the disparity between the temperature of 

 the external and internal atmospheres. 



There exists, however, another drain of moisture, constantly affect- 

 ing all hothouses, however perfectly constructed, and however cau- 

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

 the expenditure is capable of pretty accurate calculation. It has been 

 ascertained by experiment, that each square foot of glass will cool 1 J 

 cubic feet 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 the 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 suspension, 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. 



In a parched atmosphere the roots of plants are apt to suffer, a? well 

 as the tops, from rapid evaporation through the sides of the pots, and 

 over active exhalations through the leaves. 



The custom of lowering the temperature of fluids in hot climates, 

 by placing them in coolers of wet porous earthenware, is well known, 

 and the common garden pot is as good a cooler for this purpose as can 

 be made. Under the common circumstances of the atmosphere of a 



