40 Heat, Moisture, and Evaporation. 



mean temperature, and the lower shaded line the mean temper- 

 ature of the dew point . This species of tabular picture is valu- 

 able, because we can so easily observe not only the whole range, 

 but also the range of any particular period. From Mr. Cald- 

 cleugh's observations at Villa Rica, the mean evaporation was 

 double that in this country, and the quantity of moisture in the 

 air also nearly double. 



It will at once be perceived, that the progress of evaporation 

 must depend on the quantity of water the air is capable of taking 

 up to saturate it, and when it is perfectly saturated, evaporation 

 must cease. 



Also, since a supply of heat competent to form the vapour 

 produced is absolutely necessary to the process, the evaporation 

 must be limited by the supply of heat, and proportional to it. 

 Hence, in the progress of evaporation, there will be cold 

 produced, till the difference between the temperature of the 

 surface affording vapour and that of the surrounding medium 

 be equivalent to produce the proper flow of heat. It must 

 obviously depend on the nature of the conductors which supply 

 it, but must be constant under the same circumstances ; and, 

 thei'efore, the hygrometer of Dr. Hutton is founded on true 

 principles. 



But the surface affording vapour being cooled below the 

 general temperature, the quantity of moisture the air is ca- 

 pable of taking up must be estimated from the temperature 

 of the evaporating surface. 



In ordinary circumstances, the ultimate depression of tem- 

 perature is about one degree, by an evaporation of 1 50 grains 

 per hour, from a surface one foot square, or 2*5 grains per 

 minute. 



It also appears from Mr. Dal ton's experiments (Nicholson's 

 Journal, vol. vii. p. 7.) that in a still atmosphere, where the heat 

 is supplied in the quantity necessary to produce the vapour, 

 the evaporation per minute, from a surface one foot in area, is 

 exactly equal to twice the quantity of vapour in a cubic foot of 

 saturated air of the same temperature, or 120 times that quantity 

 in an hour. 



From these conditions we have the easy means of forming a 

 table to exhibit the evaporation from an area of one foot of 

 water at any temperature. For this purpose, I will take the 

 weight of vapour which a cubic foot of dry air takes up at dif- 

 ferent temperatures, when saturated, from my work on Warming 

 and Ventilating, and add the columns for showing the rate of 

 evaporation and temperature of the atmosphere. 



The first column shows the temperature of the evaporating 

 fluid at its surface; the second column the weight of water in 

 grains that would saturate a cubic foot of dry air at that tem- 



