552 LECTURE LVII. 



exist, without coming within the reach of their mutual cohesion, is the 

 same, whatever other particles may be scattered through the interven- 

 ing space. It appears, however, more consistent with some experiments, 

 to suppose, that the presence of air of the usual density allows the particles 

 of water to approach a little nearer together without cohering, so that the 

 utmost quantity of moisture that can be contained in a cubic foot of air 

 at a given temperature is not exactly the same as would make a cubic foot of 

 pure vapour, but always in a certain proportion to it ; and it seems to fol- 

 low,, from the experiments of Saussure, compared with those of Pictet, that 

 the weight of the vapour contained in a cubic foot of air is about one half 

 greater than that of a cubic foot of pure vapour at the same temperature. 



When the air, in the neighbourhood of the surface of the water, has 

 become thus saturated with moisture, the evaporation proceeds very slowly, 

 the vapour being precipitated as soon as it rises : but if the air be continu- 

 ally changed, so that the moistened portion may be removed, and dry air 

 substituted for it, the process will be greatly expedited ; and such a change 

 may be effected either by wind, or by the natural circulation, occasioned 

 by any elevation of temperature communicated by the water to the neigh- 

 bouring air ; but when this circulation is prevented, the evaporation is 

 much diminished, although the temperature may be considerably elevated. 

 In moderate exposures, the depth of the quantity of water, evaporating in 

 24 hours from any surface, is expressed, according to Mr. Dalton's experi- 

 ments, by the height of the column of mercury equivalent to the force of 

 steam at the given temperature, deducting, however, the effect of the elas- 

 ticity of the moisture already existing in the air. 



Since the quantity of moisture, which the air [or rather a given space^ 

 is capable of receiving, is greater as its temperature is greater, we may 

 obtain a natural measure of the quantity which it contains by reducing 

 it to the temperature at which the moisture begins to be deposited. Thus, 

 if we take a glass of cold water, and add to it some common salt, or some 

 muriate of lime, we may cool the air near it so much as to cause it to 

 deposit a part of its moisture on the glass : and by measuring the tempera- 

 ture of the water when the precipitation begins, Mr. Dalton estimates the 

 true state of the air with respect to moisture. Thus, if the glass begins to 

 be moistened when the water is at 40, he infers from the known elasticity 

 of steam at that temperature, that the quantity of moisture contained in 

 the air is equivalent to the pressure of a column of mercury about a 

 quarter of an inch in height ; and if the actual temperature of the air be 

 50, the corresponding elasticity of steam being a little more than one third 

 of an inch, the daily evaporation in such air will amount to about one 

 ninth of an inch, making 40 inches in the whole year. In fact, however, 

 the air is* usually moister than this, and the mean evaporation of all 

 England is, according to Mr. Dalton,* about 23 inches only. 



In hotter climates, and in particular situations, the evaporation may be 

 considerably greater. The Mediterranean Sea, being surrounded by land, 

 is more heated than the ocean, and the winds which blow ovens^t are 

 drier ; consequently its evaporation is greater than that of the Atlantic, 

 and its specific gravity is increased by the increased proportion of salt ; 

 * Manch. Mem. v. 346. 



