elevated Rain-gauge, as caused by Wind. 429 



=0-042 of its weight, = ^th of the quantity to be accounted 

 for." 



20. But, in reality, Sir J. Herschel's suppositions are far too 

 favourable for the opinion which he opposes. In the first place, 

 he makes no allowance for the heat derived from the gaseous air 

 in addition to that received from the condensation of vapour. 

 To estimate the amount of this, we may fairly make the assump- 

 tion that has been found to give very exact results in the theory 

 of the dry- and wet-bulb hygrometer. We may assume that the 

 indefinitely thin film of air surrounding the drop of rain always 

 takes the temperature of the drop, and yields up to it both the 

 excess of its own sensible heat and the latent heat of the con- 

 densed aqueous vapour (the sensible heat of the aqueous vapour 

 may be neglected as very minute). Then according to the for- 

 mulae of M. Auguste, — 



Let w = weight of a volume of air equal to that of the film 

 at 0° (Cent.). 



a = coefficient of dilatation of a gas per degree of tem- 

 perature. 



t = temperature of the air. 



t'= temperature of the drop. 



/ = elastic force of aqueous vapour at temperature t, the 

 air being supposed saturated. 



f= elastic force of aqueous vapour at temperature P. 



h— height of the barometer. 



8 = specific gravity of aqueous vapour. 



y= specific heat of dry air. 



\= latent heat of aqueous vapour. 



Then 



-f 



C-0.7 



\ + a.t' 760 



will nearly represent the sensible heat given out by the film of 

 air in cooling from t to t', and 



will be the amount of latent heat given out by the vapour con- 

 densed. The ratio of these is 



which varies with the value of t'. In Sir J. HerschePs example, 

 let us suppose the lowest 213 feet of air to have the temperature 

 of 60° F. Then t' is at first 20° F., and the above formula (insert- 

 ing for S its value -6235; for y, -2669; for X, 640-*'; for h, 

 •760; and for /'and/' their values from the common tables of 



