332 On the Hygiomeier. 



}wur of some unknown tension is contained. The volume of 

 combined air and vapour is then supposed to sink in tempera- 

 ture, each successive portion of heat disengaged in so doing be- 

 ing entirely employed in adding to the quantity of vapour con- 

 tained in the air, until the saturation be completed. 



The equation deduced from these data is somewhat compli- 

 cated. The complexity, however, arises chiefly from the condi- 

 tion that the mass to be cooled is composed of variable propor- 

 tions of air and vapour, having different specific heats, and also 

 from taking into account the difference of volume corresponding 

 to the difference between the original temperature and that of the 

 dew-point. But as these conditions, from the minuteness of the 

 quantities which they introduce, do not sensibly affect the result 

 of the formula. Sir James Ivory adopts a more simple equation, 

 which may be investigated as follows, on the supposition that 

 the air from which the heat is evolved is dry, and that its origi- 

 nal volume remains unchanged. 



Suppose a given volume of air at the temperature ^, and un- 

 der the pressure B, to contain a quantity of moisture which 

 would saturate it at the temperature f. Further, that the 

 temperature of the given volume of air is reduced from t to 

 t — D = ^, and that the water converted into vapour by the 

 heat thus evolved, is exactly sufficient to raise the point of satu- 

 ration of the given volume from -^' to i'. It may be shewn, 

 1> 



that if ^^ represent the weight of the given volume of air, the 

 30 • 



weight of water to be converted into vapour will be very nearly 



/— / 



1. ^, where ^ andy\, denote the tensions of vapour at the 



temperatures f and f respectively. 



Again,the specific heatof air under a constant weight, and when 

 the pressure is B, being =: a', and the latent heat of steam being 



T> 



= Z, then a weight of dry air = — , in cooling D degrees, will 

 communicate to a weight of water = 1, sufficient heat to raise 

 its temperature degrees. Further, the same weight of wa- 



