MR. W. N. SHAW ON HYGROMETRIC METHODS. 



115 



" The dew-points in the sixth column do not differ very materially from observa- 

 tion ; but the temperatures from which they were computed had first to be corrected 

 for the barometric pressure being different from 30 inches. The precise rule for 

 estimating such a correction is as yet unknown ; but it appears that, for the same 

 temperature of the moist bulb, the difference between it and the dry thermometer, 

 when the pressure amounts to 30 inches, is to their difference under any other pres- 

 sure, B, nearly in the inverse ratio of 57 to 27 + B. On this supposition, t t', the 

 observed depression in the fourth column, before being used in the formula, lias been 

 multiplied by (27 + B)/57, and the difference between the product and t t' has like- 

 wise been applied, with its sign changed, as a correction to t, the temperature of the 



air. 



Passing by a discussion of the theory of the instrument, which may require modifi- 

 cation in the light of subsequent progress in the theory of heat, and a graphic method 

 of reducing observations proposed by Mr. MEIKLE, the other points of interest in the 

 article are a brief account of DE LA RIVE'S suggestion (since developed by Mr. WILD- 

 MAN WHITEHOUSE) to apply the rise of temperature produced in a thermometer 

 moistened with sulphuric acid, to determine the hygrometric state of the air. 



"While the sulphuric-acid hygrometer displays considerable ingenuity, the other 

 instrument, the wet-bulb, is on several accounts so decidedly preferable that the 

 invention of M. DE LA RIVE is not likely ever to come into general use. Water can 

 be more readily obtained everywhere, and is much more safe and portable than 

 sulphuric acid. Besides, owing to sulphuric acid freezing at an uncertain or variable 

 temperature, depending on its strength, such an instrument would be apt to give 

 doubtful results at low temperatures. For, whatever be the strength of the acid 

 at first, it will continue to decrease in nn uncertain manner on the bulb by gradually 

 absorbing moisture. However, the heat derived from the condensation of the vapour 



Q2 



