Obgervat'to)is mi the Hygrovieter. 2T5 



ing the latter an absolute measure of the former, still it only be- 

 comes so accidentally. 



With respect to Professor Leslie's hygrometer, it must be 

 confessed, that there seems still to remain a degree of uncer- 

 tainty about its theory ; indeed nothing but the belief that some 

 such defect existed, could have prevented the universal adop- 

 tion of this the most easily constructed, and most convenient, of 

 all hygrometers. The remaining observations will consist of a 

 brief review of the theories of this mstrument which have been 

 proposed, and of the formulas for calculating the dew-point de- 

 rived from these, so far as they are known to the writer. 



Theory of Professor Leslie's Hygrometer^ 



Professor Leslie''s own theory is*, that the degree of cold pro- 

 duced by evaporation, is proportional, under the same pressure, 

 to the dryness of the air when its temperature is reduced to 

 that of the evaporating surface. This, according to Mr H. 

 Meikle, is also Mr Ivory's view of the subject. 



The only other theory with which the writer is acquainted, 

 is that proposed by Dr Anderson -|-, namely, that the degree of 

 cold is nearly proportional to the rate of evaporation under the 

 same circumstances. Or, in other words, that the degree of cold 

 is nearly proportional to the dryness of the air at its unreduced 

 temperature. 



But our ideas respecting these theories, and the formulas de- 

 duced from them, will be rendered more precise by employing 

 the following symbols. 



t = The temperature of the air, (in degrees of Fahrenheit). 



a = The temperature of the evaporating surface. 



i' z=. The temperature of the dew-point. 



60 



L = — X (^ — ^') = The indication of Leslie's Hygrometer. 



Ti-t — t:, 



^ j f The force of vapour, as measured by a column of mer- 

 •^ f ~ ( cury, at the temperatures ^, H and t' respectively. 



• See Article Meteorology in the Supplement to the Encyclopaedia Bri- 

 tannica. 



f See Article Hyffrometry in the Edinburgh Encyclopaedia. The origi- 

 nal idea of this, however, is due to Dr James Hutton, the geologist. See 

 Danielle Essays, page 100. 



