526 



BESSEL ON BAROMETRICAL 



and thence the Influence of the aqueous vapour 



H'-H_ 



0-002561 



(H'-H)10'''^^ + ^-^> . io«T-cr.. . (12.) 



If we assume the pressure in this formula at the height A = 1, 

 or the height of the barometer there = 336j^-905j and k = 

 0*00375, we find the quantities to be raultipUed into a for dif- 

 ferent values of H' — H and T as they are given in the follow- 

 ing table. 



T = |(r + r'). 



From these numbers we may judge of the influence on the 

 result which may be occasioned by an uncertainty in the value 

 of a in any occurring case. 



4. 



Since the invention of Daniell's Hygrometer and of August's 

 Psychrometer, we have the means of ascertaining at all times, 

 with ease and sufiicient exactness, the quantity of aqueous 

 vapour contained in the atmosphere. The observation of the 

 psychrometer at both elevations, in addition to those of the 

 barometer and thermometer, is readily made, and dispenses 

 with any arbitrary supposition in regard to the moisture, as 

 that of the thermometer does in regard to the temperature of 

 the air. I will, therefore, examine the rules of calculation 

 which are appUcable in cases where the psychrometer has been 

 observed. 



The psychrometer rests on the comparison of the heights r, 

 and T of two thermometers, one with a moistened bulb, and the 

 other with a dry bulb. If the greatest pressui-e which aqueous 

 vapour at the temperature t can exert be denoted by ^ t, and 

 the height of the barometer in Parisian lines by b, the existing 

 pressure of the vapour 



_ _ 0-558 (r - r) b 



~ ' 336-905 (m - r/ 

 ■where, if the value of r be positive, m = 640 ; and if negatire 



