HYGROMETRIC RESEARCHES. 



547 



From the dry and wet bulb records the mass of water vapour has been calculated 



by the formula — 



m = m— \ (t — f), 



where m is the mass per cubic metre required, m' that of saturated water vapour at t'. 



The following table shows the differences between these calculated values and those 

 directly determined at low levels : — 





Total Deviation, neglecting Signs. 





No. 



Amount. 

 Grammes per 



cubic metre. 



Fort-William 



Montpellier ..... 



109 

 38 



0-17 

 0-46 



The differences are not much altered by a pressure correction, which is very much 

 smaller than the fluctuations observed from hour to hour between calculated and 

 determined values ; e.g., with no change of pressure, these fluctuations were at three 

 successive experiments at Fort- William +0-17, +0*20, -0'04. 



Theoretically, if the conception of an atmosphere of water vapour existing inde- 

 pendently of an atmosphere of other gases be allowed, no such correction should appear 

 in measuring the mass of vapour in a cubic metre of space ; but there might be a 

 small correction due to the slightly greater ease of vaporation in the lower pressures of 

 higher levels. It has been shown by Hann and others that this conception is not 

 tenable. 



In order to reduce observations at Ben Nevis Observatory from dry and wet bulb 

 readings of an aspiration-psychrometer and a table of masses of saturated water vapour 

 at wet-bulb temperatures and standard pressures, the formula 



m = m — 



t-t' 



P 



755 



was employed. That a pressure correction is necessary is shown by such records as 

 those of experiments 187-189, 191, 192, where m — (-g-J is a negative quantity with- 

 out. The following table shows the results then obtained, the values derived from the 

 gravimetric determinations, and the differences between the two : — 



