llVGKOMirrRICAL TABLES. XXXIX 



The formula and tables here given for obtaining the vapor pressure and 

 dew-point from observations of the whirled or ventilated psj^chrometer are 

 those deduced by Prof. Wni. Ferrel {^Annual Report Chief Signal Officer, 

 iSS6, Appendix 24) from a discu.ssion of a large number of observations. 



Taking the psychromctric formula in metric units, pressures being 

 expressed in millimetres and temperatures in Centigrade degrees, Prof. 

 Ferrel derived for A the value 



A = 0.000656 (i + 0.0019 /,) 



In this expression for A, the factor depending on /, arises from a similar 

 term in the expression for the latent heat of water, and the theoretical value 

 of the coefficient of /, is 0.00115. Since it would require a very small 

 change in the method of observing to cause the difference between the 

 theoretical value and that obtained from the experiments. Prof. Ferrel 

 adopted the theoretical coefficient o. 001 15 and then recomputed the obser- 

 vations, obtaining therefrom the final value 



A = 0.000660 (i + o.ooi 15^,). 



With this value the psychrometric formula in metric measures becomes 



/=/, — 0.000660 v5 (/ — /,) (i + 0.00115/,) 



In order to adapt the formula to convenient tabulation. Prof. Ferrel 



substituted / — /, for /, in the last factor, a modification which produces 



appreciable error only in extreme cases. The error in the computed vapor 



pressure will be 



ii= 0.00000076 j9 (/— /,) (/— 2/,). 



Expressed in English measures, the formula is 



•/=/, — 0.000367 B {t— /,) (i + 0.00064/,) 



and with the same modification in order to render the formula more con- 

 venient for tabulation, we have 



/=/,— 0.000367 i? (/—/,) (i -1-0.00064 (/—/,)). 



^ a which f^= Vapor pressure in inches. 



/, = Vapor pressure in saturated air at temperature /, . 

 / =^ Temperature of the air in Fahrenheit degrees. 

 /, = Temperature of the wet-bulb thermometer in Fahrenheit degrees. 

 ^ — Barometric pressure in inches. 



TABLES ao, 41. 



Reduction of Psychrometric Observations — English tneasures. 



Table 40. Pressure of aqueous vapor. 



Table 41. Values of 0.000367 i9 (/—/,)( i -\ ^) 



■ 1571/ 



These two tables provide for computing the vapor pressure and dew- 

 point from observations of ventilated wet- and dry-bulb Fahrenheit ther- 

 mometers. 



