82 



VII. 



PSYCHROMETRICAL TABLES, 



GIVING, IN ENGLISH INCHES OF MERCURY, THE ELASTIC FORCE OF VAPOR CONTAINED 

 IN THE AIR, AND ITS RELATIVE HUMIDITY IN HUNDREDTHS; 



DERIVED FROM THE INDICATIONS OF THE WET AND DRY BULB THERMOMETERS, 

 IN DEGREES OF FAHRENHEIT. 



By a. Guyot.* 



M. V. Regnault, in his Etudes sur VHygrometrie Annales de Chimie et de Phy- 

 sique., 3™"^ serie, Tom. XV. p. 129, after having discussed the theoretical bases of the 

 psychronietric formula given by August, and modified the numerical values of some 

 of its coefficients, adopts the formula 



0.480 (t — f) , 



for temperatures above the freezing-point ; and when the temperature of the wet ther- 

 mometer is below the freezing-point, the bulb being covered with a film of ice, 



- 0.480 (t — t') , 



^=f es9-t' ^'^ 



* While „his table was going through the press, a similar one, prepared by Prof. J. H. Coffin for 

 his private use, was published by the Smithsonian Institution, in order to meet an urgent demand 

 from many quarters. Being based on the same formula, it gives the same results, except, perhaps, in 

 degrees below 14° Fahrenheit, where the tables show slight discrepancies. These unimportant 

 differences arise from the fact that Prof. Coffin's table was computed from Regnault's tensions, as 

 given in the first edition of this collection, while the author's table is based on the table of tensions as 

 given in this second edition, in which the values below 14° Fahrenlieit have been somewliat modified, 

 for reasons givenabove. The following table gives also the relative humidity with one more decimal, 

 which makes the interpolations more easy ; and a column of differences for finding the values for 

 fractions of t'. A table for reducing the results to another barometric height is added at the end of 

 thq table. 



B 46 



