84 PSYCHROMETRICAL TABLES. 



ARRANGEMENT OF THE TABLES. 



The same arrangement as is found in the Psychrometrical for the Centigrade scale 

 has been adopted. 



The first column at the left contains the indications of the wet-bulb thermometer, 

 from 31 to 105 Fahrenheit. 



The second column gives the differences of the force of vapor for each tenth of a 

 degree, between each two consecutive full degrees in the first column. It enables the 

 observer easily to find the values for the fractions of degrees of the wet thermometer. 



The following double columns furnish the forces of vapor and the relative humidity 

 corresponding to each full degree of the wet-bulb thermometer given in the first 

 column in the same horizontal line, and to the difference of the two thermometers, 

 or t t', found at the head of each column, for every half-degree from to 26. 5. 

 The relative humidity, or the fraction of saturation, is given in hundredths, which is 

 near enough for meteorological purposes ; but one decimal more has been added, 

 though separated by a point, in order to facilitate the interpolations. 



At the bottom of each page is found the mean difference, for each tenth of a degree, 

 between the forces of vapor on the same line. It gives the means of finding the 

 values for the intermediate differences of t ', not found in the tables. 



USE OF THE TABLES. 



Enter the tables with the difference of the two thermometers, or / 2', and the 

 temperature of the wet-bulb thermometer, given by observation. 



In the column headed by the observed difference of the thermometer, t ', and 

 on the horizontal line headed by the observed temperature of the wet thermometer, t', 

 are found the force of vapor, and the relative humidity corresponding to these tem- 

 peratures. 



For the fractions of degrees of the wet thermometer, multiply the decimal fraction 

 by the number placed in the second column between the full degree and the next, 

 and add the product if the temperature is above, and subtract it if it is below zero 

 Fahrenheit. 



The intermediate values of t t' not given in the table are found by subtracting 

 the number in the line at the bottom of the page, multiplied by the number of addi- 

 tional tenths, from the value given in the table. This correction, being always very 

 small, can usually be neglected. 



For the relative humidity, interpolations at sight will generally suffice. 



Examples. 



1. Dry thermometer, t = 50 F. 



Wet thermometer, /' = 43 F. 



Difference, or t t 1 7 F. 



Page 58, we find for t t 1 =- 7 in the third double column, and for t 1 = 43 in 

 the first column 



Force of vapor in the air = 0.186 inch. 



Relative humidity in hundredths = 51 

 B 48 



