PSYCHROMETRICAL TABLES. 



49 



ative humidity is indicated in per cent, of the full saturation of the air at the corre- 

 sponding temperature of the dry-bulb thermometer /. 



The first vertical column contains the indications of the wet-bulb thermometer t 1 , 

 beginning with the temperatures below the freezing point, when the bulb is covered 

 with ice, from 35, and continuing from the freezing point up to -(-35 centigrade, 

 the bulb being simply wet. 



The second column gives the differences of the force of vapor for each tenth (0 1 ) 

 of a degree, between each full degree of the first column. It enables the observer to 

 find out the correction for any fraction of a degree of the wet-bulb thermometer. 



The following double columns give immediately the force of vapor and the relative 

 humidity, corresponding to each degree of the wet-bulb, placed in the first column, 

 on the same horizontal line, and to differences of the two thermometers, or to t /', 

 taken at every two tenths of a degree. 



The horizontal column at the bottom indicates the mean difference, for each tenth 

 of a degree, of the force of vapor contained in the same horizontal line. It gives the 

 correction for the intermediate differences of the thermometers; 0.1, 0.3, 0.5, 0.7, 

 0.9, &c., &c. 



To meet the wants arising from the extreme climate of North America, the tableo 

 of Mr. Haeghens have been extended from 15 to 35 centigrade, and from 

 -{-30 to -(-35 of temperature of the wet-bulb, and to -}-40 of temperature of the 

 dry-bulb thermometer. The forces of aqueous vapor of Regnault, as given in 

 Table I., have been used for the calculations. 



Use of the Tables. 



Enter the tables with the difference of the two thermometers, or t t', and with 

 the temperature of the wet-bulb thermometer ', taking the first three pages, when the 

 temperature of the wet-bulb is below the freezing point ; and the following ones when 

 it is above the freezing point. 



Seek first the column at the head of which you find the difference of the thermom- 

 eters ; go down as far as the horizontal line, at the beginning of which you see the 

 temperature of the wet-bulb thermometer; there you find the force of vapor, and the 

 relative humidity corresponding to your observation. 



Two corrections for fractions may be required for a complete calculation of the 

 force of vapor ; one for the fractions of degrees of the wet-bulb thermometer ; anoth- 

 er for the intermediate differences of the two thermometers, viz. for 0.1, 0.3, 0.5, 

 0.7, &c. 



The first correction for fractions of degrees of the wet-bulb thermometer is found by 

 multiplying the decimal fraction by the number placed in the second vertical column 

 next to the whole degree, which number is the value of a tenth of a degree. The 

 product must be added to the value of the full degree given in the table, when the tem- 

 perature of the wet-bulb is above the freezing point : it must be subtracted when the 

 temperature is below the freezing point, and receives the sign . This correction is too 

 important to be neglected. 



The second correction, less important, for the intermediate differences of the ther- 

 B 13 



