PSYCHROMETRICAL TABLES. 



in which 



X represents the force of vapor in the air at the time of the observation ; 



t, the temperature of the air in Centigrade degrees, indicated by the dry 



thermometer ; 

 t', the temperature of evaporation given by the wet thermometer ; 

 f, the force of vapor in a saturated air at the temperature t' ; 

 h, the height of the barometer. 



Substituting the Fahrenheit scale for the Centigrade, the formula, for temperatures 

 above the freezing-point, reads 



_ r 0.480 X I (< — O _ r 0.480 (/ — t') 



^ — J ~ GIG — f («'— 32°) '^ ~J ^1130 — V ' 



and below the freezing-point, 



0.480 X I - ^0 , _ r_ 0-480 {I - t') 

 J 689 — I («'— 32°) -^ 1240.2— «' 



Making, further, h = 29.7 English inches, these formulce become 



I 



and 



_ 0-48 (t - t') „Q ^ _ . 14.256 {t - V) 



'^-J— 1130 -«' ^y-^-/— 1130 _</ 



0.480 O-O 14.256 (t - t') 



•^ 1240.2 — «' -^ 1240.2 — «' 



The mean barometric pressure for which the table has been computed, viz. 29.7 

 inches, is, within a small fraction, the same as that adopted in Haeghens's Tables, 

 No. II., which is 755 millimetres = 29.725 Eng, inches. As that slight difference 

 in the barometric pressure cannot cause, in the most extreme cases, a difference ex- 

 ceeding two thousandths of an inch in the elastic forces, the results in the two tables 

 may be considered identical. 



That barometric pressure, corresponding, in our latitudes, to a mean altitude of 250 

 to 300 feet above the sea, is likely to suit, without correction, the largest number of 

 meteorological stations. Should the mean height of the barometer, in consequence 

 of the elevation of the station, much differ from that adopted in the table, a constant 

 correction can be determined, to be applied to the numbers in the table. At the end, 

 page 72, will be found a table which furnishes that correction for barometric heights 

 between 20 and 31 inches, and for values of < — /' between 2° and 26° Fahrenheit. 



The effect of the irregular variations of the barometer at the same station can, in 

 most cases, be neglected ; for the error due to that cause will scarcely ever exceed 

 those which may arise from the uncertainty of the very elements on which the tables 

 are based. 



B 47 



