PSYCHROMETRICAL TABLES. 83 



in which 



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



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



thermometer ; 



$', the temperature of evaporation given by the wet thermometer ; 

 y, the force of vapor in a saturated air at the temperature V ; 

 7i, 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 f (t ?') __ f 0.480 (t V) ^ 



610 -f (t! '62) "1 130 t' 



and below the freezing-point, 



0.480_X f (t - /Q 0.480_(<_-0 



J 689| (t' 32) J 1272.2 t' 



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



_ _ 0.480 (tQ _ 14.256 i_g _Q 



J 1180*' 1130 V 



and 



0.480 (f-Q = _ __ 14.256 (t - t') 



J 1272.2 Z' ^ 1272.2 t' 



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 requiring a correction, the largest num- 

 ber of meteorological stations. Should the mean height of the barometer, in conse- 

 quence of the elevation of the station, much differ from that adopted in the table, a con- 

 staut correction can be determined, to beapplied to the numbers in the table. Attheend, 

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

 between 20 and 31 inches, and for values of t t' 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 



