I 



PSYCHROMETRICAL TABLES. §3 



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 



_ , 0. 480 X f (/ — i') . _ r _ 0^0 (^tj- V) 

 * — / 610 — I («' — 32°) J 1130 — <' ^ ' 



and below the freezing-point, 



_ ,_ 0.480 X f (< — V) J _ r_ 0.480 (t — t') 

 ^ — / 689 — 10'— 32°) * ~ -^ 1272.2— <' 



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



_ 0.480 (t - i) oq 7 _ f 14.256 {t - V) 



""-J— 1130 — <' ^^''-J— 1130 — <' 



and 



0.480 O-O _ 14.256 O-O 



•^ 1272.2 — i' '*^-' — ^ 1272.2 — «' 



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 abovethesea,islikely 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- 

 stant 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 t — if 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 ta,bles 

 are based. 



B 47 



