140 



XII. 



PSYCHROMETRICAL TABLE, 



giving the temperature of the dew-point, the force and the weight of 

 vapor in the atmosphere, and its relative humidity, deduced from the 

 indications of the psychrometer, or dry and wet bulb thermometers. 



' By James Glaisher. 



This elaborate table, first published in London, in 1847, in pamphlet form, by J. 

 Glaisher, of the Royal Observatory at Greenwich, is based on the tables of elastic 

 forces of vapor deduced from Dalton's experiments, and given above. Table XI. 



The weight of a cubic foot of dry air at 32° Fahrenheit, and under the barometric 

 pressure of 30 inches, which has been adopted by Glaisher, and from which the 

 weight of vapor in a cubic foot of air is derived, is the mean of the determinations 

 obtained by Shuckburgh and by Biot and Arago, which is 563.2154 grains Troy; 

 563 being the number actually used in the calculations. See Preface to the Table, 

 p. 13, and also the Greenwich Meteorological Observations for 1842, p. xlvi. 



The coefficient of the expansion of air which has been employed is that deter- 

 mined by the experiments of Gay-Lussac, according to which the air expands 0.00375 

 of its bulk for 1° Centigrade, or ^^ ^ for 1° Fahrenheit. 



All these values, as may be seen by comparing Tables VI. and XI. of the elastic 

 forces, and also page 92, materially differ from those more recently determined with 

 great care by Regnault, and on which are based the Psychrometrical Tables given 

 above, page 50 et seq. This will account for the no inconsiderable differences often 

 found between the results in the two tables derived from the same data. A few 

 examples, taken from various parts of the tables, may be given here, in order to 

 enable the meteorologist to judge of the amount of the discrepancies which may occur 

 in the results when computed from different hygrometrical constants. 



1. Suppose the temperature of the air indicated by the dry thermome- 

 ter to be =10° F. 

 The temperature of evaporation indicated by the wet thermometer = 9° F. 



Difference 1° F. 



Then, Glaisher's table gives. 



The Force of Vapor = 0.065 inch. 



The Relative Humidity = 0.730 

 Guyot's table gives. 



The Force of Vapor = 0.054 inch. 



The Relative Humidity — 0.791 

 B 104 



