XXXVlll . INTRODUCTION. 



Table 38* gives the weight of aqueous vapor in grains in a cubic foot of 

 saturated air for dew-points given to every o.°5 from — i9.°5 to 115' F., the 

 values being computed to the thousandth of a grain. 



The computation of Tables 38 and 39 has been furnished by Prof Wm. 

 N^^Libbey, jr. 



REDUCTION OF OBSERVATIONS WITH THE PSYCHROMETER AND 

 DETERMINATION OF RELATIVE HUMIDITY. 



The psychrometric formula derived by Maxwell, Stefan, August, 

 Regnault and others is, in its simplest form, 



f=A-AB{t-Q, 

 in which / = Air temperature. 



/, = Temperature of the wet-bulb thermometer. 

 /= Pressure of aqueous vapor in the air. 



A — Pressure of aqueous vapor in saturated air at temperature /,. 

 B = Barometric pressure. 



A = A. quantity which, for the same instrument and for certain 

 conditions, is a constant, or a function depending in a 

 small measure on /, . 



The important advance made since the time of Regnault consists in 

 recognizing that the value of A differs materially according to whether the 

 wet-bulb is in quiet or moving air. This was experimentally demonstrated 

 by the distinguished Italian physicist, Belli, in 1830, and was well known 

 to Espy, who always used a whirled psychrometer. The latter describes 

 his practice as follows : ' ' When experimenting to ascertain the dew-point 

 by means of the wet-bulb, I always swung both thermometers moderately 

 in the air, having first ascertained that a moderate movement produced 

 the same depression as a rapid one." 



The principles and methods of these two pioneers in accurate psychrom- 

 etry have now come to be adopted in the standard practice of meteor- 

 ologists, and psychrometric tables are adapted to the use of a whirled or 

 ventilated instrument. 



The factor A depends in theory upon the size and shape of the ther- 

 mometer bulb, largeness of stem and velocity of v^eutilation, and different 

 formulae and tables would accordingly be required for different instruments. 

 But by using a ventilating velocity of three metres or more per second, the 

 differences in the results given by different instruments vanish, and the same 

 tables can be adapted to any kind of a thermometer and to all changes of 

 velocity above that which gives sensibly the greatest depression of the wet- 

 bulb temperature ; and with this arrangement there is no necessity to 

 measure or estimate the velocity in each case further than to be certain that 

 it does not fall below the assigned limit. 



*The table has been computed with the factor ir.7449, which results from Clarke's 

 value for the conversion of the metre, instead of with the value 11.7459 above derived. 



