222 PKOGRESS OF METEOROLOGY IN 1889. 



of dry air, >S^i the specific heat of raoist air referred to unit mass of dry 

 air, a the specific gravity of vapor referred to air at the same tempera- 

 ture aud pressure, 



B—pi Si m "1 





1 [a] 



a jjeueral aud strict formula for the psych rometer under the conditious. 

 Tliis reduces to August's formula ou iutroduciug the assumption that 

 the whole of the air which is reduced in temperature becomes at the 

 same time saturated by the evaporation, in other words, that mi=m. 



(2) The heat derived from the cooling of m' is assumed to represent 

 all the heat derived from conduction, radiation, local convection, and 

 the independent general motion of the air. By reckoning separately 

 that part of the heat supplied to the bulb by radiation in time Z as 

 equal to ZOR {t—ti) (0 being the area of the surface of the bulb, and E 

 the coefficient of radiation), we get 



, , m' nil 



where ^=z' ^'^ Z 



In order to express the effect of the velocity of motion of the air, 

 assume that q^ and (^ are linear functions of the velocity v with a coeffi- 

 cient f, that they are equal when v is infinite, and that their values, 

 when X) is zero, are ^^i and q^j respectively. 



Substituting, we get the following general formula for the psychrora- 

 eter, in ujoving air, with spherical bulb, radius r: 



, ,4:7tB 8 [ (5) 



where ?V= ? ~; ^= -f 



The values of IS, A, c, and B are known, and can be substituted (A= 

 G06.5— 0.G1)5 t, for water-covered bulbs, and 085.5—0.095 f, for ice-cov- 

 ered bulbs). 



The values of q^i and q'n are not known a priori., but they may be 

 regarded as constant for a given velocity, so ^ and ^ can be deter- 

 mined from observations with the psychrometer upon air of known 

 humidity moving with known velocity, and thus a numerical formula 

 of reduction obtained. It is assumed that the radiation effect is the 

 same in moving air as in still air. 



(3) From this general equation the formulas hitherto employed can be 

 deduced by the introduction of the special assumptions upon which they 

 are respectively based. To obtain August's formula (corrected for ra- 

 diation) K' must be put equal to X>, and (i equal to (i,. In Maxwell's 

 formula p and ft, are both infinite. Ferrel's formula for moving air is 



