518 SCIENTIFIC RECORD FOR 1883. 



If this is aii influence of radiation, as undoubtedly it is with dry mus- 

 lin, the consequence would be that this influence would increase 

 with increasing difference {t—t'). But the fact is that this influence 

 disappears in psychrometer comparisons more and more in proportion 

 as {t—t') increases at least with the air in motion. This behavior de- 

 mands still more thorough investigation. I will, however, now mention 

 that I consider this phenomenon as attributable to a "sluggishness" 

 of the wet bulb in the neighborhood of the point of atmospheric satu- 

 ration, so that with very damp air the evaporation does not proceed 

 fast enough in proportion to the passing current of air. The experi- 

 ments also confirm this view.* If one would in the formula take ac- 

 count of this "sluggishness" of the psychrometer, then a correction to 

 the psychrometer differences t—t' can be so applied that it shall be a 

 maximum for t—t'=0 and be inversely proportional to this difference. 

 If we call v the maximum value, then this correction will in general be 



v 

 — — ; if, with Kamtz, we take v=0°.o centigrade, then will this 



correction have a sensible influence only up to a depression {t — t') of 0° 

 C, in fact only up to those of 6° C. 



I have applied this correction to my observations and obtain, then, on 

 the average A=0.000943 instead of 0.0010415. [Pernter here alludes to 

 his thirty observations of dry and wet bulbs and dew-point on the sum- 

 mit of the Obir, spoken of in the first part of this paper. In these obser- 

 vations t—t' ranged from 3°.6 C. to 0°.5 C, and its values, corrected by 

 this new formula for sluggishness, ranged from .'P.7 O. to 0°.8 C] 



There remains now only an investigation of the constant a in our 

 psychrometer formula. The psychrometer factor is [see (E) and (I)J 



Aff 



— + 



If the air is motionless, then will a=l. For a definite velocity of venti- 

 lation and equal air-pressures a increases to a value that is constant under 

 these conditions. If however the rate of ventilation remains the same 

 and the barometric pressure varies, then we must investigate whether or 

 not a depends ou the pressure of the air. This we attain in the follow- 



K' 

 ing way; for absolute calm at both high and low pressures is equal 



to unity. In the change from calm to constant rate of ventilation K' 

 and D increase uniformly (at least very nearly so) whether the change 

 goes on at high or low pressures; «K' will always remain equal to aD. 

 Undoubtedly, however, aK' will not be so large at low as at high press- 

 ure, since certainly iu the latter case the mass of the arriving air is 



*Wullner and Grotriau fouud that fluids iu the neighborhood of their points of 

 saturation almost entirely cease to evaporate, and that even when quite far from it, 

 between glass rods (c. g., drops of water), dissipate only very slowly and with diffi- 

 culty, and. this certainly also applies to the meshes of muslin. (See Wiedemann, 

 Annalen, xi, p. 553 to 555 ; also Macaluo, Grimaldi, Gazetta Chimica Italiana, vol. xn, 

 1881.) 



