532 



MR A. J. HERBERTSON ON 



temperature of the wet bulb t', the air temperature is t, and p is the barometric- 

 pressure.* 



The following table shows that vapour pressures calculated from Regnault's formula 

 do not closely agree with those founded on Glaisher's factors in low temperatures and 

 low humidities. 



Table. — Comparing Vapour Pressures, in inches of Mercury, and Relative Humidities calculated 



by Glaislier's and by Guy of s Tables. 



The Temperatures are in F°. 



Wet Bulb. 

 Dry - Wet. 



15° 

 1° 



•060 

 •074 



2 



3° 



30° 



40° 



1° 



2° 



1° 



3° 



5° 



7° 



1° 



3° 



5° 



7° 



10° 



Vapour Pressure — 



Glaisher, 



Guyot, . 



Glaisher-Guyot, . 



Relative Humidities— 



Glaisher, 

 Guyot, . 



Glaisher-Guyot, . 



•078 

 •096 



059 



•085 



•148 

 •155 



•129 

 •132 



■118 

 109 



•107 

 •086 



•235 

 •235 



•215 

 •208 



•197 

 •182 



•181 

 •156 



•162 

 •117 1 



-•014 



-•018 



-•016 



-•007 



-•003 



+ •009+ -021 





+ •007 



+ •015 



+ •025 



+ •045 



67 



83 



70 

 85J 



50 

 72 



85 

 89 



69 

 70 



58 49 

 54 39 



92 

 91 



78 

 75 



66 

 61 



56 



48 



45 

 32 



-16 



-15* 



-22 



-4 



-1 



+ 4 | + 10 



+ 1 



+ 3 



+ 5 



+ 8 



+ 13 



Above 40° F. (say 4 0, 5 C.) these differences are not so marked, especially when the 

 air is not very dry. 



A graphic representation shows this difference at low temperatures even more 

 vividly. In fig. 1 the mass of water vapour per cubic foot is used instead of vapour 

 pressure, and the dot and dash line for - 87 grain per cubic foot has been plotted from 

 data found in Guyot's tables (Tables VII. and X.). No correction has been made 

 for pressure. 



The results of the preliminary Observations at Ben Nevis have already been 



discussed. t From them a diagram was drawn in which the abscissae represent 



temperatures of the dry-bulb thermometer, the ordinates the difference between 



the temperatures of dry and wet bulb thermometers, and the mass of water is noted 



for each experiment. It was found that they could be used for the construction of 



three straight lines drawn through points representing 0*5, TO, and 2*0 milligrammes 



of water vapour per litre of air. (See fig. 1.) 



* Except in the comparison of Glaisher and Guyot's tables, the barometric and vapour pressures are 

 expressed in millimetres of Mercury, and the temperatures are given in C°. 



t See Proc. R.S.K., xx. pp. 176-184 ; Journ. Scot. Met. Soc, x. pp. 41-49. 



