90 THE ROYAL SOCIETY OF CANADA 
In order to use the apparatus as a hygrometer it was necessary 
to make comparisons over a large range, as the proportionality was 
not so complete for lower vapour pressures. This was to be expected 
not only because the theory (incomplete) gives no indication of a true 
linear law but also because the influence of certain constant factors 
on the evaporation from the suspended paper was appreciable. A 
change in the rate of flow of the air, for example, would alter a reading 
sometimes by as much as three per cent. The following table gives a 
‘sample series of a number of comparisons of this hygrometer with 
other methods, after it had been calibrated. The results aie given 
as vapour pressures in equivalent cms. of mercury, calculated in the 
case of the dew-point and the wet-and-dry bulb instruments from 
Marvin's psychrometric tables. The advantage of the new method 
is more apparent at the lower humidities, but the agreement through- 
out is much better than that of an ordinary series taken without 
special precautions. 
TABLE OF ATMOSFHERIC VAPOUR PRESSURES ON VARIOUS DATES, AS DETERMINED 
BY THE DIFFERENT METHODS. 






| | | 
| | _ Absolute Methods. 
| | | 
Tempera- Stationary Sling | Dew-point | Weighing | Absorption New 
ture. Wet-and- Psy- | Method. | (old (new Weighing 
dry Bulb. chrometer.| | method). | method). | Method. 
| | | 
24.1°C 1-73 cms. | 1:70 cme. | 1-71 cms. | 1:716cms.| 1-73 cms. | 1-72 ems. 
Ons 0:89 “ 0.8675 0 86 Does OS 08720 
AO D'ACME | One A Oni Ih Oars DRE OSEO PIRE 
20-0 “ O82 ae NO ga Ubi a oer Mas a ees [OC 7LT CHOSES 
22-9)“ O69 9 54 0 SO EN DES ie glee eee. 0:57 “ ~| O-5ome. 
Dio sje 049€ 0-36 “ DOS | RS OR 0-290 | 0295 = 
205% OMA Oe See ADS TRIO STE | Se [HOPPER 0-28 “ 



A convenient form of the apparatus can be constructed by 
hanging the moistened paper from a delicate spring in a long vertical 
glass tube open at the ends, a small pointer indicating on a scale 
the vapour pressure corresponding to the various extensions. A 
steady circulation of the room air can be obtained by burning a very 
small “pilot light,” gas or electric, just inside the top of the tube.! 

1 The idea of using a steady convection current in this way instead of the more 
cumbrous suction method, was suggested to the writer by Dr. L. V. King of McGill 
University. 
