536 



MR A. J. HERBERTSON ON 



prevent any radiation affecting the bulbs, and the apparatus may be held by the hand 

 just over the bulbs without affecting the temperature reading, but care must be taken 

 that the current of air drawn past the bulbs is not heated in any abnormal way. 



Professor Sprung of Berlin, by means of direct dew-point and gravimetric 

 determinations, has arrived at the following formula for reducing observations made by 

 the Assmann aspiration-psychrometer : — 



/=/ - 0-5 (t- 



■if) P ~ 



which is practically identical with that given by Professor Ferrel. 



The tables calculated from these formulae may be used for expressing dry and wet bulb 

 readings as vapour pressures, provided the air was moving at least at 2^ metres per second. 

 In the reduction of observations made with stationary instruments hung in a Stevenson's 

 screen, the same tables could be used for readings made in windy weather ; but, in more 

 or less calm conditions, a considerable correction would need to be applied to the 

 departure of the wet bulb before any approximately accurate determination of vapou 

 pressure could be calculated. The rapid variation of this correction in almost calm 

 air makes it doubtful if the ordinary estimates of wind speed are sufficiently 

 minute to give complete confidence in the results of its application under such 

 conditions. 



Comparison of Dry and Wet Bulb Records in Stevenson Screen and Aspiration- 

 Psychrometer under Different Weather Conditions. 



The observations made at Ben Nevis and Fort- William during the course of the 

 winter investigations have been tabulated under different wind forces, the differences 

 in 0° between the readings of the dry and wet bulb thermometers exposed in the 

 ordinary Stevenson screen and those of the aspiration-psychrometer, at different forces 

 of the wind, are summarised in the following table : — 



Force of the Wind (1-12.) 







0-1 



1 



1-2 



2 & over. 



C° 



- 0-21 



- 0-16 



- 0-05 



4-0-17 

 + 0-28 



- 011 



Ben Nevis Observatory — 



Dry (Stevenson — Aspiration) Readings . 

 Wet do. do. 



Departure of Wet from Dry do. 



C° 



4- 0-31 

 + 0-62 

 - 0-31 



C° 



4- 0-27 

 4- 0-39 

 - 0-12 



C° 



- 004 

 + 0-14 



- 0-18 



C° 



- 0-09 

 4-0-17 



- 0-26 



Fort- William Observatori/- 



Dry (Stevenson — Aspiration) Readings . 

 Wet do. do. 

 1 )ry-Wet do. do. 



4- 0-51 

 4- 0-61 



- o-io 



+ 0-49 

 4- 0-61 



- 0-12 



+ 0-34 

 + 0-44 

 - 0-10 



+ 0-32 

 + 0-44 

 - 0-12 



