

HYGROMETRIC RESEARCHES. 537 



This table shows that both dry and wet bulb thermometers give lower readings in 

 the aspiration-psychrometer when the air is still or comparatively so, and that this 

 affects the wet bulb more than the dry bulb. On Ben Nevis, however, both the 

 Stevenson screen instruments gave lower readings than those of the aspiration- 

 psychrometer with winds over force 2, and the dry bulb with winds of force 1 and over. 

 This suggests that thermometers should, when tested, be compared with standard 

 instruments in air currents of different speeds as well as in a calm room. 



Wind, however, is not the only modifying factor to be considered. In foggy weather 

 at the summit of Ben Nevis the two thermometers give similar records even in calm 

 weather, but when the air is dry and the Sunshine considerable, differences of 

 temperatures are registered. 



The following table has been prepared from the more continuous hourly 

 observations at both summit and base stations. The differences between the readings 

 of the dry bulb thermometers in the Stevenson screen and of the aspiration-psychrometer 

 are shown at different hours, and the fog, force of wind (scale — 12), and Sunshine 

 (scale — 4), are noted. 



The influence of wind in promoting uniformity of records is clearly shown in this 

 table. Compare the Ben Nevis records on the 12th, 17th, and 18th, after 10 hours on 

 the 1 0th September 1894. On these days the Sun was unclouded. On the 12th, when 

 the air was still, the Stevenson screen thermometer indicated a temperature averaging 

 0° - 34 C. above that of the aspiration-thermometer. On the other days, when the wind 

 varied from force 1 — 2 to force 4, the temperature indicated by the Stevenson screen 

 instrument averaged 0*10 C. below that of the aspiration-psychrometer. 



The same results are obtained at Fort- William. Compare, for instance, the 

 calm sunny 11th September 1893 with the equally sunny 18th May 1894, when the 

 wind force varied from 2 — 3 to 3 — 4. The average difference of temperature readings 

 in Stevenson screen and aspiration-psychrometer was + 0'70 C. on the calm day, but 

 only + - 20 C. on the windy one. 



The influence of the Sun on the differences of the readings is equally notable unless 

 the wind is strong. When observations made in fog or in winds over force 2 are 

 eliminated, an almost smooth curve of differences is obtained with a maximum excess 

 in the Stevenson screen readings at 13 hours. At both stations higher temperatures 

 are most marked in the Stevenson screen between 11 and 17 hours, and can be 

 explained by the heating of this screen. In the early morning and towards and after 

 sunset the differences between the two differently situated thermometers are not so 

 great at Fort- William, but that in the Stevenson still is the higher, whereas at Ben 

 Nevis the Stevenson screen thermometer reads lower than that in the aspiration- 

 psychrometer. This may be a function of radiation. 



During foggy or misty weather the two thermometers give very similar readings at 

 all hours, even when the air is calm. 



Eliminating the effects due to the heating or cooling of the Stevenson screen, the 



TRANS. ROY. SOC. BDIN. — VOL. XLIII. 3 Y 



