METEOROLOGY OF BEN NEVIS. xxiii 



diminution with height are intimately connected with the cyclones and anti- 

 cyclones of north-western Europe, and form data, as valuable as they are 

 unique, in forecasting storms. 



But the wet bulb thermometer furnishes observations even more remark- 

 able than those which indicate simply the temperature of the air. Indeed, the 

 most striking feature of the climate of Ben Nevis is the repeated occurrence 

 of excessive droughts. Of these droughts the following thermometric readings, 

 made on July 30, 1885, afford a good example : — 





Dry. 



Wet. 



Diff. 



Dew-Point. 



1 A.M. . 



. 48°-8 



46°-9 



r-9 



44°-8 



2 „ . • 



. 48°-3 



45°-3 



3°0 



42°-0 



3 „ • 



. 50°-3 



36°2 



14°-1 



21°3 



4 „ • 



. 49°-7 



36°-2 



13°5 



21°-8 



Such low humidities and dew-points, sharply marked off from high humidi- 

 ties and dew-points, frequently occur. Several remarkable instances which 

 occurred during the summer and early autumn of 1885 form a valuable 

 contribution to the science, especially when viewed in connection with the 

 irregular geographical distribution of the disastrous frosts which occurred in 

 some districts, but not in others, on subsequent evenings. The observations 

 show comparatively brief continued states of the atmosphere characterised by 

 clear skies and low humidities to have repeatedly passed the Observatory, which 

 were closely preceded and followed by states of the atmosphere showing- 

 large humidity. Now it is evident that, during the night when temperature 

 falls through the effects of terrestrial radiation, those parts of the country will 

 suffer most from frosts over which these very dry states of the air pass or rest ; 

 whereas those districts over which a more humid atmosphere hangs will escape. 

 On the night of August 31 to September 1 of 1885, the potato crop was totally 

 destroyed, and grain crops in flower at the time were also destroyed by 

 frost, in various localities in the valleys of Tweed, Nith, Clyde, and Spey. 

 Over middle and upper Speyside, temperature on the ground fell to 15°'0, with 

 results most disastrous to these crops ; whereas at Dalnaspidal, in the district 

 immediately contiguous, potatoes were scarcely if at all blackened. 



But by far the greatest drought recorded at the Observatory was in March 

 1886, which commenced at 1 a.m. on the 11th, and ended at midnight of the 

 12th, thus extending over forty-eight hours. The mean humidity, calculated 

 from Apjohn's formula, of the first twenty-four hours was 19, and of the 

 second twenty-four hours 15, the least humidity being 6 at 8 p.m. of the 12th. 

 The three consecutive hours of greatest dryness were the following : — 



