520 MR OMOND ON THE 



these times — certainly solar radiation is strongest when the sun is highest, viz., at local 

 noon, — and the explanation of the abnormal time of the maximum is clearly that in the 

 middle of the day the cloud canopy drawn up by the sun's heat is thicker, and shades the 

 summit of the hill. That this is the case may be inferred from the three-day means 

 given in the table — first for three cloudless days in June 1891, and then for three days 

 of continuous fog which immediately succeeded them. On June 10th, 11th, and 12th 

 there was no cloud above Ben Nevis, except a trace of cirrus one morning ; and on June 

 13th, 14th, and 15th there was continuous fog, but only one slight shower of rain. 

 The shade temperature on the mean of the three fine days ranged from 44° "5 to 40°'7, 

 and on the foggy days from 37°'3 to 34°'6, taking the hourly averages of the two 

 periods. Thus the shade temperature did not differ very much in the two periods, 

 and its diurnal range was in both cases small. The solar action was practically 

 the same throughout, the days being consecutive and near the summer solstice ; but 

 during the first period the summit was freely exposed to solar radiation, and during 

 the latter it was densely shaded by fog. Observations at sea-level show that on 

 the three clear days there was some cloud below the level of the hill top, the sky 

 being on the average about one-fourth covered, while on the foggy days it was overcast, 

 or very nearly so, all the time. On the three fine days the Black Bulb, as appears 

 from the table, not only shows a much greater range than on the foggy days, but rises 

 with a steady sweep to its maximum of about 81° above the shade temperature about 

 midway between noon and 1 p.m., or just at local noon, and then falls again as regularly 

 in the afternoon. The curve is remarkably smooth, considering it is only the average of 

 three days ; but any individual day of the three would show almost as regular a pro- 

 gression. In utter contrast to this is the curve of the three foggy days. Up to 7 a.m., 

 indeed, and after 4 p.m., the rise and fall is pretty regular ; but between these hours the 

 readings vary wildly, indicating, however, the early maximum and the secondary rise 

 in the afternoon characteristic of the monthly averages. This breaking down of the 

 curve in the middle of the day can only be due to the greater thickness or greater 

 opacity of the clouds then, and the similarity of the monthly curves to the foggy days' 

 curve points to the same conclusion ; for the majority of the days in each month are 

 either foggy or cloudy overhead. There is practically no diurnal range in the number 

 of hours during which the hill is enveloped in fog, and very little in the total amount 

 of cloud ; what there is points the same way as the Black Bulb readings, — the cloudi- 

 ness is greatest in the afternoon; but the change is so small that, if the Black Bulb 

 readings are at all proportional to the intensity of radiation, one would expect this 

 effect to be due rather to a thickening or increased density of cloud than to more 

 frequent cloudiness at these hours. However, this is pushing the record beyond its 

 legitimate functions ; we do not know what the scale of the Black Bulb means, or what 

 the relation between it and the intensity of solar radiation is, so many elements of 

 uncertainty are introduced by the differences in diathermancy of the glass jacket and 

 in the absorptive power of the blackened bulb for different intensities of radiation. It 



