1896 - 97 .] T. S. Muir on Observing Station on Ben Nevis. 287 
On seven days well spread over the twenty -three, Ben Nevis, 
so far as visible from the station, was either all day or part of the 
day clear of fog. (I may remark that I could see to within 500 
or 600 feet of the summit.) There was no day, however, when fog 
was not observed at some hour or other, but the 13th and 21st 
were almost wholly clear. 
It frequently happened that the station was like an island 
surrounded by a sea of fog, which tried in vain to envelop it. 
The area thus unaffected was about 500 or 600 yards long and 100 
to 200 yards broad. This is quite a distinct case from thin fog. 
It seemed as if the station were between two fog-systems, for fre- 
quently a view could be obtained betwixt them to the island of 
Lismore, thirty-two miles distant. The cause of this phenomenon 
is uncertain. The wind was always northerly when it occurred. 
Perhaps the shoulder of the mountain served to give the fog an 
upward and oblique slant, which carried on to Meall-an’t-Suidhe, 
thus leaving the station clear. There were eight days when this 
was well defined. Once or twice it happened that fog coming 
down Glen Nevis, below the level of the station and apparently 
from the south, was met at the lake by the north wind and carried 
backwards and upwards, or dissipated (1st and 2nd September). 
In another case, when the wind was variable and light, fog coming 
from the north, between 2000 and 3000 feet, was met by west 
wind at the head of the corrie below and dissipated. It was very 
curious to see a cloud rolling along up to a certain point only to 
vanish into thin air. 
Table Y. — The amount of cloud, . as distinct from fog or mist, 
was large. Taking a hundred to represent an overcast sky, the 
mean amount at 9 h. was 85 ; at 10 h., 81 ; at 12 h., 85 ; at 
II h., 88 ; at 16 h., 80; at 18 h., 86 ; and at 21 h., 75. The 
maximum was at II h. and the minimum at 21 h. It happened 
frequently that a starry night succeeded a cloudy day. On each 
of the three occasions when the intermediate temperature approxi- 
mated to that of Fort- William, the sky was either wholly clear, 
as on the night of the 10th, or almost cloudless. On the other 
hand, when the middle temperature was very close to that of the 
summit, the sky was overcast, or nearly so. 
