842 
Proceedings of the Boyal Society 
subtracting the co-ordinates of the mean wind for the whole 
day from those of the mean wind at any hour, we get what may 
be called the difference wind of that hour ; that is to say, the 
wind which must be added to the mean wind of the day to produce 
the wind of that hour. The difference winds thus got show a 
well-marked diurnal variation. From 3 to 8 a.m. there is a 
northerly wind of about miles an hour, and from 11 a.m. to 
2 p.m. there is a south or S.S.E. wind of about 3 miles an hour. 
At the other hours the difference winds are small and variable in 
direction, except about midnight, when there is an indication of a 
moderate difference wind in the same direction as the mean daily 
wind. This last may be due to the daily variation in velocity 
(independent of direction) which is such a well-marked feature on 
Ben Nevis. When sufficient data are collected, I hope to be able 
to get a clue as to how far this diurnal variation in velocity is due 
to a horizontal wind and how far to vertical currents. It may be 
noted in passing, that the hours at which these difference winds 
change their direction have no connection with the hours of change 
of watch at the Observatory, so they cannot be due to any difference 
in the estimation of the wind by different observers. 
The most marked features in the diurnal variation are the northerly 
winds shortly after sunrise, and the southerly ones about noon. I 
think the most obvious explanation of their cause is to be found in 
the shape of Ben Nevis. The ridge of the hill top runs east and 
west ; on the north side is a deep gorge with precipitous sides, into 
which the sun only penetrates for a few hours after sunrise in 
summer ; the south side consists of a steep slope going down into 
Glen Nevis at an angle of about 30 degrees. In the early morning 
the sun’s rays slanting down the northern valley would warm the 
air in it, while the south side of the hill was still cold from the 
night’s radiation, and thus cause a northerly wind across the top. 
As soon as the sun swings round to the south the northern valley 
is in shade, while the bare stony slope to southward gets warmed in 
its turn, and a reverse or southerly wind is set up. 
If the above explanation is correct, these winds are purely local, 
and not connected with any general system of land and sea breezes. 
To attempt to explain them on the latter hypothesis, I believe it 
would be necessary to assume that Ben Nevis was in the back- 
