206 Proceedings of Royal Society of Edinburgh. [sess. i 
observations were made. This rapid variation in the numbers was 
no doubt due to the cross currents mixing masses of upper and 
lower air, so that sometimes one and sometimes the other passed 
the instrument. This seems to be the probable explanation of the 
variations sometimes noticed here, as they were too small to be 
due to local impurities. 
The table shows that the purity of the air, even at this isolated 
situation, varied from day to day, being as high as 4000 particles per 
c.c. on the 4th and as low as 205 per c.c. on the 6th. The higher 
numbers were generally got with southerly winds and the lower 
with northerly ones. This relation of direction of wind to number 
of particles would seem to indicate that the higher numbers were 
due to pollution from the inhabited parts of the country, as all to 
the east and south of this situation is densely populated, whilst in 
the other directions there are but few dwellings. It is true that the 
distance to the densely populated parts is very considerable, yet 
from the extreme slowness with which these fine particles settle, and 
the short time required for the wind to bring them to the place of 
observation, there seems to be a possibility of their being carried as 
far as this. When we remember the number observed on the top 
of Finouillet when the wind blew direct from Toulon, we can 
understand how it is possible for the dust of the densely populated 
parts of the lowlands being brought to the wilds of the Highlands. 
It may be remarked that an examination of the weather charts for 
these dates - T shows that much reliance cannot be placed on this 
conclusion, as the winds observed may have been and sometimes 
were quite local, and the distribution of pressure was such that the 
winds were light and the direction of the general circulation 
frequently doubtful. 
Ben Nevis . 
The Council of the Scottish Meteorological Society having pro- 
posed to acquire dust-counting apparatus for the observatory on the 
top of Ben Nevis, for the purpose of carrying on a regular series of 
observations on the dust in our atmosphere at that situation, it 
became necessary for me to make an ascent of the mountain for the 
purpose of seeing what arrangements would be required for the 
carrying out of this investigation under the conditions existing 
