200 Proceedings of Royal Society of Edinburgh . [sess. 
on will have something further to say with regard to certain meteoro- 
logical phenomena observed here. I arrived at the top of the moun- 
tain about midday on the 21st May. On the way up we entered 
a dense fog at about 2000 feet from the top. This fog continued all 
the way up, and at the top it was impossible to see more than 50 
yards in front. I began work at 2 o’clock, selecting the top of the 
hill as the most suitable place for observing. The result was not 
satisfactory. The number of particles was found to vary greatly, 
going up to 1300 per c.c. and falling to under 500 in a short 
interval. This at once suggested local contamination, from which 
I was unsuccessful in freeing the observations, by change of position, 
as the thickness of the air prevented me seeing from what direc- 
tion the polluted air might come • I had therefore to return to the 
hotel for information as to the direction of all possible sources 
of local pollution. I was then informed that the gas-work, engine- 
house, and other buildings connected with the hotel were situated 
on the slope of the hill to the windward, and within a few hundred 
yards of the place of observation. Having ascertained the direction 
of all possible sources of local pollution, a new situation was selected, 
well clear of all possible contamination, and a number of tests 
made. In this situation the number of particles was fairly constant, 
being 285 per c.c. when the testing began, and falling to 210 an 
hour later. 
The morning of the 22nd opened bright and fine. The fog had 
cleared off the hill top, the clouds having sunk to a lower level and 
cleared away a good deal to the south ; but to the north the whole 
country was covered with a nearly uniform layer of clouds, the 
upper surface of which was about 1000 feet below the top of the 
mountain. In the morning the number of particles was about 800. 
per c.c., at 10.30 a.m. the number fell to about 500, and in the 
afternoon the number was 1450 and gradually increased to 2300. 
During the afternoon observations an occasional cloud passed over 
the hill top. 
As will be seen from the table, the number of particles on the 
varied greatly during the morning observations, the successive 
tests made within an hour varying from about 500 per c.c. to 1100 
per c.c. This want of uniformity in the passing air may have been 
due to the strong wind blowing at the time, bringing imperfectly 
