196 Proceedings of Royal Society of Edinburgh. [sess. 
the district surrounding Hy&res on account of its position and 
shape. The hill is situated near the centre of a plain ; on its south, 
west, and north sides it rises with steep slopes to a rather fine 
point. Its top rises therefore into what we might expect to he the 
pure air of the district. An examination, however, of the table 
will show that no very low number was obtained at this situation, 
never less than 3500 per c.c. The reason for this would appear to 
be that the hill is surrounded on three sides by a highly cultivated 
plain, which is dotted all over with the houses of the peasantry, and 
at many places there are villages. The air of this district is there- 
fore very much polluted with the products of combustion, and 
though the situation where the observations were made is nearly 
1000 feet high, and the hill rises abruptly from the plain, yet it is 
evident that the polluted lower air came up to the top of the hill, 
being driven up the slopes by the wind. This ascent of the lower 
air to the tops of hills has been frequently observed, and will be 
referred to later. The rapid change in the number of particles 
observed at short intervals, showed that the lower air was rising 
to the hill top. On the 23rd and 25th the numbers varied greatly 
owing to the impure lower air being imperfectly mixed with the 
purer air above. It is no doubt possible that some of this variation 
may have been due to change of direction of wind bringing air from 
more or less polluted parts. 
It will be observed that on the 4th of April the number of 
particles at this situation was remarkably great, the number being 
as high as 25,000 per c.c. This high number was fairly steady 
during all the tests made on that day. When I had tested the 
different parts of the dust counter and found no reasonable cause 
for doubting its indications, it became necessary to look for some 
outside cause for this very high number. On looking in the direc- 
tion of Toulon, distant about nine miles, it was seen that the wind 
was blowing direct from that town, and bringing the products of 
combustion to the place of observation ; the smoke being traced for 
some distance from the town coming in a straight line towards 
Finouillet. 
On the 29th March observations were made at La Plage with 
the view of examining the air resting on the sea. These observa- 
tions were made at three different places near the shore. The first 
