1889 - 90 .] Mr J. Aitken on Dust Particles. 203 
conditions. If the hill is a long ridge, and the wind blowing at 
right angles to its length, the probability is the impure air of the 
valley will come almost unmixed with pure air to the hill top. 
Balloons might be used to carry the observer free from the pollution 
of the lower air, but they are evidently unsuitable for observations 
of this kind. The Eiffel Tower, however, looks as if it had been 
specially constructed for the purpose ; only it does not go high 
enough and is not movable. The wind-driven air passes through the 
open framework of the tower, and the structure has no tendency to 
cause the lower air to rise and mix with the upper. Its position in a 
city, however, confines any investigation done on it, to the special con- 
ditions, which are only in a secondary way meteorological. Though 
not suitable for general meteorological work, the tower is evidently 
very suitable for investigating the vertical distribution of the im- 
purities rising into the air over cities. As it is indeed the only place 
where satisfactory work of this kind can be carried on, I determined 
to return by Paris and if possible make some observations on the top 
of this tower. Owing to the kindness of M. Eiffel I was enabled to 
get access to the tower before it was opened to the public, M. 
Eiffel very kindly accompanying me to the first stage by the lift. 
Erom there I ascended to the top by the stairs, the upper lifts not 
being then finished. 
The observations on the Eiffel Tower were made on the 29th 
May. The day was cloudy, with passing showers and strong 
southerly wind. On my way up the tower I stopped for a short 
time at the platform about 100 m. from the top, and tested the air 
at this elevation. The number at this height was 41,000 per c.c., 
showing the air to be very much polluted to a height of 200 m. I 
then ascended to the top, passing through the top platform and 
ascending some distance higher, and made the test on the small 
gallery just under the electric lamp, at about 300 m., or a little 
under 1000 feet from the ground. Tests were made at this eleva- 
tion from 10.15 a.m. till 1 p.m. 
My intention before starting was to make a series of tests at 
different elevations to try and find the rate at which the city im- 
purities decreased with the height above the ground. A few 
observations on the top soon showed this plan to be impracticable, 
that with anything like the limited series of tests I would be able 
