204 Proceedings of Royal Society of Edinburgh. [sess. 
to make it would be impossible to find any regular diminution at 
the different heights. I therefore confined my attention and gave 
most of my time to the variations taking place at the top, as a series 
of observations made there seemed to promise to give more informa- 
tion than a few taken at different heights. 
It will be seen from the table that the numbers varied greatly 
from time to time, the changes taking place very quickly ; indeed, 
it was often impossible to change the quantity of air used quick 
enough to meet the changing conditions. For instance, when the 
particles were few for a short time, they would suddenly increase to 
such a number that the quantity of dusty air which gave a con- 
venient number of particles for counting gave in the next test a 
number so great they formed a fine fog, so dense it was impossible 
to count the particles. The two extremes which were counted with 
any degree of certainty were 104,000 per c.c. and 226 per c.c. The 
last and low number remained fairly constant during the taking of 
the ten tests of which the above is the average. This small number 
was obtained while a shower of rain was passing over the tower. As 
this rain was quite local, falling only over a small breadth of the 
city, the descending drops would strike down the impure city air 
and at the same time draw the pure upper air downwards. It seems 
probable that while this shower was passing, the top of the tower 
was in nearly pure air, probably of a level higher than its top. 
An examination of the Eiffel Tower figures show that the impure 
air of the city must rise to a much greater height than 1000 feet. 
They also show that this air does not diffuse itself uniformly 
upwards, but rises in great masses, the rapid rise and fall in the 
numbers showing that the hot city air rises in columns through the 
purer air above, and that at considerable elevations over a city the 
air is of a very different character at places quite close to each other. 
It seems possible that a sensitive quick-acting thermometer might 
show these changes in the passing air by the rise and fall in the 
temperature. This want of homogeneity, apart from the dust and 
impurities, is one of the causes of the unsuitableness of the air 
near a city for astronomical observations. 
On the way down from the top of the tower a halt was made at the 
lower platforms, and tests made at a height of 200 m. and at 115 m. 
As will be seen from the table, the results of these tests are of little 
