Mr J. Aitken on Dust Particles. 
195 
1889 - 90 .] 
nent and in this country. In the table is entered the number of 
dust particles, per cubic centimetre, observed in the air of the place, 
at the hour and date given, each of the numbers given is an average 
of ten tests made in rapid succession. The direction and force of the 
wind is also entered, as well as the temperature and the humidity. 
The humidity is indicated by the amount to which the wet bulb 
was depressed below the dry one at the time the tests were made. 
These records of humidity are not very satisfactory, owing to the 
difficulty of getting suitable positions for making the observations 
at the different places. In a few cases the observations were 
made with wet and dry bulb thermometers placed at a window, 
and in shade and protected as much as possible from radiation. 
In some cases the humidity was taken with a hygroscope, and its 
readings converted into wet bulb depressions by comparative read- 
ings of the two instruments. In the second last column of the table 
is entered the state of the air with regard to transparency. These 
transparency observations are far from satisfactory, as no special 
means have been adopted of measuring it. It has simply been 
judged by the unaided eye, which is a very rough method of estimat- 
ing it. There are many things which alter the apparent trans- 
parency of the air, and make the estimate taken in this way very 
unsatisfactory. For instance, when the sky is clouded all over the 
air looks more transparent than under a cloudless sky, because the 
haze in the air between us and the distant scene is not illuminated 
by direct sunlight, and therefore does not show so much. Again, 
when looking in the direction away from the sun, the air appears 
clearer than when looking in the opposite direction. Only a very 
rough scale of clearness has therefore been attempted. Some special 
I instrument would require to be devised before more definite records 
can be obtained. 
It will be seen from the table that a great number of particles 
was observed in the air on the 19th March, at the open window of 
the hotel at Hyeres. This high number was evidently due to local 
contamination, the wind at the time blowing from the town. On 
I the 21st the wind was towards the town, and, as will be seen, the air 
was much freer from dust, the number per c.c. falling from 46,000 
to 5000. 
The top of Finouillet was selected for testing the air of 
