1890-91.] Mr Aitken on Water Particles in a Fog. 
261 
was 30 drops per min. per sq. mm., that is 3000 per sq. cm., or 
19,350 per square inch per min. This high number never lasted 
for long, and in the intervals the number fell as low as 300 per sq. 
cm., or to one-tenth ; the temperature at the time was 29°. Two 
days later, that is on the 21st of February, the air was again very 
foggy, about the same thickness as it was on the 19th at 10 a.m., 
and all exposed surfaces were dry; the number counted was 13 
per sq. mm. per min., that is 1300 per sq. cm. or 8385 per sq. in. 
per min. This number remained fairly constant on this occasion, 
and slowly diminished as the fog cleared away. The temperature 
at the time was 31°. On both of these occasions the temperature 
had not risen more than |° above the night minimum. The number 
of dust particles in the air was also counted on these occasions, and 
on both days the number was very high, varying from 45,000 to 
80,000 per c.c. 
The number of water particles in a fog, as given by these observa- 
tions, seems to be very large, and it is difficult to imagine how they 
evaporate so quickly that exposed surfaces are not wetted by them. 
It must, however, be remembered that they are very small, so small 
that they are not felt falling on the hands or face of the observer. 
Indeed, it is probable they never touch the skin. These fog drops 
are very similar in size to the little drops artificially produced in 
the “Dust-counter,” and it is found that, if the stage of the “Dust- 
counter ” is slightly heated, the drops never reach its surface, 
but are evaporated in the slightly heated layer of air over it. 
If we knew the size of these drops we might be able to calculate 
the velocity of their fall, and from that obtain the number per given 
volume of air. As it would be more satisfactory to obtain this 
number from direct observation, the second form of the instrument 
has been designed. It is constructed on the same principle as the 
other one, but an arrangement is made by means of which the 
number of particles that fall from a known height are counted. 
My first attempts in this direction were not satisfactory, owing to 
using a magnifying lens for observing. This limited the height of 
air out of which the drops fell to little more than 1 cm. Another 
instrument has since been constructed in which this difficulty is 
overcome. In place of a short focussed lens, a low power microscope 
is used. This enables us to get easily 5 cm. of air over the stage. 
VOL. XVIII. 1/6/91 z 
