A FOG-COUNTER
Next to the enumeration of the dust-particles in the atmosphere is the
marvellous accuracy of counting the number of particles in a fog. The same
ingenious inventor has constructed a fog-counter for the purpose; and the
number of fog-particles in a cubic inch can be ascertained. This
instrument consists of a glass micrometer divided into squares of a known
size, and a strong microscope for observing the drops on the stage. The
space between the micrometer and the microscope is open, so that the air
passes freely over the stage; and the drops that fall on its surface are
easily seen. These drops are very small; many of them when spread on the
glass are no more than the five-hundredth of an inch in diameter.
In observing these drops, the attention requires to be confined to a
limited area of the stage, as many of the drops rapidly evaporate, some
almost as soon as they touch the glass, whilst the large ones remain a few
seconds.
In one set of Dr. Aitken's observations, in February 1891, the fog was so
thick that objects beyond a hundred yards were quite invisible. The number
of drops falling per second varied greatly from time to time. The greatest
number was 323 drops per square inch in one second. The high number never
lasted for long, and in the intervals the number fell as low as 32, or to
one-tenth.
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 in a cubic inch.
An ingenious addition is put to the instrument in order to ascertain this
directly. It is constructed so as to ascertain the number of particles
that fall from a known height. Under a low-power microscope, and
concentric with it, is mounted a tube 2 inches long and 1-1/2 inch in
diameter, with a bottom and a cover, which are fixed to an axis parallel
with the axis of the tube, so that, by turning a handle, these can be slid
sideways, closing or opening the tube at both ends when required. In the
top is a small opening, corresponding to the lens of the microscope, and
in the centre of the bottom is placed the observing-stage illumined by a
spot-mirror. The handle is turned, and the ends are open to admit the
foggy air. The handle is quickly reversed, and the ends are closed,
enabling the observer to count on the stage all the fog-particles in the
two inches of air over it.
The number of dust-particles in the air which become centres of
condensation depends on the rate at which the condensation is taking
place. The most recent observations show that quick condensation causes a
large number of particles to become active, whereas slow condensation
causes a small number. After the condensation has ceased, a process of
differentiation takes place, the larger particles robbing the smaller ones
of their moisture, owing to the vapour-pressure at the surface of the
drops of large curvature being less than at the surface of drops of
smaller curvature.
By this process the particles in a cloud are reduced in number; the
remaining ones, becoming larger, fall quicker. The cloud thus becomes
thinner for a time. A strong wind, suddenly arising, will cause the
cloud-particles to be rapidly formed: these will be very numerous, but
very small--so small that they are just visible with great care under a
strong magnifying lens used in the instrument. But in slowly formed clouds
the particles are larger, and therefore more easily visible to the naked
eye.
Though the particles in a fog are slightly finer, the number is about the
same as in a cloud--that is, generally. As clouds vary in density, the
number of particles varies. Sometimes in a cloud one cannot see farther
than 30 yards; whereas in a few minutes it clears up a little, so that we
can see 100 yards. Of course, the denser the cloud the greater the number
of water-particles falling on the calculating-stage of the instrument.
Very heavy falls of cloud-particles seldom last more than a few seconds,
the average being about 325 on the square inch per second, the maximum
reaching to 1290. This is about four times the number counted in a fog.
Yet the particles are so very small that they evaporate instantly when
they reach a slight increase of temperature.