or 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

when spread on the glass are no more than

ed area of the stage, as many of the drops rapidly evaporate, some almost

invisible. The number of drops falling per second varied greatly from time to time. The greatest number was 323 drops per squar

able to calculate the velocity of their fall,

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 o

tion 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 large

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 gre

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 w

uare inch per second, the maximum reaching to 1290. This is about four times the number counted in a fog. Yet