1890-91.] Mr Aitken on Water Particles in a Fog. 
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On a Method of Observing and Counting the Number of 
Water Particles in a Fog. By John Aitken, Esq. 
(Read May 4, 1891.) 
The phenomena known as haze, fog, mist, and rain are in a 
general way but the successive development of the same process, and 
the line which divides the one from the other is very indefinite. 
Dust in the atmosphere produces a haze, and the thickness of a 
haze of this kind depends principally on the amount of dust present 
when the relative humidity of the atmosphere is very low. But as 
the humidity increases the effect of the vapour increases also ; the 
dust particles attract the water vapour which becomes deposited on 
them, thus increasing their size and their hazing effect, till at last 
when the air is nearly saturated it becomes very thick, and forms 
what we call a fog ; when in this condition, the thickness of the 
atmosphere depends principally on the degree of saturation. 
Between the haze and the fog, however, there is no recognised dis- 
tinction in kind, it is principally one of degree. After the air is 
saturated and the conditions are such as to tend to cause super- 
saturation, then a change takes place in the condensation. A few 
of the dust particles have water deposited on them, and after a 
time they grow and become little drops of water, in which the 
original dust nucleus hears a very small proportion to the total 
weight. At this stage it is still called a fog, hut after more water 
is deposited on the small drops they grow and become what is 
known as mist, and when the mist drops combine and fall they are 
called rain-drops. 
The instrument to he described will, it is hoped, in addition to 
enabling us to count the water drops, also give us a means of finding 
by observation the boundary line between a dry fog and a wet one, the 
latter being the name often given to the first stages in the formation 
of a mist, when the condensation is taking place at the level of the 
observer. At present two forms of apparatus are being developed 
for observing these water particles. The first and simplest is an 
instrument for observing whether there are any water particles in 
