which water is drawn for industrial purposes. I have been for many years
in a position to observe its effects and the conditions under which it
is formed.

The essential conditions are, that the temperature of the water is at
its freezing point, and that of the air below that point; the surface of
the water must be exposed to the air, and there must be a current in the
water.

The ice is formed in small needles on the surface, which would remain
there and form a sheet if the surface was not too much agitated, except
for a current or movement in the body of water sufficient to maintain
it in a constant state of intermixture. Even when flowing in a regular
channel there is a continued interchange of position of the different
parts of a stream; the retardation of the bed causes variations in the
velocity, which produce whirls and eddies and a general instability in
the movement of the water in different parts of the section--the result
being that the water at the bottom soon finds its way to the surface,
and the reverse. I found by experiments on straight canals in earth and
masonry that colored water discharged at the bottom reached the surface
at distances varying from ten to thirty times the depth.[1] In natural
water courses, in which the beds are always more or less irregular, the
disturbance would be much greater. The result is that the water at the
surface of a running stream does not remain there, and when it leaves
the surface it carries with it the needles of ice, the specific gravity
of which differs but little from that of the water, which, combined with
their small size, allows them to be carried by the currents of water in
any direction. The converse effect takes place in muddy streams. The mud
is apparently held in suspension, but is only prevented from subsiding
by the constant intermixture of the different parts of the stream; when