forms and motions of clouds. The same
cloud which to one observer may be glowing with light, to another may be
enveloped in shadow. That which appears to be its summit may be only a
portion of its outer edge, while that which seems to be its lower bed may
really be a portion of its further border. A spectator, on the summit of
a tall cliff, may observe what he takes to be a single cloud; while a
second spectator, on lower ground, will perceive that there are two
clouds. The motions of clouds are so deceptive, that they often seem to
be moving in a curve over the great concave of heaven, while they are in
fact advancing in nearly a right line. Suppose, for example that a cloud
is moving from the distant horizon towards the place where we stand, in a
uniform horizontal line without changing either in size or form. Such a
cloud, when first seen, will appear to be in contact with the distant
horizon, and consequently much nearer to us than it really is. As it
advances towards us, it will seem to rise into the sky, and to become
gradually larger till it is almost directly overhead. Continuing its
progress, it will then seem again to descend and to lessen in size as
gradually as it had before increased; till at length it disappears in the
distant horizon at a point exactly opposite to that at which it was first
seen. Thus the same cloud, without varying its motion in the least from
a straight line, and remaining throughout of the same size and form,
would seem to be continually varying in magnitude; and the line of its
motion, instead of being straight, would appear to be curved. This is
one of the most simple cases that can be supposed: but the clouds as they
exist in nature do not remain of the same magnitude, but are constantly
changing in form, in size, in direction, and in velocity; so that it is
quite impossible to form an accurate idea of their shape and size, or to
explain their motions. Clouds, at different elevations, may often be
seen to move in different directions under the influence of different
currents of wind.

[Picture: Different appearance of the same clouds to different observers]

The distribution of light and shade in clouds is most striking. The
watery particles of which they are composed, yielding constantly to
changes in temperature and moisture, are always changing; so that a most
beautiful cloud may alter in figure and appearance in an instant of time;
the light parts may suddenly become