e rafts are brought near together,
but do not actually touch each other. They are possibly kept apart by
electrical repulsion. In this state of association without union the
divided water may undergo the curiously modified aggregations which
give us the varied forms of clouds. As yet we know little as to the
cause of cloud shapes. We remark the fact that in the higher of these
agglomerations of condensed vapour, the clouds which float at an
elevation of from twenty to thirty thousand feet or more, the masses
are generally thin, and arranged more or less in a leaflike form,
though even here a tendency to produce spherical clouds is apparent.
In this high realm floating water is probably in the frozen state,
answering to the form of dew, which we call hoar frost. The lower
clouds, gathering in the still air, show very plainly the tendency to
agglomerate into spheres, which appears to be characteristic of all
vaporous material which is free to move by its own impulses. It is
probable that the spherical shape of clouds is more or less due to the
same conditions as gathered the stellar matter from the ancient
nebular chaos into the celestial spheres. Upon these spherical
aggregations of the clouds the winds act in extremely varied ways. The
cloud may be rubbed between opposite currents, and so flattened out
into a long streamer; it may take the same form by being carried off
by a current in the manner of smoke from a fire; the spheres may be
kept together, so as to form the patchwork which we call "mackerel"
sky; or they may be actually confounded with each other in a vast
common cloud-heap. In general, where the process of aggregation of two
cloud bodies occurs, changes of temperature are induced in the masses
which are mixed together. If the temperature resulting from this
association of cloud masses is an average increase, the cloud may
become lighter, and in the manner of a balloon move upward. Each of
the motes in the cloud with its charge of vapour may be compared with
the ballast of the balloon; if they are warmed, they send forth a part
of their load of condensed water again to the state of invisible
vapour. Rising to a point where it cools, the vapour gathers back on
the rafts and tends again to weight the cloud downward. The ballast of
an ordinary balloon has to be thrown away from its car; but if some
arrangement for condensing the moisture from the air could be
contrived, a balloon might be brought into the adjust