at all seasons of
the year, when the N. W. wind is prevailing; differing in appearance
according to the season. Let these, as well as the storm scud, be
carefully observed and studied by the reader, and let no opportunity to
familiarize himself with their appearance be lost. A brief glance at
each recurrence of easterly or north-westerly wind will suffice.

[Illustration: SUMMER CUMULI.]

The _cumuli_ appear in isolated clouds of every size, or in vast clouds
composed of aggregated masses, as the peculiar cloud of the thunder
shower. They form as low down as the scud or fair-weather cloud of the N.
W. wind, which, for convenience, I will call N. W. _scud_; and often in
violent showers, and particularly in hail storms, extend up as far as the
density of the atmosphere will permit them to form. Professor Espy thinks
he has measured their tops at an altitude of ten miles. Others have
estimated their height, when most largely developed, at twelve miles; but
it is very doubtful whether the atmosphere can contain the moisture
necessary to form so dense a cloud at that elevation. It is their immense
height, however, whether it be six, or eight, or ten miles, together with
the sudden and violent electric action, condensing suddenly all the
moisture contained in the atmosphere within the space occupied by the
cloud, which produces such sudden and heavy falls of rain or hail. As the
rain drops or hail, when formed at such an elevation, in falling through
the partially condensed vapor of the cloud must necessarily enlarge by
accretion from the particles with which they come in contact, and probably
also by attraction, their size when they reach the earth, though
frequently very considerable, is not a matter of astonishment. The cumulus
is represented in the general plate with sufficient accuracy to show its
peculiar character.

In summer, when the air is calm, the weather warm, and no storm is
approaching, there is always, in the day time, a tendency to the formation
of cumuli. This tendency exhibits itself about ten o'clock in the
forenoon, and they gradually form and enlarge until about two in the
afternoon; and after that, if they do not continue to enlarge and form
showers, they melt away and disappear before nightfall. Sometimes in July
and August the atmosphere will be studded with them at mid-day, floating
about three-quarters of a mile from the earth (in a level country), gently
and slowly away to the eastward. At t