led with the smoke of London, it produced there the memorable _dark
day_ of the 24th of February, 1832, and at various other times has
produced others of like character. (See Howard's Climate of London, vol.
iii. pp. 36, 207, 303.) These fogs have been so dense there that every
kind of locomotion was dangerous, even _with lanterns, at mid-day_.

The next in order, ascending, are the storm scud, which float in the
north-east or easterly, south-east or southerly wind, before and during
storms.

These, as the reader will hereafter see, are, _practically_, very
important forms of cloud condensation--although they have found no place
in any practical or scientific description given of the clouds, and are
not upon the cuts. They are patches of foggy seeming clouds of all sizes,
more or less connected together by thin portions of similar condensation,
often passing to the westward, south-westward, north-westward, or
northward with great rapidity. Their average height is about half a mile,
but they often run much lower. They are usually of an "ashy gray" color.
The annexed cut shows one phase of them, from among many taken by
daguerreotype. The arrows pointing to the west show the scud distinguished
from the smooth partially formed stratus above. This view was taken a few
hours prior to the setting in of a heavy S. E. rain storm. It is a
northerly view.

[Illustration: Fig. 7.]

At about the same height, but in a _different state of the atmosphere_,
float the peculiar fair-weather clouds of the N. W. wind. They usually
form in a clear sky, and pass with considerable rapidity to the S. E.
Sometimes they are quite large, approaching the cumulus in form, and
white, with dark under surfaces, and at others, in the month of November
particularly, are entirely dark, and assume the character of squalls and
drop flurries of snow; and then resemble the nimbus of Howard. They assume
at different times and in different seasons, different shapes like those
of the scud, the cumulus, or the stratus.

[Illustration: Fig. 8.]

They form and float in the peculiar N. W. current which is usually a
fair-weather wind, and are never connected with storms. In mild weather
they are usually white, and in cold weather sometimes very black, and at
all times differ _in color_ from the ashy gray scud of the storm. This
variety is not represented upon the general cuts. The annexed diagram
shows one phase of them, but they are readily observable