gh which they pass. But it may happen that
the cloud, in descending, arrives in a warmer region than that in which
it was formed: in this case, the condensed moisture may again become
vapour, and ascend again to a region where condensation may again take
place.

Mr. Daniell's explanation of the formation of rain differs from the above
in some of its particulars, which are not sufficiently elementary to be
given here; but it may be instructive to give a few of Mr. Howard's
illustrations respecting the formation of the various clouds. If hot
water be exposed to cool air, it _steams_--that is, the vapour given off
from the surface is condensed in mixing with the air; and the water thus
produced appears in visible particles, the heat of the vapour passing
into the air. This effect may be seen about sunrise, in summer, on the
surface of ponds warmed by the sun of the previous day, and also with
water newly pumped from a well. But the small cloud formed in these
instances usually disappears almost as soon as formed, the air being too
dry to allow it to remain. But in the wide regions of the atmosphere the
case is different, on account of the vast supply of vapour, and the
ascent and descent of the cloud to regions which allow it to remain
tolerably permanent. In the fine evenings of autumn, and occasionally at
other seasons, mists appear suddenly in the valleys, gradually filling
these low places, and even rising to a certain height, forming a foggy
atmosphere for the following day. These collections of visible vapour
resting on the earth, and often cut off so as to form a level surface
above, so nearly resemble a sheet of water, as to have been occasionally
mistaken for an inundation, the occurrence of the previous night. Such
is the origin and appearance of the _stratus_: it constitutes the fog of
the morning, and sometimes, as at the approach of a long frost, occupies
the lower atmosphere for several days. But the sun, we will suppose, has
broken through and dissipated this obscurity, and cleared the lower air.
On looking up to the blue sky, we see some few spots showing the first
formation of a cloud there: these little collections increase in number,
and become clouds, heaped, as it were, on a level base, and presenting
their rounded forms upwards; in which state they are carried along in the
breeze, remaining distinct from each other in the sky. This is the
_cumulus_, or _heap_.

By and by, if the clouds cont