inue to form, and enough vapour is supplied
from above, these heaps are seen to grow over their base like a mushroom
or cauliflower. Perhaps a flat top is seen forming separately, and this
afterwards joins the simple heap of cloud; or the flat forms and the
heaps become mixed irregularly among each other, occupying the spaces
everywhere, till the sky becomes overcast, and presents the usual
appearance of dense clouds. This is the _cumulo-stratus_, or _heaped and
flat cloud_. It is not productive of rain, and it forms, both in summer
and in winter, the common scenery of a full sky.

On examining minutely the higher regions of the air, especially after the
sky has been clear for some time, the spectator will probably see the
cirrus descending from above in the form of _threads_ or _locks_ and
_feathers_, which go on increasing until they fill the sky. They are
more commonly seen above the two former kinds, which float upon the clear
air below. On continuing to watch the cirri, they will be seen to pass
to the intermediate form of cirro-cumulus, consisting of smaller rounded
clouds attached to each other, or simply collected together in a flat
aggregate, and forming the mottled or dappled sky.

The cumulo-stratus is more dense and continuous in its structure; thick
in the middle, and thinned off towards the edges. Over-head it is a mere
bed of haze, more or less dense. In the horizon, when seen sideways, it
often resembles shoals of fish, as already noticed; but it is liable to
put on the most ragged and patchy appearances, making a very ugly sky.

The nimbus, or rain cloud, is seen to the greatest advantage in profile,
in the horizon, and at a great distance, when it often resembles a lofty
tower raised by its greater height to a conspicuous place among the dark
threatening clouds, and catching the sun's last rays upon its broad
summit and sides. In its nearer approach, it may always be known by
being connected below with an obscurity caused by the rain it lets fall,
and which reaches down to the horizon.

In ascending from the lower valleys to the tops of lofty mountains,
clouds may be traced through six modifications, the cirrus being seen
from the loftiest summits, while the other forms are only skirting the
sides of the mountains. Mr. Mason remarks, that clouds occasionally lie
so low, that before the balloon seems to have entirely quitted the earth,
it has been received between their limits, and entirely e