there often follows an immediate and great addition to the quantity
of cloud. At the same time the darkness becomes less, because the
arrangement, which now returns, gives free passage to the rays of light;
the lower broken clouds rise into cumuli, and the upper sheets put on the
various forms of the cumulo-stratus, sometimes passing to the
cirro-cumulus.

The various phenomena of the rain-cloud are best seen in a distant
shower. If the cumulus be the only cloud at first visible, its upper
part is seen to become tufted with cirri. Several adjacent clouds also
approach and unite at its side. The cirri increase, extending upwards
and sideways, after which the shower is seen to commence. At other
times, the cirro-stratus is first formed above the cumulus, and their
sudden union is attended with the production of cirri and rain. In
either case the cirri spring up in proportion to the quantity of rain
falling, and give the cloud a character by which it is easily known at
great distances, and which has long been called by the name of _nimbus_.

When one of these arrives hastily with the wind, it brings but little
rain, and frequently some hail or driven snow.

Since rain may be produced and continue to fall from the slightest
obscuration of the sky by the nimbus, while a cumulus or a
cumulo-stratus, of a very dark and threatening aspect, passes on without
discharging any until some change of state takes place; it would seem as
if nature had destined the latter as reservoirs, in which water is
collected from extensive regions of the air for occasionally irrigating
particular spots in dry seasons; and by means of which it is arrested, at
times, in its descent in wet ones.

Although the nimbus is one of the least beautiful of clouds, it is,
nevertheless, now and then adorned by the splendid colouring of the
rainbow, which can only be seen in perfection when the dark surface of
this cloud forms for it a background.

The small ragged clouds which are sometimes seen sailing rapidly through
the air, are called _scud_. They consist of portions of a rain-cloud,
probably broken up by the wind, and are dark or light according as the
sun shines upon them. They are the usual harbingers of rain, and, as
such, are called by various names, such as _messengers_, _carriers_, and
_water-waggons_.

* * * * *

In attempting to explain the production of clouds and rain, it is
necessary to observe th