mference. They are sometimes separate, and sometimes in
groups. (Figures 4, 5, 6.)

The third is the _Cumulo-Stratus_, which is made up of the cirro-stratus
blended with the cumulus. (Figure 8.)

The fourth is the _Cumulo-Cirro-Stratus_, or _Nimbus_. This is the true
_rain-cloud_, or system of clouds from which rain is falling. (Figure
9.)

The term _modification_ applies to the structure or manner in which a
given mass of cloud is made up, and not to its precise form or size,
which in most clouds varies every instant. Mr. Howard remarks, that it
may be at first difficult to distinguish one modification from another,
or to trace the narrow limits which sometimes separate the different
modifications; but a moderate acquaintance with the subject will soon
enable any one to point out the various forms, and to a great extent to
judge of the state of the weather by them. In order, therefore, to
assist the reader in gaining a certain amount of knowledge on this
interesting subject, it may be useful to state more fully the various
phenomena of the different forms of clouds already enumerated.

[Picture: The Cirrus, or curl-cloud]

The Cirrus occurs in very great variety, and in some states of the air is
constantly changing. It is the first cloud that appears in serene
weather, and is always at a great height. The first traces of the cirrus
are some fine whitish threads, delicately-pencilled on a clear blue sky;
and as they increase in length others frequently appear at the sides,
until numerous branches are formed, extending in all directions.
Sometimes these lines cross each other and form a sort of delicate
net-work. In dry weather the cirrus is sharp, defined, and fibrous in
texture, the lines vanishing off in fine points. When the air is damp
this cloud may be seen in the intervals of rain, but is not well defined,
and the lines are much less fibrous. Such cirri as these often grow into
other varieties of cloud, and are frequently followed by rain.

The cirrus may last a few minutes only, or continue for hours. Its
duration is shortest when near other clouds. Although it appears to be
stationary, it has some connexion with the motions of the atmosphere; for
whenever, in fair weather, light variable breezes prevail, cirri are
generally present. When they appear in wet weather, they quickly pass
into the cirro-stratus.

According to Dalton, these clouds are from three to five miles ab