This cloud appears to be formed from the fibres of the cirrus sinking into a horizontal position, at the same time that they approach each other sideways. This cloud is to be distinguished by its flatness and great horizontal extension, in proportion to its height; a character which it always retains, under all its various forms. As this cloud is generally changing its figure, and slowly sinking, it has been called the _wane-cloud_. A collection of these clouds, when seen in the distance, frequently give the idea of shoals of fish. Sometimes the whole sky is so mottled with them, as to obtain for it the name of the _mackerel-back sky_, from its great resemblance to the back of that fish. Sometimes they assume an arrangement like discs piled obliquely on each other. But in this, as in other instances, the structure must be attended to rather than the form, for this varies much, presenting, at times, the appearance of parallel bars or interwoven streaks, like the grain of polished wood. It is thick in the middle and thinned off towards the edge. [Picture: The Cirro-Stratus, or wane-cloud] These clouds precede wind and rain. The near or distant approach of a storm may often be judged of from their greater or less abundance and duration. They are almost always to be seen in the intervals of storms. Sometimes the cirro-stratus, and the cirro-cumulus, appear together in the sky, and even alternate with each other in the same cloud, presenting many curious changes; and a judgment may be formed of the weather likely to ensue, by observing which prevails at last. The cirro-stratus most frequently forms the solar and lunar halo. Hence the reason of the prognostics of bad weather commonly drawn from the appearance of halos. THE CUMULO-STRATUS, OR TWAIN-CLOUD. [Picture: The Cumulo-Stratus, or twain-cloud]