extensive downward motion of the atmosphere, and a slow horizontal movement of air over the upper surface of the cloud. The first agency may be said to determine the possibility of the cloud's formation, the second, the degrees and characters of its structural arrangement. In reference to the first it should be stated that the occurrence of the cloud marks the tropical belts of high barometric pressure, the shifting and non-periodic areas termed anticyclones, the winter season, and the nocturnal hours. In reference to the second it is to be remarked that amount of interfret determines whether the cloud be an almost uniform sheet or present a broken and knotted appearance due to the separation and agglutination of nubecules. When nearly uniform the cloud slightly resembles Nebula. Nowhere, when the velocities of superimposed currents differ most, does its structural arrangement rise to the complex symmetry of Stratus Castellatus, the ponderous architecture of Cumulus and Cumulo-nimbus, or to the silken embroidery of the cirriforms.

36. The mean altitude of Stratus Quietus is about 3100 feet (945 m.) above mean sea-level, as given in Table III., and the variation between the extreme altitudes is not very great; neither is the difference between the level of the cloud above the tropical calm belts and above the transitory systems of atmospheric circulation very considerable. The vertical thickness of the cloud is small, on the average only 760 feet (231 m.), yet the cloud obstructs much light, the particles being coarse and abundant.

Illuminated by the sun, the cloud earth impure orange and red orange.

reflects to the

In shadow it is seen of an impure neutral or of a bluish-gray tint. Wherever there are large gaps in the quiet cloud at a distance from the zenith, if the atmosphere below is devoid of much haze, these colours, viewed nearly opposite to the sun's position, are often very distinctly marked, and the slightly rugged upper surface of the patches, whose irregularity is due to the process of interfret, is plainly visible. Seen from above in the beautiful atmosphere most commonly prevailing over a canopy of this cloud, the upper surface, which here reflects much sunlight, is seen to be composed of innumerable sloping ridges of no great height; and it is over and among the corries of the cloud that the current of air to which their figuration is due travels with slight or with very moderate velocity, the vertical component of which current is observationally almost inappreciable.

When Stratus Quietus occupies a layer of air. near the lowest level at which it usually exists (Table III.), and when the portions of cloud are not compactly welded together, it presents an appearance similar to that represented in Plate II. When, on the other hand, the cloud is near its highest elevation, when it is distributed in a rather compact canopy, either totally unbroken, or having small apertures visible only near the zenith, the under surface of the cloudbank exhibits irregular clusters and agglomerations. But, in any case, since the cloud is not arranged in real strips or bands, the apparent belts of deeply

shadowed and of more illuminated portions appear to the eye nearly parallel with the horizon. An exception worthy of notice occurs when the exterior edge of a bank of this cloud approaches the observer. Such an edge is sometimes approximately rectilinear, and therefore in perspective appears curved. This is notably the case when in the higher latitudes the outside edge of a sheet of this cloud approaches the observer on a moonlight night, and this is an apparent confirmation of the popular belief that moonlight clears the sky of cloud.

37. Very important are the connections between the occurrence of this cloud and special types of weather, and somewhat strange is the fact that little attention has been given to these. Part of this subject will be dealt with when we speak of the cloudforms of the "Anticyclonic" systems, § 166. Here we may observe that even in interspaces between cyclonic disturbances, and, in short, wherever there is any downward movement of the air, there is a tendency to the formation of this cloud; further, that the gradual or sudden disappearance of the cloud after a long spell of its occurrence in the higher latitudes deserves the most careful attention on the part of the local observer, since this disappearance is the forerunner of a change from settled to unsettled weather, and of a change from calm to windy conditions. It is also worthy of remark that the rather uncommon occurrence of this cloud in the summer of higher latitudes in concurrence with clouds of Inclination at a great altitude above it is, for reasons which will appear