We can say little with certainty as to the formation of this fleecy sky, though, in a general way, there seems to be little doubt that both the woolly look and the striation are due to the contact and rolling friction of two layers of air moving in different directions. Fleecy clouds, though apparently so different in form, are really not very far removed from wispy cirro-stratus. We often see in England wispy clouds develop rapidly into fleecy ones for a few minutes, and then back again into wisps and curls; but, as a rule, cirro-stratus develops into strato-cumulus, and is practically a sign of worse weather than fleecy cirro-cumulus.

We know by observation that fleecy cirro-cumulus is chiefly formed in the temperate zone on the edges of anticyclones, and also before thunderstorms and some forms of non-isobaric rain. These are both cases in which there would be upper currents varying much in direction from the surface-winds, while the rapidity of motion would depend upon circumstances. This enables us to explain the following set of widely reputed prognostics.

"If woolly fleeces spread the heavenly way,
Be sure no rain disturbs the summer's day."

Or the provincial French saying, "El ciel pecoun promête un bel matin." But, on the other hand, Virgi (“Georg.” i. 397) considers it a sign of rain if it should happen that

“Tenuia ... lanæ per cælum vellera ferri.”

And so in the neighbourhood of Pisa they say, “Cielo a pecorelle, Acqua a catinelle;" and in the Tyrol, "Sind Morgens Himmelschäflein, wird's Nachmittags hageln

oder schnei'n;" and in France they have a proverb contrary to the one we have first quoted:

"Temps pommelé, fille fardée,

Ne sont pas de longue durée."

The term "dappled sky" (ciel pommelé) is a little equivocal, and might refer to the other form of cirrocumulus, known in Northern Europe as "mackerel sky."

Anyhow, we have to reconcile an apparently contradictory set of prognostics. The reason appears to be that in Northern Europe rain is chiefly cyclonic, and therefore rarely preceded by fleecy cirro-cumulus, so that the appearance of that cloud denotes the edge of an anticyclone, and fine weather for a day at least. In Central and Southern Europe, on the contrary, fleecy clouds are usually formed in front of secondaries, thunderstorms, and non-isobaric rains, so that their cirro-cumulus is a sign of approaching rain. We can readily imagine that, both at the edges of anticyclones and in front of secondaries, thunderstorms, etc., we have upper currents moving in very different directions to those on the surface, with a layer of cloud between them, though the origin of the condensed vapour is not the same. In anticyclones the vapour probably rises from evaporation, till it reaches an altitude where the temperature falls to the dew-point; in secondary cyclones, etc., the upward impulse is due to the dynamical properties of cyclonic or other motion.

This is exactly analogous to the difference between the wispy cirrus formed of an evening at the edges of anticyclones in fine weather, and the same cloud which precedes a dangerous storm.

In practice the surroundings are so different that the

apparent similarity of names rarely misleads the most ordinary observer.

The second chief variety of cirro-cumulus is composed of rounded and isolated nubicules without any fleecy texture.

This is the well-known "mackerel sky" of Northern Europe; and when the cloudlets are a little angular, we get a form called "mackerel-scales." We may call this hard cirro-cumulus, to distinguish it from the fleecy form of the same generic name. While fleecy cloud is one of the commonest, mackerel is one of the rarest skies, so that we have not got a sufficient number of observations to correlate these isolated cloudlets with any particular form of isobars or kind of rain.

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However, all weather-lore connects mackerel with fine weather, for even in rainy Ireland we find the saying, 'Mackerel sky, twelve hours dry." Why this should be the case we are unable to say, but there is no doubt about the fact.

In a still rarer form of cirro-cumulus, the lower surface of the general cloud-stratum exhibits very small pendulous protuberances, resembling sacks or bags, by which a part or even the whole sky is festooned. Ley calls this cirro-velum mammatum, but we may call it festooned cirro-cumulus. When, near sunset in the tropics, these festoons take up a rosy tint, and hang like pink grapes in a serene sky, these clouds can scarcely be surpassed for beauty.

Sometimes a more compact form of fleecy cirrocumulus is found at a lower level, when the cloud may be more appropriately reported as cumulo-cirrus, so as to indicate its lower level. This apparent multiplication

of cloud-names is forced on us by the necessity of giving some idea of height in reports of the motion of the upper currents. For instance, on the west edge of an anticyclone low cumulo-cirrus might be moving from south, whilst the higher cirro-cumulus would come from the south-west ; so that observations which reported cirro-cumulus and cumulo-cirrus indiscriminately would lead to a discordant or erroneous view of the general circulation of the air in an anticyclone.

STRATO-CUMULUS.

Another of the great series of compounds is stratocumulus. By this we mean a large mass of cloud, forming a layer, which is not sufficiently uniform to be called stratus, and not sufficiently rocky to be called cumulus. This is the cumulo-stratus of Fitzroy. Howard's cumulostratus is not a true variety of cloud at all, but a compound of a thin patch of cirro-stratus, resting either on the top of a cumulus or crossing an isolated lump of cumulus, as in Fig. 13, a. The origin of the name is obvious. The general mass of the cloud is a layer, and therefore the name must contain the word strato, while the components are lumpy, and it must therefore contain the word cumulo.

This form of cloud is typical of a cyclone-front in Great Britain. We can trace its gradual development in all stages. Cirrus-stripes first get thicker and lower, so as to form cirro-stratus. As we get nearer the rainy portion of the cyclone, the cirro-stratus loses its fibrous texture, becomes still denser and nearer the earth's

surface, till at last all trace of structure is lost in the irregular, shapeless masses of cloud which cover the whole sky. Still later, the cloud gets even lower and blacker, till rain ultimately begins to fall. Then the cloud would be called nimbus, because it forms a layer and precipitates rain. Sometimes, when the sky breaks for a moment, we get a glimpse at the composition of this cloud; we then see that it differs much from pure rocky cumulus, by reason of its flatness and comparative thinness. We must, in fact, look at strato-cumulus as a development of cirro-stratus, and not as an ally or hybrid of cumulus, though we have to use the word "cumulus" in composition.

The point which we cannot altogether explain is, why in front of the cyclone's trough the clouds should have such a marked tendency to form stratus, while in rear the rising currents take the form of well-defined columns, and produce rocky cumulus. This points to some difference of symmetry between these two portions of a cyclone, and the only suggestion which we can make is, that perhaps it may be partly due to the upper currents in front of the trough being much more opposed to those on the surface than those in rear of the centre, which are nearly parallel to the lower winds; and partly to the forward motion of the cyclone, as a whole, meeting the incurving winds in front, and running away from them in rear of the disturbance.

Another form of strato-cumulus is very common in the tropics. The component masses of cloud are more isolated than in Great Britain, and so thin that when seen in perspective each only looks like a dark thin bar,