to form a fibre of cirrus is a thin thread of damp air rising slowly into a current different in speed or direction from that in which the air started.

If the rising impulse is merely the effect of the sun heating air near the ground, the resulting wisp is a fineweather cirrus; but if, on the contrary, the ascent of air is due to the upward impulse of a cyclone, then the bundle of cirrus-fibres indicates wind and rain.

Others consider that the advent of damp upper currents in front of a cyclone induce the condensation of vapour at a high level into icy particles, which latter are drawn into wisps of cirrus as they descend into lower strata.

We believe that cirrus may be formed by both methods, but it is impossible to pronounce definitely on the subject, till we know more of the mechanism of a cyclone.

CIRRO-STRATUS.

We now come to the composite forms of clouds, and here, unfortunately, we find the utmost confusion in the words applied by different meteorologists to the same clouds. We will first begin with cirro-stratus. By this we mean a thin stratum of cloud which, instead of being uniform like pure stratus, is composed of fibres of cirrus in any complexity, but not of streaked, fretted, or speckled nubecules.

Sometimes the fibres of cirrus interlace, and give this cloud a reticulated appearance like a woven cloth, and the variety of forms is unlimited. The cloud we call cirro-stratus is practically identical with what Howard

and Hildebrandson call "cirro-stratus," and almost coextensive with the cirro-velum of Ley. As to the origin of cirro-stratus, we can say little with certainty. As a matter of observation, it is usually formed in front of cyclones or secondaries. When the sun or moon shine through it, we generally find that a halo is formed, and then we may conclude with certainty that it is composed of frozen particles of vapour. The difficulty is to explain the innumerable forms which it assumes, and the rapid changes which it undergoes. Though we are obliged to employ the word strato to describe this cloud, because it forms a thin layer, it is extremely doubtful whether its formation has much in common with that of pure stratus, which we have seen is due to the radiation of anticyclones. It does, however, seem to have something in common with pure cirrus, and still more with cirrus-stripes; but we cannot say why the front of a cyclone should develop stratiform, and the rear cumulo-form, clouds.

Sometimes cirro-stratus is formed lower down, and more compact in structure, when it should be called strato-cirrus. This is unknown in Scandinavia, but quite common in some parts of the tropics.

ORIGIN OF STRIÆ.

With regard to the stria which we find both in cirrusstripes and in cirro-stratus, the only reasonable suggestion which has been proposed to account for their formation is, that a stripe, or a thin stratum of ice-dust, may sometimes be supposed to be relatively at rest to a wind more rapid than itself, which may strike it suddenly. Then

we can conceive that a smooth layer of cloud might be furrowed into small waves at right angles to the wind; but this, of course, would only account for striæ square to the stripe, and not for oblique markings. We often see an apparently structureless patch of cirro-stratus suddenly become striated, as if a cat's paw of wind had blown on it like a gust on a pond.

But, as a matter of fact, striæ are as often as not oblique to the lie of a stripe, and to the direction of the motion of cirro-stratus. Here also the only rational suggestion is that the oblique striations are in some way the effect of an upper current, which moves in a different direction to that on the surface, and forms cloud rolls.

There is certainly something to the eye about the sideways motion of some cirrus-stripes that is not the same as the drive of a detached cumulus before the wind. If one is really propagated by a dynamical disturbance, while the other merely floats in an air-current, the difference would probably be explained. If this can ever be satisfactorily worked out, we should get the motion of higher currents more accurately than at present, for now we always assume the motion of a cloud is the same as that of the wind which drives it along.

Sometimes a succession of rising threads of air, one behind the other, form nearly vertical parallel fibres of cirrus, which must not be mistaken for horizontal striæ. All observers are agreed that the fact of striation, or reticulation, is of no practical importance in forecasting weather from clouds, so that we do not make definite varieties of these forms.

CIRRO-CUMULUS.

The next great class of compounds is cirro-cumulus. By this we mean a broken layer of cloud, at a high or middle level, of which the component masses are not fibrous like cirro-stratus, but more or less rounded or rolled, though without any of the rocky look of pure cumulus.

For this reason the term cirro-cumulus is to a certain extent unfortunate; but we are almost obliged to use the word, so as not to introduce new expressions, and, so long as it is conventionally recognized what kind of cloud is meant by cirro-cumulus, it does not so much matter if the word is not quite logical. The misfortune of the word "cirro-cumulus" is that, even excluding the small high cumulus that sometimes grows out of hairy cirrus, and which we have described as linear, or high, cumulus, there are still two rather distinct forms, to either of which the definition we have given of cirrocumulus applies.

The first kind, and far the commoner all over the world, is composed of rolled masses of cloud, with a fleecy appearance, that are universally known in different languages as "wool-pack," "sheep," "lambs," or by similar terms. This is the cirro-cumulus of Fitzroy, Weilbach, Hildebrandson, and of Howard. The clouds called nubes hiemales by Weilbach, are a variety of this type that is formed with great persistency over Scandinavia and Northern Europe during the cold season. The thin layer of cloud is then at a moderate altitude, and tends to arrange itself in long parallel bands of quickly moving, fleecy masses.

It is extremely difficult to render that kind of cloud in an engraving. Fig. 18 is, however, a moderately successful attempt to reproduce a photograph of a fleecy sky. There, as always, the cloud has a more or less pronounced tendency to arrange itself along two lines

one for the length of the bands; the other for the lie of the striæ. Sometimes the effect of these two crossing lines is to give the individual nubicules which compose the whole a square or lozenge shape, and the whole sky the appearance of a gigantic chess-board.