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severe is advancing and may be looked for within 30 or 48 hours. When the storm seems past and the sky has cleared, should a few fine cirrus clouds be seen slightly blown back at their eastern extremities, the storm has in all likelihood really past, and fair weather may with some confidence be expected, since the dry polar current has already begun to prevail overhead.

374. But if, instead of this, innumerable groups and streaks of cirrus cover the sky, crossing each other in all directions, and presenting the appearance of skeins of yarn inextricably tangled together, we may be sure that a second storm will shortly follow the one already past.

375. Cumulus.-This name is applied to convex or conical heaps of clouds increasing upwards from a horizontal base. They are usually of a very dense structure; are formed in the lower regions of the atmosphere; and are carried along in the current next the earth. The cumulus has been well called the cloud of the day, being caused by the ascending currents of warm air which rise from the heated ground. Its beginning is the little cloud not bigger than a man's hand, which is the nucleus round which it increases. The lower surface remains roughly horizontal, while the upper rises into towering heaps, which may continue comparatively small, or swell into a size far exceeding that of mountains.

376. Saussure attributes their conical shape to the way in which they are formed. When one fluid is poured through another it makes its way in curved lines; thus ink poured into water is diffused through it like clouds. The steam from an engine poured into the air diffuses itself as a cloud; and in like manner the vapour poured upward into the air by the heated currents as they ascend is diffused, and, being condensed, forms the cumulus.

377. When they are of moderate height and size, of a welldefined curved outline, and appear only during the heat of the day, they indicate a continuance of fair weather. But when they increase with great rapidity, sink down into the lower parts of the atmosphere, and do not disappear towards

evening, rain may be expected. If loose fleecy patches of cloud begin to appear thrown out from their surfaces, the rain is near at hand.

378. Stratus.-The stratus is a widely-extended, continuous sheet of cloud, increasing from below upwards. It is properly, and as its name implies, a continuous layer of cloud. It is, besides, the lowest sort of cloud, its lower surface commonly resting on the earth. The stratus may be called the cloud of night, since it generally forms about sunset, grows denser during the night, and disappears about sunrise. It is caused by the vapours which rise during the day, but towards evening fall to the earth with the falling temperature; and since during night the cooling of the air begins on the ground and thence proceeds upwards, the stratus first appears like a thin mist floating near the surface of the earth; it thence increases from below upwards as successive layers of the air are reduced below the point of saturation. It includes all those mists, already described, which in the calm evening of a warm summer day form in the bottom of valleys and over low-lying grounds, and then spread upwards over the surrounding country like an inundation.

379. When the sun has risen and begun to shine on the upper surface of the stratus cloud, it begins to be agitated and to heave up in different places into the rounded forms of the cumulus, while at the same time the whole of its lower surface begins to rise from the ground. As the heat increases it continues to ascend, and becomes broken up into detached masses, and soon disappears altogether. These appearances indicate a continuance of the finest and serenest weather.

380. Cirro-cumulus. - This cloud is composed of welldefined, small, roundish masses, lying near each other, and quite separated by intervals of sky. It is formed from the cirrus cloud by the fibres breaking, as it were, and collapsing into small roundish masses, thus destroying the texture but retaining the arrangement of that cloud. The change takes place either over the whole cloud at once, or it begins at one

extremity and proceeds slowly to the other; and while the change takes place it generally descends to a lower position in the atmosphere.

381. This very beautiful cloud is commonly known as a "mackerel sky;" it occurs frequently in summer, and is attendant on dry and warm weather. It is also sometimes seen between showers, its graceful form and slow easy motion presenting a striking contrast to the dark, heavy rain-clouds below, which drift hurriedly across the sky. But in this case the cirro-cumulus will be found wanting in the settled order which it wears in fine weather.

382. Cirro-stratus.-The cirro-stratus partakes partly of the characteristics of the cirrus and stratus. It consists of horizontal or slightly-inclined masses thinned towards a part of the circumference, bent downwards or undulated, and either separate or in groups. Their form and relative position sometimes resemble shoals of fishes. In distinguishing this cloud, attention must be paid, not so much to the form, which is very variable, but to the structure, which is dense in the middle and thin towards the edges.

383. The cirro-stratus is markedly a precursor of storms; and from its greater or less abundance and permanence, it gives some indication of the time when the storm may be expected. It may generally be seen between storms, occasionally with the cirro-cumulus, and, from what then takes place, important information may be learned regarding the continuance or non-continuance of the stormy weather then prevailing. For if the cirro-cumulus give way or pass into the cirro-stratus, thus leaving it, as it were, in possession of the field, more wind and rain may be confidently expected ; but if the cirro-stratus yield, and the cirro-cumulus prevail, the storm is past, and fair weather may be looked for.

384. Since the cirro-stratus possesses great extent and continuity of substance, with little perpendicular depth, it is the cloud which most frequently and completely fulfils the conditions requisite for exhibiting Parhelia or mock-suns, Paraselenæ or mock-moons, Coronce, and Solar and Lunar Halos.

385. Cumulo-stratus.-This cloud is formed by the cirrostratus blending with the cumulus, either among its piled-up heaps, or spreading underneath its base as a horizontal layer of vapour. It sometimes appears indistinctly in the intervals of showers. The distinct cumulo-stratus is formed when the cumulus becomes surrounded with small fleecy clouds just before rain begins to fall, and also on the approach of thunderstorms. Tennyson has finely described it as it rises in the west:

"The wild unrest that lives in woe

Would dote and pore on yonder cloud,
That rises upward always higher,

And onward drags a labouring breast,
And topples round the dreary west,
A looming bastion fringed with fire."

386. Cumulo-cirro-stratus, or Nimbus.-This is the wellknown rain-cloud, consisting of a cloud, or system of clouds, from which rain is falling. The rain-cloud often has its origin in the cumulo-stratus, which increases till it overspreads the sky, and becomes black or bluish-black in colour; but this colour soon changing to grey, the nimbus is formed, and rain begins to fall.

387. Its name, cumulo-cirro-stratus, suggests more accurately the manner of the formation of the rain-cloud. At a considerable height a sheet of cirro-stratus cloud is spread out, under which cumulus clouds drift from the windward; these rapidly increasing unite at all points, forming one continuous grey mass, from which the rain falls. It is evident from this that the whole body of air under the upper sheet of cloud into which the clouds drift must be completely saturated. The breaking-up of the lower grey mass indicates that the rain will soon cease.

388. When a rain-cloud is seen approaching at a distance, cirri appear to shoot out from its top in all directions; and it has been observed that the more copious the rainfall the greater is the number of the cirri thrown out from the cloud. 389. Observing Clouds.-In observing clouds, the kind, the direction in which they are carried both in the lower and

upper regions of the atmosphere, and the proportion of the sky covered with them, should be noted. In estimating the amount, that portion of the sky from the horizon half-way to the zenith should not be taken into account, because, the clouds being there foreshortened, the estimate formed would be too great. The scale generally adopted in this country is 0 to 10; 0 indicating a clear sky, 5 that is half covered, and 10 that it is wholly obscured.

390. The mean amount of cloud in the west of Scotland is 7.2 in winter, 6.4 in spring, 6.5 in summer, and 6.8 in autumn; and in the east and interior of the country the amounts are 6.0, 5.8, 6.0, and 6.1. In Shetland the amount of cloud is greater, three-fourths of the sky being the average space covered with cloud. The month most free of clouds is May, owing to the rising temperature and the dry east winds which then prevail. The cloudiest month is December, when the temperature is rapidly falling, and the south-west wind attains its maximum frequency.

391. Velocity of Clouds.-The rate of the motion of the clouds is greater than is commonly supposed, and very much greater than the velocity of the wind at the earth's surface. One day in the beginning of May 1867, happening to be on the top of Corstorphine Hill, near Edinburgh, I made some observations on the motion of the clouds. The occasion was very favourable, inasmuch as a storm which had prevailed during the previous night was breaking up, and the clouds had separated into large cumulus masses, which drifted with great apparent leisureliness across the otherwise clear sky in the direction of Donaldson's Hospital, a prominent object in the landscape. The number of seconds which elapsed between the times of the shadows of four clouds touching the top of the hill, and the same shadows touching the hospital, was noted; and the exact distance of the two places being known from the map of the Ordnance Survey, the velocity of the clouds was found to be at the rate of 72 miles an hour. At the same time the force of the wind on the surface of the earth was estimated at 3 in the scale 0 to

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