"A rainbow in the morning-
Sailors take warning;

A rainbow at night

Is the sailor's delight."

622. Coronas.-The corona is an appearance of faintlycoloured rings encircling the moon when seen behind the light fleecy cloud of the cirro-cumulus, or the otherwise invisible minute crystals referred to in par. 612. When the corona is perfect, the rings form several concentric circles, the blue prismatic colour being nearer the centre than the red. When of large dimensions it is called a brough in Scotland, and the ring has then generally a whitish nebulous appearance. The peculiar rings of the corona may be seen by looking at a light through a piece of glass upon which club-moss seed, which is very small, has been dusted; and the same appearance may be observed by looking at the gas-lamps of the streets through the window of a carriage on which moisture has been condensed. Similar appearances are seen in certain diseases of the eye, when the cornea becomes coated with minute particles of foreign matter.

623. Coronas can only be seen when the globules composing the cloud are all or nearly all of equal size; and the smaller the size of the globules the greater is the diameter of the corona. Hence the corona is a valuable prognostic; for when its diameter contracts round the moon, we know that the watery particles composing it are uniting into larger ones, which by-and-by will fall in rain; whereas if the corona be extending, the particles are growing less, thus indicating increasing dryness, and consequently fair weather.

624. Coronas are also very frequently formed round the sun; but to see them it is necessary to dim his stronger light, by looking through smoked glass, or at his image reflected from still water.

625. Glories of light, sometimes called anthelia, because formed opposite the sun, are sometimes seen when the shadow of an observer is cast on fog; and the shadow of his head is surrounded with the prismatic circles. On one occasion

Scoresby saw four coloured concentric circles around his shadow, and he observed that the phenomenon was always seen in the polar regions whenever sunshine and fog occurred at the same time.

the sun (figs. 53, 54, 55, and 56) or moon (figs. 57 and 58), but they are perfectly distinct from coronas, with which they should not be confounded. Halos are of comparatively rare occurrence; coronas, on the contrary, may be seen every time a light fleecy cloud comes between us and the sun or moon. The structure of halos, as seen from the figures, is often very complicated, circle cutting circle in the most remarkable manner, and with mathematical exactness, the diameters of the circles being generally very large; but the structure of the corona is simple, the circles concentric, the inner one small, varying from 2° to 4°, the diameter of the second circle being double that of the first, and of the third three times. In halos, the red prismatic colour is next the centre; in coronas, the blue. Halos are formed from the refraction and reflection of the rays of light by the minute snow-crystals of the cirrus cloud; while coronas arise from the interference of the rays passing on each side of the globules of vapour.

627. Parhelia and Paraselence.—At the points of intersection of the circles of the halo, images of the sun or moon generally appear from the light concentrated at these points, the images of the sun being called parhelia or mock-suns, and those of the moon paraselenæ or mock-moons, which also exhibit the prismatic colours of the halo.

628. COLOURS OF CLOUDS. The gorgeous aërial landscapes of red and golden-coloured clouds which fire the western sky at sunset, "the day's dying glory" of the poet, all admire. They are observed to be the accompaniment of cumulus clouds (the cloud of the day during fine weather), while in the act of dissolving as they sink slowly down into the lower and warmer parts of the atmosphere, consequently they disappear from the sky shortly after sunset. Such sunsets are therefore universally regarded as prognostics of fine weather.

629. Frequently small thin clouds appear high up in the eastern sky some time before sunrise, or when

"The dappled dawn doth rise;"

and when the sun has risen they disappear. They are probably caused by the sun shining on and warming the upper layers of the atmosphere before it appears above the horizon; thus small ascending currents are formed, the vapour of which, as they ascend, is condensed in small clouds, or the cirro-cumulus. Their rounded definite forms show them to be produced in the same manner as the cumulus cloud-viz., by ascending currents forcing their way through colder strata. Their consistence is thin and vapoury, and their colour generally whitish or grey. They may thus be regarded as heralding the cumulus, and as sure prognostics of fine weather.

630. A green or yellowish-green tinted sky, on the other hand, is one of the surest prognostics of rain in summer, and snow in winter. An attentive consideration of the changing tints of the evening sky after stormy weather, supplies valuable help in forecasting the weather; for if the yellow tint becomes of a sickly green, more rain and stormy weather may be expected; but if it deepen into orange and red, the atmosphere is getting drier, and fine weather may be looked forward to.

631. Some years ago, Principal Forbes showed from experiments that high-pressure steam, while transparent, and in the act of expansion, readily absorbs the violet, blue, and part of the green rays, thus letting the yellow, orange, and red pass; and he suggested that coloration may be produced in a mixture of air and vapour, when the vapour is in the intermediate state referred to above. Dr E. Lommel has shown that successive layers of air with visible vapour diffused through them act, so to speak, like sieves, which continually separate the transmitted light more and more perfectly from its more refrangible rays. Hence, in passing through different thicknesses of vapour, the blue rays are first absorbed, then the yellow rays, and finally the red rays. It is in the lower layers of the atmosphere that dust, smoke, watery vesicles, and small rain-drops are chiefly suspended. When the sun is high in the heavens, the thickness of the vapour-screen between the sun and the eye is not sufficient

to produce any perceptible action on the rays of light, which consequently appear white; but as the sun descends to the horizon the thickness of the vapour is greatly increased, and at sunset it is calculated that the light of the sun has to pass through 200 miles of the air in illuminating a cloud a mile above the earth. Hence, as the rays fall more and more obliquely on the clouds, they appear successively yellow, orange, and finally red. The varied colours often seen at sunset are caused by the clouds appearing at different heights and in different parts of the sky, so that various thicknesses of vapour are interposed between them and the sun. dawn the clouds first appear red; but as the sun rises higher, the yellow light ceases to be absorbed, and the clouds appear orange, yellow, and finally white. These successive stages of a perfect dawn are felicitously described in the Purgatorio :'

"The dawn was vanquishing the matin hour,

Which fled before it, so that from afar

I recognised the trembling of the sea.
Already had the sun the horizon reached,
So that the white and the vermilion cheeks
Of beautiful Aurora, where I was,

By too great age were changing into orange."

At

-Longfellow's translation.

Milton has accurately described the last stage of dawn in 'L'Allegro :'

"The great sun begins his state,

Robed in flames, and amber light

The clouds in thousand liveries dight."

632. It is evident that a high red dawn may be regarded as prognostic of settled weather, because the redness seen in clouds at a great height while the sun is yet below the horizon may be occasioned by the great thickness of the vapour-screen through which the illuminating rays must pass before reaching the clouds, and not to any excess of vapour in the air itself. But if the clouds be red and lowering in the morning, it may be accepted as a sign of rain, since the thickness traversed by the illuminating rays being now much