but because more air has passed over the tests, thus deepening the tint of the paper. Ozone is more abundant on the sea-coast than inland ; in the west than in the east of Great Britain ; in elevated than in low situations; with south-west than with north-east winds ; in the country than in towns ; and on the windward than on the leeward side of towns. From the observations made by the observers of the Scottish Meteorological Society, ozone is most abundant from February to June, when the average amount is 6.0, and least from July to January, when the average is 5.7. The maximum, 6.2, is reached in May, and the minimum, 5.3, in November. Thus the maximum period occurs when evaporation is greatest, and the minimum when the condensation of aqueous vapour is greatest—a result in accordance with the conclusions arrived at by Dr Berigny and M. Houzeau. It thus appears that it is most abundant where electricity is produced ; and least so, or entirely wanting, where electricity is in least quantity, and where there is much decaying vegetable and animal matter. But there are great, if not insuperable, difficulties in the way of accurately observing the ozone of the atmosphere ; for no means have yet been devised of drawing over the test-papers an ascertained quantity of air, the same at all places and times ; and of determining whether the colouring of the paper is due to ozone, to nitric or other acid present in the atmosphere.


616. Though rainbows, halos, and other optical phenomena furnish many most beautiful and surprising spectacles, yet from the very subordinate position they hold among meteorological objects, any description given here will be brief and of a popular character. For a full account of them we must refer to the common treatises on Optics, to which department of Natural Philosophy they more immediately belong.

617. The Rainbowo.— The rainbow generally consists of two arches, the inner or primary bow, and the outer or secondary bow, each composed of the seven prismatic colours, violet, indigo, blue, green, yellow, orange, and red; or rather of the three primary colours, blue, yellow, and red, blended together. In the primary bow the violet colour is on the inner side of the bow, and the red on the outer; but in the secondary bow this arrangement of the colours is reversed. Coloured bows are also sometimes seen in the interior of the primary bow, and more rarely at the exterior of the secondary bow; these are called supernumerary bows.

618. As the centre of the circle of which the rainbow is a part is in the continuation of a line drawn from the sun through the eye of the spectator, its position varies with that of the spectator, and its size with the degree of proximity of the sun to the horizon. Thus, if the sun be in the horizon the rainbow will be a semicircle ; if higher, less than a semicircle ; if about 42° high, the top of the primary bow will be just visible in the horizon; and if at greater heights, no rainbow will be formed on the sky. Rainbows have been sometimes seen from the tops of mountains forming complete circles, when the sun was high and a shower of rain was falling in the plain below. Circular rainbows were seen by Colonel Sykes on the 29th April, 9th, 11th, and 12th May 1829, on the hill-fort of Hurreechundurghur, and are thus described by him in the Transactions of the Royal Society of London, 1835:-“The stratum of fog from the Konkun on some occasions rose somewhat above the level of the top of a precipice, forming the north-west scarp of the hill-fort of Hurreechundurghur, from 2000 to 3000 feet in perpendicular height, without coming over the table-land. I was placed at the edge of the precipice, just without the limits of the fog, and with a cloudless sun at my back, at a very low elevation. Under such a combination of favourable circumstances, the circular rainbow appeared quite perfect, of the most vivid colours, one half above the level on which I stood, the other half below it. Shadows in distinct outline of myself, my horse, and my people, appeared in the centre of the circle as in a picture, to which the bow formed a resplendent frame. My

attendants were incredulous that the figures they saw under such extraordinary circumstances could be their own shadows, and they tossed their arms and legs about, and put their bodies into various postures, to be assured of the fact by the corresponding movements of the objects within the circle ; and it was some little time ere the superstitious feeling with which the spectacle was viewed wore off. From our proximity to the fog, I believe the diameter of the circle at no time exceeded 50 or 60 feet. The brilliant circle was accompanied with the usual outer bow in fainter colours.

619. When the sun is reflected from the surface of still water, extraordinary bows are sometimes formed. From the reflected image of the sun being in effect beneath the horizon, such bows are larger than a semicircle. The arrangement of the colours is the same as the primary bow; and when their summit happens to coincide with that of the secondary bow, a band of white light is formed at the place of union where they blend together.

620. Lunar Rainbows.—Rainbows are also produced by the light of the moon falling on rain-drops, exactly in the same way as solar rainbows. They are by no means of rare occurrence. Owing to the feeble light of the moon the bow is generally without colours ; but when the sky is very clear and the moon at the full, the prismatic colours appear, but in subdued splendour.

621. Since rainbows in the morning are always seen in the west, they indicate the advance of the rain-cloud from the west at the time that it is clear and bright in the east; and since the fall of rain at this time of the day when the temperature should be rising is an additional evidence of increasing moisture, a morning rainbow is regarded as a prognostic of a change to wet stormy weather. On the contrary, the conditions under which a rainbow can appear in the evening are, the passing of the rain-cloud to the east, and a clearing up in the west at the time of day when the temperature has begun to fall, thus further indicating a change from wet to dry weather. Hence the popular prognostic, —

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.

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Fig. 57.

Fig. 58. 626. Halos.—Halos are circles of prismatic colours around

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