What we name Rainbows frequently occur, and are not considered either wonderful or ominous; for they do not predict, with certainty, either rain or fair weather. It is obvious, that the rays of the sun, being projected upon a hollow cloud, the light is thrown back to the sun and is refracted[435], and that the variety of colours is produced by a mixture of clouds, air, and fire[436]. The rainbow is certainly never produced except in the part opposite to the sun, nor even in any other form except that of a semicircle. Nor are they ever formed at night, although Aristotle asserts that they are sometimes seen at that time; he acknowledges, however, that it can only be on the 14th day of the moon[437]. They are seen in the winter the most frequently, when the days are shortening, after the autumnal equinox[438]. They are not seen when the days increase again, after the vernal equinox, nor on the longest days, about the summer solstice, but frequently at the winter solstice, when the days are the shortest. When the sun is low they are high, and when the sun is high they are low; they are smaller when in the east or west, but are spread out wider; in the south they are small, but of a greater span. In the summer they are not seen at noon, but after the autumnal equinox at any hour: there are never more than two seen at once. CHAP. 61.—THE NATURE OF HAIL, SNOW, HOAR, MIST, DEW; THE FORMS OF CLOUDS. I do not find that there is any doubt entertained respecting the following points. (60.) Hail is produced by frozen rain, and snow by the same fluid less firmly concreted, and hoar by frozen dew[439]. During the winter snow falls, but not hail; hail itself falls more frequently during the day than the night, and is more quickly melted than snow. There are no mists either in the summer or during the greatest cold of winter. There is neither dew nor hoar formed during great heat or winds, nor unless the night be serene. Fluids are diminished in bulk by being frozen, and, when the ice is melted, we do not obtain the same quantity of fluid as at first[440]. (61.) The clouds are varied in their colour and figure according as the fire which they contain is in excess or is absorbed by them. CHAP. 62. (62.)—THE PECULIARITIES OF THE WEATHER IN DIFFERENT PLACES. There are, moreover, certain peculiarities in certain places. In Africa dew falls during the night in summer. In Italy, at Locri, and at the Lake Velinum, there is never a day in which a rainbow is not seen[441]. At Rhodes and at Syracuse the sky is never so covered with clouds, but that the sun is visible at one time or another; these things, however, will be better detailed in their proper place. So far respecting the air. CHAP. 63. (63.)—NATURE OF THE EARTH.