SECTION X.

Peculiar solar and lunar Irises seen in South America.

To the before-mentioned particulars of the mountainous deserts, I shall subjoin the phenomena seen there, as subjects equally meriting the curiosity of a rational reader. At first we were greatly surprised with two, on account of their novelty; but frequent observations rendered them familiar. One we saw in Pambamarca, on our first ascent thither; it was a triple circular iris. At break of day the whole mountain was encompassed with very thick clouds, which the rising of the sun dispersed so far as to leave only some vapours of a tenuity not cognizable by the sight; on the opposite side to that where the sun rose, and about ten toises distant from the place where we were standing, we saw, as in a looking-glass, the image of each of us, the head being as it were the centre of the three concentric irises: the last or most external colours of one touched the first of the following; and at some distance from them all, was a fourth arch entirely white. These were perpendicular to the horizon; and as the person moved, the phenomenon moved also in the same disposition and order. But what was most remarkable, though we were six or seven together, every one saw the phenomenon with regard to himself, and not that relating to others. The diameter of the arches gradually altered with the ascent of the sun above the horizon; and the phenomenon itself, after continuing a long time, insensibly vanished. In the beginning, the diameter of

the inward iris, taken from its last colour, was about five degrees and a half; and that of the white arch, which circumscribed the others, not less than sixty-seven degrees. At the beginning of the phenomenon, the arches seemed of an oval or elliptical figure, like the disk of the sun, and afterwards became perfectly circular. Each of the least was of a red colour, bordered with an orange: and the last followed by a bright yellow, which degenerated into a straw colour, and this turned to a green. But, in all, the external colour remained red.

On the mountains we also had frequently the pleasure of seeing arches formed by the light of the moon, particularly one on the 4th of April, 1738, about eight at night, on the plain of Turubamba. But the most singular one was seen by Don George Juan, on the mountain of Quinoa loma, on the 22d of May, 1739, at eight at night. These arches were entirely white, without the mixture of any other colour, and formed along the slope or side of a mountain. That which Don George Juan saw, consisted of three arches, touching in the same point: the diameter of the inner arch was sixty degrees, and the breadth of the white mark, or delineation, took up a space of five degrees; the two others were, in every respect, of the same dimensions.

The atmosphere, and the exhalations from the soil, seem more adapted than in any other place for kindling the vapours, meteors being here more frequent, and often very large, last longer, and are nearer the earth, than the like phenomena in other parts.

[Ulloa's Voyage to South America.

SECTION XI.

Lunar Rainbow in Derbyshire.

By Mr. Ralph Thoresby, F. R. S.

THE iris lunaris being so rarely seen, that Dr. Plot tells us that several learned and observing men never saw one in their lives; and that even Aristotle himself observed only two in above fifty years; the ensuing account, which I had from a gentleman of great veracity and ingenuity, will be the more acceptable. He was lately in Derbyshire, where, on Christmas last, he was at Glapwell Hall; and walking towards Patterson Green, about eight in the evening, he observed with great satisfaction the bow, which the moon had fixed in the clouds: she had then passed her full about Nat. Hist. of Oxford, chap. 1, sec. 7. M M

VOL. IV.

twenty-fours hours; the evening had been rainy, but the clouds were dispersed, and the moon shone pretty clear. The iris was more remarkable than that which Dr. Plot observed at Oxford the 23d of November, 1675; that being only of a white colour, but this had all the colours of the solar iris, exceedingly beautiful and distinct, only faint in comparison of those we see in the day; as must necessarily be the case, both from the different beams that cause it, and the disposition of the medium. What puzzled him the most, was the largeness of the arc, which was not so much less than that of the sun, as the different dimensions of their bodies, and their respective distances from the earth seem to require: but as to its entireness and beauty of its colour, it was surprising. It continued about ten minutes, before the interposition of a cloud hindered his further observations. [Phil. Trans. 1711.

SECTION XII.

On the 13th of February, 1780, as I was returning to Chester, and ascending, at Rhealt, the mountain which forms the eastern boundary of the Vale of Clwyd, I observed a rare and curious phenomenon. My ingenious friend Mr. Falconer has given, from my description, an exact representation of it, in a drawing which accompanies this paper.

In the road above me, I was struck with the peculiar appearance of a very white shining cloud, that lay remarkably close to the