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cious forms frequently assumed by the flakes of snow, we shall see no reason to think them inadequate to the production of all these appearances.
[Young Nat. Phil. Vol. I,
It is in his second volume that Dr. Young has favoured us with his abstract of Dr. Wollaston's observations on the quantity of bo.. rizontal refraction, which is as follows.
Dr. Wollaston notices Mr. Monge's Memoir on the Mirage, observed in Egypt, as containing facts which fully agree with his own theory formerly published. From his observations on the degree of refraction produced by the air near the surface of the Thames, it appears that the variations derived from changes of temperature and moisture in the atmosphere, are by no means cal. culable; but that a practical correction may be obtained, which, for nautical uses, may supersede the necessity of such a calculation. Dr. Wollaston first observed an image of an oar at a distance of about a mile, which was evidently caused by refraction, and when he placed bis eye near the water, the lower part of distant objects was hidden, as if by a curvature of the surface. This was at a time when a continuation of hot weather had been succeeded by a colder day, and the water was sensibly warmer than the atmosphere above it. He afterwards procured a telescope, with a plane speculum placed obliquely before its object glass, and provided with a micrometer, for measuring the angalar depression of the image of a distant oar, or other oblique object; this was some. times greater when the object glass was within an inch or two of the water, and sometimes when at the height of a foot or two. The greatest angle observed was somewhat more than nine minutes, when the air was at 50°, and the water at 63°; in general the dry. ness of the air lessened the effect, probably by producing evaporation, but sometimes the refraction was considerable, not with. standing the air was dry. Dr. Wollaston has observed but one instance which appeared to encourage the idea, that the solution of water in the atmosphere may diminish its refractive power,
In order to correct the error, to which nautical observations may be liable, from the depression of the apparent horizon, in conse. quence of such a refraction, or from its elevation in contrary cir. cumstances; and at the same time to make a proper correction for
the dip, Dr. Wollaston recommends that the whole vertical angle between two opposite points of the horizon, be measured by the back observation, either before or after taking an altitude; and that half its excess above 180° be taken for the dip: or if there be any doubt respecting the adjustment of the instrument, that it be be reversed, so as to measure the angle below the horizon, and that one-fourth of the difference of the two angles, thus determined, be taken as extremely near to the true dip. It is indeed possible, that the refraction may be somewhat different at different parts of the surface; but Dr. Wollaston is of opinion that this can rarely happen, except in the neighbourhood of land.
[Id. Vol. II. Journ. Royal Instit,
Parheliu, or Mock Suns, seen at Dantzic.
By M. Hevelius. On February 5, 1674, N.s. near Marienburg in Prussia, I saw the sun, in a sky every where serene enough, being yet some degrees above the horizon, and shining very bright, yet lancing out very long and reddish rays, 40 or 50 degrees towards the zenith. Under the sun towards the horizon, there hung a some. what dilute small cloud, beneath which there appeared a mock sun of the same size, to sepse, with the true sun; and under the same vertical, of a somewhat red colour. Soon after, the true sun more and more descending to the horizon, towards the said cloud, the spurious sun beneath it grew clearer and clearer, so as that the reddish colour in that apparent solar disk vanished, and put on the genuine solar light; and that the more, the less the geovide disk of the sun was distant from the false sun : till at length the u per true sun passed into the lower counterfeit one, and so re, mained alone.
This appearance being unusual, I took the freedom of impart. ing it unto you, especially since bere the mock san was not found at the side of the true sun, as it is wont to be in all parhelia seen by me, but perpendicularly under it; not to mention the colour, so different from that which is usual in mock suns; nor the great length of the tail, cast up by the genuine sun, and of a far more vivid and splendid light, than parhelia use to exhibit. Upon this appearance there soon followed here an exceedingly in. tense and bitter frost, whereby the whole bay was frozen up from this town of Dantzic, as far as Hela in the Baltic sea, which lasted till the 25th of March; and the bay was frozen so hard, that with great safety people run out into it with sleds and horses, for se. veral of our miles.
[Phil. Trans. 1674.
Pyramidal Appearance in the Heavens, observed near Up.
By the Rev. William Derham.
On the afternoon of Thursday, April 3, 1707, I perceived in the west, a quarter of an hour before sun set, a long slender pyramidal appearance, perpendicular to the horizon. The base of this py. ramid I judged to be the sun, then below the horizon. Its apex reached fifteen or twenty degrees above the horizon. It was through. out of a rusty red colour ; and was, when I first saw it, pretty vivid and strong; but the top part much fainter than the bottom, nearer the horizon. At what time this appearance began, whether at, or how soon after sun-set, I cannot say, being at that time in a friend's house. But after a while, it grew by degrees weaker and weaker, so that in about a quarter of an hour after I first saw it, the top part (A, L, C,) was scarcely visible. But the lower part remained vivid much longer, but yet grew by degrees shorter and shorter. I saw the remains of the lower half (F, F,) a full hour after sun-set; and should perhaps have seen it longer, had the horizon been open, instead of which it was often in my walk obstructed by trees. The whole atmosphere seemed