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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.
Parhelia, 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 genuine disk of the sun was distant from the false sun : till at length the up per true sun passed into the lower counterfeit one, and so re, mained alone.
This appearance being unusual, I took the freedom of impart. jog it unto you, especially since here 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.
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 hoor 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
hazy, and full of vapours, especially towards the sun-set. The moon and stars were bearded at that time, and succeeded with a halo about the moon afterwards. Which disposition of tbe air was probably the cause of the phænomenon. But the pyramid was doubtless imprinted on the far distant vapoors of the atmosphere, it being manifestly far off, or laying beyond some small thin clouds (C. L, C, L,) that intercepted it, and in those parts covered and bid it. I do not remember I ever saw any thing like it, except the white pyramidal glade, which is now called the aurora borealis. And it being, except in colour and length, so like that, I have thought it may perchaoce in some measure con. duce to the solution of that old phenomenon, the aurora borealis.
Parhelia at Sudbury, Suffolk.
By Mr. Petto, AUGUST 28, 1698, about eight o'clock in the morning, there was seen the appearance of three suns, which were at the brightest then, or a little after. About half an hour after eight I saw it, when there was in the east a dark, dusky, watery cloud; and be. low it towards the middle, was the true sun, shining with such strong beams, that persons could not look upon it ; on each side were the reflections, with the true sun in the middle. Elsewhere much of the firmament was of an azure light blue colour. The circles which I saw were not of rainbow colours, but white; there was also higher in the firmament, more over our heads, and towards the south, at the same time, at a considerable distance from the other, the form of a half moon ; but I think it was more than double the size of a half moon, with the borns turned upwards, and within of a fiery red colour, and more like a rainbow colour; these all faded gradually, after having continued about two hours.
[Phil. Truns, 1699.