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.

SECTION V.

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 sense, 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 upper 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 here the mock sun 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 intense 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 several of our miles.

[Phil. Trans. 1674.

SECTION VI.

Pyramidal Appearance in the Heavens, observed near Up. minster, Essex.

By the Rev. William Derham.

F

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 pyramid 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

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 the air was probably the cause of the phænomenon. But the pyramid was doubtless imprinted on the far distant vapours 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 hid 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 perchance in some measure con. duce to the solution of that old phenomenon, the aurora borealis. [Id. 1707.

SECTION VII.

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 below 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 horns 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. Trans, 1699.

SECTION VIII.

Two Mock-Suns, and an Arc of a Rain-bow inverted, with a Halo.

By the Rev. William Whiston,

ABOUT ten o'clock in the morning, on Sunday Oct. 22, 1721, being at Lyndon, in the county of Rutland, after aurora borealis the night before, wind W.S.W. I saw an attempt towards two mock. suns, as I had done sometimes formerly. About half or three quarters of an hour after, I found the appearance complete; when two plain parhelia, or mock suns, appeared tolerably bright and distinct; and that in the usual places, viz. in the two intersections of a strong and large portion of a halo, with an imaginary circle, parallel to the horizon, passing through the true sun.

I call imaginary, because it was not itself visible, as it sometimes has been at such appearances. Each parhelion had its tail of a white colour, and in direct opposition to the true sun; that to. wards the east was 20 or 25° long; that towards the west about ten or twelve degrees; but both narrowest at the remote ends. The mock-suns were evidently red towards the sun, but pale or whitish at the opposite sides, as was the halo also. Looking upward, we saw an arc of a curious inverted rainbow, about the middle of the distance between the top of the halo and the vertex. This arc was as distinct in its colours as the common rainbow; and of the same breadth. The red colour was on the convex, and the blue on the concave of the arc; which seemed to be about 90° long: its centre in or near the vertex. On the top of the halo was a kind of in. verted bright arc, though its bend was not plain. The lower part of the halo was among the vapours of the horizon, and not visible. The angles, as more exactly measured on Monday, near noon, when the same appearance returned again, but more faintly, were as follow the sun's altitude 22°; perpendicular semidiameter of the halo 23; distance of the rainbow from the top of the halo 231; semidiameter of the arc of the rainbow, if our vertex be supposed its centre, 21°. The phenomenon lasted each day for an hour and a half, or two hours. What was most remarkable on Monday was, that the wind, which on Sunday had been almost in.

sensible, was now become sensible, and changed to N.N.E. that the halo was sensibly become oval; its shorter axis parallel to the horizon; and the two mock-suns, which were then but just visible, especially that on the east, were not in the halo, but a degree or two without it, which I ascribe to the unusual shortness of the horizontal diameter; which position of the mock-suns does not appear to have been hitherto taken notice of by any, though it was now very sensible.

October 26, about nine in the morning, as I was coming in the Northampton coach towards London, the halo returned larger and clearer than before; and the two mock-suns just attempted an appearance, as on Sunday; but the air becoming thicker and thicker towards rain, I saw them no more. I add nothing to this account, but only, that August 30, before, I saw at the same place, Rutland, a remarkable halo, whose upper part had its inverted arc reddish within, and pale without, but brighter and more vivid than ever I saw before. That we had there, September 11, in the evening, the lightest and most remarkable aurora borealis, with its unaccountable motions and removals, that ever I saw, excepting that original one, March 6, 1711: that it was seen in Northamp. tonshire, at the Bath, and elsewhere: that the vertex of the coJumns which shot upwards, was not our vertex, but evidently fifteen or twenty degrees distant towards the south; and that the wind was in Rutland north, as I observed myself; at the Bath west; and in Northamptonshire south; all at the same time, which deserves particular reflection.

[Id. 1721.