An Account of Two Mock-Suns, and an Arc of a Rainbow Inverted, with an Halo, and Its Brightest Arc, Seen on Sunday and Monday, Octob. 22, & 23. 1721. at Lyndon, Comitat' Rutland, Communicated by the Rev. Mr. William Whiston, M. A. Sometime Professor of the Mathematicks in the University of Cambridge

white Tail, much narrower than the Mock-Sun, but which I took to be a Segment of the white Circle which I once saw entire in London. Had the Air been clear, I doubt not but much more of the Phenomena of the Parhelia might this Time have been observed: and I hope, that from our Neighbourhood some Member of the Society may furnish us with a fuller Relation. But how to explain these Appearances, and account for the Magnitude of these Circles, is what seems still wanting. III. An Account of two Mock-Suns, and an Arc of a Rainbow inverted, with an Halo, and its brightest Arc, seen on Sunday and Monday, Octob. 22, & 23. 1721. at Lyndon, Comitat' Rutland, communicated by the Rev. Mr. William Whiston, M. A. sometime Professor of the Mathematicks in the University of Cambridge. About Ten o’Clock in the Morning, on Sunday, Octob. 22. 1721. being at the House of Samuel Barker, Esq; of Lyndon in the County of Rutland, after an Aurora Borealis the Night before (Wind W. S. W.) I saw an Attempt towards two Mock-Suns, as I had done sometimes formerly, of which I immediately inform’d Mr. Barker, though without any great Expectations of what followed. About ½ or ¾ of an Hour after, I went to view the Heavens, and then found the Appearance compleat; and when Mr. Barker and others of the Family were call’d, we all saw it, and all saw saw indeed what we had none of us seen before; I mean two plain Parbelia, or Mock-Suns, tolerably bright and distinct; and that in the usual Places, viz. in the two Intersections of a strong and large Portion of an Halo, (Fig. 1.) with an imaginary Circle, parallel to the Horizon, passing through the true Sun. I call this Circle here imaginary, because it was not itself visible, as it sometimes has been at such Appearances. Each Parbelion had its Tail, of a white Colour, and in direct Opposition to the true Sun; that towards the East was 20 or 25 Degrees long; that towards the West about 10 or 12 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. Upon casting our Eyes upward, we saw an Arc of a curious inverted Rainbow, about the Middle of the Distance between the Top of the Halo and our Vertex. I mean this, when Allowance is made for the usual Inequality, that appears between the same Number of Degrees, nearer to and remoter from that Vertex. This Arc was as distinct in its Colours as the common Rainbow; and, with the like Allowance as before, 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 Degrees long: Its Center in or near our Vertex. On the Top of the Halo was a kind of inverted 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, especially as more exactly measured on Monday, near Noon, when the same Appearance return'd again, but more faintly, were as follows: Sun's Altitude $22^\circ \frac{1}{3}$; perpendicular Semidiameter of the Halo $23^\circ \frac{1}{3}$; Distance of the Rainbow from the Top of the Halo $23^\circ \frac{1}{3}$; Semidiameter of the Arc of the the Rainbow, if our Vertex be suppos'd its Center, $21^\circ$. The Phaenomenon lasted each Day for an Hour and an half, or two Hours. What was most remarkable on Monday was that the Wind, which on Sunday had been almost insensible, 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. On Thursday Morning, Octob. 26. as I was coming in the Northampton Coach towards London, about 9 o'Clock, the Halo returned larger and clearer than before; and the two Mock-Suns just attempted an Appearance therein, 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 Aug. 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 in my Life: That we had there, Sept. 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, 1715: That it was seen in Northamptonshire, at the Bath, and elsewhere: That the Vertex of the Columns which shot upwards, was not our Vertex, but evidently 15 or 20 Degrees distant towards the South; and that the Wind was in Rutland North, as I observed myself; at the Bath West, as Mr. Molyneux observed; and, as I am inform'd by Sir Robert Clarke, in Northamptonshire amptonshire South; all at the same Time, which deserves particular Reflection. But then, if any Reader expects here the Solution of all these Phenomena, he is to know, that as to these Northern Lights, Dr. Halley has communicated his Thoughts to the Publick in the Philosophical Transactions soon after the first Appearance; and I communicated mine about the same Time in a small Pamphlet. And as to the Halo's, Mock-Suns, Inverted Arcs of Rainbows, and other Phenomena of the like Nature, Mons. Huygens has most accurately explained them in his Posthumous Works, from p. 293 to p. 366. and Sir Isaac Newton himself has touched upon them in his Opticks, 1st Edit. p. 134. to which the Inquisitive Reader may have Recourse for his Satisfaction. Only if any enquire farther, Why the Northern Lights have of late been so unusually frequent, I must declare, I am far from having satisfied myself, and so shall not pretend to offer any Thing for the Reader's Satisfaction. London, Nov. 6. 1721, Will. Whiston.