A Letter from Mr. John Martyn, F.R.S. Prof. Botan. Cantab. to John Machin, Esq; Secr. R. S. and Prof. Astron. Gresham. concerning an Aurora Australis, Seen March 18. 1738-9. at Chelsea, Near London

from the Zenith: Out of these arose three beautiful Pyramids of Light, which extended very near the Zenith; the middle of these Pyramids were of a beautiful Sea-green, which went off gradually in lighter Shades towards the Edges, which were of a bright White; the Colour of these very much resembled the Light of Phosphorus. I observed these Columns for some Minutes, and then, going in a-doors, saw no more of the Phænomenon; and was told, half an Hour after, that it was all over; but have been informed since, that it returned again about Ten; when the Redness spread, almost universally, over the Southern Parts of the Heavens. XXIV. A Letter from Mr. John Martyn, F.R.S. Prof. Botan. Cantab. to John Machin, Esq; Secr. R. S. and Prof. Astron. Gresham. concerning an Aurora Australis, seen March 18. 1738-9. at Chelsea, near London. SIR, I now trouble you with a rude Account of the Aurora Australis, or what other Name you chuse to call it by, which was mentioned at the Society last Thursday. Sunday, March 18. 1738-9. at half an Hour past Eight, being informed, That there was a great Fire towards London, I made haste towards an upper Window that looked to the North North-east: I found an extraordinary Redness in the Air, but of too determined termined a Figure to arise from the burning of a House: A broad red Band extended itself to the Northward of the East; in the Middle of which I very plainly saw Arcturus, then about $25^\circ$ high; and its Northern Edge touched Cor Caroli. It seemed to be fixed and permanent; not radiating, or fading, as in a common Aurora Borealis. This red Band, or Arch, was bounded on the North by Streams of a greenish Blue, in the same Direction; the most Northern Edge of which touched the Star marked $\eta$ in the Tip of the Tail of Ursa Major. After I had considered this Phænomenon for some little time, I retired into my Garden, where I saw a great Brightness almost in the Zenith, but declining to the South-west; which I found to be a Centre, from which many luminous Radii, of which the red Band was much the most considerable, proceeded. This Crown, or Centre, seemed, at that time, as near as I could judge, about the Place of Cancer; for it effaced all the neighbouring Stars, and I could but just see two Stars, which I take to be those in the Heads of the Twins; when the Brightness was most faded: It would sometimes almost disappear for near a Minute, and then kindle again, and dart Rays on all Sides; but those to the West and North were short, pale, and soon disappeared. Those which shot Southward, were of a fiery Red; and the whole Southern Part of the Atmosphere was tinged with a red Brightness, which did not however reach quite down to the Horizon; for, to the South-west, where my Prospect was least confined, I could see Sirius, then about $15^\circ$ high beyond the red Light. The Houses opposite to mine hindered me from seeing, how far to the East and North-east the the red and blue Rays extended. All this while, the North and North-west Parts of the Heavens seemed dark, by reason of the great Brightness in the East and South: It was very clear however, and the Stars visible. About Nine, the red Band had covered the Tail of *Ursa major*, having moved considerably towards the North, the Centre continuing in the same Place; and by degrees it faded so as not to be distinguishable from the common Redness which was spread over so considerable a Part of the Heavens. About Ten I went to the River-side, where I had a large Prospect to the South-east; and found all that Part covered with a dusky Red, quite down to the Horizon. There were afterwards some faint Rays darted, sometimes from the Centre of this Phænomenon, which has the Appearance of a common *Aurora Borealis*. If you can gather anything material from this Description, it will be a Pleasure to, *SIR,* *Your most obedient* *humble Servant,* *John Martyn.* XXV. Part