Franklin’s Conjecture on the Foregoing
Copy:4 the Royal Society
[After August 12, 1772]
The Aurora Boreales, tho’ visible almost every Night of clear Weather in the more Northern Regions, and very high in the Atmosphere, can scarce be visible in England but when the Atmosphere is pretty clear of Clouds for the whole Space between us and those Regions, and therefore are seldom visible here. This extensive Clearness may have been produced by a long continuance of Northerly Winds. When the Winds have long continued in one Quarter, the Return is often violent. Allowing the Fact so repeatedly observed by Capt. Winn, perhaps this may account for the Violence of the Southerly Winds that soon follow the Appearance of the Aurora on our Coasts.5
4. The copy, in Fevre’s hand, is on the verso of Winn’s second letter to BF on the same subject: below, Jan. 22, 1773. The comment could have been on the two letters together, in which case it was obviously written after the later one, or only on the first, the preceding document; we act on the latter supposition because our policy is to assign the earliest feasible date. BF’s conjecture, together with an abbreviated version of Winn’s second letter, was printed as notes to the first one in Phil. Trans., LXIV (1774), 132.
5. This was not the end of BF’s speculations about the aurora; for a continuation of them years later, which embodied this paragraph, see Smyth, Writings, VII, 209–15.