25. If such an operation of nature were really performed, would it not give all the appearances of an aurora borealis?
26. And would not the auroras become more frequent after the approach of winter: not only because more visible in longer nights; but also because in summer the long presence of the sun may soften the surface of the great ice cake, and render it a conductor, by which the accumulation of electricity in the polar regions will be prevented?
27. The atmosphere of the polar regions being made more dense by the extreme cold, and all the moisture in that air being frozen; may not any great light arising therein, and passing, through it, render its density in some degree visible, during the night time, to those who live in the rarer air of more southern latitudes; and would it not in that case, although in itself a complete and full circle, extending perhaps ten degrees from the pole, appear to spectators so placed (who could see only a part of it) in the form of a segment; its chord resting on the horizon, and its arch elevated more or less above it as seen from latitudes more or less distant; darkish in colour, but yet sufficiently transparent to permit some stars to be seen through it.
28. The rays of electric matter issuing out of a body, diverge by mutually repelling each other, unless there be some conducting body near, to receive them: and if that conducting body be at a greater distance, they will first diverge, and then converge in order to enter it. May not this account for some of the varieties of figure seen at times in the motions of the luminous matter of the auroras: since it is possible, that in passing over the atmosphere, from the north in all directions or meridians, towards the equator, the rays of that matter may find, in many places, portions of cloudy region, or moist atmosphere under them, which (being in the natural or negative state) may be fit to receive them, and towards which they may therefore converge: and when one of those receiving bodies is more than saturated, they may again diverge from it, towards other surrounding masses of such humid atmosphere, and thus form the crowns, as they are called, and other figures mentioned in the histories of this meteor?
29. If it be true that the clouds which go to the polar regions, and carry thither the vapours of the equatorial and temperate regions, [have their] vapours condensed by the extreme cold of the polar regions, and fall in snow or hail; the winds which come from those regions ought to be generally dry, unless they gain some humidity by sweeping the ocean in their way. And if I mistake not, the winds between the north east and the north west, are for the most part dry, when they have continued for some time.
[In the Philosophical Transactions for 1774, p. 122, is a letter from Mr. I. S. Winn to Dr. Franklin, stating, that since he had first made the observation concerning the south or south west winds succeeding an aurora, he had found it invariably obtaining in twenty-three instances; and he adds in a note a fresh confirming instance. In reply, Dr. Franklin makes the following conjecture.]
The Auroræ Boreales, though 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 Mr. Winn, perhaps this may account for the violence of the southerly winds, that soon follow the appearance of the aurora on our coasts.
FOOTNOTES:
[12] If I mistake not, this paper was read to the Royal Academy of Sciences, at Paris, at the meeting held immediately after Easter, 1779. B. V[13].