Suppositions and Conjectures towards forming an Hypothesis, for the Explanation of the Aurora Borealis ²³ .

1. Air heated by any means, becomes rarefied, and specifically lighter than other air in the same situation not heated.

2. Air being made thus lighter rises, and the neighbouring cooler heavier air takes its place.

3. If in the middle of a room you heat the air by a stove, or pot of burning coals near the floor, the heated air will rise to the ceiling, spread over the cooler air till it comes to the cold walls; there, being condensed and made heavier, it descends to supply the place of that cool air, which had moved towards the stove or fire, in order to supply the place of the heated air, which had ascended from the space around the stove or fire.

4. Thus there will be a continual circulation of air in the room; which may be rendered visible by making a little smoke, for that smoke will rise and circulate with the air.

5. A similar operation is performed by nature on the air of this globe. Our atmosphere is of a certain height, perhaps at a medium [    ] miles: above that height it is so rare as to be almost a vacuum. The air heated between the tropics is continually rising; its place is supplied by northerly and southerly winds, which come from the cooler regions.

6. The light heated air, floating above the cooler and denser, must spread northward and southward; and descend near the two poles, to supply the place of the cool air, which had moved towards the equator.

7. Thus a circulation of air is kept up in our atmosphere, as in the room above-mentioned.

8. That heavier and lighter air may move in currents of different and even opposite direction, appears sometimes by the clouds that happen to be in those currents, as plainly as by the smoke in the experiment above-mentioned. Also in opening a door between two chambers, one of which has been warmed, by holding a candle near the top, near the bottom, and near the middle, you will find a strong current of warm air passing out of the warmed room above, and another of cool air entering below; while in the middle there is little or no motion.

9. The great quantity of vapour rising between the tropics forms clouds, which contain much electricity.

Some of them fall in rain, before they come to the polar regions.

10. If the rain be received in an isolated vessel, the vessel will be electrified; for every drop brings down some electricity with it.

11. The same is done by snow or hail.

12. The electricity so descending, in temperate climates, is received and imbibed by the earth.

13. If the clouds are not sufficiently discharged by this gradual operation, they sometimes discharge themselves suddenly by striking into the earth, where the earth is fit to receive their electricity.

14. The earth in temperate and warm climates is generally fit to receive it, being a good conductor.

15. A certain quantity of heat will make some bodies good conductors, that will not otherwise conduct.

16. Thus wax rendered fluid, and glass softened by heat, will both of them conduct.

17. And water, though naturally a good conductor, will not conduct well, when frozen into ice by a common degree of cold; not at all, where the cold is extreme.

18. Snow falling upon frozen ground has been found to retain its electricity; and to communicate it to an isolated body, when after falling, it has been driven about by the wind.

19. The humidity, contained in all the equatorial clouds that reach the polar regions, must there be condensed and fall in snow.

20. The great cake of ice that eternally covers those regions may be too hard frozen to permit the electricity, descending with that snow, to enter the earth.

21. It may therefore be accumulated upon that ice.

22. The atmosphere being heavier in the polar regions than in the equatorial, will there be lower; as well from that cause, as from the smaller effect of the centrifugal force: consequently the distance of the vacuum above the atmosphere will be less at the poles, than elsewhere; and probably much less than the distance (upon the surface of the globe) extending from the pole to those latitudes in which the earth is so thawed as to receive and imbibe electricity; (the frost continuing to lat. 80, which is ten degrees, or six hundred miles from the pole; while the height of the atmosphere there of such density as to obstruct the motion of the electric fluid, can scarce be esteemed above [    ] miles).

23. The vacuum above is a good conductor.

24. May not then the great quantity of electricity, brought into the polar regions by the clouds, which are condensed there, and fall in snow, which electricity would enter the, earth, but cannot penetrate the ice; may it not, I say, (as a bottle overcharged) break through that low atmosphere, and run along in the vacuum over the air towards the equator; diverging as the degrees of longitude enlarge; strongly visible where densest, and becoming less visible as it more diverges; till it finds a passage to the earth in more temperate climates, or is mingled with their upper air?

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:

[13] For an explanation of the signature B. V. see the note in page 399 of Vol. I. Editor.

TO DR. L. ²⁴ AT CHARLES-TOWN, SOUTH-CAROLINA.