Passy, May 1784.
FOOTNOTE:
[11] This paper is taken from the Memoirs of the Literary and Philosophical Society of Manchester, Vol. II. page 373. It was communicated by Dr. Percival, and read December 22, 1784. Editor.
[Suppositions and Conjectures towards forming an Hypothesis, for the Explanation of the Aurora Borealis][12].
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.