Air heated by any Means becomes rarefied & specifically lighter than other Air in the same Situation not heated.

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

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 there over the cooler Air till it comes to the cold Walls; there being condens’d & 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.

Thus there will be a continual Circulation of Air in the Room, which may be render’d visible by making a little Smoke; for that Smoke will rise & circulate with the Air.

A Similar Operation is perform’d 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 supply’d by northerly & southerly Winds which come from the cooler Regions.

The light heated Air floating above the cooler and denser must spread northward & southward, and descend near the two Poles to supply the Place of the cooler Air which had moved towards the Equator.

Thus a Circulation of Air is kept up in our Atmosphere as in the Room above mentioned.

That heavier & lighter Air may move in Currents of different & even opposite Direction, appears sometimes by the Clouds that happen to be in those Currents, as plainly as by the Smoke in the Expr 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, & 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 it below, while in the Middle there is little or no Motion.

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.

If the Rain be received in an isolated Vessel, the Vessel will be electrified; for every Drop brings down some Electricity with it.

The same is done by Snow, and Hail.

The Electricity so descending in temperate Climates, is receiv’d & imbib’d by the Earth.

If the Clouds are not sufficiently discharg’d by this means, they sometimes discharge themselves by striking into the Earth, where the Earth is fit to receive their Electricity.

The Earth in temperate & warm Climates is generally fit to receive it, being a good Conductor.

A certain Quantity of Heat will make some Bodies good Conductors, that will not otherwise conduct.

Thus Wax render’d fluid, & Glass softened by Heat, will both of them conduct.

And Water tho’ 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 extream.

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.

The Humidity contain’d in all the equatorial Clouds that reach the Polar Regions, must there be condens’d & fall in Snow.

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.

It will therefore be accumulated upon that Ice.

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 to the Vacuum above the Atmosphere will be less at the Poles than else where; and much less than the Distance upon the Surface of the Globe extending from the Pole to the Latitudes in which the Earth is so thaw’d as to receive & imbibe Electricity; the Frost continuing to Lat. 80, which is 10 Degrees or 600 Miles from the Pole, while the Height of the Atmosphere there can scarce be esteemed above     Miles.

The Vacuum above is a good Conductor.

May not then the great Quantity of Electricity brought into the Polar Regions by the Clouds which are condens’d there & fall in Snow, which Electricity would enter the Earth but cannot penetrate the Ice; May it not, I say, as a bottle overcharg’d, break thro’ 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?

If such an Operation of Nature were really performed, would it not give all the Appearances of an Aurora Borealis?

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, & all the Moisture contain’d in that Air being frozen, may not any great Light arising therein & passing thro’ 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, altho’ in itself a compleat and full Circle, extending perhaps 10 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, & its Arch elevated more or less above it, as seen from Latitudes less or more distant, darkish in Colour, but yet sufficiently transparent to permit the Stars to be seen thro’ it?

28. The Rays of electric Matter issuing out of a Body, diverge, unless there be some Conducting Body near to receive them. And if that Body be at a greater Distance, they will first diverge, & 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 towards the Equator, the Rays of that Matter may find in many Places Portions of Cloudy Region or moist Atmosphere under them, fit to receive them, and towards which they may therefore converge; and when that receiving Body 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, & other Figures mentioned in the Histories of this Meteor?