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.
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.