165 
The Aurora Borealis is too much connected with our subject, and so 
extraordinary a phenomenon, that we must beg farther to intrude upon the 
patience of our candid reader. 
Franklin was also the first who has attempted a reasonable hypothesis 
respecting the aurora borealis. He reasons thus. Air heated by any means 
becomes rarefied, and specifically lighter than other air in the same situation 
not heated, and when lighter it rises, and the neighbouring cooler and 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 over the cooler air till it comes to the cold walls; there being con¬ 
densed 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 descended. Thus there will be a continual circu¬ 
lation 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. 
A similar operation is performed by nature on the air of this globe. 
Above the height of our atmosphere the air 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. 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. Thus a 
circulation of air is kept up in our atmosphere, as in the room above men¬ 
tioned. 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 men¬ 
tioned; 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. 
The great quantity of vapour rising between the tropics forms clouds, 
which contain much electricity ;* some of them fall in rain, before they come 
* The meteors of the torrid zone are different from those which are found near the polar circles; for 
there the sun exerting his greatest force, raises vapours of various kinds, which form a great variety 
2 T 
