Hence the first snows remained on it unmelted, and received continual additions.
Hence perhaps the winter of 1783-4, was more severe than any that had happened for many years.
The cause of this universal fog is not yet ascertained. Whether it was adventitious to this earth, and merely a smoke proceeding from the consumption by fire of some of those great burning balls or globes which we happen to meet with in our rapid course round the sun, and which are sometimes seen to kindle and be destroyed in passing our atmosphere, and whose smoke might be attracted and retained by our earth; or whether it was the vast quantity of smoke, long continuing to issue during the summer from Hecla, in Iceland, and that other volcano which arose out of the sea near that island, which smoke might be spread by various winds, over the northern part of the world, is yet uncertain.
It seems however worth the enquiry, whether other hard winters, recorded in history, were preceded by similar permanent and widely extended summer fogs. Because, if found to be so, men might from such fogs conjecture the probability of a succeeding hard winter, and of the damage to be expected by the breaking up of frozen rivers in the spring; and take such measures as are possible and practicable, to secure themselves and effects from the mischiefs that attended the last.
[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.