154 Scient'ific Intelligence. [Fbb. 



portion of gum contained in each. The first yielded 13, and the \ 



1 1 4 per cent., of a brown transparent gum, similar in its j 



ities to gum arabic, and capable of being applied to the \ 



second 

 proper 

 same uses. 



X. Extraordinary Fog. 



Between Monday, the 27th of December, and Sunday, the 

 2d of January, 1814, a most extraordinary fog prevailed in Lon- 

 don, and it seems to have extended a great many miles round in 

 every direction. It was frequently so thick that it was impossible 

 to see across the street. Candles were burnt in most of the shops 

 and counting-houses all day long. The mean temperature of the 

 air during the week was 2/°. The thermometer was never higher 

 than 34°, nor lower than 22'5°. The fog condensed upon the 

 grass, tlie trees, and every wooden or iron railing. The grass in 

 consequence was covered with a coating of snow at least half an 

 incli thick. Below the trees in St. James's park there lay a bed of 

 snow at least an inch thick, which had fallen down from them. 

 In London, the thickness of the fog was still farther increased by 

 the smoke of the city ; so much so, that it produced a very sensible 

 cifect on the eyes, and the coal tar vapour might be distinctly per- 

 ceived by the smell. But at a distance from town, though there 

 was no smoke, the fog was very thick. Not a breath of wind was 

 perceptible during the whole week. 



To account for the existence of such a fog, at such a tempera- 

 ture, and for such a length of time, constitutes a very difficult 

 iVieteorological problem. According to the best data, air at the 

 temperature of 27° is capable of containing a quar >ty of vapour 

 weighing ^ ^ \ „ „ of tiie atmosphere ; and this quantity produces 

 perfect satuiaiion. Were we to suppose the whole atmosphere to 

 be saturated with vapour, and this vapour, by some means or 

 other, to be condensed into particles of water, its quantity is so 

 sm.ill that it could scarcely constitute a thick fog; since, allowing 

 it all to he deposited on the surface of the earth, it would not 

 amount to tljree quarters of an ounce avoirdupois for every square 

 foot. The quantity of vapour condensed into snow upon the 

 surface of the earth during the fog must have exceeded this very 

 consideiabiy. And this estimate, small as it is, must be reduced at 

 least to one half, because there is no reason to believe that the 

 upper half of the atmosphere contains vapour. 



VVere we to suppose a constant stream of warm air from the 

 soutli flowing over our atmosphere at some height above the surface, 

 it would have produced clouds, and these clouds might have fallen 

 down to the ground and constituted a fog. But I cannot conceive 

 the existence of siicli a current over still air for a week, and a 

 constant precipitation of vapour, without producing a sensible in- 

 crease of the temperature of th.e place. No such change,' however, 

 <ook place to any great degree. On hjunday, tlie :7th, a slight wind 



