216 Temperalure of the Sumir.cr arid JJ'inter Seasons, 



more northern latitudes, as may be seen in Van Swinderi** 

 general table, for it reached these towns on the 18th or 19th 

 of January. In the more northern latitudes it was felt only 

 on the 27th ; it is true its date at St. .lean de Luz, latitude 

 43°, is January 2Sth in the general table ; but this is a mis- 

 take, as may be seen p. 181, for January 19th is there said 

 to be the true date, and, p. 179, it is said that the 18th or 

 19th of January are the days on which the greatest cold 

 was observed in all places south of the Garonne ; which 

 fully confirms my former statement, that the wind which 

 produced this cold originated in the south-west, and thence 

 was gradually propagated northwards and eastwards. All 

 the minuter modifications of this cold, in places not very 

 distant from each other, may be ascribed either to recent 

 falls of snow, the proximity to which nuist affect more or 

 less the thermometers, the greater or lesser abundance of 

 vapours in the atmosphere, and other circumstances too 

 tedious and minute for insertion in this general view. 



Snow falling from some height in the atmosphere is ge- 

 nerally for some time surrounded with an atmosphere much 

 colder than the air some feet above it, as Mr. Wilson ob- 

 served, though it did not occur to him that the cold was 

 communicated to the air by the- snow ; for he thought it 

 highly remarkable that a thermometer hung 24 feet above 

 the snow was four degrees less cold than one suspended 

 25- feet above it. (Phil. Trans. 1780, p. 462.) Yet Mr. 

 Bovle has long since noiiccd a similar fact (Boyle Abridg. i. 

 p. 629), as related to him by some navigators; and Foster 

 expressly mentions, that being to the leeward of an icy 

 mountain, probably many feet distant, the thermometeK 

 sunk four degrees, and rose to its former heiglit vvhen ht> 

 had passed tiiat miountain (Observat. p. 73) ; but when 

 there is not a recent fall of snow, the air several feet above 

 the surface of the earth is generally colder (when no great 

 e^•aporation takes place) than that nearer to its surface. 

 Thus during the intense cold of January 1776, there hav- 

 ino- been no fall of snow since the 24th, Van Swinden found 

 the degree of cold on the morning of the 27th to be 8'^ 25', 

 while Camper, in the same street, whose thermometer was 

 some feet nearer to the earth, found it only 6^ 5'. (See Van 

 Swinden, sur le Froid de I'Aunee 1/76, p. £4^ 25, 28, and 

 U6.) 



XXXV. Ac- 



