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line muft frequently approach nearly to the lower line, yec they 

 never coincide, or at leaft very rarely, namely, when the cold of 

 the lower air is fo great that no vifible vapours are formed in it. 

 In the month of April, and in part of that of Oflober, this line is 

 intermediate between its height in fummer- and that in winter. 

 And during the whole year it muft be taken as a mean between its 

 heights over the oceans and over land. 



The lower term of congelation is fituated at the height at which 

 it freezes at night, though in day time, and pa'rticularly on funny 

 days, it may furpafs that point by feveral degrees within the 

 tropics ; though fewer without them, even in the fummer feafon. 

 Thus Bouguer found the heat on the ftony fummit of Pinchinca, 

 at whofe height he fixes this term, to vary 17*' of Reaumur, that 

 is 38^ of Fahr. from fun-rife until noon. Voy. p. xxxix. 



He alfo ftates the height of this line in latitude 28" at 1324O 

 feet, and Sauffure, more corredly, at about 12 170 feet ; and 1 be- 

 lieve it ftill lower. 



In the latitude of the fouth of France, (that is 43^ and 44*^) 

 Bouguer Itates the height of this line to be about 10240 and 9600,- 

 but Sauffure efteems it lower, for he makes it between latitude 42*^ 

 and 43*^, (that of Canigou) to be only 9290 feet*. 



Y y 2 'All 



* Voy. aux Alpes, p. 374. 



