153 



is but very recently, that pains had been taken 

 with some success, to solve this grand problem 

 of subterraneous meteorology. The stony stra- 

 ta, that form the crust of our planet, are alone 

 accessible to our examination ; and we now 

 know, that the mean temperature of these 

 strata varies not only with the latitudes and 

 the heights, but that, according to the position 

 of the several places, it performs also, in the 

 space of a year, regular oscillations rounol the 

 mean heat of the neighbouring atmosphere. 

 We are already far from that period, when 

 men were surprised to find, under other zones, 

 the heat of grottoes and wells differing from 

 that, which is observed in the caves of the ob- 

 servatory at Paris. The same instrument, which 

 in those caves marks 12°, rises in the sub- 

 terraneous caverns of the island of Madeira, 

 near Funchal *, to 16*2°; in Joseph's Well, at 

 Cairo-f-, to 21 -2°: in the grottoes of the island 



* At Funchal (lat. 32° 370 tne mean temperature of the 

 air is 20 4°. This is so much the more probable, as Mr. 

 Escolar finds at Santa Cruz, in Teneriffe, 21*8° (Cavendish, 

 in the Philosoph. Trans, for 1778, p. 392.) We shall 

 hereafter recur again to this remarkable difference between 

 the caves in the island of Madeira and the surrounding at- 

 mosphere. 



+ At Cairo (lat. 30° 2 y ) the mean temperature of the air 

 is 22*4°, according to Nouet. 



