ON ITS EXTERIOK. THERMAL SPRINGS. 197 



that, with very moderate differences of elevation, such 

 as four or five thousand feet for example, mountain- 

 springs would frequently be found, whose temperature 

 should be from 70 to 90 Fahr. above the mean tempera- 

 ture of the place ; and lastly, what might we not expect 

 would be the case at the foot of mountains in the torrid 

 zone, which, at a height of nearly 15,000 feet, are still 

 free from perpetual snow, and which often show no vol- 

 canic rocks, but only gneiss and mica-slate ? ( 268 ) The 

 great mathematician Fourier, the year before his death, 

 (at my solicitation, and stimulated by the consideration 

 of the topography of the eruption of Jorullo in a plain 

 where, for many hundred square leagues around, no 

 traces of unusual terrestrial heat had been discovered,) 

 engaged in theoretical investigations into the question of 

 the manner in which, in cases of upheavals and altered 

 configuration of the surface, the isothermal planes adjust 

 themselves afresh, in accordance with the new form of 

 the ground. The lateral radiation of strata, which are 

 situated at the same level but unequally covered, has 

 greater influence in this matter, than has the inclination 

 of the planes of separation of the rock where stratifica- 

 tion is discernible. 



I have already remarked elsewhere, ( 269 ) how the hot 

 springs in the vicinity of ancient Carthage, probably the 

 thermal waters of Pertusa (aquae calidse of Hammam- 

 el-Enf), led the martyr, Bishop Patricius, to a correct 

 view as to the cause of the higher or lower temperature 

 of the gushing fountains. When the pro-consul Julius 

 sought mockingly to perplex the accused bishop by the 

 question, " Quo auctore fervens hsec aqua tantum ebul- 

 liat ? " Patricius, in reply, unfolded his theory of central 



