CATARACTS AND INUNDATIONS* 107 



i i 



the way in which those mineral springs, that contain sulphureted 



hydrogen gas, receive iheir impregnation* Bui we are pretty cer- 

 tain, that such a supposition will not apply lo Bath water: first, 

 because it does not contain the notable quantity of sulphate of iron, 

 which would be necessary upon such a supposition; and, secondly, 

 because instead of sulphureted hydrogen gas, which would infallibly 

 result from such a decomposition of pyrites, there is an evolution 

 of azotic gas. This evolution of azotic gas, however, is a derisive 

 proof that the heat of Bath waters is owing to some decomposition 

 or other, which takes place within the surface of the earth ; though, 

 from our imperfect acquaintance with the nature of the mineral 

 strata, through which the water flows, we cannot give any satisfac- 

 tory information about what that decomposition actually is." 



EDITOR. 



2. On the Temperature of the Earth belozo the Surface, in re- 

 gard to Springs and Hills, and especially those oj Jamaica. 



By John Hunter, M.D. F.R.S. 



" The great difference, savs Dr. H. between the temperature of 

 the open air, and that of deep caverns or mines, has long been taken 

 notice of, both as matter of curiosity and surprize. After thermo- 

 meters were brought to a tolerable degree of perfection, and meteo- 

 rological registers were kept with accuracy, it became a problem, 

 to determine what was the cause of this difference between the heat 

 of the air and the heat of the earth ; for it was soon found that the 

 temperature of mines and caverns did not depend on any thing pe- 

 culiar to them ; but that a certain depth under ground, whether 

 in a cave, a mine, or a well, was sufficient lo produce a very sen. 

 sihle difference in the l,t at. In observations of this kind, there was 

 periiaps nothing more striking, than th;it the heal in such caves was 

 nearly the same in summer and winter ; ami this even in changeable 

 climates, that admitted of great variation be* \u-in the extremes of 

 heat in summer, and cold in winter. There is uii exn -pie of this 

 in the cave of the Royal Observatory at Paris, i'ne cxpia ations, 

 which have been attempted ot this phenomenon, have tu.iu d chiefly 

 on a supposition, that there was an internal source of heat in the 

 earth itself, totally independent of the influence of the sun*. M. 

 de Mairan has bestowed much labour on this subj t, and by obser- 



* Vid. Marline's Essays, p. 319. 



