432 



connnence, such as, it is known, happens in many mines ; for, un- 

 doubtedly, the temperature of the atmosphere descending rapidly, 

 the gravity of the external air would soon exceed that of the 

 internal column. A current would immediately commence from 

 below, and, entering from the cavern door and ascending through 

 the rents, escape at the surface. The consequence of such a current 

 would be, that the cold would soon reappear in the cave, and 

 gradually increase during the severity of the winter, and completely 

 overpower the heating influence of the thermal wave, now beginning 

 to operate on the walls of the cavern, and so prevent the warmth of 

 the cave during winter. 



An occurrence, however, now takes place which puts a stop to the up- 

 ward draught, and permits the thermal ware to have its full influence 

 on the temperature of the cavern. The winter commences with re- 

 peated falls of snow, which form a thick covering on the surface of 

 the earth, and closes up all the communications between the extre- 

 mities of the crevices and the external air, and no current can take 

 place. In this manner the influx of the intensely cold air into the 

 cavern, and its ascent through the fissures, is prevented, and then full 

 play is given to the calorific power of the wave of heat which con- 

 tinues to arrive in the cavern through its rocky sides during the 

 whole continuance of winter, and communicates the warmth i-ecorded 

 by Mr Murchison. In the beginning of summer the snows melt, and 

 the terminations and ramifications of the fissures have their commu- 

 nication with the atmosphere restored. The currents, as already de- 

 scribed, are re-established, and all the paradoxical phenomena to 

 which they give bii'th present themselves in due succession. 



2. Observations on the Temperature of the Earth in India. 

 By John Caldecott, Esq. Communicated in a Letter to 

 Professor Forbes. 



These thermometers, made by Mr Adie of Edinburgh, were sunk 

 in the ground at Trevandrum, in lat, 8° 30' 35", to depths of 3, 6, 

 and 12 French feet. Mr Caldecott says, — " I send you herewith 

 the readings of my long thermometers, which, from various causes, 

 I was not able to put into the ground until the 1st of last May 

 (1842). These two months' readings, therefore, will not, of course, 

 have the proper temperature at the respective depths, especially as 

 it has been raining more or less nearly ever since. Still, I think 



