Scientific Intelligence — Meteorology. 193 



But the rapidity of descent does not only depend upon the increas- 

 ing coldness of the air in the fissures, but is further augmented by 

 the warmth of the summer expanding the external air, so that the 

 difference of weight between the external and internal columns be- 

 comes greater. In the manner now explained, a current of cold 

 air is constantly descending and flowing through the cavern, produc- 

 ing all the surprising frigorific effects displayed within it. 



That such a current does exist, Mr Murchison gives a satisfactory 

 proof. He says, *' That, upon unlocking the frail door of the cavern, a 

 volume of air, so surpassingly keen, struck the legs and the feet, that he 

 was glad to rush into a cold bath in front of him to equalize the effect." 

 This downward current will continue the same till the close of au- 

 tumn, when its course comes to be changed ; by that time the first ap- 

 proaches of May's surface warmth will begin to be experienced, the cold 

 of the sides of the rents begins to diminish, and the temperature of the 

 external air must have fallen to nearly that of the internal current. 

 As soon as an equality between the temperatures and densities of the 

 external and internal columns shall have been established, all current 

 must cease. At this period, namely, the commencement of winter, 

 the wave of the summer's heat begins to reach both the walls of the 

 air-channels and of the cavern, and gradually communicates a warmth 

 which progressively elevates the temperature, and dissipates every 

 mark of the preceding summer's congelation. It might at first 

 be reasonably expected, that at this time the preceding order of things 

 would be reversed, and that a current in the opposite direction would 

 commence, 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 duringr winter. 



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

 ward draught, and permits the thermal wave 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 



VOL. XXXV. NO. LXIX. JULY 1843. N 



