228 LETTEES ON NATUEAL MAGIC. 



worn stones, having hollows either left entirely 

 empty, or filled up with materials of different 

 density, then the sound will be reflected in 

 passing from the loose to the dense materials, 

 and there will arise a great number of echoes 

 reaching the ear in rapid succession, and forming 

 by their union a hollow rumbling sound. This 

 principle has been very successfully applied by 

 Sir John Herschell to explain the subterranean 

 sounds with which every traveller is familiar who 

 has visited the Solfaterra, near Naples. When 

 the ground at a particular place is struck vio- 

 lently by throwing a large stone against it, a 

 peculiar hollow sound is distinctly heard. This 

 sound has been ascribed by some geologists to 

 the existence of a great vault communicating 

 with the ancient seat of the volcano, by other 

 writers to a reverberation from the surrounding 

 hills with which it is nearly concentric, and by 

 others to the porosity of the ground. Dr. Dau- 

 beny, who says that the hollow sound is heard 

 when any part of the Solfaterra is struck, accounts 

 for it by supposing that the hill is not made up 

 of one entire rock, but of a number of detached 

 blocks, which, hanging as it were by each other, 

 form a sort of vault over the abyss within which 

 the volcanic operations are going on.* Mr. 

 Forbes, who has given the latest and most inte- 

 resting description of this singular volcano,! 

 agrees in opinion with Dr. Daubeny ; while Mr. 

 ScropeJ and Sir John Herschell concur in opinion 



* Description of Volcanoes, p. 170. 



f Edinburgh Journal of Science, New Series, No. i., 

 p. 124. 



t Considerations on Volcanoes, and Edinburgh Jottrnal 

 of Science, No. xx., p. 261, and No. xiv., p. 265. 



