SUBTERRANEAN ECHO, 285 



the water and road-way, that from the latter only affects 

 the ear, the line drawn from the auditor to the water 

 being too oblique for the sound to diverge sufficiently in 

 that direction. Another peculiarity deserves especial 

 notice, namely, that the echo from the opposite pier is 

 best heard when the auditor stands precisely opposite to 

 the middle of the breadth of the pier, and strikes just on 

 that point. As it deviates to one or the other side, the 

 return is proportionally fainter, and is scarcely heard by 

 him when his station is a little beyond the extreme edge 

 of the pier, though another person, stationed (on the 

 same side of the water) at an equal distance from the 

 central point, so as to have the pier between them, hears 

 it well." 



A remarkable subterranean echo is often heard when 

 the hoofs of a horse or the wheels of a carriage pass over 

 particular spots of ground. This sound is frequently 

 very similar to that which is produced in passing over an 

 arch or vault, and is commonly attributed to the existence 

 of natural or artificial caves beneath. As such caves have 

 often been constructed in times of war as places of secu- 

 rity for persons and property, many unavailing attempts 

 have been made to discover hidden treasures where their 

 locality seemed to be indicated by subterraneous sounds. 

 But though these sounds are sometimes produced by ex- 

 cavations in the ground, yet they generally arise from the. 

 nature of the materials of which the ground is composed, 

 and from their manner of combination. If the hollow of 

 a road has been filled up with broken rock, or with large 

 water-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 



