SUBTERRANEAN ECHO 227 



water. Thus it appears that in the repercussion 

 between 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 security for persons and pro- 

 perty, 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 excavations 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- 

 Q 2 



