284 LETTERS ON NATURAL MAGIC. 



the focus of one of the reflecting surfaces was chosen for 

 the place of the confessional, and when this was acci- 

 dentally discovered, the lovers of secrets resorted to the 

 other focus, and thus became acquainted with confessions 

 of the gravest import. This divulgence of scandal con- 

 tinued for a considerable time, till the eager curiosity of 

 one of the dilettanti was punished, by hearing his wife's 

 avowal of her own infidelity. This circumstance gave 

 publicity to the whispering peculiarity of the cathedral, 

 and the confessional was removed to a place of greater 

 secrecy. 



An echo of a very peculiar character has been described 

 by Sir John Herschel in his Treatise on Sound, as pro- 

 duced by the suspension bridge across the Menai Strait in 

 Wales. " The sound of a blow with a hammer," says he, 

 " on one of the main piers is returned in succession from 

 each of the cross beams which support the road-way, and 

 from the opposite pier at a distance of 576 feet ; and in 

 addition to this, the sound is many times repeated between 

 the water and the road-way. The effect is a series of 

 sounds which may be thus written : the first return is 



Fig. 50. 



f rrrrrrrr 



sharp and strong from the road-way overhead ; the rattling 

 which succeeds dies away rapidly, but the single reper- 

 cussion from the opposite pier is very strong, and is suc- 

 ceeded by a faint palpitation repeating the sound at the 

 rate of twenty-eight times in five seconds, and which, 

 therefore, corresponds to a distance of 184 feet, or very 

 nearly the double interval from the road-way to the 

 water. Thus it appears that in the repercussion between 



