MOMENTARY DEAFNESS. 229 



that no such cavities exist. " It seems most 

 probable," says the latter, " that the hollow re- 

 verberation is nothing more than an assemblage 

 of partial echoes arising from the reflexion of 

 successive portions of the original sound, in its 

 progress through the soil at the innumerable 

 half-coherent surfaces composing it: were the 

 whole soil a mass of sand, these reflexions would 

 be so strong and frequent as to destroy the whole 

 impulse in too short an interval to allow of a dis- 

 tinguishable after-sound. It is a case analogous 

 to that of a strong light thrown into a milky 

 medium or smoky atmosphere ; the whole medium 

 appears to shine with a nebulous undefined light. 

 This is to the eye what such a hollow sound is 

 to the ear."* 



It has been recently shown by M. Savart, that 

 the human ear is so extremely sensible as to be 

 capable of appreciating sounds which arise from 

 about twenty-four thousand vibrations in a second, 

 and consequently that it can hear a sound which 

 lasts only the twenty-four thousandth part of a 

 second. Vibrations of such frequency afford only 

 a shrill squeak or chirp ; and Dr. Wollaston has 

 shown that there are many individuals with their 

 sense of hearing entire, who are altogether in- 

 sensible to such acute sounds, though others are 

 painfully affected by them. Nothing, as Sir John 

 Herschell remarks, can be more surprising than 

 to see two persons, neither of them deaf, the one 

 complaining of the -penetrating shrillness of a 

 sound, while the other maintains there is no 

 sound at all. Dr. Wollaston has also shown that 

 this is true also of very grave sounds ; so that 

 * Art. SOUND, Encycl. Metrop., 110. 



