50 ACOUSTICS. 



of vibration, which is communicated to the air ; and Euler 

 was of opinion that no sound making fewer vibrations 

 than 30 in a second, or more than 7520, is distinguish- 

 able by the human ear ; the former being the most grave, 

 and the latter the most acute tone that can be possibly 

 sounded. The vibrations of a sounding body depend on 

 its elasticity, and are governed by certain laws : if a 

 musical string be divided into two parts, the sound of 

 each half will be an octave of the whole string. 



The ear is evidently the most direct instrument for 

 the reception of sound in order to convey it to the brain, 

 which it does by means of the auditory nerve ; yet the 

 palate, teeth, and nostrils lend their assistance; so that 

 the deaf may often be made to hear by holding one end 

 of a piece of metal between their teeth while the other 

 end is in contact with the mouth of the speaker. 



Sound travels in air at the rate of 114-2 feet in a se- 

 cond, or 13 miles in a minute ; and the softest whisper 

 is transmitted as rapidly as the loudest thunder. From 

 a knowledge of the rate of velocity of sound, the distance 

 of a ship at sea, or of a thunder-cloud, may be easily 

 ascertained. If the report of a gun be heard half a mi- 

 nute after the flash* is seen, the object will be about six 

 miles and a half distant, and so in proportion : the same 

 holds good in regard to the distance of a thunder-cloud. 

 As the pulse of a full grown man in health beats seconds 

 nearly, the number of pulsations multiplied into 1 142 

 feet, will give the distance pretty accurately. 



The best conductor of sound is water. On a still night, 

 a conversation may be heard at a very great distance 

 over water. The fire of the English, on landing at Egypt 

 some years since, was heard more than 130 miles at sea ; 

 and it has been asserted that every word of a sermon was 

 distinctly heard two miles distant over water. The in- 

 fluence that water has in propagating sound, is evident 

 from the difference caused by a canal of water laid some 

 years ago under the pit floor of the theatre of Argentine, 

 at Rome, the voice having been since heard distinctly at 

 the remotest part of the theatre, where it was before 



* Light travels so rapidly, that at the distance of a few miles it may 

 be said to be instantaneous. 



