ACOUSTICS. 193 



pass is thus greatly increased, and every change of medium diminishes tho 

 strength of sound- waves. 



414. The velocity of the sound undulations 



What law gov- . . ^ . , . , . 



erns the veioc- IS uniiorm, passinfiT over equal intervals m 



ily of sound? . ' ^ ° ^ 



equal times. 



The softest whisper, therefore, flies as fast as the loudest thunder. 

 With what ye- ^15. Souuds of every kind travel, when tho 

 S travel? tcmperaturo Is at 62° Fahrenheit's thermom- 

 eter, at a rate of 1,120 feet per second, or 

 about 13 miles per minute, or 765 miles per hour. The 

 velocity of sound increases or diminishes at the rate of 13 

 inches for every variation of a degree in temperature above 

 or below the temperature of 62° Fahrenheit. 



^, , When a e;un is fired at some distance, we see the flash a 



Why <Jo we see ° 



the flash of a considerable time before we hear the report, for the reason 



CTrtheTepwt? ^^^t light travels much faster than sound. Light would go 

 round the earth 480 times while sound was traveling 13 miles. 



A knowledge of these cu'cumstances is taken advantage of for the measure- 

 ment of distances. 



How may a Thus, suppose a flash of lightning to be perceived, and on 



knowledge of counting the seconds that elapse before the thunder is heard. 

 Bound be ap- we find them to amount to 20; tlion as sound moves 1,120 

 measurement" ^'^^^ ^^ ^ second, it will foUow that the thunder-cloud must bo 

 of distances? distant 1,120X20 = 22,400 feet. 



When a long column of-soldiers are marching to a measure beaten on the 

 drums which precede them, we may obser\-e an undulatory motion transmit- 

 ted from the drummers through the whole column, those in the rear stepping 

 a little later than those which precede them. The reason of this is, that each 

 rank steps, not when the sound is actually made, but when in its progress 

 down the column at the rate of 1,120 feet in a second of time, it reaches their 

 ears. Those who are near the music hear it first, wliile those at the end of 

 the column must wait until it has traveled to their ears at the above rate. 

 Explain the 416. If two waves of water, advancing from opposite direo- 



phe:iomena of tions, meet in j5uch a way that their points of elevation coin- 

 the interference «,,,,,., ^. , • i ■^^ \ 



of sound. cide, a wave of double the height of the smgle one will be 



formed at the point of interception ; or if two wave depressions on tho sur- 

 face of water meet, a depression of double depth will be produced. If^ how- 

 ever, the two waves come into contact in sucli a manner that an elevation of 

 one wave coincides with the depression of another, both will be destroyed. 

 Such a result is termed an interference of waves. In the same manner when 

 two series of sound undulations, propagated from different sounding bodies, 

 intersect each other, a like phenomena of interference is produced — the two 

 undulations destroy each other, and silence is produced. 



9 



