146 INTENSITY OF SOUND. [SECT. xvi. 



All mere noises are occasioned by irregular impulses 

 communicated to the ear ; and, if they be short, sudden, and 

 repeated beyond a certain degree of quickness, the ear loses 

 the intervals of silence, and the sound appears continuous. 

 Still such sounds will be mere noise : in order to produce a 

 musical sound, the impulses, and consequently the undula- 

 tions of the air must be all exactly similar in duration and 

 intensity, and must recur after exactly equal intervals of 

 time. If a blow be given to the nearest of a series of broad, 

 flat, and equidistant palisades, set edgewise in a line direct 

 from the ear, each palisade will repeat or echo the sound ; 

 and these echoes, returning to the ear at successive equal 

 intervals of time, will produce a musical note. The quality 

 of a musical note depends upon the abruptness, and its 

 intensity upon the violence and extent of the original im- 

 pulse. In the theory of harmony the only property of 

 sound taken into consideration is the pitch, which varies 

 with the rapidity of the vibrations. The grave or low tones 

 are produced by very slow vibrations, which increase in fre- 

 quency as the note becomes more acute. Very deep tones 

 are not heard by all alike, and Dr. Wollaston, who made a 

 variety of experiments on the sense of hearing, found that 

 many people, though not at all deaf, are quite insensible to 

 the cry of the bat or the cricket, while to others it is pain- 

 fully shrill. From his experiments he concluded that human 

 hearing is limited to about nine octaves, extending from the 

 lowest note of the organ to the highest known cry of insects ; 

 and he observes with his usual originality that, "as there is 

 nothing in the nature of the atmosphere to prevent the exist- 

 ence of vibrations incomparably more frequent than any of 

 which we are conscious, we may imagine that animals like the 

 Grylli, whose powers appear to commence nearly where ours 

 terminate, may have the faculty of hearing still sharper 

 sounds which we do not know to exist, and that there may 

 be other insects hearing nothing in common with us, but 

 endowed with a power of exciting, and a sense which per- 



