SECT. XVI. INTENSITY OF SOUND. 131 



vibratory state. It is thus that an impulse given to any one 

 point of the atmosphere is successively propagated in all direc- 

 tions, in a wave diverging as from the centre of a sphere to 

 greater and greater distances, but with decreasing intensity, in 

 consequence of the increasing number of particles of inert matter 

 which the force has to move ; like the waves formed in still 

 water by a falling stone, which are propagated circularly all 

 around the centre of disturbance (N. 160). 



The intensity of sound depends upon the violence and extent 

 of the initial vibrations of air ; but, whatever they may be, each 

 undulation when once formed can only be transmitted straight 

 forwards, and never returns back again unless when reflected by 

 an opposing obstacle. The vibrations of the aerial molecules arc 

 always extremely small, whereas the waves of sound vary from a 

 few inches to several feet. The various musical instruments, 

 the human voice and that of animals, the singing of birds, the 

 hum of insects, the roar of the cataract, the whistling of the wind, 

 and the other nameless peculiarities of sound, show at once 

 an infinite variety in the modes of aerial vibration, and the 

 astonishing acuteness and delicacy of the ear, thus capable of 

 appreciating the minutest differences in the laws of molecular 

 oscillation. 



All mere noises are occasioned by irregular impulses com- 

 municated 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 undulations 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 impulse. 

 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 frequency as the note becomes more 



