412 DIFFERENCES IN TONES, AND IN SENSE OF HEARING. 
upon the force and extent of the vibrations communicated by 
the sounding body to the air. Thus, when we draw the 
middle of a tight string far out of the straight line, and then 
let it go, a loud sound is produced, and we can see that the 
space through which the string passes from side to side is 
considerable. As the extent of the vibrations of the string 
diminishes, the sound becomes less powerful ; and when we 
can no longer see the vibrations, but can only feel them, the 
sound is faint. The length of the undulations in the air 
corresponds with that of the vibrations in the sounding 
body ; and consequently they will strike upon the tympanum 
with more or less force, according as these are longer or 
shorter. The cause of the differences in the timbre or quality 
of musical tones,—such, for instance, as those which exist 
between the tones of a fiute, a violin, and a trumpet, all 
sounding a note of the same pitch,—are unknown ; but they 
probably depend upon the different form of the vibrations. 
525. The faculty of hearing, like that of sight, may be very - 
much increased in acuteness by cultivation ; but this increase 
depends rather upon the habit of attention to the faintest 
impressions made upon the organ, than upon any change in 
the organ itself. This habit may be cultivated in regard to 
sounds of some one particular class ; all others being heard as 
by an ordinary person. Thus the watchful North American 
Indian recognises footsteps, and can even distinguish between 
the tread of friends or foes, whilst his companion who lives 
amid the busy hum of cities is unconscious of the slightest 
sound. Yet the latter may be a Musician, capable of distin- 
guishing the tones of all the different instruments in a large — 
orchestra, of following any one of them through the part 
which it performs, and of detecting the least discord in the 
blended effects of the whole,—effects which would be, to his” 
coloured companion, but an indistinct mass of sound. In the 
same manner, a person who has lived much in the country is 
able to distinguish the note of every species of bird which 
lends its voice to the general concert of Nature ; whilst the 
inhabitant of a town hears ouly a confused assemblage of 
shrill sounds, which may impart to him a disagreeable rather 
than a pleasurable sensation.—Of the direction and distance 
of sounds, our ideas are for the most part formed by habit. ~ 
Of the former we probably judge, in great degree, by the — 
