QUALITY (TIMBRE) OF MUSICAL SOUNDS. 483 
Now, sound is the name given to the sensation experienced when 
the auditory nerves which connect the brain with the apparatus of 
the ear are affected by a peculiar motion or tremor, generally 
communicated to them by rapid changes in the atmospheric pres- 
sure upon the tympanic membrane, or drum of the ear. 
The ordinary case is this : Some body more or less removed from 
the hearer is thrown into rapid motion ; the air which envelopes 
both the ear and the moving body communicates the motion of the 
latter to the former, and produces upon it the sensation of sound. 
Thus, then, it follows that the sole difference between tones of 
various kinds must consist in the different mechanical effect which 
each has in producing alterations in the pressure upon the tym- 
panum, and that inquiries into the causes of tone must ultimately 
resolve themselves into examinations of differing forms of motion. 
It has long been known that in order that a sound may be 
musical the motion producing it must be of a vibratory character, 
and that the vibrations be isochronous; also, that the length of the 
period, i.e. the rapidity of vibration, is the condition which deter- 
mines the pitch, and the amplitude or distance of vibration the 
loudness of the note produced. What were the physical conditions 
answering to differences of tone or " timbre" was a matter of 
conjecture by the earlier acousticians, and it is only within the 
last few years that the researches of Professor Helmholtz, G. S. 
Ohm, and others, have succeeded in partially elucidating this rather 
obscure point, and in showing that " timbre " mostly depends 
upon the form of vibration or the proportions obtaining between 
the velocities of the moving body in going through its phases. 
The study of this subject is much facilitated by diagrams, which, 
by means of rectangular co-ordinates, the one of time and the other 
of displacement, show a curve which answers to the peculiar form 
of motion assumed by the moving body. 
Thus, if a pendulum be allowed to swing across a sheet of paper, 
carrying a pencil to mark it, a straight line only will result while 
the paper is still, representing the displacement of the pendulum. 
The pendulum being still, the paper has a regular rectilinear 
motion, with a constant velocity, given to it at right angles to the 
former line, another straight line will be generated, successive 
points in which will represent successive periods of time. If the 
two motions be compounded, a curved line will result, which is a 
diagramatic representation of the form of vibration of the moving 
