_An Undulation conceived to originate from the Vibration of a single
Particle, must expand through a homogeneous Medium in a spherical Form,
but with different quantities of Motion in different Parts._
For, since every impulse, considered as positive or negative, is
propagated with a constant velocity, each part of the undulation
must in equal times have past through equal distances from the
vibrating point. And, supposing the vibrating particle, in the course
of its motion, to proceed forwards to a small distance in a given
direction, the principal strength of the undulation will naturally
be straight before it; behind it, the motion will be equal, in a
contrary direction; and, at right angles to the line of vibration, the
undulation will be evanescent.
Now, in order that such an undulation may continue its progress
to any considerable distance, there must be in each part of it, a
tendency to preserve its own motion in a right line from the centre;
for, if the excess of force at any part were communicated to the
neighbouring particles, there can be no reason why it should not
very soon be equalised throughout, or, in other words, become wholly
extinct, since the motions in contrary directions would naturally
destroy each other. The origin of sound from the vibration of a
chord is evidently of this nature; on the contrary, in a circular
wave of water, every part is at the same instant either elevated or
depressed. It may be difficult to show mathematically, the mode in
which this inequality of force is preserved; but the inference from
the matter of fact, appears to be unavoidable; and, while the science
of hydrodynamics is so imperfect that we cannot even solve the simple
problem of the time required to empty a vessel by a given aperture,
it cannot be expected that we should be able to account perfectly for
so complicated a series of phenomena, as those of elastic fluids.
The theory of HUYGENS indeed explains the circumstance in a manner
tolerably satisfactory: he supposes every particle of the medium to
propagate a distinct undulation in all directions; and that the general
effect is only perceptible where a portion of each undulation conspires
in direction at the same instant; and it is easy to show that such
a general undulation would in all cases proceed rectilinearly, with
proportionate force; but, upon this supposition, it seems to follow,
that a greater quantity of force must be lost by the divergence of the
partial undulations, than appears to be consistent with the propagation
of the effect to any considerable distance. Yet it is obvious, that
some such limitation of the motion must naturally be expected to take
place; for, if the intensity of the motion of any particular part,
instead of continuing to be propagated straight forwards, were supposed
to affect the intensity of a neighbouring part of the undulation,
an impulse must then have travelled from an internal to an external
circle in an oblique direction, in the same time as in the direction
of the radius, and consequently with a greater velocity; against
the first proposition. In the case of water, the velocity is by no
means so rigidly limited as in that of an elastic medium. Yet it is
not necessary to suppose, nor is it indeed probable, that there is
absolutely not the least lateral communication of the force of the
undulation, but that, in highly elastic mediums, this communication is
almost insensible. In the air, if a chord be perfectly insulated, so
as to propagate exactly such vibrations as have been described, they
will in fact be much less forcible than if the chord be placed in the
neighbourhood of a sounding board, and probably in some measure because
of this lateral communication of motions of an opposite tendency.
And the different intensity of different parts of the same circular
undulation may be observed, by holding a common tuning fork at arm’s
length, while sounding, and turning it, from a plane directed to the
ear, into a position perpendicular to that plane.