If a vibrating tuning-fork be held close to a delicately suspended body,
the latter will approach the fork, as if impelled by some attractive
force. The experiment can be made by fastening a bit of paper about an
inch square to a straw five or six inches long, and then suspending the
straw to a thread, so that it is balanced horizontally. Bring the
vibrating tuning-fork within a quarter of an inch of the paper. In this
case the motion of approach is due to the fact that the pressure of the
air is less close to a vibrating body than at a distance from it; there
is therefore a slightly greater pressure on the side of the paper away
from the fork than on the side next to it.
If a vibrating tuning-fork be held near to the ear, and turned around,
there may be found four places in one rotation where the sound will be
heard but very faintly, while in every other position it can be heard
plainly enough. The extinction of the sound is due to what is called
interference. Each of the prongs of the fork is giving out a sound-wave
at the same time, but in opposite directions, each wave advancing
outwards in every direction. Where the rarefied part of one wave exactly
balances the condensed part of the other, there of course the sound will
be extinguished; and these lines of interference are found to be
hyperbolas, or, if considered with reference to both entire waves, two
hyperbolic surfaces.