They are based on the presence of bodies set in vibration in a liquid.
The vibrations produced by Mr. Bjerknes are of two kinds--pulsations
and oscillations. The former of these are obtained by the aid of small
drums with flexible ends, as shown to the left in Fig. 1. A small pump
chamber or cylinder is, by means of a tube, put in communication with
one of these closed drums in which the rapid motion of a piston
alternately sucks in and expels the air. The two flexible ends are
successively thrust outward and attracted toward the center. In an
apparatus of this kind the two ends repulse and attract the liquid at
the same time. Their motions are of the same phase; if it were desired
that one should repulse while the other was attracting, it would be
necessary to place two drums back to back, separated by a stiff
partition, and put them in connection with two distinct pump chambers
whose movements were so arranged that one should be forcing in while
the other was exhausting. A system of this nature is shown to the
right in Fig. 1.

The vibrations are obtained by the aid of small metal spheres fixed in
tubular supports by movable levers to which are communicated the
motions of compression and dilatation of the air in the pump chamber.
They oscillate in a plane whose direction may be varied according to
the arrangement of the sphere, as seen in the two apparatus of this
kind shown in Fig. 1. Fig. 2 will give an idea of the general
arrangement. The two pistons of the air-pumps are connected to cranks
that may be fixed in such a way as to regulate the phases as may be
desired, either in coincidence or opposition. The entire affair is put
in motion by a wheel and cord permitting of rapid vibrations being
obtained. The air is let into the apparatus by rubber tubing without
interfering with their motions.