Page:Popular Science Monthly Volume 21.djvu/264

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tions in a trough of water about six inches deep. By pulsating bodies are meant those which undergo alternate changes of volume, marked by two distinct phases, one of swelling and one of contraction. The pulsations of the, two bodies are spoken of as synchronous when the similar phases occur simultaneously in them. The oscillating bodies are constant in volume, but undergo alternate changes of place, from right to left and from left to right, or in a vertical direction.

Pulsations are communicated by means of little tambours or drums made of hollow cylinders of metal, over the ends of which are stretched flexible plates or membranes. These drums are made to swell and contract by means of pumps, with which they are connected by India-rubber tubing, that compress the air within them. Two drums are usually employed in the experiments, each connected with a separate

pump, so that the rhythm of the pulsations may be regulated at will. Thus both drums may be caused to pulsate synchronously, or with an opposite rhythm. In the simplest pulsator (Fig. 1,1) the two drum-heads

PSM V21 D264 Simple hydrodynamic pulsators.jpg
Fig. 1.

beat synchronously, or suffer dilatation and collapse together, as the pump is worked. In another disposition the drum-heads are separated by a rigid partition dividing the instrument into two chambers, each having its separate connection by a distinct tube, with a different pump, making it practicable to produce either synchronous or unsynchronous pulsations. A more common disposition is to use two simple pulsators in connection with the two pumps, one of which is held in the hand, while the other is mounted in the water, so as to be left free to move.

The two phases of pulsation are regarded by Professor Bjerknes as analogous to the poles of the electrode or magnet. The phase of dilatation may thus be likened to the north pole, that of collapse to the south