254 



THE POPULAR SCIENCE MONTHLY. 



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- 



FlG. 1. 





Mm 





m 



heads beat synchronously, or suffer dilatation and collapse together, as 

 the pump is worked. In another disposition the drum-heads are sepa- 

 rated 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 unsyn- 

 chronous 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 dilata- 

 tion may thus be likened to the north pole, that of collapse to the south 



