150 DRK. GUY BARLOW AND H. B. KEENE ON THE ANALYSIS OF SOUND. 
in a small sealed box. The diaphragm was ol tinned iron, 10 cm. diameter and 0‘3 mm. 
thick, and with a natural frequency in water of 120/sec. 
(2) Rubber Diaphragm Receiver with Adjustable Natural Frequency .—With this instru¬ 
ment the object was to obtain a much lower natural frequency. The effect of the air- 
cavity in giving an additional restoring force to the diaphragm, as already referred to 
under Sounders,” has now to be taken into account. In this receiver the diaphragm 
was made of very thin rubber, so that the restoring force was almost entirely due to 
the enclosed air. Advantage was taken of this fact to make the instrument of adjustable 
frequency by changing the volume of the air-cavity behind the diaphragm. The prin¬ 
ciple of conqDensation for hydrostatic pressure described above was again applied. 
The diaphragm, with the microphone attached at its centre, formed one end of a 
brass tube 30 cm. long and 5 cm. in diameter, the opposite end being closed by a screwed 
cap (fig. 11). Inside the brass tube was a solid brass plunger the position of which 
could be varied by means of a detachable rod. A very small hole in the plunger allowed 
equalisation of pressure on either side, while at the far end of the cylinder was a side 
tube communicating with two symmetrically placed compensating chambers. The 
total volume of the cylinder was 600 c.c., and the chambers were 500 c.c. each. This 
allowed a compensation to be made for depths down to 40 feet. For convenience in 
setting the chambers, a scale of depths was marked on the sides. 
