CHAP. V.] DEEP-SEA SOUNDING. 219 
‘Hydra,’ in recognition of its inventor and of the 
vessel in which it was first used. 
The axis of the ‘ Hydra’ (Fig. 42) is a strong brass 
tube, which unscrews into four chambers. The three 
lowest of these are closed above by conical valves 
opening upwards but not fitting absolutely tightly, so 
as to allow a little water to pass; and the lowest 
chamber B is closed by a butterfly valve also opening 
upwards. The upper (fourth) chamber a contains 
a piston, and the piston-rod c is continued upwards 
into a rod which ends in the ring to which the 
sounding-line is attached. The upper chamber in 
which the piston works has a large hole on either 
side about the middle of its length, and a small hole 
passes through the piston itself. Projecting from the 
upper part of the rod there is a notched tooth p, and 
over the tooth passes an arched steel spring, with 
a slit which allows the tooth to pass through its 
centre, and its two ends fastened moveably to the 
rod. When the spring is forcibly pushed back, it 
allows the tooth with its notch to protrude through 
the central slit. The weight consists of three or four 
cylinders of iron F, toothed and notched so as to fit 
into one another and make one mass. The weight 
used in the ‘ Porcupine’ was from two to three 
hundredweight, according to the depth. The weight 
is suspended by an iron wire sling which passes over 
the notched tooth, the spring having been pressed 
back. The weight is amply sufficient to retain the 
spring in that position. 
The figure represents the instrument prepared to 
let go, the whole weight suspended from the ring at 
the top of the piston-rod, which is thus fully drawn 
