CHAP, v.] DEEP-SEA SOUNDING. 219 



' Hydra,' ia recognition of its inventor and of the 

 vessel in wMch it was first used. 



The axis of the ' Hydra ' (Pig. 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 open- 

 ing 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 d, 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 out 



