920 THE DEPTHS OF THE SEA. [cHap. v. 
out of its cylinder. As the instrument runs down the 
water passes freely through the tube and valves, and 
pours out by the holes in the wall of the cylinder. 
When it touches the ground the piston is pulled 
down by the weight, but its progress is somewhat 
arrested by the water in the lower part of the 
cylinder, which can only escape slowly, thus giving 
the weight time to force the terminal chamber with 
the butterfly valves into the ground. The weights 
then rest upon the bottom and relieve the spring 
which throws the sling off the tooth. The tube 
comes up free with all the valves closed, and the last 
chamber filled with the substance of the bottom, and 
the other chamber with bottom water. 
In the skilful hands of Captain Calver the ‘ Hydra’ 
never once failed, and from the great weight used it 
is admirably suited for accurate soundings in deep 
water; but it is somewhat complicated, and it brings 
up very small samples of the bottom. Jn the case of 
the cruise of the ‘ Porcupine,’ where the large dredge 
was sent down at almost every sounding-station, this 
was of little consequence; but where dredging is im- 
practicable, and all information as to the condition of 
the bottom must be got from soundings, some simple 
adaptation of the ‘ Bull-dog’ scoops or the Fitzgerald 
apparatus would certainly have a great advantage. 
During the cruise of the ‘ Porcupine’ in 1869 
soundings were taken with the utmost care at ninety 
stations, and in 1870 at sixty-seven stations, and on 
every occasion the operation was conducted by Capt. 
Calver himself, whose great experience on the sur- 
veying service was in itself a guarantee of the greatest 
possible accuracy. Captain Calver told me that on 
