The hulk and weight of the body, taken along Avith
the slimness of the stem, make great care in handling
it necessary, and, in order to get good results, the
instrument must he kept perfectly clean, heing ahvays
Aviped dry Avith a clean cloth after use. Repeated
experiments Avith the same liquid give results always
agreeing Avithin one scale division. In taking specific
gravities at sea the operation is performed on a
swinging table ; the motion of the ship gives the
hydrometer just so much oscillation as to overcome
all effect of sticking, and to make the reading as
much more satisfactory than on shore as that of a
balance is, Avlien it is alloAA’ed to oscillate, in preference
to heing brought to rest Avith the tongue on the
zero. As it is A'ery difficult to place the cylinder
perfectly vertically on the SAvinging table, the hydrometer
generally has a certain list to one side or
another, wdiich also favours its freedom of motion.
The space on the port side of the main-deck
hetAveen the chart-room and the laboratory is occupied
hy such of our gear as Avould not pack into
the Avork-rooms, including the apparatus devised
hy Mr. Siemens, R.R.S. for telegraphing the temperature
from all depths ; and his photometric
apparatus for determining, hy the exposure of sensitive
paper for a certain length of time, the depth
to Avhich the chemical rays of the sun penetrate
into the Avater of the sea. Rig. 13 represents a
hydraulic pump for reproducing the pressure to
Avhich thermometers and other instruments are
subjected at great depths, and thus affording us a
means of determining their error under certain
The pump A is of the ordinary construction, only
Avith a very narrow cylinder, the diameter of the
cylinder and piston being 4 inch. The water is
pumped into the reservoir B, a cast-iron tuhe of
3 inches internal and 9 inches external diameter,
closed above hy the plug c, Avhich is held in its place
F ig . 13,—The Hydraulic Pump.
measured pressures before sending them doAvn.
by the bolt d. The instruments to be tested are
placed in b ; the plug c is inserted and made fast
hy the holt, and water is pumped in until the desired
pressure has been obtained. This is indicated hy
Avater issuing from the safety-valve e , Avhich is of