58 THE ATLANTIC. [cHaP. 1. 
in order to get good results, the instrament must be kept per- 
fectly clean, being always wiped dry with a clean cloth after 
use. Repeated experiments with the same liquid give results 
always agreeing within 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 os- 
cillation 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, when it is allowed to oscillate, in preference to being 
brought to rest with the tongue on the zero. As it is very 
difficult to place the cylinder perfectly vertically on the swing- 
ing table, the hydrometer generally has a certain list to one side 
or another, which also favors its freedom of motion. 
The space on the port side of the main-deck between the 
chart-room and the laboratory is occupied by such of our gear 
as would not pack into the work-rooms, including the apparatus 
devised by Mr. Siemens, F.R.S., for telegraphing the tempera- 
ture from all depths, and his photometric apparatus for deter- 
mining, by the exposure of sensitive paper for a certain length 
of time, the depth to which the chemical rays of the sun pene- 
trate into the water of the sea. Fig. 13 represents a hydraulic 
pump for reproducing the pressure to which thermometers and 
other instruments are subjected at great depths, and thus afford- 
ing us a means of determining their error under certain meas- 
ured pressures before sending them down. The pump A is of 
the ordinary construction, only with a very narrow cylinder, the 
diameter of the cylinder and piston being + inch. The water 
is pumped into the reservoir B, a cast-iron tube of 3 inches in- 
ternal and 9 inches external diameter, closed above by the plug 
C, which is held in its place by the bolt D. The instruments to 
be tested are placed in B; the plug C is inserted and made fast 
by the bolt, and water is pumped in until the desired pressure 
has been obtained. This is indicated by water issuing from the 
safety-valve E, which is of the ordinary construction. The ma- 
