120 THE DEPTHS OF THE SEA. [cHaP, TI. 
involve considerable difficulties. Still, the liability of the index 
to slip, and the probability that the indication of the thermo- 
meters would be affected by the great pressure to which they 
were exposed, rendered it very desirable to control their indica- 
tions by an independent method. 
“ Two plans were proposed for this purpose, one by Sir Charles 
Wheatstone, and one by Mr. Siemens. Both plans involved the 
employment of a voltaic current, excited by a battery on deck ; 
and required a cable for the conveyance of insulated wires. The 
former plan depended upon the action of an immersed Breguet’s 
thermometer, which, by an electro-mechanical arrangement, was 
read by an indicating instrument placed on deck. The latter 
plan made the indication of temperature depend on the existence 
of a thermal variation in the electric resistance of a conducting 
wire. It rested on the equalization of the derived currents in 
two perfectly similar partial circuits, containing each a copper 
wire running the whole length of the cable, the sea, and a 
resistance-coll of fine platinum wire; the coil in the one circuit 
being immersed in the sea at the end of the cable, and that in 
the other being immersed in a vessel on deck, containing water 
the temperature of which could be regulated by the addition of 
hot or cold water, and determined by an ordinary thermometer. 
“The instruments required in Sir Charles Wheatstone’s plan 
were more expensive, and would take longer to construct; and, 
besides, the Committee were unwilling to risk the loss of a 
somewhat costly instrument in case the cable were to break. 
On these accounts they thought it best to adopt the simpler 
plan proposed by Mr. Siemens ; and the apparatus required for 
carrying the plan into execution is now completed, and in use in 
the expedition. 
“Meanwhile a plan had been devised by Dr. Miller for 
obviating the effect of pressure on a minimum thermometer, 
without preventing access to the stem for the purpose of setting 
the index. It consists in enclosing the bulb in an outer bulb 
riveted on a little way up the stem, the interval between the 
bulbs being partly filled with liquid, for the sake of quicker 
conduction. The Committee have had a few minimum thermo- 
