] 40 THE DEPTHS OF THS SEA. [chap. ni. 



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 

 Wlieatstone, 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-coil 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 some- 

 what costly instrument in case the cable were to break. On 

 these accounts they thought it best to adopt the simpler plan 

 proposed by M]'. Siemens ; and the apparatus required for carry- 

 ing the plan into execution is now completed, and in use in the 

 expedition. 



" Meanwhile a plan had been devised by Dr. Sliller 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 

 rivetted 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- 



