18 THREE CRUISES OF THE “ BLAKE.” 
each arm of the bridge, and the steel-wire serves as the return 
earth connection for both. Sir William Thomson’s marine gal- 
vanometer, with a mirror and scale, is employed to determine 
the balance of the bridge. (Fig. 14.) 
The apparatus was set up on board the “Blake” in April, 
1881, and experiments were made off the east coast during 
Fig. 14. — Wheatstone’s Bridge. (Bartlett, U. S. Coast Survey.) 
August. In each series of experiments the temperatures at dif- 
ferent depths were first taken by Miller-Casella thermometers 
attached to a sounding-wire. A sinker was then fastened to 
the resistance-coil, and it was lowered by the cable to the same 
depths, when the temperature was read by means of the mer- 
curial thermometer attached to the comparison-coil. The depths 
at which readings were taken ranged from the surface down 
to eight hundred fathoms, and experiments were made in both 
rough and still water. The temperatures recorded varied from 
38.5? to 81.5? Е. In every case the readings of the electrical 
