18 



THREE CRUISES OF THE " BLAKE. 



each arm of the bridge, and the steel-wire serves as the return 

 earth connection for both. Sir WilHam 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 soundino^-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 reading's were taken rangfed 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° F. In every case the readings of the electrical 



