SCIENCE AND PRACTICE, 



401 



Mr. Smith proposes to receive signals upon very delicate 

 reflecting galvanometers, constructed for the purpose upon 

 Professor Thomson's principle, by which the currents trans- 

 mitted may be very weak, and yet sufficient to give sensible 

 deflections. Two such instruments^ of different degrees of 

 sensibility, are to be inserted in the line the less sen- 

 sitive one on board the ship, the other at the land- station 

 and are to be employed, at the same time, for the insulation 

 tests. The only battery to be used is that on board. One 

 pole of this is to be put to earth, the other through the ship's 

 galvanometer to the cable end. On shore, the other end of 

 the cable is to be connected with one side of a galvanometer 

 coil, the other side being put to earth ; and the earth wire 

 having a very great resistance. 



This will be easily understood by Fig. 182. The cable 

 A B is supposed to be partly submerged. The end A is con- 



Fig. 182. 



nected with the galvanometer G, and with the lever of the 

 key K. Between the galvanometer and earth, resistances, R, 

 equal to about a hundred millions of units, and between the 

 anvil-contact of the key and earth resistances to a much 

 less amount, are inserted. On board ship the cable conductor 

 is connected with the marine- galvanometer G f , and the bat- 

 tery B. 



As this system was used in submerging the Atlantic 

 line, we may suppose A B to represent the 2,000 knots 



D D 



