SUBMARINE TELEGRAPHY 27 



test of continuity, etc. Then blunders would arise from time 

 not being perfectly kept, or from some of the many changes 

 having been incorrectly performed, so that probably this plan 

 was practically inferior to a simple insulation test permanently 

 maintained. It was, moreover, rigid, and could not be readily 

 altered to suit the special tests required when a fault did occur. 

 All these defects were remedied for the first time during the 

 Atlantic expedition of this year. The end of the cable at 

 Valentia was not quite insulated ; it was connected with the 

 earth through an enormous resistance, so great that the insu- 

 lation test of the cable was hardly sensibly affected by the 

 small leakage through it ; but this small leakage was easily 

 perceived by an astatic Thomson's reflecting galvanometer. 

 When, therefore, an insulation test was being made on board 

 the ' Great Eastern,' the current used was perceived at Valentia, 

 where the observer could further judge of the tension or poten- 

 tial produced by the ' Great Eastern's' battery by observing the 

 current it would produce through his enormous but known 

 resistance. Any fault would lower that potential, and reduce 

 this current at Valentia. More than this, the ' Great Eastern,' 

 by slightly decreasing or increasing their battery, could cause 

 such small changes in the current observed at Valentia as should 

 serve as signals, and this without intermitting their insulation 

 test. Conversely, Valentia, by. drawing off little charges, or 

 adding them, could produce effects similar to slight changes in 

 the insulation of the cable, and those effects could be used as 

 signals from the shore to the ' Great Eastern ;' being of short 

 duration, and definitely arranged, they could not be mistaken 

 for faults. Thus simultaneous and continuous tests could be 

 made on ship and on shore. Nevertheless, conversation could 

 be carried on in either direction at any time. No fault of 

 insulation would escape detection, even during conversation, and 

 as soon as it did occur the instruments were ready arranged to 

 make those simultaneous tests by which alone its position could 

 be determined, and then to transmit that intelligence from one 

 end to the other. The merit of this admirable invention is due 

 to Mr. Willoughby Smith. The details of the arrangement ac- 

 tually adopted were worked out by him, in concert with Sir W. 

 Thomson and Mr. Cromwell F. Varley, whose valuable assistance 



Q 9. 



