1867.] Physics. 289 



perfectly distinct, the rest of the line being left almost imme- 

 diately entirely free from all traces of electricity, ready for the 

 instant production of a second signal. 



This operation of discharging has been expedited to such an 

 extent by means of the instrument known as the " curb key," that 

 the great wave can be arrested at the distant end of the cable before 

 it has arrived at ^ of its proper height, and the line left clear for 

 another signal almost immediately after. This was illustrated by 

 the Australian line. 



The lecturer then showed his invention for preventing the dis- 

 turbance arising from earth-currents, and for expediting the signals 

 through the cable, which did not record the strength of the current, 

 but " the rate of increment and decrement of potential." 



He placed a reflecting galvanometer between the distant, or 

 " Newfoundland," end of the cable and the earth (this being the usual 

 mode of connecting telegraph instruments), and also a second one 

 with a " condenser " between it and the earth. At the English end 

 there was an apparatus which produced currents through the cable 

 similar to those produced on the telegraph circuits by the Aurora 

 Borealis and other causes. These experiments were very conclu- 

 sive ; the slow but large wave of the earth-current was seen to pro- 

 duce scarcely any action upon the galvanometer with the condenser, 

 while the ordinary one was seen running 20 feet on either side of 

 the scale in consequence of the earth- currents. 



The small waves on the back of the great swell of the earth- 

 currents were, however, perfectly disentangled, by this simple con- 

 trivance, from the great slow-rising wave, and the signals were per- 

 fectly legible on the one instrument, while illegible on the other, 

 whose image ran wildly over the wall of the Institution. In addi- 

 tion, the signals were transmitted more rapidly and clearly by this 

 arrangement. 



In concluding, Mr. Yarley remarked that it was upon the data 

 furnished by this artificial cable that he had designed the present 

 Atlantic cable, and that without it he could not then have guaran- 

 teed eight words a minute without a core whose conductor and 

 insulator were each 60 per cent, greater than the present, which 

 consisted of 300 lbs. of copper and 400 lbs. of gutta-percha to the 

 mile ; and he added that it was at least some reward for the years 

 of arduous labour he had had in connection with this great enter- 

 prise, to find that everything he had predicted as to the capabilities 

 of the cables had been entirely verified. 



VOL. IV. 



