TI.I.KCKAI'II, DLECTPJC. 



721 



in>n wire, each .-(rand made of 7 wires, tv 



,'nl 1, 1 ill equally iii-ninul til-- cure, which 



bad preNhni^ly been padded with a serving of 



i hemp. Ureaking strain, 3 tons, > 



le oi' bearing its own weight in u trillo 



' depth of water. Length 



le produced, 174 nautical 



1805. 



In the cable of 1805 the conductor was a cop- 

 per strand of 7 wires, 6 laid around 1 ; weight 

 300 Ibs. per nautical mile. Embedded in Chat- 

 terton's compound. Insulation was effected 

 with gutta-percha and Chatterton's compound. 

 1 it 400 Ibs. per nautical mile. The outer 

 coat was 10 single wires, each wire surrounded 

 with tanvd Manila rope, and the whole laid 

 spirally around the core, which had previously 

 been padded with a serving of tarred jute yarn. 

 "P.reaking strain, 7 tons, 15 cwt. Capable of 

 bearing its own weight in 11 miles' depth of 

 water. Length of cable, 2,300 nautical miles. 



1806. 



The present cable has for a conductor a cop- 

 per strand of 7 wires, 6 laid around 1 ; veJgttt 

 800 Ibs. per nautical mile. Embedded for solid- 

 ity in Chatterton's compound. The insulator 

 ia 4 layers of gutta-percha, laid on alternately 

 with thinner layers of Chatterton's compound ; 

 weight 400 Ibs. per nautical mile. The outer 

 coat is 10 solid wires galvanized, each wire sur- 

 rounded separately with 5 strands of white 

 Manila yarn, and the whole laid spirally around 

 the core, which had previously been padded 

 with a serving of tarred hemp. The breaking 

 strain ia 8 tons, 2 cwt.. anil it is capable of 

 bearing its own weight in 12 miles' depth of 

 water. This length of cable is 2,~:>0 nautical 

 miles, part of which is used for completing the 

 cable which parted in is 



Mr. "Willoughby Smith, the inventor of nn 

 apparatus for securing continuous tests of the 

 insulation of the core, had taken the place of 

 Mr. De Sauty as electrician-in-chief ; while tbo 

 services of Prof. Thomson and Mr. 0. F. Var- 

 YOL. n. 46 A 



ley were, on this occasion, secured to the 



'Telegraph Construction Company." These 

 gentlemen agreed upon a system by 

 which, with tho aid of the iiMruiiK-nt-t in- 

 Vented or impn.v.-il by them respectively, 

 while messages could t any time !< Iran 

 1 in either direction over the cab!, 

 for insulation and continuity could al*> 

 bo kept up ut the same time, and constantly, 

 pi i-liaps at the mere moments of rever- 

 sal of tho current; whereas, in tbe pre< 

 expedition, the insulation test was applied only 

 every alternate half hour, the other half hour 

 being devoted to tests expressly of the r 

 ance of the conductor and of its continuity. 



The signalling instrument, devised by Prof. 

 Thomson in 1858, had since been brought by 

 him to a still higher working perfection. The 

 image of a divided scale, reflected from a sus- 

 pended mirror and viewed with a telescope, 

 was first employed by Gauss, of Germany, for 

 showing the magnetic deflection caused by 

 given currents, and so measuring their strength , 

 and Mr. J. P. Joule, of Manchester, had em 

 ployed, for galvanometers to give quick indica- 

 tions, light needles hung by single fibres of silk 

 their deflections visibly indicated by light glass 

 bars attached to them. Prof. Thompson sub- 

 stituted for these plans that of indications by 

 means of a tine ray (rather, beam) of light re- 

 flected from a minute mirror carried by the 

 galvanometer needle, this ray accordingly being 

 caused, during the deflections of the needle 

 duo to transmitting in any desired succession 

 brief direct and reversed currents, to shift its 

 place to the right and left along a horizontal 

 scale lixed about three feet in front of the 

 mirror. The latter, of microscopic glass sil- 

 vered, the inventor has reduced to a diameter 

 of three-eighths of an inch, and a weight of 

 about one grain. The ray of light to serve as 

 " index," is that admitted through a fine aper- 

 ture in the middle of the scale, its movements 

 being usually confined within, about two feet 

 in length on either side. 



In using this plan with the cable or similar 

 conductor, and before, during, or after submer- 

 sion, one of these " reflecting " or " mirror 

 galvanometers" is, by means of its helix wire, 

 connected with each end of the former. The 

 operating battery current at either end being 

 thrown into tho conductor in brief direct and 

 reverse charges, as required, a corresponding 

 succession of quick movements of the needle 

 and mirror, and hence of the indicating ray, to 

 right or left over the scale, is produced at the 

 other end of the wire. The Morse alphabet, as 

 in use in England, has been employed, the 

 das?iea being denoted by movements of the ray 

 to one hand, and the dots by those to tho oppo- 

 site ; the combinations of these required denot- 

 ing the letters ; and tho reading of these '' light 

 signals" being of course directly by the eye. 

 Til-- instrument Affords the means of compara- 

 tively rapid signalling over long submerged 

 wires, or of signalling by feeble currents 



