June 24, 1 875 J 



NATURE 



15 



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tion vary in an extreme degree. Thus there is found 

 copper and insulation in the respective proportions by 

 weight of I to I, also 3 to 4, also 3 to 2, also 2 to 3, and 

 also in the irregular proportion of 11 to 14. By these 

 figures it appears that there is no accepted ratio, and 

 every new cable seems to be constructed according to 

 the electrical views of the designer, in some cases at an 

 enormous cost, as compared with others of similar length 

 and equal efficiency in transmitting power. Thus, by re- 

 ducing the weight of material per nautical mile into an 

 average money value, assuming for copper \s. id. per lb., 

 and insulation bs. per lb., we obtain the following ratios :— 



I ICO nautical miles : copper ;,^i6 o insulation ;^6o 



1,632 „ ,,65 „ 42 



2,600 ,, ,,23 ID „ 70 



2,000 ,, ,, 16 o „ 70 



With such indiscriminate specifications there is certainly 

 something left to discover, and the next few years may 



Intermediate. Main. 



Fig. 36.— French Atlantic Cable laid between Brest and Island of Saint- 

 Pierre, 1869. 



determine with some degree of accuracy the true propor- 

 tions by weight to ;be adopted between the conducting 

 wire and the external thickness of the insulator, to obtain 

 the best practical results at the least expenditure of 

 capital on a circuit of given length, worked with one of 

 the sensitive recording instruments already brought under 

 notice. As an example of the augmentation of speed 

 upon a submarine circuit, according to the delicacy of the 

 recording instrument employed, upon the Great Northern 

 cable between England and Denmark, 365 nautical miles 

 in length, with the most improved submarine morse, an 

 average of seventeen words per minute was obtained ; 

 with the Wheatstone's automatic thirty words, and with 

 the Thompson syphon recorder fifty words per minute are 

 practically reached. 



For many years there has existed a divided opinion as 



to whether a light submarine cable, combining economy 

 of construction with mechanical facilities of laying, is 

 not the right system to adopt as against the heavy 

 and more expensive form of iron covered cable. The 

 light cable theory may be said to be based upon the 

 opinion of the late Lieut. M. F. Maury, who through 

 every opposition adhered in principle to light cables. 

 His argument may be expressed in his own words : " \'ou 

 may snap a taut rope, but you cannot break a slack 

 line." This remark may nautically be quite true, but 

 electrically far from correct, for the following reasons. In 

 submerged cables, speed is greatest upon the shortest line. 

 Now, in deep-sea telegraphy, in the only circuits upon 

 which a light cable could possibly be employed with any 

 security against mechanical interruptions, two or three 

 points come into play. Supposing a light cable were to 

 be used over, say, a circuit of 2,000 miles, with an average 

 depth of 1,500 fathoms, or about if miles. First, take 

 the specific gravity of the light cable as compared with 

 water, at what rate will it sink to the bottom ? if not 

 so adjusted as to sink at about one mile per hour 

 (looking to the enormous sweep between the paying out 

 steamer and the bottom of the ocean at the depth of if 

 miles), it is more than probable that although you cannot 

 break a " slack line," it may be so twisted and contorted 

 by surface-currents and under-currents moving at various 

 velocities or even in opposite directions as it slowly sinks 

 to the bottom by reason of low specific gravity, that a 

 very great length of cable may be paid out (as a slack 

 line). Secondly, the cost of this increased mileage must 

 be taken into account as compared with that of the heavier 

 iron-sheathed cable upon which a mechanical strain 

 can be placed to ensure more or less a " Bee " line. 

 Thirdly, the speed of transmiission through a submarine 

 cable is inversely as the square of the length. Now, if this 

 is practically correct, it is easy to determine whether the 

 best commercial results will be obtained from alight cable 

 with increased electrical resistance, although it may be 

 carried out at a less original outlay, or from a shorter 

 cable more costly per mile from increased strength 

 and weight of iron, but with greater transmitting speed, 

 and in consequence dividend earning capacity. But 

 of equal importance with any of the previous points 

 is the impossibility of grappling a light cable from any con- 

 siderable depth in cases of injury affecting the insulation. 

 To raise a cable from a depth of i| miles involves a 

 great strain upon the cable, and unless the breaking strain 

 has been calculated to meet such an emergency, any suc- 

 cessful attempt at restoration must be abandoned, and the 

 entire line is rendered useless and the capital lost. Every 

 submarine cable should be laid with a certain percentage 

 of slack, regulated according to depth of water and sur- 

 rounding circumstance. The average slack is from 8 to 

 14 per cent. 



The first Atlantic cable, 1857, between Valentia and 

 Newfoundland, is shown in elevation and section at Fig. 34. 

 This cable, from imperfect construction, remained electri- 

 cally sound for a very limited period, and very few 

 messages were successfully passed through the con- 

 ducting wire. It, however, became the pioneer to suc- 

 cess, and elucidated several important points in connection 

 with the design of the 1865 and 1866 Atlantic cables shown 

 at Fig. 35. The covering of these cables consists of ten 

 strands of Manilla hemp, each containing a homogeneous 

 steel wire. The French Atlantic iron-sheathed cable 

 between Brest and Saint-Pierre, laid in 1869, is shown at 

 Fig. 36. 



Tons. 



The weight of the main cable.'per naut is ... 1-652 



,, intermediate ,, ... 6-246 



„ shore ends „ ... 20-447 



{To be continued^ 



