128 ANNUAL OF SCIENTIFIC DISCOVERT. 



in war time, above all others, its services would be absolutely indispensable 

 to the country. 



Before the form, weight, strength, and outer covering of each portion of 

 the cable were resolved on, the Board of Trade took the utmost pains, by 

 consulting our chief electricians, to ascertain the kinds best suited for the 

 purpose and for long endurance under water. Researches into these matters 

 have led to the adoption of a cable of different thicknesses, weights, and 

 strengths, according as the depth of the water under which it will be laid 

 increases; the whole forming one continued submarine rope, which, if not 

 perfect in its mechanical arrangement, is nevertheless one which holds out 

 high prospects of ultimate success. The core or conductor is, of course, of 

 the same thickness and formation throughout from end to end, being formed 

 of seven No. 18 copper wires, in all about one-eighth of an inch diameter 

 the thickest conductor that has ever yet been made. The copper strands of 

 this, in accordance with the advice of the electricians, have been very care- 

 fully selected and tested for conducting power, as even the purest copper 

 wire, from some unknown cause, has been found to vary in electrical con- 

 ducting power as much as forty per cent. Its power of conducting heat also 

 diminishes or increases in the same proportion with its electrical sensitive- 

 ness. Yet, though the conductor with its insulating medium of gutta-percha 

 is alike in diameter throughout, the manner in which this core is protected, 

 or, we had better say, the thickness to which the outer covering is laid on, 

 differs considerably. Thus, each of the two shore ends is made to rest in 

 from 100 to 200 fathoms, and these for thirty knots each way are very 

 massive, at the rate in weight of seven tons to the mile. The next length 

 at each end is also of thirty knots, and will rest in from 200 to 400 fathoms 

 water, and is for this depth a very massive cable, weighing about five tons 

 to the mile. By the substitution of a finer gauge of wire at each two or 

 three miles or so, this gradually tapers down to meet the first deep-sea 

 length, which will be laid in from 500 to 800, or, possibly, even 1,000 fathoms. 

 The length of this portion of the cable is 940 nautical miles, its weight in 

 air two tons per mile, in water about thirty-four hundredweight. The deepest 

 deep-sea portion across the centre of the Bay of Biscay extends over 

 about 280 knots, though 300 knots are being manufactured to meet contin- 

 gencies in submerging. Here the depth averages about 2,500 fathoms, 

 equal to the very deepest parts of the cable plateau of the North Atlantic. 

 To overcome the difficulties of this vast depth of water, the cable is 

 strengthened by the introduction of steel wire in its outer covering, and 

 reduced to weigh in air only twenty-six hundredweight; in water as low as 

 thirteen. The weight of the Atlantic cable in air was one ton, and in water 

 about fifteen or sixteen hundredweight, per mile. 



The different weights of the different parts of the cable are, of course, 

 entirely due to the thickness of the outer spiral wires with which it is covered. 

 The conductor, with its threefold insulation of gutta-percha, is all served 

 round alike with yarns of tarred hemp closely bound in, and over which 

 come the outer wires of various gauges: No. 1 gauge, the thickest known, 

 being as thick as a cedar pencil, and so on up to No. 45 gauge, as fine as 

 cotton. The two heaviest shore ends, then, of thirty miles each, are covered 

 with twelve No. 3 gauge wires, which brings its weight up to seven tons a 

 mile, and its breaking strain from twenty-five to thirty tons. The second 

 land ends are enclosed in twelve No. 5 gauge wires, of five tons to the mile, 

 and equal to about fifteen tons' strain. 



