76 SUBMARINE CABLE LAYING AND KEPAIRING. 



older cables it was customary to make up about 45 to 50 per 

 cent, of the total weight with hemp and compound with only 

 about 30 to 35 per cent, of iron in order to keep the specific 

 gravity low. This facilitated the laying, as the cable sank 

 very slowly (only at the rate of about 13 ft. per minute), and 

 therefore the strain as it passed out was very moderate. For 

 instance, when paying out in 2,500 fathoms ac the rate of five 

 knots per hour with 20 per cent, slack, the strain carried was 

 about 20cwt, or 25cwt., the brake absorbing about 45 h.p. 

 After some years, however, when raised for repairs, it was found 

 that the rough hemp exterior caused considerable surface- 

 friction during picking up, and that the margin of tensile 

 strength when using only the above percentage of iron was a 

 rather low one, taking into account the natural decay of the 

 metal. These cables are now exceedingly difficult to lift in 

 deep water without breaking, even when the strain is reduced 

 by using a cutting grapnel holding one end only ; and they fre- 

 quently have to be replaced by new pieces several miles in length.. 

 In later designs therefore the weight of iron has been increased to 

 about 40 to 45 per cent, of the whole, and steel is used in many 

 cases, increasing the tensile strength of the sheath alone from- 

 something like 6^ tons to 9 tons per square inch. 



As regards the cable itself, the strain in laying is of no con> 

 sequence, as it is always less for a given depth than the weight 

 in water of a length of the cable equal to that depth, and 

 cables are always designed to bear a strain several times greater 

 than that due to their own weight in the greatest depth of water 

 in which they may be laid. The question of the strain carried 

 enters however into the consideration of the type of brake 

 employed on board, and the arrangements for keeping it cool 

 and maintaining a steady pull free from jerk or sudden stoppage 

 when paying out a cable with little fi^oatage. The difficulty 

 is of course further increased when the rate of paying out i& 

 greater and the slack less than formerly. 



While this generally speaking may be taken as the line of 

 development, it does not cover every case. The contour of the 

 ocean bed and the nature of bottom enter into the original 

 design, and affect the direction in which improvement may be 

 made. Some ocean beds are precipitous and mountainous, and 

 others are flat, with sand and mud bottoms, requiring respec- 



