80 SUBMAKINE CABLE LAYING AND KEPAIBING. 



the ratio of the weight of the suspended portion in this depth 

 to its breaking strain is 



9x20 



49 



= 3-7 



This cable would support about 9,000 fathoms of its own length 

 in water ; the utmost limit being 



9x1,014 = 9,100 fathoms. 



For the best conditions in paying out a cable should be capable 

 of sustaining 5,000 to 10,000 fathoms of its own length in water. 

 This sheath would be amply strong enough to support a larger 

 cable of, say, 25cwt. in water such as might be used with a 

 heavier core for high signalling speed. The type described is 

 laid from the deepest water to about 500 fathoms, sometimes 

 a little heavier type being laid between 500 and 1,000 fathoms 

 for the purpose of tapering the sheathing. The development 

 in construction of deep-water cables has been in the direction 

 to attain the highest tensile strength with a low specific gravity 

 and durability. 



Sections of various types to actual size, as manufactured by 

 the Telegraph Construction and Maintenance Company, are 

 shown in Fig. 39. 



For depths of about 500 to 100 fathoms a light intermediate 

 and for 100 to 50 fathoms a heavy intermediate type is generally 

 adopted. Although these types are heavier than deep-sea cables, 

 it will be evident that they are not subject to the same strain 

 in picking up, as the distance to lift to the surface is less. The 

 strain when raised on the bight from various depths is the same 

 per unit of length when the slack bears a certain relation to the 

 depth. For instance the strain per itnit length in lifting a deep- 

 sea cable from 2,000 fathoms with 11 per cent, of slack is 

 approximately the same as in lifting an intermediate type from 

 500 fathoms with 2^ per cent, of slack ; but the actual strain is 

 in proportion to the length of cable (or the depth of water) and 

 is therefore (in the case of the intermediate) only one-quarter 

 of that in raising the deep-sea type. 



It is not therefore absolutely necessary to use sheathing 

 wires of very high tensile strength for intermediates and shore- 

 ends, unless the route' abounds in coral patches. As regards 



