86 SUBMARINE CABLE LAYING AND REPAIRING. 



Inner sheath of 10 No. 6 wires ; pitch circumference 2'2in. ; 

 lay 16in. 



Increase in weight works out by same formula to 1 per cent., 

 and the weights per naut are : 



Outer Sheath, 36 No. 6 wires = 211 •4cwt. 

 Add 13 per cent. = 27*5 „ 



238-9cwt. 



Inner Sheath, 10 No. 6 wires = 58*6 „ 

 Add 1 per cent. = '58 „ 



59-18 



Both sheaths 298-0 „ 



46 No. 6 Straight wires = 270-0 „ 



or a mean increase of weight due to lay of 10| per cent, on the 

 total weight of both sheaths. 



From these examples it will be seen that the percentage in- 

 crease of weight due to lay becomes greater in the larger sizes 

 of cable, and the range of variation is from about 1^ to 2 per 

 cent, in deep-sea sizes up to about 15 per cent, in large shore- 

 ends. 



Messrs. Willoughby Smith and Granville's patent Air Space 

 Cable is shown in section in Fig. 42. The two or three-core 

 cable is constructed in an ingenious way, substantially reducing 

 the inductive capacity and producing a line of higher signalling 

 speed for telegraphy and specially suitable for telephonic com- 

 munication. The core is first made in two semicircular or three 

 crescent-shaped sections, as shown at A, afterwards laid together 

 to form a tube and then covered with layers of gutta-percha by 

 passing through an ordinary covering machine. The sections, 

 being in the form of an arch, are found to be sufficiently strong 

 to retain their form while the outer coatings are laid on. In 

 this way a longitudinal air space of considerable relative cross- 

 sectional area is left in the core and the thickness of gutta- 

 percha covering the conductors on their inner sides is very much 

 less than would be practicable by the ordinary method of 

 construction and the inductive capacity is correspondingly 

 diminished. The ordinary single-core cable — say with 1301b. 

 of gutta-percha and 130 lb. of copper per naut — has a capacity 

 of about 0'34: microfarad, or in a twin core with metallic return 

 half this, namely, 0-17 microfarad per naut. The air-space 



