PRINCIPLES OF DESIGN. 



87 



twin-core cable with metallic return has a capacity of only 0*10 

 microfarad, or an improvement of 40 per cent. 



Several lengths of this cable have been laid both in this 

 country and abroad, the longest being 60 nauts, between 

 Wales and Ireland, laid in the spring of 1898. 



No special difficulties have arisen in practice either with 

 regard to laying or maintenance, and experiments made upon 

 actual lengths have proved that this form of cable is capable 

 of withstanding without deformation of the air-space a hydraulic 

 pressure of 700 lb. per square inch. It is therefore suitable 

 for laying in the sea in depths up to 250 fathoms. 



When the air-space cable contains two conductors only it is 

 less flexible in the plane in which the wires lie than in a plane 



Fig. 42. — Willoughby Smith and Granville's Air Space Cable. 



at right angles thereto if the conductors are in a straight line. 

 In cases where this difference in flexibility is likely to be 

 prejudicial, the sections are made in the form of a helical coil 

 instead of in straight lengths. This is done by twisting the 

 sections after they have been brought together and before the 

 outer coatings are laid on. 



The two Atlantic cables laid In 1894, designed for special 

 high speed of signalling and durability, are of Interest. That 

 manufactured and laid by the Telegraph Construction and 

 Maintenance Company for the Anglo-American Telegraph 

 Company, to the designs of Messrs. Clark, Forde and Taylor, 

 the consulting engineers to the latter company, was constructed 



