_Silvertown Cables._--The following is a description of the core of an electrical submarine cable, which is used by the English government, and is supposed to contain all the advantages of the foregoing, and none of their defects. It consists of a strand conductor of four copper wires (No. 20 B. W. G.) of quality not less than 92 per cent. of pure copper, and possessing an electrical resistance of not more than 14 ohms per nautical mile. This strand is tinned and insulated with vulcanised india rubber to a diameter of ·24 inch, and then covered with a layer of felt, and the whole subjected to a temperature of 300° F. under steam pressure. This forms the core of the various kinds of cables employed in connection with a system of defence by electrical submarine mines, which are enumerated as follows:-- 1.--Single core armoured cable. 2.--Multiple cable. 3.--Circuit closer cable. 4.--Single core unarmoured cable. 5.--Special cables for firing by cross bearings. _Single Core Armoured Cable._--This form of cable is used in connection with each mine of a group or system, and also to connect forts, &c. across an arm of the sea. Over the core, which has been fully described, is laid a spiral covering of tanned, picked Russian hemp, over this are laid ten galvanised iron wires (No. 13 B. W. G.), each one of which is covered with a similar hemp, which is laid in an opposite spiral to the former similar covering, with a twist of one revolution in about thirteen inches; in order to prevent these wires from gaping when the cable is kinked, a further covering of two servings of hemp passed spirally in opposite directions is laid, and the whole passed through a hot composition of a tar and pitch mixture. Exterior diameter of this cable is 7/8 inch. Its weight in air is 27-50/112 cwt., and in water 14-40/112 cwt. per nautical mile. The breaking strain of a cable thus manufactured is 62-1/2 cwt., and its cost about £47 per nautical mile. A diagram of this cable is shown at Fig. 32.