MAKING AND LAYING SUBMARINE CABLES. 151 
the quantity of material employed; so the core of a cable that 
will transmit twenty words per minute, will weigh and cost 
four times as much as the core of another cable capable of 
transmitting only five words per minute. 
India-rubber is now often used for insulating the cables. 
In this case it is applied to the conducting wire in spiral 
bands that overlap each other in opposite directions. When a 
Fig. 93.—Splicing a Cable. 
proper thickness has been obtained the wire thus covered is 
interposed between longitudinal india-rubber bands, which are 
united together by simple contact of the freshly-cut edges on 
the application of pressure. The two halves of the covering 
are afterwards strongly pressed and kept together by a binding 
of floss silk wound round the whole length of the wire. It is 
then wound on an iron drum turning on a pivot placed in the 
bottom of a plate-iron cylinder the cover of which is screwed 
down, and the cable is here exposed to the temperature of 
about 120° C. (248° Fahr.) by which it is suitably vulcanized, 
