422 



THE ELECTRIC TELEGRAPH. 



too large as for instance if it fall but little short of the outer 

 diameter of the gutta-percha the thinness of the gutta- 

 percha coat gives rise to a greater loss of speed by increased 

 capacity than is compensated by the gain, owing to dimin- 

 ished resistance." 



According to this, a cable insulated with a coating one- 

 third the thickness of the diameter of its conductor is in the 

 most favourable condition to transmit with speed. Of all the 

 long cables yet made, the new Malta- Alexandria comes 

 nearest to this theoretical proportion ; the thickness of the 

 insulating covering of this cable being, within five per cent., 

 equal to the diameter of the conductor. From a mechanical 

 point of view, a cable which has a proportionally thinner 

 covering than this would be too much exposed to faults, 

 therefore no engineer would ever recommend it. It is never- 

 theless highly interesting to find in practice that the theory 

 is corroborated. 



Calculated by the above formula, taking the speed of 475 

 words, actually obtained on 1320 knots of the Malta- Alexandria 

 cable, as data, the speeds with which some other cables might 

 be worked through, are given in the following table : 



The calculated speed of working through the Suakin-Aden 

 section of the Red Sea cable, 12 J words per minute, agrees 

 with that which was really obtained, viz., 11 words, allowance 

 being made for the great improvements which had been 

 effected in the relays and transmitting apparatus. With 



