352 THE ELECTRIC TELEGRAPH. 



at the same place, capable of resisting an internal pressure 

 of over 10,000 pounds on the square inch, or that of a column 

 of sea-water nearly four nautical miles high. 



The core to be tested in the tank is coiled regularly upon 

 an iron drum of such dimensions as permit it to enter com- 

 fortably into the tank, and to hold as great a length of cable 

 as is possible. With this tank the core of the Mediterranean 

 cable, made in 1863-64 for Messrs. Siemens, was tested, under 

 a pressure of nearly 4,500 pounds per square inch, equal to 

 the pressure which the cable would be exposed to at the 

 greatest depth. 



The engineers of the Atlantic cable tested by this 

 method the core of the new cable, under only 600 pounds 

 pressure per square inch, although when laid it has to 

 resist a pressure of at least ten times this. The economy 

 of time and of joints was probably the principal reasons 

 which induced the scientific committee to insist on no higher 

 pressure tests, combined with a conviction that the perfect 

 manner in which the core was made, and some weak 

 points in the system of high-pressure testing itself, render 

 at the present day the testing of core in this manner almost 

 superfluous. 



If the gutta-percha coating were put on to the conductor 

 at one operation, it would be always possible, even probable, 

 that bubbles would occur which could be broken open by the 

 vacuum test, and completely filled with water when under 

 the water-pressure ; but when the gutta-percha is put on 

 the wire in two, three, or four concentric coatings, as is done 

 with all cable cores, the probabilities of such an occurrence 

 are infinitely reduced ; and it may be almost regarded as an 

 impossibility for a bubble to extend beyond a single coating, 

 especially as, between each coat of gutta-percha, an inter- 

 vening layer of compound* is put, which, in the process of 

 its application to the wire, fills up any cavities left in the 

 gutta percha. 



The weak point of the system is, that the bubbles which 



* Its ingredients are, by weight, 1 part of Stockholm tar, 1 part of resin, 

 and 3 parts of gutta-percha. Its conducting power is higher than that of the 

 best gutta-percha ; its specific gravity about the same. 



