28 SUBMARINE CABLE LAYING AND REPAIRING. 



Electricians, on November 8th, 1888, entitled "On Ocean 

 Temperatures in Kelation to Submarine Cables." Mr. Car- 

 penter's Paper, which, owing to the special and prolonged 

 study he had devoted to the subject, was of particular interest 

 was also the means of bringing to light considerable experience 

 in the use of these instruments during the discussion which 

 followed {Journal of the Society, Vol. XVII., page 658). 



Temperatures at the bottom of all the great oceans closely 

 approach freezing point, the Atlantic at 2,000 fathoms depth 

 being 36°F., and the Pacific a degree or two lower. The 

 Mediterranean Sea is in this respect an exception, on account 

 of its comparatively shallow connection with the Atlantic 

 through the Straits of Gibraltar, thus preventing circulation. 

 The temperature even at the greatest depths in this sea iS' 

 never below 55°F. 



The condition of low temperature is most favourable for 

 submarine cables. The conductivity of the copper conductor 

 and the resistance of the insulator are both thereby increased. 

 The great pressure of sea water on the ocean bed also increases 

 the insulation resistance, most remarkably so in the case of 

 gutta-percha. On this head, information obtained at the time 

 of laying a cable is most valuable in subsequent repair work in 

 the same waters. After submersion the contractors carry out 

 daily tests of the cable for one month, and amongst other 

 determinations they measure very carefully the resistance of 

 the conductor, making all corrections for temperature of testing 

 room. On comparing this with the resistance of the cable as 

 measured at the standard temperature of 75°F. in tank at the 

 factory, the mean temperature of the sea in which the cable is 

 laid is arrived at. 



While the observations for depth, temperature and nature of 

 bottom are being taken the ship's position must be determined 

 to as great a degree of accuracy as possible. For this purpose 

 the log is always used, showing the distance travelled from one 

 position to the next, but if the course of the ship is affected by 

 currents or leeway the positions arrived at by dead-reckoning 

 alone are not strictly accurate, as the compass only indicates 

 the bearing of the ship's head, not her actual course, and the 

 log the distance the ship has travelled through the water, not 

 that over ground. When the sun permits, therefore, the ship's 



