MECHANICS AND USEFUL ARTS. 103 



direct submarine line yet in existence ; tlien its track lies through 

 a part of the Atlantic which until very recently had been unex- 

 plored, and the nature of the bottom comparatively unknown ; and, 

 thirdly, we look upon it with interest because it shows that the 

 importance of submarine telegraphic communication is commend- 

 ins: itself to other countries besides our own. Hitherto, nearlv all 

 the more important submarine lines have been the direct offspring, 

 and have remained in possession of English companies, but the 

 present cable, although manufactured and laid by an English firm, 

 is the result entirely of French enterprise, and to a large extent 

 owes its existence to French capital. 



The vital part of the longer section of the cable — or technically 

 the "core" — is a copper conductor of 7 wires twisted to- 

 gether, insulated by 4 concentric coatings of gutta-percha, 

 separated from each other by an equal number of coatings of the 

 material known as " Chetterton's compound," exactly after the 

 pattern of the cores in the last Atlantic cables, the onl}'' differ- 

 ence between them being in the weight of the conductor, which 

 in the present case is 400 pounds per mile, instead of 300 pounds. 

 This increase is to compensate for the adJitional length of the 

 cable. Experiments have shown that the sjDced of signalling 

 through submarine cables varies inversely according to their 

 length, and directly as the weight of the conductor; so that, by 

 adding to the weight in due proportion to the increased length, 

 the speed obtained is the same as through a shorter cable. 



The core is surrounded with a serving of yarn, called the " wet 

 serving," allowing of the ready access of the water to the core. 

 Until comparatively recently, this serving was saturated with tar, 

 but experience shov/ed that, should a slight defect occur in the 

 gutta-percha, the tar, from the serving being in itself an insulator, 

 would sufficiently stop it up to prevent its being discovered by the 

 electrical tests, until perhaps it was too late to remedy it. The 

 present wet serving, however, containing no insulating fluid, 

 permits of the instant detection of a fault. 



Around the serving are twisted spirally 10 homogeneous iron 

 wires galvanized, each of them embedded in 5 strands of Manila 

 hemp. The cable thus completed is of a diameter of about one 

 and a quarter inches, weighing about 36 cwt. to the nautical mile, 

 and capable of bearing a strain of 7 tons. 



The core of the shorter section — St. Pierre to Boston — is of 

 the same description as that of the Brest to St. Pierre section ; 

 but owing to its much shorter length, the weights of the copper 

 conductor and insulator are only 107 pounds and 150 pounds per 

 mile respectively. This core is also covered with a wet serving, 

 and then surrounded with about a dozen iron wires oralvanized, 

 the outside covering consisting of a silicated material, known as 

 I* Clark's compound ; " the whole forming a cable of a])out one 

 inch in diameter, weighing about two and three quarter tons to ^ 



the mile. 



The Brest to St. Pierre section was manufactured at the Tele- 

 graph Construction Company's Works at Greenwich, and trans- 

 mitted piece by piece in old hulks to the Great Eastern steamship, 



