Page:Popular Science Monthly Volume 3.djvu/51
that condition until it was laid, was the Malta and Alexandria cable, laid in 1861. This cable was submerged in too shallow water, for many miles in less depth than 20 fathoms; the result was the frequent recurrence of fracture from being rolled about by the surf, and yet this cable was only finally abandoned last year; not because it could not be kept in repair, but because it was too expensive to keep in order. These and many other examples have established the principle that no cable should be laid without first obtaining an accurate survey of the approach to the coast and landing-places, with accurate soundings over the intended route, and as much knowledge as possible of the nature of the bottom. Currents and anchorage should be avoided, and, where that is impossible, the heaviest cable that can be laid should be provided. Heavy cables should be laid out to depths of 400 fathoms, where there are tide-ways. Where a current exists, a position should be sought for as far removed from it as possible. A great cause of injury to cables is the corrosion of the external wires, caused by moving water or marine vegetation, etc., and this has established the general practice of covering the external wires with tarred yarn saturated with a mixture of pitch and silica. There is still great room for improvement upon the present method of protecting the external covering; of cables, and I commend it to the further careful study of telegraph-engineers as a subject of vital importance.
Another enemy of submarine cables is the teredo of all kinds; there is one kind which has proved destructive by boring through the core, but that has only occurred in shallow water; there is another kind which destroys the hemp in a few months, and is then satisfied to fix itself upon the gutta-percha and remain there. Cables have been recovered from depths of 1,200 fathoms with all the hemp eaten away, and the core pitted with these marine animals. The recovery is then only possible by the strength of the external wires.
All the experience we. have points to the value of protection, first, of the core, then of the external covering, and, if those responsible for the safety and maintenance of submarine cables could be allowed to dictate the most desirable conditions of safety, they would select, besides the strongest possible cable to be manufactured, and laid with extreme care, a depth of water of about 500 fathoms, and a bottom of sand or mud; but, as this cannot always be secured, nothing should be omitted in the direction of strength and quality.
Lightning is still another source of injury to cables; this is, however, so readily guarded against that we no longer hear of injury from this cause: it is said to have destroyed three cables. Mr. Siemens produced before the committee a piece of the core of the Corfu cable injured by lightning; the land-line had been struck, and, from the absence of any lightning-guards, the cable was damaged. Mr. Preece described the Jersey cable as having been destroyed by lightning. Mr.
- See article, in this number, on the "Borers of the Sea."