The Story of the Atlantic Telegraph

The great problem had been to combine strength with flexibility. Mere dead weight was an objection. The new cable, though nearly twice as heavy as the old in air, when immersed in water, weighed but a trifle more; so that it was really much lighter in proportion to its size. This increased lightness was a very important matter in laying the cable, as it caused it to sink slowly. The old cable, though smaller, was heavy almost as a rod of iron, so that, as it ran out, it dropped at an angle which exposed it to great danger in case of a sudden lurch of the ship. Thus in 1857 it was broken by the stern of the Niagara being thrown up on a wave just as the brakes were shut down. Now the cable, being partially buoyed by the rope, would float out to a great distance from the ship, and sink down slowly in the deep waters. By this combination of rope and iron, a cable was secured two and a half times as strong as the old--the breaking strain of the former having been three tons five cwt., and of the latter seven tons and fifteen cwt. Or, to put it in another form, the contract strain of the former was less than five times its own weight per mile in water; so that if the cable had been laid in some parts of the Atlantic, where the ocean is more than five miles deep, it would have broken under the enormous strain. But the contract strain of the new cable was equal to eleven times its weight per mile in water, which, as the greatest depth of water to be passed was but two and a half miles, rendered the cable more than four times as strong as was required.