The first telegraph line was opened in 1844. Soon there were thousands of miles of telegraph lines on land. And before long people could send telegraph messages across oceans. Sending these messages was made possible by the laying of heavy wires, or cables, across the seas from continent to continent. Cables laid under the sea are called submarine cables. The telegraph messages sent over them are called cablegrams.
The Atlantic was the first ocean to be crossed by cable. After several failures, Cyrus W. Field, an American, succeeded in 1866 in laying a cable from Newfound-land to Ireland. The Pacific Ocean was spanned with a cable 36 years later.
In 1955 a telephone cable was laid across the Atlantic. Since then other telephone cables have been laid. Such cables make it possible to hear someone across the sea as clearly as if he were just across town.
The wire through which the electricity flows is only a small part of a submarine cable. The wire must have waterproof covering, because salt water is a fairly good conductor of electricity. The wire must be protected against sharp rocks, dragging anchors, and swordfish. The cable must be so strong that it will not break of its own weight as it is being lowered to its place at the bottom of the sea. The cable also must have an outside layer that shipworms cannot bore through.
