Jennifer Broom, age 16, of Columbia, Tenn., for her question:

WHEN WERE UNDERSEA CABLES FIRST USED?

As early as the 1840s attempts were made to lay telegraph cables under water. The early efforts were all unsuccessful.

Then English brothers Jacob and John Brett laid the first successful cable under the English Channel in 1851. An American businessman named Cyrus Field then headed a company that in 1856 laid an undersea cable under Cabot Strait from Cape Ray, Newfoundland, to Cape Breton Island, Nova Scotia.

Early undersea cables could transmit telephone signals, but they could not carry the wide frequency of vibrations that made up speech. It wasn't until coaxial cables with built in repeaters came along that long distance telephoning by cable was made possible.

The first transatlantic telephone cable was laid in 1956. It stretched 2,250 miles from Clarensville, Newfoundland, to Oban, Scotland. Voices could be heard clearly over the cable.

Other cables were laid in the next few years. Since 1959, channels on most coaxial cables have also been made available to international telegraph companies.

Transatlantic telephone cables laid before the 1960s can carry messages in only one direction. These early systems consist of two cables laid side by side. Steel wires are wrapped around the outside of the cables to strengthen and protect them. The cables have flexible one way separaters spaced about 40 miles apart.

At first, cables carried 36 telephone calls at one time. But improved terminal equipment later permitted 48 calls. By the 1960s, almost 100 calls could be made over the same cables at one time. This increase resulted from newly designed terminal equipment.

In 1963, a single two way cable was developed to provide 138 telephone circuits across the Atlantic Ocean. And in the mid 1960s, Bell Telephone Laboratories developed a transistorized two way repeater for the single cable system that can provide 720 telephone circuits.

After many attempts and great difficulty, the first Atlantic Ocean telegraph cable was completed on July 27, 1866. The cable stretched from Valencia, Ireland, to Hearts Content, Newfoundland.

About six weeks later, the cable that had been lost in an 1865 attempt was recovered from the floor of the ocean. It was repaired and put in working order. This cable was also laid between Valencia and Hearts Content.

The Atlantic cables marked the beginning of fast communication across the seas. They made it possible to send a message and receive an answer in only a few hours.

By 1900, 15 cables had been laid across the Atlantic.

By the 1930s, improved sending and receiving techniques and better cables made it possible to send up to 400 words a minute. Messages could be sent over cables at 2,500 words a minute by the mid 1960s