110
��AN OCEAN CABLE.
��ometer, and sends through it a current of electricity. The amount of resistance that the current meets upon its passage is instantly recorded before his eyes by the spot of light moving out upon the scale a certain number of degrees. This, as already explained, is caused by the "deflection" of the little mirror, whose magnetic needle is influenced by the strength of the current. Suppose, for instance, that the spot of light is deflect- ed 40 degrees upon the scale. The elec- trician then disconnects the cable, and in its place hitches on some of the resistance coils, which is simply another cable. Suppose he hitches on enough of them to represent 200 miles of real cable. He then turns on the same electric current as before, and finds that the spot of iight moves out upon the scale only 30 degrees. What is the inference? Why, simply that the interruption in the real cable is more than 200 miles from the shore, be- cause 200 miles of the artificial cable does not produce the requisite deflection. Well, suppose he hitches on another hun- dred miles of resistance coils, and then finds the light deflected exactly 40 de- grees upon the scale — what then? Why, he has found the break. It is the fourth term of the proportion ; the equality of ratios. It is thus, after taking numerous and repeated tests (the mean of the whole being taken) that the exact spot of the break is '"located," and final direc- tions given to the company's officers sent out to repair it. Every test requires the utmost exactness and nicety of manipu- lation, and the greatest care in the math- ematical calculations.
Various methods of "testing" are em- ployed, but the one described includes the general principles of the science, and the materials used. The direct com- pany's electricians use, in addition to the mirror, what is known as the "Wheat- stone bridge," by which it is claimed a greater degree of accuracy can be ob-
��tained, but the process is too complicated for an ordinary magazine articie. When- ever a ship has been sent out to repair a break in the cable, the electricians and operators sit night and day and watch for the first movement of the little mir- ror by which messages are read ; until finally the long-expected flash indicates intelligence at the other end. Messages are sent and received, tests made, the broken ends reunited, and finally the de- lighted operator finds it will again re- spond to his magic touch.
The "picking up" of the cable from the bottom of the ocean is attended with immense expense, inasmuch as it re- quires a ship specially adapted for the business. The direct cable was laid and repaired by the "Farraday," which is one of the largest ships in the world, and was built expressly for laying ocean cables, by the Siemens Brothers, con- tractors, and is owned by the firm. She cost a million and a half dollars, meas- ures over five thousand tons, carries a crew of three hundred men, and is let to repair defunct cables for the modest sum of five thousand dollars per day I
She is supplied with all the appliances that modern science and ingenuity can invent, and is a marvel of machinery from end to end. The "hunting for a needle in a hay-mow," which was the tradition- al impossibility of a quarter century ago, is thrown completely in the shade by this five thousand ton monster of marine architecture that gropes amid the tem- pests of the North Atlantic, hunting for — and finding — a rope of wire no larger than a whip-stock, and that, too, in water two and a half miles deep ! That a wire can be laid across the ocean com- pletely impervious to the element in which it is submerged, and become a ve- hicle of thought, is a triumph of man's ingenuity that must forever remain as one of the foremost wonders of the nine- teenth century.
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