THE BOARD OF TRADE TELEGRAPH EXAMINATION. 35 lines, I do not believe that there is to be found in operation a com- mercial circuit of 1000 miles. The lines are divided into circuits, the length adopted for each depends upon the extent of the inter- ruptions above mentioned—submarine and air-lines respectively. Air-line circuits are usually 300 to 500 miles long, and but seldom longer. It is usual to overcome distance by coupling two or more circuits together with an apparatus called a “translator” or “ repeater.” Without these instruments the distance cannot be overcome, except by re-writing with the hand. Sometimes, how- ever, auxiliary batteries are placed at different parts of the same circuit, but in submarine telegraphy -we can have batteries only at the ends of the cable, and the cable can only be charged by a battery at the station of transmission. We cannot have co-operating batteries at both ends at the same time. On submarine lines the circuits are arranged as thus described, but the speed of transmis- sion is dependent upon the length of line and the retarding force of the Leyden jar. On the many thousand miles of air-lines on the American continent we have no circuit as long as 1000 miles. In submarine telegraphy there is no circuit of that length. The inteiTuptions occasioned to air-lines are but equals to those common to submarine telegraphs, excepting with this difference, that on air-lines the electric force is taken from the conductor or increased in charge by atmospheric electricity, as daily experienced in warm climates, and on submarine lines it is held in suspen- sion. The results, however, are the same. The electric force cannot be scaled to intelligible measurement. The pulsations of the electi’ic current must be even, regular, and known as equal, at both ends of the line respecting duration. If the current is irre- gular the signals transmitted will be questionably received. Accord- ing to a written opinion given by Sir Charles Bright, a distin- guished telegi’aph engineer, the retardation of the electric current, transmitted through a subterranean line of copper wire No. 16, was at the rate of one third of a second for 500 miles, and one second for a distance of 1000 miles; and according to that progression, it would be about nine seconds for 2000 miles, if that length of circuit could be brought to commercial telegraphy. It is possible that a larger conductor and a greater thickness of insulation will lessen the retardation. To w’hat extent this latter oi’ganisation of a cable would prove effective, on a long submarine circuit, remains a theory; and every practical telegrapher knows that theory in the mysterious workings of the electric telegraph cannot, in all cases, be depended upon. In the application of theory to practice new developments in physics appear, and, in many in- stances, wholly defeat that which theory seemed to demonstrate as certain results. The old Atlantic line was 2050 miles long, in one continuous d 2

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