So intimately has the electric telegraph become associated with our daily life, that any invention which will have a tendency to increase the facilities of intercommunication becomes to all of us, more or less, a matter of private concern. When land lines bad become generally established, and messages were flashed actors Europe, or a network of wires stretched from one end of England to the other, it seemed as if telegraphic progress had censed for a time. With the laying of the first submarine cable, a new era was Cables @ ab once ex-limited in their powers of transmission, and when, more especially, the bold stroke was made of laying one across the Atlantic, it became evident that some more refined and perfect method of trans needed than bad proved 5 roughly constructed land lines. The conductive power of long lengths of insulated cable was soon found to be limited, while the proximity of the conductor to the earth and water caused induced currents, whose nature was but little understood. The first early attempts to get over these difficulties by sheer force resulted in. signal failure, and the first Atlantic cable remains as. a record to show how little were the Jaws of electric resistance comprehended in those days. Even with improved instruments, and working with as low charges as possible, deep sea cables are subject to many maladies from which the more robust land lines are exempt and we here, in China, depending as we do, for our telegraphic news on submarine lines, are particularly interested in the progress of ocean telegraphy. The difficult questions to be solved have engaged the attention of the ablest men of science, and hence it, is not surprising that, gradually, ocean telegraphy is becoming fixed on surer basis, Marine cables, as we know, are subject to many difficulties and dangers, some external, some intimately connected with the cables themselves. The dangers of breakage, or chafing along rocky ground; those incident to marine accidents, such as fouling of anchors or destruction by the attacks of marine avimals—form one class. Of another sort are those incident to imperfect insulation, to induced currents, to resistance and to that gradual but silent deterioration which as yet seems the inevitable fate of submarine A third class of difficulties arises from the want of capacity; for marine cables take a definite time to report a signal, and any attempt to make them jog faster only results in inevitable confusion. Cate in manufacture. and judgment in laying ave reduced the first class of accidents; and the substitution of india-rubber for gutta percha has consider. ably reduced the difficulties of the second order, Still, as the capacity of cable seemed to be strictly limited, its powers of earning were palpably constricted, and this necessitated the keeping down the first cost to the lowest limits. Any improvement, therefore, which would enable the cable to carry more messages in a given time, would increase its powers of earning and permit of more elaborate precau tions in the first instance, or a good many years the thoughts of electricians have been turned towards the means by which such a desideratum could be accomplished. One of the most ingenious of these was what has become known asthe "duplex" system, whereby messages could seemingly be sent from each end at the same time, We say seemingly, because, in effect, the signal was given not by the passage of the messages through the wire from opposite ends, but by the. stoppage of the currents upsetting the equilibrium of a galvanometer at either extremity. If the stream of a mill-race be divided into two equal parts, and made to impinge on the opposite sides of a wheel, it is evident that equilibrium will result. If theria check be given to the current flowing in one channel, the current in the other will be increased and the wheel will revolve.
So, in the duplex system of telegraphy,
Text read from the printed page and cleaned; old type leaves the occasional error.