The letters _A_ _B_ represent the wires making communications between the batteries _D_ and _E_, and the telegraph instruments _I_ _O_ at the receiving station. The electricity from the copper end of the battery _D_ would be conducted along _A_ through the instrument _I_, and by the wire _K_ to the earth-plate _H_. It would be then transmitted through the earth on its return to the battery, in the direction of the arrows, to the other earth-plate _G_, and thence it would find its way to the zinc pole of the battery _D_, and complete the circuit. In the same manner, the electric current from the copper end of the battery _E_ would be transmitted through the wire _B_, and would complete its current also by means of the earth-plates _G_ _H_, and would traverse the course indicated by the arrows, and return to the zinc end of _E_. Though both electric currents traverse the same wire from the instruments _I_ _O_ to the earth-plate _H_, and are thence transmitted through the earth to a single plate, _G_, at the transmitting station, there is no mingling of currents, the electric current of each battery being kept as distinct as if separate wires were used both for the transmitted and the return current. It would, indeed, be as impossible for the separate currents transmitted from the two batteries to be mingled together, as it would be for the written contents of two letters enclosed in the same mail-bag to intermix.[8] The length of telegraph lines at present laid down by the several telegraph companies in Great Britain, exceeds 10,000 miles. To complete those lines required 40,000 miles of wire, and there are 3,000 persons engaged in the transmission of telegraphic intelligence. In North America there is a direct communication from New York to New Orleans, a distance of 2,000 miles, through the whole length of which wires messages can be transmitted without any break. Wires have also been suspended on lofty posts across the Indian Peninsula, where no railways have been yet laid down. Lines of insulated wire, partly submerged in the sea, partly buried underground, and partly suspended on posts in the air, place London and Vienna in direct communication; and other telegraph lines are in the course of construction, which will unite London with Africa: and a complete net-work of telegraph wires is spreading over the face of Europe.