The cost of constructing a telegraphic line has greatly diminished with the increased facilities of insulating the wires, and since the expiration of patents, which conferred a monopoly on certain plans of doing so. The cost to the Great Western Railway Company for a line of six wires to Slough, was £150 per mile, with comparatively low and slender posts and very imperfect insulation. The cost of the same number of wires at the present day would not be one-half that sum, with thicker wires and better insulation. It is customary in England to restrict the suspension of telegraphic wires to railways, from the notion that the protection of railways is necessary to prevent wilful damage to the wires; and as the Electric Telegraph Company have made exclusive arrangements with all the railway companies out of London, the competing telegraph companies have preferred to lay their wires underground rather than incur the supposed risk of damage to the wires if suspended from posts on common roads, though by this means the cost of construction is at least quadrupled. The protection which railways afford is, however, more imaginary than real, for any one inclined to interrupt the communication could easily do so; and if on common roads proper precautions were taken in fixing the posts, and a heavy penalty were imposed on wilful offenders, the common roads and open fields would, there can be little doubt, offer as safe a course for the telegraphic wires as railways. The conducting power of the earth is now employed by all electric telegraph companies for one-half of every circuit. Thus, whether a communication be sent from London to Liverpool, to Edinburgh, Paris, or Brussels, the moist earth serves to complete one-half of the communication. In the telegraphic circuit between London and Liverpool, for example, the insulated wire is connected at each end with the earth by being soldered to a copper plate, which is buried a few feet underground, so as to insure its being always surrounded with moisture. To improve the connection of this plate with the earth, it is customary to bury with it a quantity of sulphate of copper, the solution of which surrounds the earth-plate with a better conducting liquid than water, and thus extends the connecting surface. The gas pipes or water pipes are sometimes employed for the attachment of the wires instead of an earth-plate, but the latter is generally preferred.