All interruptions are not so costly, for in shallow waters, with favorable conditions of weather, a repair may be only a matter of a few hours, and it is in such waters that the majority of breaks occur, but still a large reserve fund must be laid aside for the purpose.
As an ordinary instance it has been stated that the cost of repairing the direct United States cable up to 1900 from its submergence in 1874 averages £8,000 per annum.
Nearly all the cable companies possess their own steamers of sufficient dimensions, and specially equipped for making ordinary repairs, but for exceptional cases where a considerable quantity of new cable may have to be inserted, it may be necessary to charter the service of one of the larger vessels owned by a cable manufacturing company at a certain sum per day, which may well reach £200 to £300.
This fleet of cable ships now number forty, ranging in size from vessels of 300 tons to 10,000 tons’ carrying capacity.
The life of a cable is usually considered to continue until it is no longer capable of being lifted for repair, but, in some cases the duration and frequency of interruptions as affecting public convenience with the loss of revenue and cost of repairs, must together decide the question of either making very extensive renewals or even abandoning the whole cable. It is a well ascertained fact that the insulator—gutta percha—is, when kept under water, practically imperishable, so that it is only the original strength of the sheathing wires and the deterioration allowable in them that have to be considered.
Cables have frequently been picked up, showing after many years of submergences, no appreciable deterioration in this respect, while in other cases ends have been picked up which in the course of twelve years had been corroded to needle points, the result, no doubt, of metalliferous deposits in the locality.
The experience gained in the earlier days of ocean telegraphy from the failure and abandonment of nearly 50 per cent. of the deep-sea cables within the first twelve years, placed the probable life of a cable as low as fifteen years, but the weeding out of unserviceable types of construction and the general improvement in materials, have, by degrees, extended that first estimate until now the limit may be safely placed at not less than forty years.
In depths beyond the reach of wave motion and apart from the suspension across a submarine gully which will sooner or later result in a rupture of the cable, the most frequent cause of interruption is seismic or other shifting of the ocean bed, while in shallower waters and near the shore the dragging of anchors or fishing trawls have been mostly responsible.
Since by international agreement the wilful damage of a cable has been constituted a criminal offence and the cables have avoided crossing the fishing banks or have adopted the wise policy of refunding the value of anchors lost on their cables, the number of such fractures have been greatly diminished.