The greatest tunnels that we know of connected with mining (irrespective of galleries for working in mines) are the great drainage galleries at the mines of Clausthal, in the Hartz, 11,377 yards, or 6½ miles long, and in many parts 900 feet below the surface of the superjacent mountain. The second is the _Great Adit_, which drains several of the important mines in the parish of Gwennap, Cornwall. It is from 30 to 60 feet below the surface, and is 30 miles long. There is an adit level of 10,000 yards to the celebrated silver-mines of Norway.
There are two modes by which it is proposed to carry a subway between France and England,[134] the first is by means of iron tubes laid on the bed of the ocean, the other is by actual tunnel. At the present time no less than three competitors present themselves for the honour of constructing the former, and there is only one whose scheme is before the public for the latter. The three advocates for the tubular system are Mr. James Chalmers, Mr. B. Hilmer, and M. Thome de Gammond, of Paris. Each has his mode of laying down and connecting the tubes together, but the great and distinguishing feature of the plan of M. De Gammon, is that he proposes to construct a great oceanic station, which is to be a kind of half-way halting house between the two shores. Here also is to be a harbour and three ship’s basins, so that any one returning from a long voyage and being in a hurry to get either to London or to Paris, or to any other place—it signifies not where—in England or the Continent, might land and at once proceed upon the _terra firma_ portion of his journey. In the centre of the harbour there is to be a huge shaft 330 yards in diameter, which would serve the double purpose of ventilating the tunnel, and of providing means of ingress and egress between the _Islet de Varne_ station and the upper and outer world. All these great advantages—tunnel, shaft, and railway—are to obtained at the cost, as estimated by M. De Gammond, of £7,200,000!
The advocates for the tubes insist that theirs is the right system, in consequence of its having been publicly stated that Mr. Hawkshaw has satisfied himself by many borings that the bottom of the English channel between Dover and Cape Grinez, has too many and too deep “faults” to permit of tunnelling. Mr. George Remington, C.E., however, considers he gets over the difficulty by avoiding the line originally selected for the Anglo-French tunnel. He therefore proposes Dungenness as his English starting point. The depths of the tunnel is, says Mr. Remington, to be from 90 to 130 feet below the bed of the channel, and there are to be three main shafts, the first at the point of Dungenness, the second on the shoal in mid-channel, where there are only eleven feet at low water spring tide, and the third at Cape Grinez. These shafts are to be 100 feet diameter, and being carried up considerably above the sea are to act as lighthouses. It would be inconsistent with the character of this work to enter into an account of the technical details which Mr. Remington proposes to adopt. We shall, therefore, limit ourselves to saying that in addition to the three intended permanent shafts, it is likewise proposed to put down ten temporary shafts, the cost of each of which is not to exceed £20,000. With these thirteen shafts, says the editor of _Engineering_, “the tunnel may be carried on in twenty-six sections, and the distance from shore to shore being twenty-six miles, gives only one mile for each section, or two miles for a shaft, and assuming an advance of only one yard a day for each headway, the whole distance may be accomplished in about five and a-half years,” exclusive of the couple of years required for sinking the shafts.