The quantity of excavation and brickwork or concrete in each case will be as follows: Single tunnel: 30 ft. diameter lining, 3 ft. thick, with the brickwork forming the air passage = to 36.5 cubic yards per yard forward. Excavation to outside of brickwork 36 ft. diameter = to 113 cubic yards per yard forward. Three tunnels 17 ft. diameter and 18 in. brickwork. Brickwork lining for three tunnels = 24.5 cubic yards per yard forward. Excavation outside brickwork for the same 105 cubic yards per yard forward. It is assumed that three 17 ft. tunnels are stronger, more conveniently formed, and involve less risks in construction than one of 30 ft. diameter; at the same time there is no difficulty in making the latter. The above shows the saving in the three tunnels of 23 per cent. in brickwork, and about 7 per cent. of earthwork, compared with one of 30 ft. With regard to ventilation, it is well known that the power required to force air along passages is practically as the cube of the velocity; and as the area of the air passages in the single tunnel is 106 ft. with speed ten miles per hour, and that of one of the 17 ft. diameter is 227 ft., or rather more than double, giving only five miles per hour velocity, it follows that the power for this portion would be eight times less. That for the working tunnels would be practically the same, the velocities being nearly alike in both cases, which would be about 2½ miles per hour--the 30 ft. having an area of 470 ft., the two single ones together about 450 ft. Upon the face of it the system deserves a trial. A full consideration of the scheme by engineers preparing plans for new tunnels would no doubt throw further light upon the subject and be of interest wherever such work is contemplated.--_Contract Journal_.
Every one who has the slightest regard for historical monuments, who values mediæval architecture, or cares in the least degree for the beautiful and the picturesque, must heartily sympathize with M. Victor Hugo in his protest against the proposed scheme for uniting the wonderful island of Mont St. Michel with the mainland by means of a _causeway_, and possibly a _railway_!
Those who know Mont St. Michel well, and, like the writer, have spent several days upon the island, cannot but feel that such a scheme would not only be a frightful disfigurement, but would entirely destroy all the associations and the poetry of the place. Practical people will say, "Modern improvement cannot stop in its march forward to consider poetical associations and mere artistic whims and fancies." Now, this would be a possible argument if Mont St. Michel were a busy, thriving town, a commercial port, or the seat of great industries; but in a case where the only trade is that of touting, the only visitors sightseers, the only "stock-in-trade" mediæval remains, surely, from a practical point of view, anything which will injure these antiquities will really destroy the importance of the island, as its _only_ value consists in its wonderful historic and artistic associations.