PASSAGE OF THE MOUNT CENIS.

BY GEORGE NEUMANN,

MEMBER OF THE INST. OF CIVIL ENGINEERS, AND CHEVALIER DE LA LEGION D'HONNEUR.

DURING a long period of years suggestions had been made for the purpose of diminishing the time, cost, and the fatigue of the journey across the Alpine range separating France and Switzerland from Italy. One of the most frequented and direct routes from Paris into Italy is that which passes through Chambery in Savoy, and up the valleys of the Isere and Arc as far as Lanslebourg, whence it ascends the Mount Cenis, in a zig-zag direction, to an elevation of 6,900 feet above the level of the sea. It then descends by a variety of contours into the valley of the river Dora in Piedmont, at the head of which the town of Susa is situated. Having thus briefly given a general idea of the carriage road across this chain of mountains, I will now describe the newly completed works, and those in course of construction.

The railway system has gradually been extended across France to St. Michel, a small town situated in the valley of the Arc, and also throughout the north of Italy to the town of Susa above mentioned; so that at the present time this one link between St. Michel and Susa is only wanting to unite, by an iron road, Rome and Naples with Paris and other capital cities in northern Europe. To England, the Mount Cenis pass is of very great importance, as the establishment of a railway in that direction enables passengers on their way to India to embark at Brindisi for Alexandria, instead of following the usual route via Marseilles; and thus saving twenty-four hours in their journey.

In 1852 and the following years I laid out the railway line in Savoy on the French side of the Alps, sixty miles of which were constructed under my superintendence; and in 1854 I was charged by the Victor Emmanuel Railway Company to report as to the shortest and best direction for a railway to connect their line with the Italian one, commencing at Susa.

The line then selected was near to that previously proposed by Mr. Maus, a Belgian engineer, and on which a long tunnel would be required. The tunnel as proposed by me was slightly curved so as to pass it under some of the lowest ground, and obtain thereby the advantage of sinking a few shafts for the first three miles at each end of it. The Railway Company was not however sufficiently powerful in a financial point of view to undertake such a work; and during the summer of 1857 a law was passed by the Parliament at Turin, authorizing the government to make a straight tunnel nearly eight miles in length, and to construct and employ a system of drilling machinery invented in England. In 1864 I presented a paper to the institution of Civil Engineers in London by which I demonstrated that eight years of time might have been saved by the adoption of shafts as I had suggested.

THE TUNNEL.

On the 31st of August, 1857, the construction of a tunnel 26 feet wide, 24 feet high, and 7 miles long was formally commenced; the northern end being 3,945 feet above the level of the sea, and the southern end 4,379 feet. By an agreement made between the French and Italian governments each state is to participate in the cost of the works; and the year 1887 is fixed as the latest period for the completion of the tunnel with its approaches extending from St. Michel to Susa. The tunnel is only being pierced from each end, and at the end of June last the length completed was about 5% miles. During the month of June 122 yards were pierced, the number of men employed in and about the tunnel being about 2500.

DRILLING MACHINERY.

At each working face of the tunnel there is a leading gallery or heading 10 feet square, in which is placed a frame carrying 10 drilling engines, each of which is worked separately by atmospheric pressure. In the course of six to eight hours these engines pierce 80 holes, each 3 feet deep; four of the holes being 3 inches in diameter, and the remainder 1 inches. The larger holes are placed in the centre of the gallery, and used only to give effect to the explosion; the smaller holes are divided over the surface, and charged with gunpowder cartridges, those near the large centre holes being first fired. Previous to the blasting the frame carrying the ten drilling engines is withdrawn to a certain distance

from the face, and as soon as the explosions are accomplished the loose material is cleared away. This process is repeated once in every ten or fifteen hours, as follows: six to eight hours adjusting, drilling, and removing the engines; one-anda-half to two hours charging and firing; and three to five hours removing the débris.

The widening out of the gallery to the full size of the tunnel is accomplished by manual labour in the usual manner. The drilling engines are each provided with two atmospheric cylinders, one containing a piston and piston rod, at the end of which a steel pointed drill is fixed. This with the piston is carried backwards and forwards by the admission of the air, and it performs 200 to 250 strokes per minute, striking the rock at each stroke. The second cylinder is used to work a rack and pinion wheels, by which a forward movement is given to the first cylinder as the drilling advances, and a rotary motion to the drill after each stroke. A jet of water is kept constantly playing into the holes for the purpose of clearing out the débris. By means of an experimental machine made in England I have seen a two-inch hole drilled in hard limestone two feet deep, in eight minutes; this however far exceeds the average of the work done at the tunnel.

AIR COMPRESSING APPARATUS.

The system employed until recently at the south end of the tunnel for compressing the air was by means of hydraulic rams, worked by a vertical column of water 85 feet high, obtained by diverting a mountain stream. At the north end of the tunnel water power is also employed in the following manner for compressing the air: On the river Arc, which passes at a distance of one-third of a mile from the mouth of the tunnel, and 300 feet below it, a number of large overshot water-wheels are placed, each working two atmospheric pumps; these force the air into large iron reservoirs, compressing it to seven or eight atmospheres. These pumps are surrounded by cold running water so as to prevent them becoming hot by the heat thrown off from the air whilst being compressed. The temperature in the compressers is 40° centigrade (= 105° Fahrenheit), but in the receivers the same as the adjacent atmosphere. The air is conveyed from the reservoirs to the tunnel mouth in 74 inch iron pipes, and thence to the working gallery, where it is found to have nearly the same pressure as in the lower reservoirs.

STRATA, COST OF THE TUNNEL, AND TIME OF COMPLETION.

The material through which the tunnel has been pierced is of a schistose nature, and largely mixed up with quartz. It is very variable as to hardness, and the strata at the south end is more slaty and of a softer nature than that at the north.

There are now 5 miles of tunnel completed, and the total length is 7 miles; 2 are not yet pierced. Allowing the progress to be at the rate of 122 yards per month at the two faces, the whole length will be pierced by the spring of 1871. The cost of the Mount Cenis tunnel is not yet well ascertained, but it cannot be taken at less than £200 per lineal yard. This (compared with the cost, say of £30 per yard, of rock tunnels mined in the usual manner by manual labour) is excessive, and it cannot be attributed solely to the use of machinery. It has, however, been proved at this tunnel that the rate of progress is much greater than it would have been if drilling machinery had not been employed at all.

TEMPORARY RAILWAY OVER THE MOUNT CENIS.

An English company (having taken into consideration the probable time which may elapse before the great tunnel and the thirty-four miles in length of costly railway works required to connect it with the present lines are completed) has constructed a temporary railway, 48 miles long, over the top of the Mount Cenis. This line extends from the Italian station at Susa to the French station at St. Michel, and it surpasses any other yet made, both as regards the steep gradients, the sharp curves, and the great elevation it attains. It follows the direction of the carriage-road, being generally laid on the outside edge of it, except at the sharp turns, where deviations are made in order to obtain curves of not less than 44 yards radius. The line is a single way, with a guage of 3ft. 7 in., sufficient sidings being provided at the stations. The summit level is 6907 feet above the sea, and the rise from Susa on the first 17 miles is 5225 feet, averaging a rise of 307 feet per mile. The steepest gradients are 1 in 12 for many miles in length; that is, such an inclination as we should allow our horses to walk up on a turnpike road. In the districts most exposed to avalanches of snow, or to the fall of mountain débris, the line is protected by strong masonry arches; and for several miles where the snow is most subject to drift a corrugated iron roof has been constructed over it.

This railway was projected by Mr. Fell, to whom great credit is due for the application of a centre rail placed on its side, and about nine inches above the ordinary rails. The engines are provided with four horizontal wheels, which work against the centre rail, and are set in motion by the same cylinders which drive the vertical wheels. A very great additional traction power is thus obtained without materially increasing the weight of the engine. In descending the inclines, not only are ordinary breaks used, but the horizontal wheels on the engines can be employed for the same purpose, and the carriages are also each provided with a pair of horizontal friction wheels in addition to the common breaks. At all road crossings an ingenious plan has been adopted by which twenty or thirty feet of the central rail can easily be lowered so as to allow carts, &c., to pass. The engines weigh 22 tons with fuel and water, and the passenger trains, consisting of four carriages and a van, weigh about 17 tons each.* In October last Mrs. Neumann and I, accompanied Mr. Brassey, the constructor of the works, in the first passenger train, and I could hardly imagine, whilst we were looking up the mountain side into the fir plantations, clad with snow and thousands of feet above us, that we were actually trying to see an approaching railway engine descending towards us. At last, however, we obtained an occasional glimpse of it as it passed round some of the precipitous rocks, and in half an hour it arrived at our feet. In June last the line was opened to the public, and the trains have since run regularly, accomplishing the distance from St. Michel to Susa in rather more than four hours. It may be interesting to some of those present to know, that near the summit of the pass, and at an elevation of 6365 feet above the level of the sea, there is a lake about 1 mile long and three-quarters of a mile wide, containing very good trout.

* I have just received a letter from St. Michel, stating that the engines are now drawing 32 tons.