As has already been pointed out, the excavation of the gallery at both ends had already been in operation, by ordinary means, since the latter part of the year 1857; this work continued without interruption until the machinery was ready; and the progress made in that time affords a valuable standard by which to measure the effect of the new machinery. In the interval between the end of 1857 and that to which we have now arrived, namely, the end of 1858, many important works had been pushed forward. At Bardonnêche, the communications had been opened, and bridges and roads constructed for facilitating the transport of the heavy machinery. Houses for the accommodation of the workmen had been rapidly springing up, together with the vast edifices for the various magazines and offices. The canal, more than a mile and a half in length, for conveying water to the air-compressing machines, was constructed, and the little Alpine village had become the centre of life and activity. At Fourneaux, works of a similar character had been put in motion; only here the transport of the water for the compressors was more costly and difficult, the water being at a low level. At first, a current derived from the Arc was used to raise water to the required height, but afterward it was found necessary to establish powerful forcing-pumps, new in their details, which are worked by huge water-wheels driven by the Arc itself. Early in the month of June, 1859, the first erection of the compressing machinery was commenced at Bardonnêche. The badness of the season, however, and [{64}] the Italian campaign of this year, delayed the rapid progress, and even caused a temporary suspension of this work. The results obtained by the experiments which had previously been made on a small scale at St. Pier d'Arena, failed completely in supplying the data necessary to insure a practical success to the first applications of the new system; numberless modifications, both in the compressing-engines and in the perforating-machines, were found necessary; and several months were consumed in experimenting with, modifying, and improving the huge machinery; so that it was not before the 10th of November, 1860, that five compressors were successfully and satisfactorily at work. On the 12th, however, two of the large conducting-pipes burst, and caused a considerable amount of damage, without causing, however, any loss of life. This accident revealed one or two very serious defects in the manner of working the valves of the engine; and in order to provide against the possibility of future accidents of the same nature, further most extensive modifications were undertaken.
By the beginning of January, 1861, the five compressors were again at work; and on the 12th of this month the boring-engine was introduced for the first time into the tunnel. Very little useful result was, however, obtained for a long and anxious period, beyond continually exposing defects and imperfections in the perforators. The pipes conducting the compressed air from the compressing-machines to the gallery gave at first continued trouble and annoyance; soon, however, a very perfect system of joints was established, and this source of difficulty was completely removed. After much labor and patience, and little by little, the perforating-machines became improved and perfected, as is always the case in any perfectly new mechanical contrivance having any great assemblage of parts. Actual practice forced into daylight those numberless little defects which theory only too easily overlooks; but there was no lack of perseverance and ingenuity on the part of the directing engineers; one by one the obstacles were met, encountered, and eventually overcome, and the machines at last arrived at the state of precision and perfection at which they may be seen to-day. About the month of May, 1861, the work was suspended for about a month, in consequence of a derangement in the canal supplying water to the compressors; and it was considered necessary to construct a large reservoir on the flank of the mountain, to act as a deposit for the impurities contained in the water, and which often caused serious inconvenience in the compressors. In the whole of the first year 1861, the number of working days was two hundred and nine, and the advance made was but one hundred and seventy metres (five hundred and fifty feet), or about eighteen inches per day of twenty-four hours, an amount less than might have been done by manual labor in the same time. In the year 1862, however, in the three hundred and twenty-five days of actual work, the advance made was raised to three hundred and eighty metres (one thousand two hundred and thirty-five feet), giving a mean advance of 1.17 metres, or about three feet nine inches per day. In the year 1863, the length done (always referring to the South or Bardonnêche side) was raised to above four hundred metres; and no doubt this year a still greater progress will have been made.
At the Fourneaux or Northern end of the tunnel—owing to increased difficulties peculiar to the locality—the perforation of the gallery was much delayed. A totally different system of mechanism for the compression of air was necessitated; and it was not before the 25th of January, 1863, that the boring-machine was in successful operation on this side, or two years later than at Bardonnêche. The experience, however, gained at this latter place, and the transfer of a few skilful workmen, soon raised the advance [{65}] made per day to an amount equivalent to that effected at the Southern entrance. Thus, on the South side (omitting the first year, 1861) since the beginning of 1862, and on the North side since the beginning of 1863, the new system of mechanical tunnelling may be said to have been in regular and successful operation.
In the beginning of September of this year were completed in all three thousand five hundred and seventy metres of gallery. From this we deduct sixteen hundred metres done by manual labor, leaving, for the work done by the machines, a length of nineteen hundred and seventy metres. From this we can make a further deduction of the one hundred and seventy metres executed in the first year of experiment and trial at Bardonnêche, so that we have eighteen hundred metres in length excavated by the machines in a time dating from the beginning of 1862 at the South end, and from the beginning of 1863 at the North end of the tunnel. Thus, up to the month of September, 1864, we have in all four years and six months; and eighteen hundred metres divided by 4.5 gives us four hundred metres as the rate of progress per year at each side, or in total, eight hundred metres per year. Basing our calculation, then, on this rate, we find that the eight thousand six hundred and fifty metres yet to be excavated will require about ten and a half more years; so that we may look forward to the opening of the Mont Cenis tunnel at about the year 1875. The directing engineers, who have given good proof of competency and skill, are, however, of opinion that this period may be considerably reduced, unless some totally unlooked-for obstacles are met with in the interior of the mountain. As has been indicated above, sixteen hundred metres in length of the tunnel was completed by manual labor before the introduction of the mechanical boring-engines, in a period of five years at the North and three years at the South side, equal to four years at each end; and eight hundred metres in four years gives us two hundred metres per year, or just one-half excavated by the machine in the same period.
In using the machines, up to the present time, a perfect ventilation of the tunnel has been secured by the compressed air escaping from the exhaust of the boring-engines; or by jets of air expressly impinged into the lower end of the gallery to clear out rapidly the smoke and vapor formed by the explosion of the mine. It should be remembered, moreover, that in working a gallery of this kind, where vertical shafts are impossible, by manual labor, a powerful and costly air-compressing apparatus would have been necessary for the ventilation of the tunnel alone, so that the economy of the system, as applied at the Mont Cenis over the general system of tunnelling in hard rock, is evident. I propose, in the second portion of this article, to give a short description of the machinery employed and the system of working adopted, both at the South and North ends of the Mont Cenis gallery.
II.
Travellers who are given to pedestrian exercises may easily visit the works being carried on for the perforation of the tunnel through the Alps, both at Bardonnêche and at Modane, passing from one mouth of the tunnel to the other by the Colle di Frejus; and in fine weather, the tourist would not repent the eight hours spent in walking from Bardonnêche to Susa—a distance of about twenty-five miles. The road descends the valley of the Dora Ripari, and abounds in beautiful scenery. The railway to be constructed along this narrow defile will be found to tax the skill of the engineer as much as any road yet attempted. Its total length, from the terminus at Susa to the mouth of the Mont Cenis tunnel, will be forty kilometres, [{66}] or about twenty-four miles; and the difference of level between these two points is about two thousand five hundred feet, the line having a maximum gradient of one in forty, and a minimum of one in eighty-four. There will be three tunnels of importance, having a total length of about ten thousand feet; three others of lesser dimensions, having a total length of five thousand five hundred feet; and twelve other small tunnels, of lengths varying from two hundred and twenty to eight hundred and fifty feet, their total length being five thousand four hundred feet. Thus, the total length of tunnel on these twenty-four miles of railway will be nearly twenty-one thousand feet, or about four miles—just one-sixth of the whole line. There will also be several examples of bridges and retaining walls of unusual dimensions.
The works being carried on at Bardonnêche are on a larger scale than at Modane; so we will, with our readers' permission, suppose ourselves arrived in company at the former place, and the first point which we will visit together will be the large house containing the air-compressing machinery. Before entering, however, we will throw a glance at the exterior of the building. We find before us, as it were, two houses, in a direct line one with the other—one situated at the foot of a steep ascent; and the other at about seventy or eighty feet above it, on the side of the mountain. These two houses are, however, but one, being joined by ten rows of inclined arch-work. Along the summit of each row of arches is a large iron pipe, more than a foot in diameter. These ten pipes, inclined at an angle of about forty-five degrees, come out of the side of the upper house, and enter the side of the lower house, and serve to conduct the water from the large reservoir above to the air-compressing machinery, which is arranged in the house below, exerting in this machinery the pressure of a column of water eighty-four feet six inches in height. On entering the compression-room, we have before us ten compressing-machines, precisely the same in all their parts—five on the right hand, and five on the left, forming, as it were, two groups of five each. In the centre of these two groups are two machines, in every respect like a couple of small steam-engines, only they are worked by compressed air instead of steam, and which we will call aereomotori. Each of these aereomotori imparts a rotary motion to a horizontal axis extending along the whole length of the room, and on which are a series of cams, which regulate the movements of the valves of the great compressors. This axis we will call the "main shaft." One group of five compressors is totally independent of the other, and has its aereomotore with its main shaft; but still, with one single aereomotore, by means of a simple connecting apparatus, it is possible to work one or the other group separately, or both together; also, any number of the ten compressors can be disconnected for repairs without affecting the action of the rest, or may be injured without conveying any injury to the others. In front of each of the ten compressors are placed cylindrical recipients, in every respect like large steam-boilers, except that they have no fire-grate or flues, each having a capacity of seventeen cubic metres, or five hundred and eighty-three cubic feet. These recipients are put into communication one with the other by means of a tube similar to a steam-pipe connecting a series of steam-boilers; and each connection is furnished with a stop-valve, so that any one recipient can be isolated from the rest.
Let us now examine the end and action of this machinery. As the aereomotori which work the valves of the machines for forcing air into the recipients are themselves worked by compressed air coming from the recipients, it is evident that before we can put the compressing-machines in motion, we must have already some supply of compressed air in the [{67}] cylindrical vessels. This supply of air, compressed to a pressure of six atmospheres, is obtained in the following manner: Each group of five recipients, filled with air at the ordinary atmospheric pressure, is put in communication with a large pipe which enters into a cistern placed in the side of the mountain at about one hundred and sixty-two feet above the floor of the compressing-room. The first operation, then, is to open the equilibrium valves placed at the bottom of the two pipes (one from each group of recipients); water then rushes into the vessels, compressing the ordinary air therein contained to about a pressure of six atmospheres. A communication is now opened between this compressed air and the cylinders of the aereomotori, which commence their action precisely as a steam-engine would do on the admission of steam; a rotary motion is given to the main shaft; and the equilibrium valves, placed in chambers at the bottom of each of the ten pipes coming from the cistern of water placed in the house above, are opened. We will observe the operation in one of the ten lines of action, as it were, consisting of the pipe conducting the water from the cistern, the compressing-machine, and the cylindrical recipient. The equilibrium valve at the bottom of the pipe being opened in the manner above explained, the water, with its head of eighty-four feet six inches, rushes past it, along a short length of horizontal pipe (in which is an exhaust valve, now closed), and begins to mount a vertical column or tube of cast-iron about ten feet high and two feet in diameter: the air in this column undergoes compression until it has reached a pressure sufficient to force open a valve in a pipe issuing from the summit of the tube, and connecting it with the recipient. This valve being already weighted with the pressure of the air compressed to six atmospheres by the means previously explained, a certain quantity of air is thus forced into the vessel; at this moment, another revolution of the main shaft causes the equilibrium valve at the bottom of the conducting-pipe to be shut, and at the same time opens the exhaust valve at the foot of the vertical column. The head of water being now cut off, and the exhaust open, the water in the vertical column begins to sink by its own gravity, leaving a vacuum behind it, if it were not for a small clack-valve opening inward in the upper part of the compressing column, which opens by the external pressure of the air, so that by the time all the water has passed out of the exhaust valve, the compressor is again full of atmospheric air; the valve in connection with the recipient being closed by the compressed air imprisoned in the vessel. The aereomotori continue their motion, another revolution of the main shaft shuts the exhaust and opens the equilibrium or admission valve; the column of water is again permitted to act, and the same action is repeated, more air being forced into the recipient at each round or pulsation of the machine. Now, supposing no consumption of the compressed air to take place beyond that used for driving the aereomotori, it seems evident that the water in the vessels would be gradually forced out, owing to the growing pressure of the air inside, above the pressure of the column of water coming from the higher cistern; but the communication with this higher cistern is always kept open, the column of water acting, in fact, as a sort of moderator or governor to the compressing-machine, rising or falling according to the consumption of the compressed air, and always insuring that there shall be a pressure of six atmospheres acting against the valve at the summit of the vertical column. A water-tube placed on the outside of each group of recipients, with a graduated scale marked on it, indicates at a glance the consumption of air. If the perforating-machines in the tunnel cease working, the pressure augments in the recipients, and the water in them falls until an equilibrium is established, [{68}] between the pressure of the column of water and the force of the compressors, until, in fact, these work without being able to lift the valve at the summit of the vertical compressing column. On the other hand, if more air than usual be used for ventilating the tunnel, or by an accidental leakage in the conducting-pipes, the water rises rapidly in the recipients, and consequently in the water-gauge outside, and in thus creating an equilibrium, indicates the state of things. By this means a continual compensation of pressure is kept up, which prevents any shock on the valves, and causes the machine to work with the regularity and uniformity of a steam-engine provided with a governor. In every turn of the main shaft, a complete circle of effects take place in the compressors; and experience has shown that three turns a minute of the shaft—that is, three pulsations of the compressing-machine per minute—are sufficient. It will thus be seen that a column of water, having the great velocity due to a head of eighty-four feet six inches, acts upon a column of air contained in a vertical tube; the effect of this velocity being to inject, as it were, a certain quantity of air into a recipient at each upward stroke of the column, and at each downward stroke drawing in after it an equivalent quantity of atmospheric air as a fresh supply. The ten recipients charged with air compressed to six atmospheres (ninety pounds on the square inch) in the manner above explained, serve as a reservoir of the force required for working the boring-engines in the tunnel, and for ventilating and purifying the gallery. The air is conducted in pipes about eight inches in diameter, having a thickness of metal of about three-eighths of an inch. Much doubt had previously been expressed as to the possibility of conveying compressed air to great distances without a very great and serious loss of power. The experience gained, however, at the Mont Cenis has shown that, conveyed to a distance of thirteen English miles, the loss would be but one-tenth of the original force; and that the actual measured loss of power in a distance of six thousand five hundred feet, a little more than a mile and a quarter, was less than 1-127th of the original pressure in the recipients.
The mouth of the tunnel is but a few hundred yards from the air-compressing house—we will now proceed thither. For nearly a mile in length the gallery is completed and lined with masonry. At the first view, we are struck with the bold outline of its section and its ample dimensions. Excepting, perhaps, the passage of an occasional railway-truck, laden with pieces of rock and rubbish, we find nothing to remind us of the numbers of busy workmen and of the powerful machines which are laboring in the tunnel. All is perfectly quiet and solitary. Looking around us as we traverse this first and completed portion, we observe nothing very different from an ordinary railway-tunnel, with the exception of the great iron pipe which conveys the compressed air, and is attached to the side of the wall. At the end of about a quarter of an hour we begin to hear sounds of activity, and little lights flickering in the distance indicate that we are approaching the scene of operations. In a few moments we reach the second division of the tunnel, or that part which is being enlarged from the comparatively small section made by the perforating-machine to its full dimensions, previously to being lined with masonry. In those portions where the workmen are engaged in the somewhat dangerous operation of detaching large blocks of stone from the roof, the tunnel is protected by a ceiling of massive beams, under which the visitor passes—not, however, without hurrying his pace and experiencing a feeling of satisfaction when the distance is completed. Gradually leaving behind us the bee-like crowd of busy miners, with the eternal ring of their boring-bars against the hard rock, we find the excavated gallery [{69}] getting smaller and smaller, and the difficulties of picking our way increasing at every step; the sounds behind us get fainter and fainter, and in a short time we are again in the midst of a profound solitude.