Volume 1 — 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.

The little gallery in which we are now stumbling our way over blocks of stone and rubbish, only varied by long tracts of thick slush and pools of water, is the section excavated by the boring-machine—in dimension about twelve feet broad by eight feet high. The tramway which has accompanied us all the way is still continued along this small section. In the middle portion underneath the rails is the canal, inclined toward the mouth of the tunnel, for carrying off the water; and in this canal are now collected the pipes for conveying the compressed air to the machines, and the gas for illuminating the gallery. At the end of a few minutes, a rattling, jingling sound indicates that we are near the end of our excursion, and that we are approaching the perforating-machines. On arriving, we find that nearly the whole of the little gallery is taken up by the engine, the frame of which, mounted upon wheels, rests upon the main tramway, so that the whole can be moved backward or forward as necessary. On examining the arrangement a little closely, we find that in reality we have before us nine or ten perforators, completely independent of one another, all mounted on one frame, and each capable of movement in any direction. Attached to every one of them are two flexible tubes, one for conveying the compressed air, and the other the water which is injected at every blow or stroke of the tool into the hole, for the purpose of clearing out the debris and for cooling the point of the "jumper." In front, directed against the rock, are nine or ten tubes (according to the number of perforators), very similar in appearance to large gun-barrels, out of which are discharged with great rapidity an equal number of boring-bars or jumpers. Motion is given to these jumpers by the direct admission of a blast of compressed air behind them, the return stroke being effected by a somewhat slighter pressure of air than was used to drive them forward. We will suppose the machine brought up for the commencement of an attack. The points most convenient for the boring of the holes having been selected, the nine or ten perforators, as the case may be, are carefully adjusted in front of them. The compressed air is then admitted, and the boring of the holes commences. On an average, at the end of about three-quarters of an hour, the nine or ten holes are pierced to a depth of two feet to two feet six inches. Another ten holes are then commenced, and so on, until about eighty holes are pierced. The greater number of these holes are driven toward the centre of the point of attack, and the rest round the perimeter. The driving of these eighty holes to an average depth of two feet three inches, is usually completed in about seven hours, and the second operation is then commenced.

The flexible tubes conveying the compressed air and the water are detached from the machines, and placed in security in the covered canal. The perforating-machine, mounted on its frame or truck, is drawn back on the tramway behind two massive folding-doors of wood. Miners then advance and charge the holes in the centre with powder, and adjust the matches; fire is given, and the miners retire behind the folding-doors, which are closed. The explosion opens a breach in the centre part of the front of attack. Powerful jets of compressed air are now injected, to clear off the smoke formed by the powder. As soon as the gallery is clear, the other holes in the perimeter are charged and fired, and more air is injected. Then comes the third operation. Gangs of workmen advance and clear away the debris and blocks of stone detached by the explosion of the mine, in little wagons running on a pair of rails placed by the side of the main tramway. This done, the main line is {70} prolonged to the requisite distance, and the perforating engine is again brought forward for a fresh attack. Thus, we have three distinct operations—first, the mechanical perforation of the holes; secondly, the charging and explosion of the mine; and thirdly, the clearing away of the debris. By careful registers kept since the commencement of the work, it is found that the mean duration of each successive operation is as follows: for the perforation of the holes, seven hours thirty-nine minutes; for the charging and explosion of the mine, three hours twenty-nine minutes; for the clearing away of the debris, two hours thirty-three minutes; or, in all, nearly fourteen hours. Occasionally, however, the three operations may be completed in ten hours, all depending upon the hardness of the rock. It has been found practically more expeditious to make two series of operations in twenty-four hours.

Whatever may be the nature of the rock, if it is very hard, the depth of the holes is reduced; that is, the perforation is only continued for a certain given time—about six and a half hours—which, for the eighty holes with ten perforaters, gives us about three-quarters of an hour for each hole. The rock is generally of calcareous schist, crystallized, and exceedingly hard, traversed by thick veins of quartz, which often break the points of the boring-tools after a few blows. Each jumper gives about three blows per second, and makes one-eighteenth of a revolution on its axis at each blow, or one complete revolution every six seconds. Thus, in the three-quarters of an hour necessary to drive a single hole to the depth of twenty-seven inches, we have four hundred and fifty revolutions of the bar, and eighteen hundred violent blows given by the point against the hard rock, and that under an impulse of about one hundred and eighty pounds. These figures will give us some idea of the wear and tear of the perforating-machines. It is calculated that on an average one perforating-machine is worn out for every six metres of gallery, so that more than two thousand will be consumed before the completion of the tunnel. The total length completed at the Bardonnêche side at the present time is just two thousand three hundred metres, or nearly a mile and a half.

At the north or Modane end, the mechanical perforators are precisely the same as at Bardonnêche, as also is the system of working in the gallery. The machinery for the compression of air, however, is very different, more simple, and in every way an improvement upon that at the South end. Not finding any convenient means of obtaining a head of eighty-four feet of water sufficient in quantity for working a series of compressors, as at Bardonnêche, there has been established at Modane a system of direct compression, the necessary force for which is derived from the current of the Arc. Six large water-wheels moved by this current give a reciprocating motion to a piston contained in a large horizontal cylinder of cast iron. This piston, having a column of water on each side of it, raises and lowers alternately these two columns, in two vertical tubes about ten feet high, compressing the air in each tube alternately, and forcing a certain quantity, at each upward stroke of the water, to enter into a cylindrical recipient. There is very little loss of water in this machine, which in its action is very like a large double-barreled common air-pump. It is a question open to science whether the employment of compressed air for driving the perforating engines in a work such as is in operation at the Mont Cenis, could not be advantageously and economically exchanged for the employment of a direct hydraulic motive force, the ventilation of the tunnel being provided for by other means. The system, however, employed at Modane has many advantages, which it is impossible to overlook, and its complete success has given a marked and decided impulse to the modern science of tunnelling through hard rock.

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