Part 43

Air has also been made the medium for conveying intelligence in another manner than by shooting written messages through tubes, for its property of transmitting pressure has been applied to produce at a distance signals like those made use of in the electric telegraph system. A few years ago, an apparatus for this object was contrived by Signor Guattari, whose invention is known as the “Guattari Atmospheric Telegraph.” In this there is a vessel charged with compressed air by a compression-pump, and the pressure is maintained by the same means, while the reservoir is being drawn upon. A valve is so arranged that the manipulator can readily admit the compressed air to a tube extending to the station where the signals are received, at which the pressure is made to move a piston as often as the sender opens the valve. This movement is made to convey intelligence when a duly regulated succession of impulses is sent into the tube—the receiving apparatus being arranged either to give visible or audible signals, or to print them on slips of paper, according to any of the methods in use with the electric telegraph. Certain advantages over the electric system are claimed for this pneumatic telegraph—as, for example, greater simplicity and less liability to derangement. The tubes, which are merely leaden piping of small bore, are also exempt from the inconvenient interruptions which electric communication sometimes suffers from electrical disturbances in the atmosphere. The pneumatic system is easily arranged, and from its great simplicity any person can in a few hours learn to use the whole apparatus, while it is calculated that the expense of construction and working would not be above half of that incurred for the electric system. For telegraphs in houses, ships, warehouses, and short lines, this invention will doubtless prove very serviceable; but for long lines a much greater force of compression would be required, and the time needed for the production of an impulse at the distant ends of the tubes would be considerably increased. [1875].

ROCK BORING.

Allusion has already been made to one great characteristic of our age, namely, the replacement, in every department of industry, of manual labour by machines. A brief notice of even the main features of the various contrivances which have been made to take the place of men’s hands would more than occupy this volume. Accordingly, we must omit all reference to many branches of manufacture, although the products may be of very great utility, and the processes of very high interest; and in taking one example here and another there, we must be guided mainly by the extent and depth of the influence which the new invention appears destined to exert. This consideration has, with scarcely an exception, decided the selection of the topics already discussed, and it has also determined the introduction of the present article, which relates to machines of no less general importance than the rest, although at first sight it might seem to enter upon the details of merely a special branch of industry. But so general are the interests connected with the subject we are about to lay before our readers, that we are not sure it would not have been more logical to have placed the present article before all the rest. For whence comes the iron of which our steam engines, tools, rails, ships, cannon, bridges, and printing presses are made?—whence comes the fuel which supplies force to the engines?—whence come, in fine, the substances which form the _matériel_ of every art? Plainly from the earth—the nurse and the mother of all, and in most cases from the bowels of the earth, for her treasures are hidden far below the surface—the coal, and the ores of iron and other metals, are not ready to our hand, exposed to the light of day. The railways also, and the canals, can be made only on condition that we cut roads through the solid rocks, and pierce with tunnels the towering mountains. Hence the tools which enable us to penetrate into the substance of the earth present the highest general interest from a practical point of view, and this interest is enhanced by the knowledge of the structure and past history of our planet acquired in such operations.

The operations by which solid rocks are penetrated in the sinking of shafts for mines, or in the driving of tunnels, drifts, headings, galleries, or cuttings for railways, mines, or other works, are easily understood. In the first place a number of holes—perhaps 3 ft. or 4 ft. deep and 2 in. or 3 in. in diameter—are formed in the rock. The holes are then charged with gunpowder or other explosive materials, a slow-burning match is adjusted, the miners retire to a safe distance, the explosion takes place—detaching, shattering, and loosening masses of the rock more or less considerable; and then gangs of workmen clear away the stones and _débris_ which have been detached by the explosion, and the same series of operations is renewed. The holes for the blasting charges are formed by giving repeated blows on the rock with a kind of chisel called a _jumper_—the end of which is formed of very hard steel, so that the rock is in reality chipped away. The _débris_ resulting from this operation is cleared away from time to time by a kind of auger or some similar contrivance. But for many purposes it is necessary to drill holes in rocks to great depths, hundreds of feet perhaps, as for example, in order to ascertain the nature of underlying strata, or to verify the presence of coal or other minerals before the expense of sinking a shaft is incurred. These bore-holes were commonly formed in exactly the same manner as the blast-holes already mentioned, by repeated blows of a chisel or jumper, which was attached to the end of a rod; and as the hole deepened, additional lengths of rod were joined on, and the rods were withdrawn from time to time to admit of the removal of the _débris_ by augers, or by cylinders having a valve at the bottom. The reciprocating movement is given to the chisels and rods either by hand or by steam or water power. When the length of the rods becomes considerable, of course the difficulty of giving the requisite blows in rapid succession is greatly increased, for the whole length of rods has to be lifted each time, and if allowed to fall with too much violence, the breaking of the chisel or the rods is the inevitable result. The time requisite for drawing out the rods, removing the fragments chipped out, and again attaching the rods and lowering, also increases very much as the bore gets deeper. Messrs. Mather and Platt, the Manchester engineers, have, in order to obviate these difficulties, constructed machines in which the chipping or cutting is done by the fall of a tool suspended from a rope, the great advantage resulting from the arrangement being the facility and rapidity with which the tools used for the cutting and for the removal of the _débris_ are lowered to their work and drawn up. It is necessary in using the jumper, whether in cutting blast-holes or bore-holes, to give the tool a slight turn after each blow, in order that the rock may be chipped off all round, and the action of the tool equalized. Many attempts have been made to drill rocks after the fashion in which iron is drilled—that is, by drilling properly so called, in which the tool has a rapid rotary motion. But even in comparatively soft rock, it is found that no steel can sufficiently withstand the abrading action of the rock, for the tool becomes quickly worn, and makes extremely slow progress. We shall have presently to return to the subject of bore-holes; but now let us turn our attention to an example which will illustrate the nature and advantages of the machinery which has in recent times been applied to work the jumpers by which the holes for blasting are formed.

_THE MONT CENIS TUNNEL._

The successful construction, by the direction of Napoleon, of a broad and easy highway from Switzerland into Italy, crossing the lofty Alps amid the snows and glaciers of the Simplon, has justly been considered a feat of skill redounding to the glory of its designers. But we have recently witnessed a greater feat of engineering skill, for we have seen the Alps conquered by the stupendous work known as the Mont Cenis Tunnel. This tunnel is 7½ English miles in length; but it is not the mere length which has made the undertaking remarkable. The mountain which is pierced by the tunnel is formed entirely of hard rock, and what added still more to the apparently impracticable character of the proposal when first announced was the circumstance that it was quite impossible to sink vertical shafts, so that the work could not, as in the usual process, be carried on at several points simultaneously, but must necessarily be continued from the two extremities only, a restriction which would occasion a vast loss of time and much expense, to say nothing of the difficulties of ventilating galleries of more than three miles in length. The reader must bear in mind that the importance of this question of ventilation depends not simply on the renewing of the air contaminated by the respiration of the workmen, but on the quick removal of the noxious gases produced in the explosions of the blasting charges. A work surrounded by such difficulties would probably have never been attempted had not Messrs. Sommeiller and Co. invited the attention of engineers to an engine of their invention, worked by compressed air, and capable of automatically working “jumpers” which could penetrate the hardest rock. These rock-boring machines, having been examined by competent authorities in the year 1857, were pronounced so efficient that the execution of the long-spoken-of Alpine tunnel was at once resolved upon, and before the close of that year the work had actually been commenced, after a skilful and accurate survey of the proposed locality had been made, and the direction of the tunnel set out. The tunnel does not pass through Mont Cenis, although the post road from St. Michel to Susa passes over part of Mont Cenis, which gives its name to the pass. The mountain really pierced by the tunnel is known as the Grand Vallon, and the tunnel passes almost exactly below its summit, but at a depth the perpendicular distance of which is as nearly as possible one mile. The northern end of the tunnel is near a village named Fourneaux.

Pending the construction of the Sommeiller machines, and other machinery which was to supply the motive force, the work of excavation was commenced at both ends, in 1857, in the ordinary manner, that is, by hand labour, and in 1858 surveys of the greatest possible accuracy were meanwhile made, in order that the two tunnels might be directed so that they would meet each other in the heart of the mountain. The reader will at once perceive that the smallest error in fixing on the direction of the two straight lines which ought to meet each other would entail very serious consequences. The difficulties of doing this may be conceived when we remember that the stations were nearly 8 miles apart, separated by rugged mountains, in a region of snows, mists, clouds, and winds, over which the levels had to be taken, and a very precise triangulation effected. So successfully were these difficulties overcome, and so accurately were the measurements and calculations made, that the junction of the centre lines of the completed tunnel failed by only a _few inches_, a length utterly insignificant under the conditions.

The work was carried on by manual labour only, until the beginning of 1861, for it was found, on practically testing the machinery, that many important modifications had to be made before it could be successfully employed in the great work for which it was designed. After the machinery had been set to work, at the Bardonnêche end, breakages and imperfections of various parts of the apparatus, or the contrivances for driving it, caused delay and trouble, so that during the whole of 1861 the machines were in actual operation for only 209 days, and the progress made averaged only 18 in. per day, an advance much less than could have been effected by manual labour. The engineers, not disheartened or deterred by these difficulties and disappointments, encountered them by making improvement after improvement in the machinery as experience accumulated, so that a wonderful difference in the rate of progress showed itself in 1862, when the working days numbered 325, and the average rate of advance was _three feet nine inches per day_.

At the Fourneaux extremity more time was required for the preparation of the air-compressing machinery, and the machines had been at work in the other extremity, with more or less interruption, for nearly two years before the preparations at Fourneaux were completed.

The illustration at the head of this article, Fig. 178, represents the Sommeiller machines at work, the motive power being compressed air, conveyed by tubes from receivers, into which it is forced until the pressure becomes equal to that of six atmospheres, or 90 lbs. per square inch. The compression was effected by taking advantage of the natural heads of water, which were made to act directly in compressing the air; the pressure due to a column of water 160 ft. high being made to act upwards, to compress air, and force it through valves into the receivers; then the supply of water was cut off, and that which had risen up into the vessel previously containing air was allowed to flow out, drawing in after it through another valve a fresh supply of air; and then the operations were repeated by the water being again permitted to compress the air, and so on, the whole of the movements being performed by the machinery itself. The compressed air, after doing its work in the cylinders of the boring tools, escaped into the atmosphere, and in its outrush became greatly cooled, a circumstance of the greatest possible advantage to the workmen, for otherwise, from the internal warmth of the earth, and that produced by the burning of lights, explosions of gunpowder, and respiration, the heat would have been intolerable. At the same time, the escaping air afforded a perfect ventilation of the workings while the machines were in action. At other times, as after the explosion of the charges, it was found desirable to allow a jet of air to stream out, in order that the smoke and carbonic acid gas should be quickly cleared away. Even had the work been done by manual labour alone, a plentiful supply of compressed air would have been required merely for ventilation, so that there was manifest advantage in utilizing it as the motive power of the machines.

The experience gained in the progress of the work suggested from time to time many improvements in the machinery and appliances, which finally proved so effectual that the progress was accelerated beyond expectation. At the end of 1864, when the machines had been in work about four years, it was calculated that the opening of the tunnel might be looked for in the course of the year 1875. But in point of fact it happened that on the 25th December, 1870, perforator No. 45 bored a hole from Italy into France, by piercing the wall of rock, about 4 yards thick, which then separated the workings from each other. The centre lines of the two workings, as set out from the different sides of the mountain, failed to coincide by only a foot, that set out on the Fourneaux side being this much higher than the other, but their horizontal directions exactly agreeing. The actual length of the tunnel was found to be some 15 yards longer than the calculated length, the calculation having given 7·5932 miles for the length, whereas by actual measurement it was found to be 7·6017 miles. The heights above the sea-level of the principal points are these:

Feet. Fourneaux, or northern entrance 3,801 Bardonnêche, or southern entrance 4,236 Summit of tunnel 4,246 Highest point of mountain vertically over the tunnel 9,527

The tunnel is lined with excellent brick and stone arching, and it is connected with the railways on either side by inclined lines, which are in part tunnelled out of the mountain, so that the extremities of the tunnel referred to above are not really entered by the trains at all; but these lateral tunnels join the other and increase the total distance traversed underground to very nearly 8 miles, or more accurately, 7·9806 miles. The time required by a train to pass from one side to the other is about 25 minutes. What a contrast is this to the old transit over the Mont Cenis pass by “diligence”! We have the scene depicted in Fig. 179, where we perceive, sliding down or toiling up the steep zigzag ascents, a series of curious vehicles drawn by horses with perpetually jingling bells.

The cost of the Mont Cenis Tunnel was about £3,000,000 sterling, or upwards of £200 per yard; but as a result of the experience gained in this gigantic work, engineers consider that a similar undertaking could now be carried out for half this cost. It is supposed that the profit to the contractors for the Mont Cenis Tunnel was not much less than £100 per yard. The greatest number of men directly employed on the tunnel at one time was 4,000, and the total horse-power of the machinery amounted to 860. From 1857 to 1860, by hand labour alone, 1,646 metres were excavated; from 1861 to 1870 the remaining 10,587 metres were completed by the machines. The most rapid progress made was in May, 1865, in which month the tunnel was driven forward at one end the length of 400 feet. When the workings were being carried through quartz, a very hard rock, the speed was greatly reduced—as, for example, during the month of April, 1866, when the machines could not accomplish more than 35 ft.

The perforators used in the Mont Cenis Tunnel were worked by compressed air, conveyed to a small cylinder, in which it works a piston, to the rod of which the jumper is directly attached. The air, being admitted behind the piston, impels the jumper against the rock, and the tool is then immediately brought back by the opening of a valve, which admits compressed air in front of the piston, at the same time that the air which has driven it forward is allowed to escape, communication with the reservoir of compressed air having previously been closed behind it. The whole of these movements are automatic, and they are effected in the most rapid manner, four or five blows being struck in every second, or between two and three hundred in one minute. Water was constantly forced into the holes, so as to remove the _débris_ as quickly as it was formed. A number of these machines were mounted on one frame, supported on wheels, running on the tramway which was laid along the gallery. The perforators had no connection with each other, for each one had its own tube for the conveyance of compressed air, and its own tube to carry the water used for clearing out the hole, and the cylinders were so fixed on the frames that the jumpers could be directed in any desired manner against any selected portion of the rock. They were driven to an average depth of about 2½ ft., and the process occupied from forty to fifty minutes. When a set of holes had thus been formed, the cylinders were shifted and another series commenced, until about eighty holes had been bored, the formation of the whole number occupying about six or seven hours, and the holes being so arranged that the next operation would detach the rock to the required extent. The flexible tubes, which conveyed the air and water to the machines from the entrances, were then removed from the machines and stowed away, the frame bearing the perforators was drawn back along the tramway, workmen advanced whose duty it was to wipe out the holes, charge them with powder, and fix the fuses ready for the explosion. When the slow-burning match was ignited, all retired behind strong wooden barricades, at a safe distance, until the explosion had taken place; and after the compressed air had been allowed to stream into the working, so as to clear away all the smoke and gas generated by the explosion, the workmen ran up on a special tramway the waggons which were to carry away all the detached stones; and when this had been done, the floor was levelled, the tramways were lengthened, and the frame bearing the drilling machines was brought up to begin a fresh series of operations, which were usually repeated about twice in the course of every twenty-four hours. A great part of the rock consists of very hard calcareous schist, interspersed with veins of quartz, one of the hardest of all rocks, which severely tries the temper of the steel tools, for a few blows on quartz will not unfrequently cause the point of a jumper to snap off.

_ROCK-DRILLING MACHINES._

Several forms of rock-drills, or perforators, have been constructed on the same principle as that used in the Mont Cenis Tunnel, and a description of one of them will give a good notion of the general principle of all. We select a form devised by Mr. C. Burleigh, and much used in America, where it has been very successfully employed in driving the Hoosac Tunnel, effecting a saving in the cost of the drilling amounting to one-third of the expense of that operation, and effecting also a still greater saving of time, for the tunnel, which is 5 miles in length, is to be completed in four years, instead of twelve, as the machines make an advance of 150 ft. per month, whereas the rate by hand labour was only 49 ft. per month. These machines are known as the “Burleigh Rock Drills,” and have been patented in England for certain improvements by Mr. T. Brown, who has kindly supplied us with the following particulars: