Chapter XX: Introduction (18)

The means which is afforded by hydraulic power of applying enormous pressures has been taken advantage of in a great many of the arts, of which, indeed, there are few that have not, directly or indirectly, benefited by this mode of modifying force. An illustration, taken at random, may be found in the machinery employed at Woolwich for making elongated rifle-bullets. The bullets are formed by forcing into dies, which give the required shape, little cylinders of solid lead, cut off by the machine itself from a continuous cylindrical rod of the metal. The rod, or rather filament, of lead is wound like a rope on large reels, from which it is fed to the machine. It is in the production of this solid leaden rope or filament that hydraulic pressure is used. About 4 cwt. of melted lead is poured into a very massive iron cylinder, the inside of which has a diameter of 7½ in., while the external diameter is no less than 2 ft. 6 in., so that the sides of the cylinder are actually 11¼ in. thick. When the lead has cooled so far as that it has passed into a half solid state, a ram or plunger, accurately fitting the bore of the cylinder, is forced down by hydraulic pressure upon the semi-fluid metal. This plunger is provided with a round hole throughout its entire length, and as it is urged against the half solidified metal with enormous pressure, the lead yields, and is forced out through the hole in the plunger, making its appearance at the top as a continuous cylindrical filament, quite solid, but still hot. This is wound upon the large iron reels as fast as it emerges from the opening in the plunger, and these reels are then taken to the bullet-shaping machine, which snips off length after length of the leaden cord, and fashions it into bullets for the Martini-Henry rifle. The leaden pipes which are so much used for conveying water and gas in houses are made in a similar manner, metal being forced out of an annular opening, which is formed by putting an iron rod, having its diameter of the required bore of the pipe, in the middle of the circular opening. The lead in escaping between the rod and the sides of the opening takes the form of a pipe, and is wound upon large iron reels, as in the former case.

FIG. 167.—_Section of Hydraulic Lift Graving Dock._
]

FIG. 168.—_Section of Column._
]

Another interesting application of hydraulic power is to the raising of ships vertically out of the water, in order to examine the bottoms of their hulls, and effect any necessary repairs. The hydraulic lift graving dock, in which this is done, is the invention of Mr. E. Clark, who, under the direction of Mr. Robert Stephenson, designed the machinery and superintended the raising of the tubes of the Britannia Bridge, where a weight of 1,800 tons was lifted by only three presses. The suitability of the hydraulic press for such work as slowly raising a vessel was doubtless suggested to him in connection with this circumstance, and the durability, economy, and small loss of power which occurs in the action of the press, pointed it out as particularly adapted for this purpose. The ordinary dry dock is simply an excavation, lined with timber or masonry, from which the tide is excluded by a gate, which, after the vessel has entered the dock at high water, is closed; and when the tide has ebbed, and left the vessel dry, the sluice through which the water has escaped is also closed. In a tideless harbour the water has to be pumped out of the dock, and this last method is also adopted even in tidal waters, so that the docks may be independent of the state of the tides. The lift of Clark’s graving dock is a direct application of the power of the hydraulic press, and we select for description the graving dock constructed at the Victoria Docks for the Thames Graving Dock Company, whose works occupy 26 acres. Fig. 167 is a transverse section of this hydraulic lift graving dock, in which there are two rows of cast iron columns, 5 ft. in diameter at the base, where they are sunk 12 ft. in the ground, and 4 ft. in diameter above the ground. The clear distance between the two rows is 60 ft., and the columns are placed 20 ft. apart from centre to centre, sixteen columns in each row, thus giving a length of 310 ft. to the platform, but vessels of 350 ft. in length may practically be lifted. The bases of the columns, one of which is represented in section in Fig. 168, are filled with concrete, on which the feet of the hydraulic cylinders rest. The outer columns support no weight, but act merely as guides for the crossheads attached to the plungers. The height of the columns is 68½ ft., and a wrought iron framed platform connects the columns at the top. In order that any inequalities in the height of the rams may be detected, a scale is painted on each column, to mark the positions of the crossheads. The hydraulic cylinders, which are within these columns, have solid rams of 10 in. diameter, with a stroke of 25 ft., and on the tops of these are fastened the crossheads, 7½ ft. long, made of wrought iron, and supporting at the ends bars of iron, to the other ends of which the girders of the platform are suspended. The girders are, therefore, sixteen in number, and together form a gridiron platform, which can be raised or lowered with the vessel upon it. The thirty-two hydraulic cylinders were tested at a pressure of more than 3 tons per square inch. The water is admitted immediately beneath the collars at the top (this being the most accessible position) by pipes of only ½ in. diameter, leading from the force-pumps, of which there are twelve, of 1⅞ in. diameter, directly worked by a fifty horse-power steam engine. The presses are worked in three groups—one of sixteen, and two of eight presses,—so arranged that their centres of action form a sort of tripod support, and the presses of each group are so connected that perfect uniformity of pressure is maintained. The raising of a vessel is accomplished in about twenty-five minutes, by placing below the vessel a pontoon, filled in the first instance with water, and then raising the pontoon with the vessel on it, while the water is allowed to escape from the pontoon through certain valves; then when the girders are again lowered, the pontoon, with the vessel on it, remains afloat. Thus in thirty minutes a ship drawing, say, 18 ft. of water is lifted on a shallow pontoon, drawing, perhaps, only 5 ft., and the whole is floated to a shallow dock, where, surrounded with workshops, the vessel, now high and dry, is ready to receive the necessary repairs. The number of vessels which can thus be docked is limited only by the number of pontoons, and thus the same lift serves to raise and lower any number of ships, which are floated on and off its platform by the pontoons. With a pressure in the hydraulic cylinders of about 2 tons upon each square inch, the combined action of these thirty-two presses would raise a ship weighing 5,000 tons.

Hydraulic power has been used not only for graving docks, as shown in the above figures, but also for dragging ships out of the water up an inclined plane. The machinery for this purpose was invented by Mr. Miller for hauling ships up the inclined plane of “Martin’s slip,” at the upper end of which the press cylinder is placed, at the same slope as the inclined plane, and the ship is attached, by means of chains, to a crosshead fixed on the plunger. Hydraulic power has also been used for launching ships, and the launch of the _Great Eastern_ is a memorable instance; for the great ship stuck fast, and it was only by the application of an immense pressure, exerted by hydraulic apparatus, that she could be induced to take to the water. Water pressure is also applied to hoists for raising and lowering heavy bodies, and in such cases the pressure which is obtained by simply taking the water supply from an elevated source, or from the water-main of a town, is sometimes made use of, instead of that obtained by a forcing pump. The lift at the Albert Hall, South Kensington, by which persons may pass to and from the gallery without making use of the stairs, is worked by hydraulic pressure in the manner just mentioned. In such lifts or hoists there is a vertical cylinder, in which works a leather-packed piston, having a piston-rod passing upwards through a stuffing-box in the top of the cylinder. The upper end of the piston-rod has a pulley of 30 in. or 36 in. diameter, attached to it, and round this pulley is passed a chain, one end of which is fixed, and the other fastened to the movable cage or frame. So that the cage moves with twice the speed of the piston, and the length of the stroke of the latter is one-half of the range of the cage.

Sir William Armstrong has applied hydraulic power to cranes and other machines in combination with chains and pulleys. His hydraulic crane is represented by the diagram, Fig. 169, intended to show only the general disposition of the principal parts of this machine, which is so admirably arranged that one man can raise, lower, or swing round the heaviest load with a readiness and apparent ease marvellous to behold. Here it is proper to mention once for all, that the pressure for the hydraulic machines is obtained not only by natural heads of water, or by forcing-pumps worked by hand, but very frequently by forcing-pumps worked by steam power. It is usual to have a set of three pumps with their plungers connected respectively with three cranks on one shaft, making angles of 120° with each other. A special feature of Sir W. Armstrong’s hydraulic crane is the arrangement by which the engines are made to be always storing up power by forcing water into the vessel, _a_, called the “accumulator.” The accumulator—which in the diagram is not shown in its true position—may be placed in any convenient place near the crane, and consists of a large cast iron cylinder, _b_, fitted with a plunger, _c_, moving water-tight through the neck of the cylinder. To the head of the plunger is attached by iron cross-bars, _d d_, a strong iron case filled with heavy materials, so as to load the plunger, _c_, with a weight that will produce a pressure of about 600 lbs. upon each square inch of the inner surface of the cylinder. The water is pumped into the cylinder by the pumping engines through the pipe, _f_, and then the piston rises, carrying with it the loaded case, guided by the timber framework, _g_, until it reaches the top of its range, when it moves a lever that cuts off the supply of steam from the pumping engine. When the crane is working the water passes out of the cylinder, _a_, by the pipe, _h_, and exerts its pressures on the plungers of the smaller cylinders; and the plunger of the accumulator, in beginning its descent again, moves the lever in connection with the throttle-valve of the engine, and thus again starts the pumps, which therefore at once begin to supply more water to the accumulator. The latter is, however, large enough to keep all the several smaller cylinders of the machine at work even when they are all in operation at once. Fig. 169 shows a sketch elevation and a ground plan of the crane as constructed to carry loads of 1 ton, but the size of the cylinders is somewhat exaggerated, and all details, such as pipes, guides, valves, rods, &c., are omitted. The hydraulic apparatus is entirely below the flooring—only the levers by which the valves are opened and closed appearing above the surface. The crane-post, _i_, is made of wrought iron: it is hollow and stationary; the jib, _k_, is connected with the ties, _l_, by side-pieces, _n_, which are joined by a cross-piece at _m_, turning on a swivel and bearing the pulley, _u_. The jib and the side-pieces are attached at _o_ to a piece turning round the crane-post, and provided with a friction roller, _p_, which receives the thrust of the jib against the crane-post; the same piece is carried below the flooring and is surrounded with a groove, which the links of the chain, _q_, fit. This chain serves to swing the crane round, and for this purpose the hydraulic cylinders, _r_, _r´_, come into operation. The plungers of these have each a pulley, over which passes the chain _q_, having its ends fastened to the cylinders, so that when, by the pressure of the water, one plunger is forced out, the other is pushed in, and the chain passing round the groove at _s_ swings the jib round. The cylinders are supplied with water by pipes—omitted in the sketch, as are also those by which the water leaves the cylinders. These pipes are connected with valves—also omitted on account of the scale of the diagram being too small to show their details—so that the movement of a lever, _t_, in one or the other direction at the same time connects one cylinder with the supply and the other with the exit-pipe. When the crane is swinging round, the sudden closing of the valves would produce an injurious shock, and to prevent this relief-valves are provided on both the supply and exit-pipes communicating with each cylinder. When, therefore, the valves are closed, the impetus of the jib and its load acting on the chain, and through that on the plungers, continues to move the latter, the motion is permitted to take place by the relief-valves opening, and allowing water to enter or leave the cylinders against the pressure of the water. There is also a self-acting arrangement by which, when these plungers have moved to the extent of their range in either direction, the valves are closed. The chain of the crane rests on guide pulleys, and passing over the pulley _u_, goes down the centre of the crane-post to the pulley _v_, and thence passes backwards and forwards over a series of three pulleys at _w_ and two at _x_, and is fastened at its end to the cylinder, _y_. As there are thus six lines of chain, when the plunger of the lifting cylinder comes 1 ft. out, 6 ft. of chain pass over the guide pulley, _u_. The plunger, when near the end of its stroke in either direction, is made to move a bar—not shown—which closes the valve. When the crane is loaded, the load is lowered by simply opening the exhaust-valve, when the lift-plunger will be forced back into its cylinder by the pull on the chain. But as the chain may require to be lowered when there is no load upon it, although a bob is provided at _z_ to draw the chain down, it would be disadvantageous to increase the weight of this to the extent required for forcing back the lifting plunger. A _return_ cylinder is therefore made use of, the plunger of which has but a small diameter, and is connected with the head of the lift-plunger, so that it forces the latter back when the lift-cylinder is put in communication with the exhaust-pipe. The water is admitted to the lifting cylinder from the accumulator by a valve worked by a lever, which, when moved the other way, closes the communication and opens the exhaust-pipe, and then the pressure in the return cylinder, which is constant, drives in the plunger of the lifting cylinder. The principle of the accumulator may plainly be used with great advantage even when manual labour is employed, for a less number of men will be required for working the pumps to produce the effect than if their efforts had to be applied to the machine only at the time it is in actual operation, for in the intervals they would, in the last case, be standing idle. Apparatus on the same plan has been used with advantage for opening and shutting dock gates, moving swing bridges, turn-tables, and for other purposes where a considerable power has to be occasionally applied.

FIG. 169.—_Sir W. Armstrong’s Hydraulic Crane._
]

FIG. 170.—_Raising Tubes of the Britannia Bridge._
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FIG. 171.—_Press for Raising the Tubes._
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FIG. 172.—_Head of Link-Bars._
]

A famous example of the application of hydraulic power was the raising of the great tubes of the Britannia Bridge. As already stated, the tubes were built on the shore, and were floated to the towers. This was done by introducing beneath the tubes a number of pontoons, provided with valves in the bottom, so as to admit the water to regulate the height of the tube according to the tide. The great tubes were so skilfully guided into their position that they appeared to spectators to be handled with as much ease as small boats. The mode in which they were raised by the hydraulic presses wall be understood from Fig. 170, where A is one of the presses and C the tube, supported by the chains, B. The tubes were suspended in this manner at each end, and as the great tubes weighed 1,800 tons, each press had, therefore, to lift half this weight, or 900 tons. The ram or plunger of the pump was 1 ft. 8 in. in diameter, and the cylinder in which it worked was 11 in. thick. Two steam engines, each 40 horse-power, were used to force the water into the cylinders. These cylinders were themselves remarkable castings, for each contained no less than 22 tons of iron. Notwithstanding the great thickness of the metal, an unfortunate accident occurred while the plungers were making their fourth ascent, for the bottom of one of the cylinders gave way—a piece of iron weighing nearly a ton and a half having been forced out, which, after killing a man who was ascending a rope ladder to the press, fell on the top of the tube 80 ft. below, and made in it a deep indentation. The accident occasioned a considerable delay in the progress of the work, for a new cylinder had to be cast and fitted. Such an accident would assuredly have caused the destruction of the tube itself but for the foresight and prudence of the engineer in placing beneath the ends of the vast tube as it ascended slabs of wood 1 in. thick, so that it was impossible for the tube to fall more than 1 in. It must be stated that as the tube was lifted each step, the masonry was built up from below, and then as the next lift proceeded inch by inch, a slab of wood was placed under the ends. Although by the giving way of the cylinder of the hydraulic press the end of the tube fell through no greater space than 1 in., the momentum was such that beams calculated to bear enormous weights were broken. At the time of the accident the pressure in the cylinder did not exceed that which it was calculated to bear or that which is frequently applied in hydraulic presses for other purposes. Some scientific observers attributed the failure of the cylinder to the oscillating of the tube. It had been found when the similar tubes of the bridge over the Conway were being raised, that when the engines at each end made their strokes simultaneously, a dangerous undulation was set up in the tube, and it was therefore necessary to cause the strokes of the engines to take place alternately. The chains by which the tubes were suspended were made of flat bars 7 in. wide and about 1 in. thick, being rolled in one piece, with expanded portions about the “eye,” through which the connecting-bolts pass. The links of the chain consisted of nine and eight of these bars alternately—the bars of the eight-fold links being made a little thicker than those of the nine-fold, so as to have the same aggregate strength. The mode in which the hydraulic presses were made to raise the tubes is very clearly described by Sir William Fairbairn in his interesting work on the Conway and Britannia Bridges, and his account of the mode of raising the tubes is here given in his own words, but with letters referring to Fig. 171: “Another great difficulty was to be overcome, and it was one which presented itself to my mind with great force, viz., in what manner the enormous weight of the tube was to be kept suspended when lifted to the height of 6 ft., the proposed travel of the pump, whilst the ram was lowered and again attached for the purpose of making another lift. Much time was occupied in scheming means for accomplishing this object, and after examining several projects, more or less satisfactory, it at last occurred to me that, by a particular formation of the links (of the chain by which the tubes were to be suspended) we might make the chains themselves support the tube. I proposed that the lower part of the top of each link, immediately below the eye, should be formed with square shoulders cut at right angles to the body of the link (Fig. 172). When the several links forming the chain E were put together, these shoulders formed a bearing surface, or “hold,” for the crosshead B attached to the top of the ram A of the hydraulic pump. But the upper part of this crosshead, C C, was movable, or formed of clips, which fitted the shoulders of the chain, and were worked by means of right- and left-handed screws, and could be made either to clip the chain immediately under the shoulders when the ram of the pump was down and a lift about to be made, or be withdrawn at pleasure. Attached to the large girders F were a corresponding set of clips, D D, which were so placed and adjusted as to height that when the ram of the pump was at the top there was distance between the two sets of clips equal to twice the length of the travel of the pump, or the length of the two sets of the links of the chain. To render the action of the apparatus more clear, suppose the tube resting on the shelf of masonry in the position that it was left in after the operation of floating was completed, and the chains attached, and everything ready for the first lift, the ram of the pump being necessarily down. The upper set of clips attached to the crosshead are forced under the shoulders of the links, and the lower set of clips attached to the frames resting upon the girders are drawn back, so as to be quite clear of the chain; the pumps are put into action simultaneously at both ends of the tube, and the whole mass is slowly raised until it has reached a height of 6 ft. from its original resting-place. The clips attached to the crosshead, B, have so far been sustaining the weight, but it will be observed that by the time the pump has ascended to its full travel, the square shoulders of another set of links have come opposite to the lower clips on the girders, D, and these clips are advanced under the shoulders of the links, and the rams being allowed to descend a little, they in their turn sustain the load and relieve the pumps. The upper clips being withdrawn, the rams are allowed to descend, and after another attachment, a further lift of 6 ft. is accomplished; and thus, by a series of lifts, any height may be attained. The fitness of this apparatus for its work was admirable, and the action of the presses was, as Mr. Stephenson termed it, delightful.”

FIG. 173.—_Apparatus to prove Transmission of Pressure in all
directions._
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FIG. 174.—_Pneumatic Tubes and Carriages._
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PNEUMATIC DISPATCH.

When the use of the electric telegraph became general, it was found necessary to establish in all large towns branch stations, from which messages were conveyed to the central station, or to which they were sent, either by messengers who carried the written despatch, or by telegraphing between the central and branch stations. The latter had the disadvantages of rendering the original message liable to an additional chance of incorrect transmission, and when an unusually great number of despatches had to be sent to or from a particular branch station, there was necessarily great delay in the forwarding of them. The plan of sending the written messages between the central stations by bearers was unsatisfactory on account of the time occupied. These inconveniences led to the invention of a system for propelling, by the pressure of air, the papers upon which the messages were written through tubes connecting the stations. This was first carried into practice by the Electric and International Telegraph Company, who, in this way, connected their central station in London with their City branch stations. The apparatus was designed and erected by Mr. L. Clark and Mr. Varley in 1854. The first tube laid down was from Lothbury and the Stock Exchange—a distance of 220 yards. This tube had an inside diameter of only 1½ in.; but a larger tube, having a diameter of 2¼ in. was, some years afterwards, laid between Telegraph Street and Mincing Lane—a distance of 1,340 yards—and was used successfully. In these tubes the carriers were pushed forward by the pressure of the atmosphere, a vacuum having been produced in front by pumping out the air. The plan of propelling the carrier by compressing the air behind it was also tried with good results, and, in fact, with a gain of speed; for, while a carrier occupied 60 or 70 seconds in passing from Telegraph Street to Mincing Lane when drawn by a vacuum, it accomplished its journey in 50 or 55 seconds when it was shot forwards by compressed air, the difference in pressure before and behind it being the same in each case. A great deal of trouble was occasioned when the vacuum system was used, by water being drawn in at the joints of the pipes. This water sometimes accumulated to such a degree, especially after wet weather, that it completely overcame the power of the vacuum to draw the air through it, by lodging in the vertical portions of the tube, where they passed to the upper floors of the central station. This was remedied by improving the construction of the joints, and by arranging a syphon for drawing off any water which might be present. The best construction of the carrier was another matter which required some experience to discover. It was found that gutta-percha, or papier maché covered with felt, was the most efficient material. The tubes found by Mr. Varley to give the best results were formed of lead covered externally with iron pipes. The joints were made perfectly smooth in the inside by means of a heated steel mandrel, on which they were formed, so that the tube was of one perfectly uniform bore throughout. An ingenious arrangement was also adopted by which the air itself was made to do the work of opening and closing the valves, and even that of removing the carrier from the tube: when, by a telegraphic bell, rung from the distant station, it was announced that a carrier was dispatched, the attendant at the receiving station had only to touch for a second a knob marked “receive,” which put the tube in communication with the vacuum, in which condition it remained until the arrival of the carrier, which, by striking against a pad of india-rubber, released the detent, and thus cut off the vacuum. The carrier then fell out of the receiver and dropped into a box placed to catch it. When a carrier was sent, it was placed in the tube, and a button marked “send” was touched, by which a communication was opened with a vessel of compressed air and the end of the tube behind the carrier was immediately closed by a slide, the movements being all performed by the air itself. On the arrival of the carrier, the boy at the receiving station rang an electric bell to signal its reception; and the sender then touched another knob marked “cut off,” which caused the supply of compressed air to be cut off, and the slide to be withdrawn from the end of the tube, which was then ready either to receive or send carriers. By this arrangement there was no waste of power, for the reservoirs of compressed air or of vacuum were only drawn upon when the work was actually required to be done.

The tubes laid down by the Telegraph Company are still in active operation; but at the new Central Telegraph Station the automatic valves of Messrs. Clark and Varley appear to be dispensed with, and the attendants perform the work of closing the tube, shutting off the compressed air, &c., by a few simple movements.

In December, 1869, Messrs. Siemens were commissioned by the Postmaster-General to lay tubes on their system from the General Post Office to the Central Telegraph Station; and the work having been accomplished in February, 1870, and proving perfectly satisfactory after six weeks’ trial, it was decided to connect in the same manner Fleet Street and the West Strand office at Charing Cross with the Central Station. The system proposed by the Messrs. Siemens consisted in forming a circuit of tubes, through which the carriers might be continually passing in one direction. The diagram, Fig. 175, will give an idea of the manner in which it was designed to arrange the tubes between the Central Telegraph Station and Charing Cross. The arrows indicate the direction in which the air rushes through the tubes; A is the piston in the cylinder, and valves are so arranged as to pump air out of the chamber V, and compress it into the chamber P. This plan has been departed from, so far as regards the Charing Cross Station, for want of space there prevented the tube being curved with a radius large enough to convey the carriers without their being liable to stick, and consequently, these are not carried round in the tube. The passage of carriers being stopped here, there are, in point of fact, two tubes: an “up” tube and a “down” tube. But these are connected by a sharp bend, so that though the tube is continuous as regards the air current, it is interrupted as regards the circulation of the carriers. The tubes are of iron, 3 in. internal diameter, made in lengths of about 19 ft.; and for the turns and bends, pieces are curved with a radius of 12 ft. Both lines are laid side by side in a trench at about a foot depth below the streets. The ends of the adjacent lengths form butt joints, so that the internal surface is interrupted as little as possible, and there is a double collar to fasten the lengths together. Arrangements are also made for removing from the inside of the tubes water or dirt, or matter which may in any manner have got in.

FIG. 175.—_Diagram of Tubes, &c._
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FIG. 176.—_Sending and Receiving Apparatus.—Transverse Section._
]

One special feature of Messrs. Siemens’ invention is the plan by which the carriers are introduced into and removed from the tube at any required station without the circulation of the air being interfered with. The simple yet ingenious mechanism by which this is effected will be understood from the sections shown in Figs. 176 and 177. The figures represent the position of the apparatus when placed to receive a carrier; A´ is the receptacle into which the carrier is shot by the air rushing from A towards A´´. This receptacle is ᗜ-shaped, the curve of the Ⅾ corresponding with that of the tube, and the upper flat part admitting of a piece of plate glass being inserted, through which the attendant may perceive when a carrier arrives. The progress of the carrier is arrested by a perforated plate, B, which allows the air to pass. The ends of this receptacle are fixed in two parallel plates, F F´, which also receive the ends of the plain cylinder, having precisely the same diameter as the tube, A. These plates are connected also by cross-pieces, D E, the whole forming a sort of frame, which turns upon E as a centre; and according as it is put in the position shown by the plain line in Fig. 176, or in that indicated by the dotted lines, causes the receiving tube or the hollow cylinder to form part of the main tube, the cross-piece, D, serving as a handle for moving the apparatus. It should be remarked that the plates are made to fit the space cut out of the main tube with great nicety, otherwise much loss of power would result from leakage. When the hollow cylinder is in a line with the main tube, it is plain that the carrier will not be stopped, as the tube is then continuous and uninterrupted. In this hollow cylinder also the carrier to be sent is deposited after the rocking frame has been placed on it, Fig. 177; then, on drawing the handle, the hollow cylinder is brought into the circuit, and the carrier at once shoots off. To stop a carrier, the receiving-tube is put in by another movement of the handle, and when the carrier arrives, it is removed by bringing the open cylinder, or _through tube_, into the circuit, and thus making the receiver ready for having the carrier pushed out of it by a rod which is made to slide out by moving a handle. In order to avoid the obstruction to the movement of the air which would be caused by the carrier while in the receiving-tube, a pipe, G, is provided, through which the air chiefly passes when the perforations of the plate, B, are closed by the presence of a carrier. In this pipe at H is a throttle-valve, which is opened by tappets, K, on the rocking frames when the receiver is in circuit, and again closed when the open tube is substituted. The current thus suffers no interruption by the action of the apparatus.

FIG. 177.—_Receiving Apparatus.—Longitudinal Section._
]

The carriers are small cylinders of gutta-percha, or papier maché, closed at one end, and provided with a lid at the other. They are covered with felt or leather, and at the front they are furnished with a thick disc of drugget or leather, like the leathers of a common water-pump, but fitting quite loosely in the tube. Such a carrier, being placed in the tube at the Central Station, Fig. 175, will be carried by the current in the direction of the arrows to the Charing Cross Station, where its progress will be interrupted; but according to the original plan it would continue its journey until it again reached the Central Station, where it would be intercepted by the diaphragm, Fig. 175. But the carrier is stopped, if at any station the receiving-tube is placed in circuit, and this is done when an electric signal indicates to the station that a carrier intended for it has been dispatched. The tubes are worked on the “block system,” that is, each section is known to be clear before a carrier is allowed to enter it, and a bell is provided, which is struck by a little lever, moved by each carrier in its passage through, so that the attendant at each station knows when a carrier has shot along the “through tube” of the station. This mode of working the tubes renders the liability to accidents much less, but their carrying power might be increased by dispatching carriers at regular and very short intervals of time, when the limit would be only in the ability of the attendants to receive a carrier and open the circuit in sufficient time to allow the next following one to proceed without stoppage. The length of the lines of tube laid down on this system, with the times required for the carriers to traverse them, are stated below, the pressure and the vacuum being respectively equal to the absolute pressures of 22 lbs. and 5½ lbs. on each square inch of the reservoirs during the experiments:

┌────────────────────────────────────────┬──────┬──────┐
│ │Yards.│M. S. │
│Telegraph Station to General Post Office│ 852│ 1 54│
│General Post Office to Temple Bar │ 1,206│ 2 28│
│Temple Bar to General Post Office │ 1,206│ 2 10│
│General Post Office to Telegraph Station│ 852│ 1 13│
│ │ —————│ —————│
│ │ 4,116│ 7 45│
└────────────────────────────────────────┴──────┴──────┘

When the air was not compressed, but the vacuum only was used, the air being allowed to enter the other end of the tube at the ordinary atmospheric pressure, the time required for the carrier to traverse the circuit was 10 minutes 23 seconds. In this case the vacuum was maintained, so that the air was constantly in movement; but when the experiment was tried by allowing the air in the tube to become stationary, placing a carrier at one end, and then opening communication with the vacuum reservoir at the other, the carrier required 13½ minutes to complete the journey. This is explained by the fact of the greater part of the air having to be exhausted from the tube before the carrier could be set in motion.

The utility and advantage of the pneumatic system is well seen when its powers are compared with the wires. Thus, a single carrier, which may contain, say, twenty-seven messages, can be sent every eight minutes; and since not more than one message per minute could be transmitted by telegraph wire, even by the smartest clerks, the real average being about two minutes for each message, it follows that only four messages could be sent in the time required for a single carrier to traverse the up tube, and to do the work which could be done by the tube seven wires and fourteen clerks would be required.

Mr. R. S. Culley, the official telegraph engineer, states as his experience of the relative wear and tear of the carriers in these iron tubes and in the smooth lead tubes, that it had been found necessary to renew the felt covering of eighty-two dozen of the carriers used for three months in the iron tubes, while in the same period only thirty-eight dozen of those used in the lead tubes required to be recovered. The numbers of carriers sent and received by the pneumatic tubes on the 21st of November, 1871, between 11 a.m. and 4 p.m., were:

Iron tubes │ 135

2¼ in. lead tubes 1,170│
│1,697
1½ in. lead tubes 527│

The mileage of the carriers sent was much greater in the lead than in the iron pipes, although the total lengths of each kind were respectively 5,974 yards and 6,826 yards. The result is remarkable, as showing the effect of apparently slight differences when their operation is summed up by numerous repetitions.

The circuit at Charing Cross having been divided on account of the difficulty mentioned above, the tubes act as separate pipes—one for “up” traffic (_i.e._, to Central Telegraph Station), the other for “down” (_i.e._, from the Central Station). The air, however, still accomplishes a circuit, being exhausted at one end and compressed at the other. A very noticeable and curious difference is found between the times required by the carriers to perform the “up” and the “down” journeys:

An “up” carrier requires 6·5 minutes
A “down” carrier requires 12·5 minutes
————
Together 19·0 minutes

When two pipes were separated at Charing Cross so that the air no longer circulated from one to the other, but both were left open to the atmosphere, while the “up” pipe was worked by a vacuum only and the “down” pipe by pressure only, the times were for

An “up” carrier 8·5 minutes
A “down” carrier 11·3 minutes
————
Together 19·8 minutes

The time, therefore, for the whole circuit was practically the same—whether the tubes were worked by a continuous current of air or separated, and one worked by the vacuum and the other by pressure. It was also seen that when the tubes were connected so that the air current was continuous, and the pump producing a vacuum at one end and a compression at the other, the neutral point where the pressure was equal to that of the atmosphere was not found midway between the two extremities—that is, at Charing Cross Station—but much nearer the vacuum end. When the tubes were disconnected, it appeared, as already shown by the figures given above, that there was a gain of speed on the down journey, and a loss of speed on the up journey; and as the requirements of the traffic happened to require greater dispatch for the down journeys, the tubes have been worked in this manner.

It has been proposed to convey letters by pneumatic dispatch between the General and Suburban Post Offices, and the Post Office authorities have even consulted engineers on the practicability of sending the Irish mails from London to Holyhead by this system. It was calculated, however, that although the scheme could be carried out, the proportion of expense for great speeds and long distances would be enormously increased. A speed of 130 miles per hour was considered attainable, but the wear and tear of the carriers would be extremely great at this high velocity, and it was considered doubtful whether this circumstance might not operate seriously against the practical carrying out of the plan. The prime cost would be very great, for the steam power alone which would be requisite would amount to 390 horse-power for every four miles. We thus see that very high velocities would introduce a new order of difficulties in the practical working. The case as regards the velocity with which electric signals can be sent round the world is very different.

An amusing hoax appears to have been perpetrated by some waggish telegraph clerk on an American gentleman at Glasgow, with regard to the pneumatic system of sending messages; for the gentleman sent to the “Boston Transcript” a letter, in which he relates that having sent a telegraphic message from Glasgow to London, he received in a few minutes a reply which indicated a mistake somewhere, and then he went to the Glasgow telegraph office, and asked to see his message.

“The clerk said, ‘We can’t show it to you, as we have sent it to London.’ ‘But,’ I replied, ‘you must have my original paper here. I wish to see that.’ He again said, ‘No, we have not got it: it is in the post office at London.’ ‘What do you mean?’ I asked. ‘Pray, let me see the paper I left here half an hour ago.’ ‘Well,’ said he, ‘if you must see it, we will get it back in a few minutes, but it is now in London.’ He rang a bell, and in five minutes or so produced my message, rolled up in pasteboard.... I inquired if I might see a message sent. ‘Oh, yes; come round here.’ He slipped a number of messages into the pasteboard scroll, popped it into the tube, and made a signal. I put my ear to the tube and heard a slight rumbling noise for seventeen seconds, when a bell rang beside me, indicating that the scroll had arrived at the General Post Office, 400 miles off. It almost took my breath away to think of it.”

In the journal called “Engineering,” into which this curious letter was copied, it is pointed out that to travel from London to Glasgow, a distance of 405 miles, in seventeen seconds, the carrier must have moved at the rate of 24 miles per second, or 5 miles a second faster than the earth moves in its orbit, and the carrier would have in such a case become red hot by its friction against the tube before it had travelled a single second.

A plan of conveying, not telegraph messages, but parcels, was proposed and carried into effect some time ago, and more recently has been applied to lines of tubes in connection with the General Post Office. These tubes pass from Euston Station down Drummond Street, Hampstead Road, Tottenham Court Road, to Broad Street, St. Giles’s, whence, with a sharp bend, they proceed to the Engine Station at Holborn, and then to the Post Office. The tube is formed chiefly of cast iron pipes of a ⌓-shaped section, 4 ft. 6 in. wide and 4 ft. high, in 9 ft. lengths. There are curves with radii of 70 ft. and upwards, and at these parts the tube is made of brickwork and not of iron. The carriages run on four wheels, and are so constructed that the ends fit the tubes nearly, and the interval left is partly closed by a projecting sheet of india-rubber all round. The carriages are usually sent through the tube in trains of two or three, and the trains are drawn forward by an exhausting apparatus formed by a fan, 22 ft. in diameter, worked by two horizontal steam engines having cylinders 24 in. in diameter and a stroke of 20 in. The air rushes by centrifugal force from the circumference of the fan, and is drawn in at the centre, where the exhaust effect is produced. The tubes which convey the air from the main tube open into the latter at some distance from its extremities, which are closed by doors, so that after the carriage passes the entrance of the suction tube, its momentum is checked by the air included between it and the doors, which air is, of course, compressed by the forward movement of the carriage. At the proper moment the doors are opened by a self-acting arrangement, and the carriage emerges from the tube. There are two lines of tube—an “up” and a “down” line—and means are provided for rapidly transferring the carriages from one to the other at the termini. The time occupied in the transit is about 12 minutes. Some of the inclines have as much slope as 1 in 14, yet loads of 10 or 12 tons weight are drawn up these gradients without difficulty. The mails are sent between Euston Station and the Post Office by means of these tubes. Passengers have also made the journey as an experiment by lying down in the carriages. Fig. 174 shows one of the carriages and the entrance to the tubes.

Great expectations have been formed by some persons of the applications of pneumatic force. Some have suggested its use for moving the trains in the proposed tunnel between England and France. But calculations show that for long distances and large areas such modes of imparting motion are enormously wasteful of power. Thus, in the tunnel alluded to it must be remembered that not only the train, but the whole mass of air in the tunnel would have to be drawn or pushed forward. The drawing of a train through by exhausting the air would be very similar to drawing it through by a rope; in fact, the mass of air may be regarded as a very elastic rope, but by no means a very light one, or one that could be drawn through without some opposing force which has a certain resemblance to friction coming into operation. Indeed, it has been calculated that in the case named, only five per cent. of the total power exerted by the engines in exhausting the air could possibly produce a useful effect in moving the train.

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].

FIG. 178.—_The Sommeiller Boring Machines._
]

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.