Part 18

The work of raising the tube to its position, 100 feet above high water mark, was a much slower operation, and was attended with serious difficulties. Hydraulic presses were used for the purpose, placed at the top of the piers; two smaller ones, which had served to raise the Conway Bridge, being at one end, and a much larger press, made for the occasion, being fixed at the other. The immense tube was lifted by chains fixed to the heads of the presses, and two steam engines, of 40-horse power each, were employed to force the water into the cylinders. The diameter of the ram of the largest hydraulic press was 20 inches, and the pressure upon it was equal to 2¼ tons on each circular inch. The tube was raised by successive lifts of 6 feet each, and, as it was lifted, the space was built in with masonry for its ultimate bearing. During the operation of lifting, the bottom of the cylinder of the large hydraulic press burst out, and fell on the top of the tube, in which it made a considerable indentation. Mr. Stephenson had provided against the possibility of such accident, by having blocks of wood, an inch thick, introduced under the tube as it was elevated, and these blocks arrested its fall, or it would otherwise have been dashed to pieces. Even the small fall of an inch did considerable injury. This accident caused some delay, but the other tubes were in the meantime progressing, and the completed bridge was opened for public traffic on the 21st of October, 1850.

The strength of the bridge was tested before passenger trains were allowed to pass through it, by placing in the centre of the longest tubes twenty-eight waggons, loaded with 280 tons of coal, and two locomotives, and by afterwards sending those heavy trains through the bridge at full speed. The deflection of the tubes in the centre amounted to only three-quarters of an inch in each cell; it being rather less when the trains were at full speed than when stationary. The strongest gusts of wind to which the bridge has been exposed have not caused a vibration of more than one inch. The total cost of construction was £601,865; of which sum £3,986 was for experiments, and £158,704 for masonry.

Another Tubular Bridge of rival magnitude to the one across the Menai Straits is now in the course of construction by Mr. Brunel across the Tamar, at Saltash, for the South Devon and Cornwall Railway. As no rock presented itself conveniently halfway across whereon to erect the central pier, Mr. Brunel was obliged to work at a great depth below the surface of the water in making the foundation of the Royal Albert Bridge. In the plan of making the foundation, as well as in the structure of the bridge itself, Mr. Brunel adopted a course altogether original. Instead of attempting to construct a coffer-dam by piles, which would have been almost impracticable at such a depth, and very costly, he caused a large iron tube to be put together, thirty-six feet in diameter, and ninety-six feet long, to reach to the bed of the river. This monster tube was lowered perpendicularly in the middle of the river, and the water being pumped out of it, the men could work at the bottom in safety. In this manner, after much labour, the rock was prepared to receive the blocks of granite, which were laid one on the other, till they rose above the surface of the water. On that granite pedestal a cast-iron pier was raised to a height of 100 feet, the level of the roadway of the rails.

The cast-iron pier consists of four octagon columns, 10 feet in diameter. They stand about 10 feet apart, forming a square, and they are bound together by massive lattice-work of wrought iron, to prevent any lateral movement. Each of these columns weighs 150 tons; and when the full weight of the bridge rests on the foundation of the central pier, the pressure will be equal to 8 tons on the square foot, or double the pressure of the Victoria Tower on its base.

In the structure of the bridge, Mr. Brunel availed himself of the results of the experiments made by Mr. Fairbairn on the strength of iron tubes, but he adopted a very different plan from that of Mr. Stephenson. Instead of constructing a large tube for the trains to pass through, Mr. Brunel made tubular arches, consisting of iron plates curved and riveted together, to serve as rigid supports, from which the roadway is suspended by chains and by connecting iron bars.

The placing of the first of the tubular arches in position between the pier near the shore at Saltash and the central pier, which took place on the 1st of September, 1857, excited great interest, and at least 50,000 persons were assembled from places far and near to witness the operation. The tube, with the roadway and suspension chains, was floated from the yard where it was put together on four pontoons; and it was thus conveyed, and safely deposited on the piers at a height of 30 feet above high water mark. It was afterwards gradually raised by hydraulic presses to the top, a height of 100 feet. The work of raising it commenced on the 25th of November, and was completed on the 19th of May last.

The following lively description of the Royal Albert Bridge, and its surrounding scenery, extracted from a recent article in the _Times_, gives a very good idea of the magnitude of the structure, by comparison with well-known objects:--"Though, probably, our readers may care little and have heard less about Saltash proper, it is likely henceforth to receive a fair share of general attention, and we can safely say, to those who will journey down to see the bridge, that the viaduct requires indeed to be a fine one to attract their attention from the lovely scenery of the valley of the Tamar, which it crosses. The banks of this noble river narrow in considerably as the stream reaches Saltash, and, hemmed in there to half a mile or so, suddenly widens out into as fine a sheet of water as any of its kind in the kingdom, its distant banks covered with cottages, and fringed with undulating woodlands down to the very edge. Across this narrow part of the channel, where Saltash, in picturesque dirt and disarray, straggles up the banks on one side, and a steep hill, covered with rock and rock-grown underwood, forms the other, the viaduct stretches high in air. The briefest general way of describing it is to say that it consists of nineteen spans or arches, seventeen of which are wider than the widest arches of Westminster Bridge; and two, resting on a single cast-iron pier of four columns in the centre of the river, span the whole stream at one gigantic leap of 910 feet, or a longer distance than the breadth of the Thames at Westminster. The total length of the structure from end to end is 2,240 feet,--very nearly half a mile, and 300 feet longer than the entire stretch of the Britannia Bridge. The greatest width is only 30 feet at basement; its greatest height from foundation to summit no less than 260 feet, or 50 feet higher than the summit of the Monument. The Britannia Bridge, both in size, purpose, and engineering importance, seems to offer the best comparison with that of Saltash, but the similarity between the structures is far from being as great as might be at first supposed. The Britannia tube is smaller, and cost nearly four times the price of the Saltash Viaduct, though the engineers had natural facilities which Mr. Brunel, for his Cornish bridge, certainly had not."

The form of the tubes is an oval, 17 feet in its longest diameter, and 12 feet in its shortest. They are bent into an elliptical curve, with a rise in the middle of twenty-eight feet. With the roadway and suspension chains attached, each tube weighs 1,100 tons. The total weight of wrought iron in the bridge, when completed, will be 2,650 tons; of cast iron, 1,200; of masonry and brickwork there will be about 17,000 cubic yards; and of timber, about 14,000 cubic feet.

The second tube, which is in every respect like the first, was completed on the 30th of June last, and on the 10th of July was successfully placed in position between the central pier and the Devonshire side of the river. The operation of elevating it began on the 9th of August, and it has now reached nearly the level of the first one, the tube being raised six feet in a week.

The engraving on the other side is a view of this wonderful structure in its completed form. Its appearance is far more light and elegant than that of the Britannia Bridge, but it remains to be seen whether it will be equally steady under a gale of wind, and whether any vibration of the suspended roadway will interfere with the rapid motion of the trains. As the South Devon Railway has only one line of rails for the greater portion of its length, but a single roadway is provided on the Royal Albert Bridge.

The progress of railway locomotion has not only given rise to the construction of new kinds of bridges, but it has directed mechanical science to devise better means of applying the strength of materials. On the South Devon and Cornwall Railways are to be seen wooden viaducts, carrying the line over valleys at great heights, constructed with such slender timbers, that, to an inexperienced eye, they seem frightfully frail for the support of heavy railway trains.

We must not omit to notice, among the remarkable bridge erections connected with railways, the viaduct across the valley of the Boyne, which passes over the river close to the town of Drogheda, at a height of 95 feet. The central portion of the viaduct is supported on four piers, 90 feet above high water mark, with a span in the centre of 250 feet, and on each side of 125 feet. This elevated portion of the work is approached on the southern side by twelve arches, of 60 feet span each, and on the north by three similar arches. The viaduct is constructed of limestone and iron lattice-work, and is calculated to bear 7,200 tons.

During the erection of this viaduct the railway trains were carried over the valley on a wooden platform, without side railings, supported by scaffold-poles; and the crackling of the timbers, as the carriages passed over it, and the dizzy height at which they were carried through the air, produced a sensation of terror in nervous passengers, that was fully justified by the apparent danger.

SELF-ACTING ENGINES.

The manufacturing progress of this country has depended, in a great degree, on the facility possessed of making machinery of all kinds by the aid of powerful engines worked by steam power. These engines, most of which appear to be self-acting, forge and roll and cut and bore beams of iron, boiler plates, and cylinders of immense size, which it would be impossible to make by hand; and they do the work with a rapidity and mechanical accuracy that would be otherwise unattainable. In the progress of manufacturing invention, the small steam engine first made by manual labour created the power to make other steam engines of large size; and those more powerful engines supplied the means of making still larger shafts and cylinders for engines that were to be employed in the construction of machines of various kinds, to be worked by the power thus accumulated.

The important advantages derived from the invention and application of self-acting machinery, not only by the community at large, but even by the workmen whose labour they for a time superseded, were forcibly stated by Mr. Whitworth, in his opening address at the Institution of Mechanical Engineers, in September, 1856:--"I congratulate you," he observed, "on the success which in our time the mechanical arts have obtained, and the high consideration in which they are held. Inventors are not now persecuted, as formerly, by those who fancied that their inventions and discoveries were prejudicial to the general interest, and calculated to deprive labour of its fair reward. Some of us are old enough to remember the hostility manifested to the working of the power-loom, the self-acting mule, the machinery for shearing woollen cloth, the thrashing machine, and many others. Now the introduction of reaping and mowing machines, and other improved agricultural machinery, is not opposed. Indeed, it must be obvious, to reflecting minds, that the increased luxuries and comforts which all more or less enjoy, are derived from the numerous recent mechanical appliances and the productions of our manufactories. That of our cotton has increased during the last few years in a wonderful degree. In 1824, a gentleman with whom I am acquainted sold on one occasion 100,000 pieces of 74-reed printing cloth at 30s. 6d. per piece of 29 yards long; the same description of cloth he sold last week at 3s. 9d. One of the most striking instances I know of the vast superiority of machinery over simple instruments used by hand, is in the manufacture of lace, when one man, with a machine, does the work of 8,000 lace makers on the cushion. In spinning fine numbers of yarn, a workman in a self-acting mule will do the work of 3,000 hand-spinners with the distaff and spindle.

"Comparatively few persons, perhaps, are aware of the increase of production in our life-time. Thirty years ago, the cost of labour for turning a surface of cast iron, by chipping and filing with the hand, was 12s. per square foot--the same work is now done by the planing machine at a cost for labour of less than one penny per square foot: and this, as you know, is one of the most important operations in mechanics; it is, therefore, well adapted to illustrate what our progress has been. At the same time that this increased production is taking place, the fixed capital of the country is, as a necessary consequence, augmented; for in the case I have mentioned, of chipping and filing by the hand, when the cost of labour was 12s. per foot, the capital required for tools for one workman was only a few shillings; but now, the labour being lowered to a penny per foot, a capital in planing machines for the workman is required which often amounts to £500, and in some cases more."

Notwithstanding the great economy of labour by the self-acting machines now employed for doing all kinds of work, it is gratifying to find that it has not had the effect of throwing men out of employ; for the increased demand, consequent on the facility of production, has more than compensated for the substitution of automaton mechanism for handicraft.

It is extremely interesting to visit a large engineering factory, and to witness the ease with which the masses of crude metal are wrought in various ways, and converted by a number of seemingly self-acting engines into other engines and machines which are, in their turn, to become the agents of the further development of the skill and ingenuity of man. In the new Government factory at Keyham, near Devonport, which we believe to be one of the largest establishments of the kind in the world, most of those powerful engines of the best construction may be seen in operation. The completeness of the arrangements redounds much to the credit of Mr. Trickett, the chief engineer, under whose supervision they were made; and a walk through the factory, which is thrown open to public inspection, will well repay a journey of many miles. A detailed description of all its machinery would fill a volume, but we must now limit ourselves to a bare enumeration of some of the most remarkable features.

Numerous machines of the largest size, placed under the cover of an extensive and lofty roof, are employed in doing everything requisite for the fitting out of the largest steam-ships in the British navy. Shears, put in continuous motion by steam power, are seen moving steadily up and down, and cutting through the thickest boiler plates without the least apparent effort, the chisel-shaped knives that cut the metal moving just the same whether they be dividing the air or shearing iron. Punching engines, in like manner, force holes through iron plates an inch thick. Shaping and planing machines pare off the tough iron as if it were not harder than cheese. Riveting machines of different kinds bind together the plates of monster boilers with marvellous rapidity; whilst machines for boring, for drilling, for forging, and for doing every variety of smaller work, are to be seen in operation in various parts of the factory.

Among the smaller self-acting engines, the forging machine for making bolts attracts attention by the rapidity of its action. It consists of a series of hammers placed side by side, so constructed as to shape small bars of iron into any required form, according to the mould of the swages beneath them, representing miniature anvils. It is interesting to watch how readily the hot iron receives its shape under the action of the hammers, which make about 700 strokes per minute, the work being transferred from one to another to be progressively finished. There is a circular saw that cuts through bars of iron as thick as railway rails, by making upwards of 1,000 revolutions per minute. A rivet-making machine forms the rivet, and shapes the head to the requisite size, with great accuracy and quickness. There are compound drilling machines, in which six drills are acting simultaneously; hydraulic presses, that force parts of machines together, and a great variety of other engines for the saving of time and labour.

Not the least curious of the smaller contrivances is an apparatus which deserves notice as a useful application of magnetism to manufacturing purposes. Several horse-shoe magnets are attached to two endless chains, moving over suitable wheels, and inclined at an angle of 30 degrees. These magnets at the lower end of the chain, dip into a tub containing the mixed brass and iron turnings and filings from the lathes and other tools, and the pieces of iron, being attracted by the magnets, are carried away and brushed off into a box, leaving the brass behind to be remelted.

In one department of the building are immense foundry furnaces, where metals are melted and cast, the blast of the fires being maintained by large rotating fans, kept in action by a powerful steam engine, by which also the other machines are worked. The foundry is most conveniently contrived for casting works of any required size, fixed and travelling cranes being so stationed and arranged as to carry the ladles of liquid metal to any part of the floor.

In another department is the smithy, where the iron to be wrought into shape is heated in forges; and near to the forges stand the Steam-Hammers--those gigantic Cyclops of modern times, that strike blows, compared with the force of which the blows of the fabled Cyclops of antiquity were but as the fall of a feather.

Ranged in a row there are four of these ponderous engines, of various sizes; the largest hammer being so heavy as to require the power of four tons to lift it, and when falling from a height of 6 feet nothing can withstand its crushing blow. Yet the force of this mighty giant is so completely under control, and may be brought to act so gently, as scarcely to crack a nut placed to receive its fall.

The invention of the steam-hammer was the result of necessity. The shaft of a steam engine having to be made larger than usual, no hammer then in action by water power was capable of forging it, and Mr. James Nasmyth was applied to, to give his aid in contriving the means of removing the difficulty. It was then that the idea of lifting the hammer-block by the direct action of steam occurred to him, and by a succession of extremely ingenious devices, he at length perfected the steam-hammer, which has been pronounced to be one of the most perfect artificial machines, and one of the noblest triumphs of mind over matter that modern English engineers have yet developed.

The accompanying woodcut represents the largest of the four steam-hammers in Keyham factory. The hammer-block, _a_, weighing four tons, is guided in its ascent and fall by grooves in two massive uprights, which hold the whole together. The hammer-block is lifted by the piston-rod of the steam cylinder above it, which is made of such diameter, that the pressure of the steam on the surface of the piston may considerably overbalance the weight of the hammer-block, and overcome the friction of the connecting mechanism. The cylinder of the largest steam-hammer at Keyham is 18 inches diameter, which gives an area of 254 square inches; and the pressure of the steam generally used being fifty pounds on the square inch, the total steam pressure tending to force the piston up, when the whole of it is brought to bear, is equal to five tons and a half. The force of the blow of the hammer, when falling from its greatest height, is equal to 144 tons.

By the arrangements of levers, screws, and pipes and valves, shown in the engraving, the steam is first admitted under the piston, and thus acts directly in forcing it up, with the heavy hammer-block attached to the piston rod. When the block has been raised to the required height, it strikes against the end of a lever, which then shuts off the steam, and allows it to escape; whereupon the hammer falls with its full force vertically on the anvil. The end of the lever which turns off the steam may be adjusted at any height, according to the required force of the blow, so that the hammer may fall from a height of six feet, or be merely raised a few inches.

The steam-hammer, in the early stages of its invention, required an attendant to turn on the steam again at the end of each stroke, but Mr. Nasmyth ingeniously contrived the means of rendering the engine altogether self-acting, by causing the force of the collision to release a spring that holds down the slide-valve; and by this contrivance a continued and regular succession of blows is maintained without any assistance.

Not only can the force of the blow be regulated by the height to which the hammer is lifted, but the ponderous mass may be arrested in its descent by admitting the steam under the piston, so that a skilful manipulator can stop it within the eighth of an inch from the anvil.

The Steam Engine itself, by which all the self-acting mechanisms of a large factory are put in motion, is, perhaps, after all, the most wonderful of inventions; but it does not strictly come within our province, for Watt had perfected his great work before the close of the last century. It was, however, not much used, excepting for mining purposes, until after the commencement of the present; and the inventor himself had but a faint idea of the value and vast importance of the motive power he had placed at the command of man. So little, indeed, was the value of steam power appreciated in the early years of its application, that no notice is taken of the steam engine in Beckmann's History of Inventions, though Watt had completed his condensing engines several years before that work was published; and Newcomen's steam engine had been at work at least sixty years.

The history of the steam engine affords a striking example of the gradual development of an invention from vague and chimerical notions, into an accomplished fact of astonishing magnitude. As in the electric telegraph the dreams of the alchemist are fully realized by the applications of scientific discovery, so in the wonder-working powers of the steam engine one of the visionary schemes sketched in the "Century of Inventions" is practically extended far beyond the conceptions of its fanciful projector. How little could Beckmann have supposed that an invention, which he considered too insignificant to be mentioned, would, in the course of fifty years, have revolutionized the world! It may possibly be the same, before this century is closed, with inventions that are now neglected or despised.

* * * * *