The Wonderful Lamp.
"Know the great genius of this land
Has many a light aërial band,
Who all, beneath his high command,
Harmoniously,
As arts or arms they understand,
Their labours ply."—Burns.
Genii, afrits, and ghouls, have long since lost their terrors, but the wonderful stories told about them will continue to charm the youthful mind for centuries to come. Chief among these stories is that of Aladdin, the poor boy, who became the fortunate possessor of a wonderful lamp, which gave him control over a powerful race of genii. By merely rubbing the lamp he summoned these superhuman servants, who waited on him hand and foot, brought him untold wealth, transported him from place to place, and fulfilled his wildest desires. Upon this beautiful Arabian romance we ground our concluding fairy tale of science.
Our wonderful lamp is merely a poetical image of Science. The lamp of science dispels intellectual darkness, and floods the world with its all-penetrating light. The night-prowling ghouls, Ignorance and Superstition, dare not encounter its glancing rays, and descend shrieking into the abyss, while Industry toils in the glare, and seems to acquire new vigour whenever the flame increases in brilliancy.
The attendant genii of this wonderful lamp are those powers of the material world which have been subjugated by man—the Aladdin of our story.
Among these genii the almost omnipotent agent Steam ranks first. The miracles wrought by this slave of the lamp transcend all the wonders conceived by the Oriental romancists. “By its agency,” says Dr. Lardner, “coal is made to minister in a variety of ways to the uses of society. By it coals are taught to spin, weave, dye, print, and dress silks, cottons, woollen and other cloths; to make paper, and print books on it when made; to convert corn into flour; to press oil from the olive and wine from the grape; to draw up metal from the bowels of the earth; to pound and smelt it, to melt and mould it, to forge it, to roll it, and to fashion it into every form that the most wayward caprice can desire. Do we traverse the deep, they lend wings to the ship, and bid defiance to the natural opponents, the winds and the tides! Does the windbound ship desire to get out of port to start on her voyage, steam throws its arms around her, and places her on the open sea! Do we traverse the land, steam is harnessed to our chariot, and we outstrip the flight of the swiftest bird, and equal the fury of the tempest!"
We may form an idea of the versatile powers of steam if we consider the manufacture of this volume. It was printed by steam upon paper made by steam. The rags of which the sheets were formed were woven by steam, their separate threads having been previously spun by steam. Moreover, by steam the types were cast in metal, that the same agent had raised from the mine; by steam, too, the mill-board and cloth which form the cover were fabricated, and the thread which fastens the sheets together was twisted.
The author we have quoted above gives the following excellent illustrations of the power of steam:— A train of coaches weighing about 80 tons, and transporting 240 passengers with their luggage, has been taken from Liverpool to Birmingham, and back from Birmingham to Liverpool; the trip each way taking about four hours and a quarter, stoppages included. The distance between these places by railway is 95 miles. The double journey of 190 miles was effected by the mechanical force produced in the combustion of four tons of coke, the value of which is about five pounds. To carry the same number of passengers daily between the same places, by stage-coaches on a common road, would require twenty coaches and an establishment of 3800 horses, with which the journey in each direction would be performed in about twelve hours, stoppages included.
The circumference of the earth measures 25,000 miles; if it were begirt with an iron railway, such a train as above described, carrying 240 passengers, would be drawn round it by the combustion of about thirty tons of coke, and the circuit would be accomplished in five weeks.
In the drainage of the Cornish mines, a bushel of coals usually raises 40,000 tons of water afoot high; but it has, on some occasions, raised 60,000 tons of water the same height. Let us take its labour at 50,000 tons raised one foot high. A horse worked in a fast stage-coach pulls against an average resistance of about a quarter of a hundred weight. Against this he is able to work at the usual speed through about eight miles daily; his work is therefore equivalent to 1000 tons raised one foot. A bushel of coals, consequently, as used in Cornwall, performs as much labour as a day's work of fifty such horses.
The Great Pyramid of Egypt stands upon a base measuring 700 feet each way, and is 500 feet high, its weight being 12,760 millions of pounds. Herodotus states that in constructing it 100,000 men were constantly employed twenty years. The materials of this Pyramid would be raised from the ground to their present position by the combustion of about 480 tons of coal.
The Menai Bridge consists of about 2000 tons of iron, and its height above the level of the water is 120 feet. Its mass might be lifted from the level of the water to its present position by the combustion of four bushels of coal.[1] The reader will hardly require to be informed that the above illustrations show what might be done by the steam generated during the combustion of certain quantities of coal, provided its entire strength could be applied to the fulfilment of the required results.
Let us now briefly consider some of the real achievements of Steam, and other genii, over which man, as the holder of the lamp of science, has absolute control.
The Great Eastern, or Leviathan, that stupendous product of engineering daring, is a structure immeasurably more wonderful than Aladdin's palace. While this ship was in course of construction, the genii of the lamp had no rest, and their Cyclopean labours excited the wonder of all beholders. Although building in the midst of the largest collection of seafaring people in the world, the Leviathan was a puzzle to them all.
None of the old-accustomed sights and sounds of ship-building attended the growth of this monster of the deep. The visitor to the works of Scott Russell and Co., at Millwall, looked in vain for the merry ship-carpenters, caulking away with monotonous dead-sounding blows; for the artisans chipping with their adzes, rearing up huge ribs, or laying the massive keel; and for the bright augers gleaming in the sun as sturdy arms worked out the bolt-holes.
What he did see might well excite his surprise. He saw the giant arm of Steam welding huge shafts, and punching inch-plates of iron as quickly and as noiselessly as a lady punches cardboard for a fancy-fair ornament. Steel, urged by the same potent genie, was seen showing its mastery over iron; while the huge lathes revolved, and the planing-machine steadily pursued its resistless course; whilst, in place of the shavings of the carpenter, long ringlets of a dull grey metal cumbered the ground. The ship-carpenter was transmuted into a brawny smith, and the civil engineer had taken the place of the marine architect.
The Leviathan is essentially an iron ship, more completely so perhaps than any vessel hitherto built. Iron plates, angle irons, and iron rivets form the sinews, muscles, and bones of this monster of the age of iron. The plates vary in thickness from half an inch to an inch; the rivets are about an inch in diameter, and it is these that hold the vast fabric together.
In fastening the plates, the mighty genie Heat lent his aid. When the holes in the plates to be held together had been brought into exact opposition, bolts at a white heat were one by one introduced, and firmly riveted whilst in that condition by three men, one holding the bolt in its position by placing a hammer against its head on the inside of the ship, whilst the other two with alternate blows produced the rivet-head on the outside. The rivets contracted in cooling, and drew the plates together with the force of a vice. Before the ship could swim, no less than two millions of these bolts had to be made secure.
We will not attempt to give a minute description of this steam-made vessel, but will confine our observations to those points in which the Leviathan differs from other ships.
Let us first consider the form of the great ship. Viewed end-wise, its outline is nearly square, for the bottom is perfectly flat throughout a breadth of forty feet, without a keel or any other protuberance. Its broadside is almost a perfect quadrangle, quite horizontal at the top, and very nearly vertical at the two ends. But although the general outline of the Leviathan is formed of nearly straight lines, this ship has curves of wondrous delicacy—curves that bring the bow to the sharpness of a wedge, by gradations which the eye can scarcely follow, while the stern below the low-water line has convexities and hollows gradually melting into each other.
The Leviathan is constructed on the wave-line principle; that is to say, there is a certain similarity between the curves of the hull and the curves of a wave. The best form of a ship, which should force its way through the water so as to meet with the least resistance from the fluid, was until recently unknown. The head and breast of a fish, and the breast of a duck or swan, were the favourite models for the ship's bow. These forms were somewhat modified by experience, but they still remained the types.
Some five-and-twenty years ago, Mr. Scott Russell, then an unknown ship-builder, ventured to question the fitness of these two forms. The fish form would be the best and most perfect, undoubtedly, provided the ship swam under water like a fish, instead of half in and half out; and the duck's-breast bow might prove faultless, if a vessel were merely required to float along the surface like a duck, and not to swim with speed. But he saw that the best constructed ships heaped up a mass of water before them, and that the resistance of this anterior wave could not be overcome without an unprofitable expenditure of power.
Every vessel in passing over the sea displaces a certain amount of water, proportional to its size and draught, and then the water closes in behind her to fill up the hollow. Scott Russell at length discovered the form of ship that would offer the least resistance to the water. He found that the lines or curvature of the bow of a ship ought to resemble the curvature of the wave of displaced water, and that the stern should be curved like the wave of replacement. The mark that still-water makes on the hull of a ship floating on it is called the water-line. Scott Russell called his curve the wave-line, because he found it precisely the same as the line which the wave of displaced water marked along the side of the ship, by which it harmlessly glided without impeding its motion. To test the merits of the wave-line principle, one hundred and fifty models were constructed, and no less than 20,000 experiments were made, which all tended towards one result—the desirability of assimilating the form of a ship in certain parts to the shape of waves.
The great point in practical navigation is to obtain a passage for a ship by removing or displacing the particles of water as quietly as possible, and to no further distance on either side than the greatest width of the vessel.
On one occasion Scott Russell caused a model boat, 75 feet long, to be drawn along a canal at a very high speed, and made the prow pass between two oranges floating on the water. These oranges, which represented on a large and visible scale two particles of water, were observed merely to touch the sides of the vessel until they got amidships, where they remained quiescent until they closed in behind the stern.
The first boat constructed on the new principle was called the Wave. This little yacht, some seventy feet long, and seven and a-half tons burden, verified all the inventor's predictions, and may be said to have heralded in a new era of ship-building. The Leviathan, as far as its lines are concerned, is but a magnified copy of the little Wave boat; and there is little doubt that it will eclipse all other vessels in speed, as well as in vastness, whenever it has a chance of displaying its powers.
We have dwelt upon the wave-line principle, as man is solely indebted to the wonderful lamp for its discovery. The form of least resistance could never have been discovered by accident. The old ship-builders jumped at the conclusion that the fish's head and the duck's breast were the only perfect types of a vessel's bow; but the magical wave-line could not be introduced into naval architecture until science had revealed the true laws of fluid motion and resistance.
We have said that the hull of the Leviathan is formed of unyielding plates of inch iron; also that this gigantic hull has innumerable curves, which die away into each other by insensible gradations. At the first glance these two statements appear irreconcilable. How can these delicate curves be produced by any aggregation of rectilinear pieces of flat boiler-plate? In ordinary wooden ships the planking by its elasticity allows itself to be modelled to the ribs; but here there are no ribs, in the true sense of the word, and the form of the vessel must depend upon the inclination given to each separate piece of iron before the fastening process is commenced. And such in fact is the case. Every individual plate, before being fixed in its proper position, was the subject of a separate study to the engineer. Of the thirty thousand plates that compose the hulk of this great ship, only a few situated in the midship section are alike either in size or curve. For each a model in wood, or "template," as it is technically called, had originally to be made, and by these patterns the plates were cut into their required shape by the huge steam-shears, in exactly the same manner as a tailor cuts out various portions of a garment. The "list," or inclination given to each plate, was the next process; and this was produced by passing the plate through a system of rollers, which could be so reversed in their action, and so adjusted, as to give any required curve.
The Leviathan was not built in the usual manner; there was no skeleton to indicate what it was about to become. The reason of this was, that on account of the enormous length of the ship, it was necessary to make use of a different mode of construction to that generally pursued in building ships, and for this purpose the tubular principle, so successfully carried out by Robert Stephenson in the Menai Bridge, was adopted.
The framework of the ship may be described as consisting, primarily, of thirty-five horizontal webs or ribs of iron plate, each nearly three feet wide, and immensely strengthened at all the points of junction. They extend from end to end of the vessel side by side at the bottom, and one over the other at the sides, at distances varying from three to five feet apart. On either side the uppermost web is about five feet above low-water mark. These webs are crossed by huge partitions of a similar construction placed just sixty feet apart. Plates of the best and toughest iron are riveted on each side of the thirty-five longitudinal webs or ribs, so as to form a double skin to the ship, or a dermis and epidermis; the Leviathan is therefore two ships, one within the other. The whole framework forms a system of cells, which, like the Menai tube, combines extreme lightness with great strength.
So thoroughly close are the joints of this framework—we quote, with some modification, the words of a competent authority—that any one cell would hold water without its running into the adjoining cells; and water is actually to be admitted to some of them, to assist in ballasting or in "trimming" ship, or in giving it a "lift" or tilt-up when the bottom needs repair, taps and valves being arranged for that purpose. Above the level already named, five feet higher than low- water mark, the hull is formed of bars and plates as below; but it is not cellular, being only one layer in thickness. The various decks, whole and partial, are mostly formed of iron. The upper deck is so strong that it is calculated that the whole weight of the vessel might be suspended from it; like the lower part of the hull it has a cellular structure, and will help to maintain the bulging sides in their places, at the same time that it supports the visible wooden deck.
At the bow, or head of the vessel, the decks and partitions, the walls and casings, the supports and angle-irons are so numerous that the whole forms a mass nearly as strong as solid iron. To strengthen the interior of the mighty ship, to define its shape, and to separate it into water-tight compartments, the ten bulkheads or cross-walls of thick iron plate, already alluded to, extend from side to side, and from bottom to top, with no openings whatever below the level of the passenger saloons. So impermeable are these walls that according to the view of the builders any one of the twelve compartments into which the ship is thus divided might be filled with water without flooding those adjacent to it; and, accordingly, a hole rent in the hull would, so to speak, only have one-tenth part of a chance in sinking the vessel. Besides these transverse walls, there are two longitudinal iron walls running along rather more than half the length of the vessel; it will thus be seen that the hollow as well as the shell of the vast fabric is cellular.
What with the two iron decks, the two longitudinal iron walls, and the ten transverse iron walls, besides partial decks, and walls of smaller size, the interior is made into a series of sixty or eighty vast iron boxes, a honeycomb of quadranglar cells, the walls of which give strength mutually one to another. Let a strain come in whatever direction it may, there is an iron wall ready to baffle it. The engineers may possibly be too sanguine, but they believe the Leviathan will prove the taughtest, trimmest, driest ship ever built, irrespective of its more important qualities. They comfort those who dread sea-sickness with the hope that a ship too long to pitch and too flat to roll, will be bearable even to "the gentlemen of England who live at home at ease;" and they talk of the ship being buoyant even if chopped into ten ships—like those animals which seem to have ten lives instead of one.[2]
It is not easy to form, an adequate idea of the dimensions of this iron monster. When we recollect that the Great Western, which twenty years ago was regarded as a marvel of vastness, is 236 feet long; the Great Britain, the first ocean screw steamer, is 322 feet long; and that the majestic Himalaya is 370 feet long we may get, by comparison, a rough notion of the magnitude of the Leviathan, which is 680 feet long between the perpendiculars, and 691 feet on the upper deck. The breadth of the hull is 83 feet, the extreme breadth across the paddle-boxes 118 feet, and the depth from deck to keel 58 feet. In the construction of the hull 30,000 iron plates were used, and these plates were fastened with 2,000,000 rivets. The weight of iron in the hull amounts to 8000 tons, and the weight of the entire vessel when voyaging with its passengers, crew, coals, and cargo on board, will be from 25,000 to 30,000 tons.
Many ingenious comparisons have been made to enable the mind to form a true conception of the value of the above figures. The London streets and squares have frequently been selected as familiar illustrations of the Leviathan's dimensions. Thus it has been said that if any gigantic power could transport the monster to Pall Mall, or street, or St. James's-street, the hull would not sink to the roadway, as its sides would rest on the opposite parapets. Even Regent-street would not receive it without the paddle-boxes; and with those appendages, the broadest street in London, Portland-place, would barely afford it room. The paddle-wheels alone are higher than any but the highest houses. If stretched over Russell-square, one end would rest on the house-tops of the north side, and the other on those of the south.
Everything relating to the Leviathan has a magnitude proportional to that of the vast hull; thus Alexis Soyer, the celebrated chef de cuisine, made a calculation that one hundred persons could dine in one of its funnels, and actually proposed that a banquet should be spread for five hundred guests in the five chimneys before they were fitted to the ship.
Let us now briefly consider the arrangements that have been made to give the iron monster life and motion. Mr. Brunel decided not to trust so precious a human freight, and so vast an amount of cargo as his big ship is designed to carry to any single propelling power, but resolved to supply it with three—the screw, the paddle, and the sail.
The paddle-wheels, which are considerably larger than the circus at Astley's, are to be propelled by monster engines, the motive-power of which will be generated by four boilers each weighing about fifty tons, and containing forty tons of water. These engines, the largest ever constructed with oscillating cylinders, are nevertheless inferior to those devoted to the screw-propeller.
This screw is twenty-four feet in diameter, and weighs thirty-six tons. Its four fans, which were cast separately, and afterwards fitted into a large cast-iron boss, have been aptly compared to the blade-bones of some huge animal of the pre-Adamite world. Besides being pulled along by the paddles, and pushed along by the screw, the Leviathan will also be propelled by the wind when exceptional circumstances render such aid desirable. There are six masts, five of iron and one of wood, and on these masts will, or may be, spread about 6500 square yards of canvass. Under ordinary circumstances the Leviathan will go faster than the wind, and sails will prove an impediment rather than an assistance to the ship's progress. It is not probable, therefore, that they will be much resorted to except for the purpose of steadying or of helping to steer the huge vessel. The steam-power will be truly enormous; it has been stated that, were everything put to work at its fullest, the whole series of engines would work up to 11,500 horse-power. This power would suffice to raise 200,000 gallons of water to the top of the Monument in less than a minute, or to work all the cotton mills of Manchester.
When all the engines are in full work, the great source of power, coal, will be needed to the extent of 250 tons each day. For a voyage to Australia and back, 12,000 tons at the very least will be required, yet such is the capacity of the Leviathan for fuel, that this immense quantity can be stowed away in the coal-bunkers without encroaching at all on the space set apart for machinery, cargo, passengers, and crew.
The great ship will carry twenty little ships, all fitted with masts and sails complete. In addition to these, two small screw-steamers will hang astern abaft the paddle-boxes, each of which will be 100 feet long, 16 feet beam, 120 tons burthen, and 40-horse power. These will be raised and lowered by auxiliary steam-engines, and will be used for landing and embarking passengers, with their luggage. They will look like toy-steamers when suspended at the sides of the sea monster, though they will be considerably larger than most of the above-bridge Thames steamers.
The passenger-arrangements are on a corresponding scale with everything else. There are ample means for accommodating 4000 guests in this floating city, besides the crew of 400. The iron partitions we have already described divide the interior capacity of the hull into separate compartments or boxes; and into each box we may suppose a large house to be let down. A clever writer has thus filled up in imagination five of these great boxes:—"If we were to take the row of houses belonging to Mivart's, and drop them down one gulf; take Farrance's, and drop it down a second; take Morley's, at Charing Cross, and fit it into a third; and adjust the Great Western Hotel, at Paddington, and the Great Northern, at King's Cross, into apertures four and five, we should get some faint idea of the nature of the accommodation in the Great Eastern."
We have only adverted to a few of the wonders of this leviathan—this floating palace of Aladdin, which owes its existence to the potent genii of the lamp of science. Although this crowning marvel of our wondrous age still rests in the Thames like a giant spell-bound, we cannot doubt that it has a mighty future before it. All who watched the Leviathan's growth, and followed its progress along the launching ways, must long to see it "walk the waters like a thing of life," and show its mastery over those waves which it so closely resembles in its graceful curves. In justice to the wise men Brunel and Russell, who have wrought such miracles in the subjugation of the powers of nature, the genii of our wonderful lamp, we trust that the merits of their daring achievement in ship-building will soon be tested.
Let us now glance at another marvellous product of science, which rivals all the magical fabrics described in the Arabian Nights. We refer to the Britannia Bridge across the Menai Straits.
The deep chasm which separates the Isle of Anglesey from the mainland had long been a serious obstacle to the modern Aladdin, who could not brook the delay which attended the use of ferry-boats. He could not rest satisfied until he had bridged-over the intervening strip of sea; and he therefore summoned the potent genii of the lamp, who helped him to form a magical roadway in mid-air. This cobweb-like structure is known as the Suspension Bridge of Telford. In course of time, however, Aladdin began to wish for a more substantial fabric, across which he might urge his steam-drawn chariot. To obtain such a bridge as he desired, he sought the aid of a potent magician, who had long been famed for his power over the genii of the lamp.
In plain language, a railway bridge across the Menai Straits was required, and its construction was left to Mr. Robert Stephenson.
The seven labours of Hercules were insignificant tasks compared with that which the railway authorities set before the great engineer, perfectly satisfied that he would accomplish it by some means or other. Yet the difficulties which Stephenson had to contend with seemed insurmountable, and a less daring genius would have shrunk from encountering them.
Those captive princesses of fairy lore who were doomed to draw water from a well without a bucket, to catch fish without a net, and to spin a thread without either wheel or distaff, were not more unfortunately situated than was Robert Stephenson, though he has never yet been made the hero of a romantic story.
"You must build a bridge," said his employers, "that the heaviest trains may pass over in safety at any speed. This bridge may have any form you please; but we wish you to remember that its rupture would be attended with most disastrous consequences, and we therefore urge upon you the necessity of making it strong enough to resist every strain."
"If you build a railway bridge across the Straits," said the Lords of the Admiralty, "you must not interfere with the navigation. Your viaduct must be at least one hundred feet above the level of the water, so that ships may pass beneath, and it must be constructed without the aid of scaffolding."
Even the elements seemed to set their face against the proposed bridge. The Straits are above twelve miles in length, the shores throughout being rocky and precipitous. The water that fills the passage is never at rest, and the fall of the tide is from twenty to twenty-five feet. Moreover, the wind blows through the Straits with such violence, that a bridge must be strong indeed to withstand its rude shocks.
Imagine an enchanted engineer with such a task before him as the construction of a bridge a hundred feet above the tumultuous waters, without scaffolding of any kind, and you will be able to get a faint idea of the difficulties which he had to overcome before a railway train could pass from Carnarvon to Anglesey.
We will not allude to the various plans which Stephenson conceived and discarded before the idea of a tubular bridge took possession of his mind. This last project, destined to prove so successful, has been well compared to a beam along which a man scrambles when escaping from a fire. Stephenson was bent upon crossing the Straits; but as he could not build an ordinary bridge, when under such extraordinary restrictions, he resolved to span the waters with a huge makeshift in the shape of a hollow beam of iron. Each tube of the Britannia Bridge is literally a beam, so constructed that it combines the maximum of strength with the minimum of weight; in other words, it is a beam from which every portion of metal that does not add to its strength has been carefully removed.
We will now endeavour to explain the simple principle upon which a beam, whether of wood or iron, is enabled to support the weight imposed upon it.
For want of a few moments' reflection most people, in looking up at a common ceiling-girder, consider that its upper and lower parts suffer equally in bearing the weight of the roof; but these upper and lower strata suffer from causes as diametrically opposite to each other as the climates of the pole and of the equator. The top of the beam throughout its whole length suffers from severe compression, the bottom from severe extension, and thus, while the particles of the one are violently jammed together, the particles of the other are on the point of separation; in short, the difference between the two is precisely that which exists between the opposite punishments of vertically crushing a man to death under a heavy weight, and of horizontally tearing him to pieces by horses.
This theory, confused as it may appear in words, can at once be simply and most beautifully illustrated by any small straight stick freshly cut from a living shrub.
In its natural form the bark or rind around the stick is equally smooth throughout; but if the little bough, held firmly in each hand, be bent downwards so as to form a bow, or in other words to represent a beam under heavy pressure, two opposite results will instantly appear. The rind in the centre of the upper part of this stick will be crumpled up, while that on the opposite side will be severely distended; thus denoting, or rather demonstrating, what we have stated—namely, that beneath the rind the wood of the upper part of the stick is severely compressed, while that underneath is as violently stretched: indeed, if we continue to bend the bow until it breaks, the splinters of the upper fracture will be seen to interlace or cross each other, while those beneath will be divorced by a chasm.
But it is evident, on reflection, that these opposite results of compression and extension must, as they approach each other, respectively diminish in degree, until in the middle of the beam, termed by mathematicians its neutral axis, the two antagonist forces, like the celebrated Kilkenny cats, destroy each other. It therefore appears that the main strength of a beam consists in its power to resist compression and extension, and that the middle is comparatively useless, so that to obtain the greatest amount of strength, the given quantity of material to be used should be accumulated at the top and bottom, where the strain is greatest; or, in plain terms, the middle of the beam, whether of wood or iron, should be bored out. All iron girders, all beams in houses—in fact all things in domestic or naval architecture that bear weight—are subject to the same law.
A hollow beam of iron having been fixed upon as the form which the projected bridge should take, an extensive series of experiments were undertaken with a view to ascertain the shape capable of sustaining the greatest weight. A rectangular tube, with a height considerably greater than its breadth, and strengthened at the top and bottom, was eventually selected. The genii of the lamp were now set to work, and the quiet folk of North Wales witnessed similar wonders to those which have since astonished the Londoners. The principal tubes were constructed on piles at high-water mark, and were formed of wrought-iron plates riveted together with white-hot iron bolts.
A system of longitudinal tubes or cells gave the required strength to the top and bottom of each fabric, these cells being quite as effectual as solid metal. Every means was taken to make the tubes as light as possible, as it was known that the strength of the bridge depended on its lightness. This fact sounds rather paradoxical; but if the reader will reflect a moment he will find that a bridge has to support itself, as well as the things passing over it. A beam of solid iron, of the dimensions of the Britannia Bridge, would be useless if placed across the Straits, as it would infallibly break down under the enormous pressure of its own weight. Stephenson's beam, as we have already intimated, has all the elements of strength, but none of the elements of weakness of a common beam.
While the monster tubes were being constructed, the masons were heaping up sandstone and marble into the huge piers upon which they were to rest. The central pier or tower was built upon a little rock in the middle of the stream. This rock, which was only exposed at low-water, had long been a trouble to sailors and nothing else, but it is now world-famous as the Britannia Rock, the chief support of Stephenson's magic aërial galleries. Two other piers were constructed, one on the Anglesey, and the other on the Carnarvon shore, each at a distance of 472 feet from the Britannia tower.
The bridge was to consist of two tubes, placed side by side, one for the down and the other for the up trains. Each tube was formed in four lengths, and when completed these lengths had to be joined together, like the pieces of a huge dissected puzzle. A huge puzzle, indeed! When these immense tubes were finished, how could they be thrown across the Straits a hundred feet above the level of the water? The reader will open his eyes in astonishment when we inform him that the four principal tubes, each 472 feet in length, were floated into the centre of the Strait, and then pumped up to their present elevated position. Said we not that science had brought the powers of nature under man's control—that the genii of the lamp had become the willing slaves of the modern Aladdin?
Each tube was supported on pontoons—huge lifebuoys if you will—and dragged from its resting-place by chains connected with a monster windlass stationed on the opposite bank. This operation was performed at high-tide, and when the water sank, the delighted spectators beheld the tube resting in its proper position, between its two towers. We need scarcely say, that we refer to the direction of the tube, but not to its height, when we here speak of its proper position. The mass of iron had yet to be lifted high into the air.
Among the genii of the lamp there is one called Fluid Pressure, and to this power the task of raising the tubes was committed. The hydraulic-press gave direction to the mighty efforts of this genie. This engine consists essentially of a strong metallic cylinder, in which is inserted a solid piston or ram, and a pump, by means of which water can be forced into the main cylinder. Many of these machines were employed in raising the different lengths of the bridge; but one of them deserves particular mention on account of its stupendous magnitude.
The cylinder of this Cyclopean engine was nine feet long, twenty-two inches in internal diameter, ten inches thick, and weighed fifteen tons. Allowing for the waste, twenty-two tons of fluid incandescent iron were required for this enormous casting. After having been left for seventy-two hours in the mould in which it was cast, the mould was detached from it. It was still red hot! It was then left to cool, but it was ten days before it was sufficiently cool to be approached by operatives well-inured to heat, in order to detach from it some of the sand of the mould which still adhered to it.
This vast machine was fixed upon an iron stage, near the summit of one of the towers, and to the cross-head of the ram were attached massive chains, which descended to the level of the water, and embraced the tube to be raised.
The greatest weight lifted by the press was 1144 tons, but it was capable of raising 2000 tons. The quantity of water injected into the great cylinder, in order to raise the ram 6 feet, was 8112 gallons. When a lift of six feet was effected, the lifting chains were seized by a set of clamps, under the lowest point to which the cross-head descended, and while they were thus held suspended, the water was discharged from the great cylinder, and the ram, with its cross-head, made to descend. Meanwhile, the lengths of the chain above the clamps were removed, and the chains thus shortened attached to the cross-head by other clamps, and all was prepared for another lift. In the practical operation of the machine each lift of six feet occupied from thirty to forty-five minutes.[3]
The towers were formed of three massive piers of solid masonry, so that each tube just filled up the space between the inner and an outer pier. As the tubes were elevated by the action of the press the vacant spaces beneath were closely packed with blocks of wood. It was very fortunate that this course was adopted, as an accident occurred, which must have resulted in the destruction of one of the tubes had the packing process been omitted. The water contained in one of the presses, not content with lifting the tube, thought fit to make a display of its power by thrusting the bottom out of the cylinder, thereby killing an unfortunate workman. The monster tube fell one inch, but was prevented from falling any further by the packing beneath; had it fallen six feet it would have been shivered into atoms.
When all the tubes were elevated to their permanent position the great work was completed, and Aladdin gazed at the new wonder with delighted eyes. These aërial galleries, nearly fifteen hundred feet in length, are marvellously strong, each being capable of bearing, spread over its whole surface, the enormous weight of 4000 tons—a weight nine times greater than it can ever be required to sustain. The hollow beam is not deflected more than an inch from the horizontal line by the passage of the heaviest luggage-train, and it is scarcely affected at all by the highest wind.
The enchanted engineer, whom we whilom saw beset with difficulties of no ordinary kind, can now point to the twin tubes across the Menai Straits, and say proudly, "My task is performed, the bridge has been constructed without scaffolding, and little Mona is no longer separated from her mighty sister." We need scarcely say that Mr. Stephenson is treated quite as badly as the ogre-guarded princess, for no sooner has he performed one task than the ogre, called "Nineteenth Century," finds him another still more impossible to all appearances than the last.
Let us not forget that although the human mind may plan a Britannia Bridge or a Great Eastern, the human hands could never construct such wonderful fabrics without the assistance of those mighty powers of the material world which man by industry and patient observation has succeeded in enslaving. Steam, heat, light, electricity—indeed, every agent that is known to exert power in the natural world, can be made to labour in the world of art. These forces, then, are the genii that attend the lamp of science. This lamp, like that of Aladdin, must be rubbed before the genii will appear; in plain language, science will not reveal its mighty powers unless the student works diligently.
Our artist has pictured the lamp of science as a luminous hand. What is the meaning of this curious emblem? Reflect for a moment, and you will detect a deep truth hidden in this fancy. Science, dear reader, is the magical hand that points out truth and strikes down falsehood; and, more than that, it is the magical hand which fashions the crude materials of the world in objects of beauty, which constructs and moves all kinds of machinery, which performs Herculean feats of strength, and executes works of marvellous delicacy.
But what has Science to do with the wolf and the hog at the bottom of the emblem? Nothing, indeed, except to keep them out of mischief! The wolf stands for the lawless man who preys upon his fellow mortals and lives by crime; the hog for the ignorant glutton who wallows in the mire of indolence, devouring everything that comes in his way. We trust that these brutes in human form will one day become extinct, and that the chains which depend from our wonderful lamp will be no longer needed; at present, however, it is absolutely necessary to restrain the wolf from interfering with those who labour in the light of science, and the hog from devouring their well-earned food.
Having thus "pointed a moral" in the emblem that "adorns our concluding tale," we have now to bid the reader farewell.
An unpleasant task is this leave-taking, dear reader. We have journeyed together for some time, and now we feel as though we were parting from an old friend. We have treated you very rudely, we fear. We have dragged you hither and thither without once asking you whether you liked such wandering habits. We have led you through the ancient forests; have soared with you to the confines of space; have plunged with you into the sea; and, in fine, have taken you everywhere. We trust that you bear us no malice, and will not think that time wasted which was spent in listening to our FAIRY TALES OF SCIENCE.
THE END.