BRIDGES.

The earliest efforts of the civil engineer were in all probability directed to the construction of roads, but it would be evident that unless they could be made to cross rivers and water-courses, they could continue but a short distance in any required direction. Bridges of some sort must therefore be constructed; small brooks were doubtless bridged over with timber laid across them, and “fords” or shallow parts of rivers taken advantage of when the stream was too wide to be spanned in this way. That these fords were once very numerous in England, is shown by the constant recurrence of such names as Stratford, Brentford, &c., and indeed fords are now in use in many places where the stream is shallow, and the traffic too little to pay for constructing a bridge. The next improvement in all probability consisted in placing stones in the fords to enable passengers to step from one to the next, and cross the stream dry-footed, which were called “stepping stones;” but the earliest bridges properly so called, were probably stones piled upon each other and united by beams of timber.

The Chinese, Romans, and several other nations were acquainted with the use and proper mode of constructing bridges, many centuries ago, when no such works had been attempted in England. The materials chiefly now in use for bridges are stone, brick, iron, and wood, and by far the greatest number, till within the last half century, were constructed of stone, but iron seems to be rapidly taking its place, especially since the mode of constructing suspension bridges has been more perfected. There are several ways of forming the foundation for the piers, or those parts on which the bridge rests. Old London bridge and many others of considerable size were based upon “piles,” these piles consist of great pieces of timber pointed at one end and driven deep down into the bed of the river close together, so as to form a solid mass on the top of which the stones for the piers were laid; these piles have been found scarcely altered after several centuries of immersion in the mud, but at the parts exposed to the action of the water they soon become decayed and eaten away by its inhabitants. For many bridges, when there is good foundation, the stonework is laid at once on the bed of the river, but in order that this may be got at, a double ring of piles is driven close together around the part where the pier is to be built; these piles are of a flat form, and the two rows inclose a space of about a foot between them, which space is filled with clay, well rammed in, and the water within the barrier pumped out, this forms what is called a “coffer-dam,” and is an expensive proceeding. The bottom of the river thus exposed can be built upon in the usual way. Another way of forming a foundation for piers consists in lowering large wooden frames or boxes filled with concrete which hardens and forms a solid basis, these are called “cassons.” Wooden bridges are now seldom made of any great size, but are very numerous in country places for crossing narrow streams, on account of their cheapness and the facility with which they can be constructed. Bridges of brick are not much used, but “viaducts” of this material often occur in the lines of railways, and very handsome arches of brick-work are used to span streets in towns where railways pass. Iron girder bridges also, are frequently employed for this purpose; they are made with girders of wrought or cast iron which generally pass in a straight line over the roadway from pier to pier. Some of them are made of iron-plate rivetted together, while others are strong girders of cast iron having small arches of brick-work built on them, running the length of the girders and resting upon ridges in them, so as to fill up the space between each girder.

Iron bridges are of almost every form and construction conceivable; some are tubes of iron-plate, as the Britannia Bridge, rivetted together; some of solid cast iron work as Southwark Bridge, others called suspension bridges are hung by rods of wrought iron, from large chains suspended from pier to pier, as Hungerford Bridge. In many places bridges of boats are used; the boats are moored to the river’s-bed and have timber-work connecting them. Another kind, which can hardly lay claim to the name of bridge, called the “floating-bridge,” is used for the conveyance of carts, horses, passengers, or merchandise, from one side of a river to the other. It consists of a sort of platform guided by chains laid down for the purpose, which pass over wheels or drums on the side of the bridge. The motive power necessary for turning these wheels, is supplied by a steam-engine within the structure itself.

TUNNELS.

Tunnels are underground passages made for the purpose of continuing roads, railways, or canals through hills or elevated parts of the ground. In slight elevations “cuttings” are generally made, and it becomes a question of expediency for the engineer as to which of the two shall be chosen, as there will be a much greater quantity of earth to remove from a deep cutting than from a tunnel; in fact, the question is often at once determined by the demand for this material, for if it should so happen that the level of the road is such, that many or large embankments are required, the earth taken from the cuttings will be wanted to construct them. As a general rule, tunnels are only made where a hill has to be penetrated which is too high to be cut through; another object sought by tunnelling is to cause the road to pass beneath a canal, river, buildings, or roads, where the level of the road or railway is far below them. The Thames Tunnel is the most remarkable example of this kind of work.

Tunnels were constructed by the ancient Greeks and Romans, chiefly for the purpose of forming aqueducts, and for draining lakes. A few years back, tunnels were not very frequently undertaken, those that were made being chiefly on the lines of canals, but they are now far more numerous, on account of the great number of railways, on which they frequently occur. Before a tunnel is undertaken, the nature of the soil should be thoroughly examined by means of “borings,” sunk down to the intended level, some soils being much less favourable than others for this purpose. Quicksand, or sand percolated by water, is one of the greatest impediments to tunnelling, while rock (contrary to what would at first be thought) is one of the least, as in this case there is no occasion to line the tunnel with brickwork, which more than saves the extra expense of excavation, as that can be rapidly done by blasting, where it is too hard to dig, while in the former case the difficulty of draining off the water, and the cost of brickwork of sufficient solidity to support such loose earth, often prevent the undertaking. Tunnels of great length are sometimes begun at several different parts by means of shafts sunk down to the required level, and each made to unite into one, but this depends upon where the earth is to be deposited; on railways it is common that the “level” is so chosen that the earth from tunnels and cuttings is always wanted to fill up hollows in their immediate vicinity (see “Railways”), and when this is the case a series of “trucks” drawn by an engine on temporary tramways remove the earth from the tunnel as it progresses, to the next embankment, both works proceeding at the same time.

When a small portion of the tunnel is excavated, the casing of brickwork is begun from the mouth of it, and continued a small piece at a time, following closely the excavation. The brickwork is generally begun at each side, and carried up to the height at which the arch springs; an inverted arch is then constructed from one side to the other to form the floor, and “centreings” resting upon uprights are placed, to “turn” the arch upon. These centreings are only about two feet deep, and span the arch from side to side; the men work with their backs to the part being excavated, and are able to reach across the narrow centreing to lay the bricks. When completed all round, and the spaces between the brickwork of the arch and the surface of the excavation filled up with earth, the centreing is shifted further in and a fresh ring of brickwork commenced, the excavation going on at the same time. In many tunnels, large perpendicular shafts are sunk down for the purposes of ventilation and lighting.

DRAINS.

Drains are constructed for the purpose of carrying off from the surface of the ground all superfluous fluid, whether it may arise from want of a proper exit for the rain, from springs rising in the ground to be drained, or from water coming from some higher level. Drains are made in towns to carry off the rain from the surface, and all liquid refuse from the houses. When the surface of any portion of land has to be drained of its accumulated water, where the soil is of an impervious nature, like clay, it is usual to cut deep furrows from the higher to the lower parts and fill these up with gravel stones or brushwood; the moisture which collects in these is made to flow into the lowest part, and there form a pond, from which it evaporates as fast as the drains supply it; or if the quantity be too great, a main drain or ditch carries it onwards to some stream or river. The furrows before being filled up with any porous substance are often laid down with “drain-tiles;” tiles bent into the form of half a cylinder, and placed with two edges on a flat tile at the bottom of the drain, or pieces of tubing made of tile. The water finds its way readily into these through the crevices. In some cases, water can be kept from flowing over lands and forming marshes, by embankments. These are more especially useful in land lying below the level of rivers and lakes; immense tracts are thus preserved in Holland. When these embankments are very extensive, there is a constant filtration of the water through them; drains are therefore constructed to collect it into certain spots, and mills erected to pump it over the banks. These are usually small windmills, working a large wooden screw in a cylindrical barrel, causing the water to rise up the cylinder into a trough, from which it is poured over the embankment; these mills require little or no attention, and work at all times when the wind blows. When water accumulates in valleys lying above the sea, deep cuttings or even tunnels may be necessary to carry off the water; at NeufchÂtel in Switzerland, very extensive tracts of land have been reclaimed, by boring a tunnel through the surrounding hills, and thus letting off the water from the lakes formed by the rain.

Sometimes land may be drained by boring through the clay or other impervious soil, down to a porous stratum, such as sand or gravel. A knowledge of the structure of the whole district is necessary to determine whether this is likely to be of service or not. By boring deep pits in the drains cut in peat mosses, the weight of the mosses will cause the water to be squeezed out, which rising in these holes flows off by the drain, producing the same effect as if it were cut to the depth of the pits. Drains are frequently made by means of narrow curved spades, but the draining plough effects the purpose more rapidly, and is very generally used.

The drainage of London and other large towns consists of small drains from each house, made of earthenware tubes leading to a sewer of brickwork, running along each street and uniting with larger ones. The question of effectually draining London is a very difficult one, schemes of various kinds being at this time propounded, which would cost several millions of pounds to put into practice; at present all the sewage runs into the river Thames, and it is a matter of serious importance to obviate this, by carrying it off to some other place.

ARTESIAN WELLS.

The construction of artesian wells depends upon the fact that water, being at liberty to flow, will always sink to a level; by which it is meant that the parts which are highest press upon those that are lower, and tend to raise them, the higher parts sinking in the same proportion that the lower parts are elevated; this continues until both are upon a level. Suppose a bent glass tube of the form of fig.1, be partly filled with water, the surfaces of the water in both arms of the tube will each be upon a level with the other; suppose now another such tube to have a small hole atA, the water sinking in each arm will force out a jet of water, and if a tube be inserted into this hole it will represent an artesian well, and the water will rise in it till all three are on a level (fig.2). Instead of these tubes there is a layer of some porous material, as gravel, at some distance beneath the surface of the earth, rising at each end and forming a sort of basin (fig.3), which is bounded above and below by some impervious substance as clay or stone; the well being sunk at any part (as ata,b, orc) below the level of the gravel, where it forms the surface of the earth, must cut through the upper stratum of clay or stone, and thus form a tube into the porous gravel which holds the water; this water is obtained from the rain, which, falling on the surface of the earth, drains through the gravel and fills its lower part. The water will rise in the bore at a height according to circumstances, if the gravel at each side of the bore rises to higher ground, a jet of water will be forced out, if not so high, the well will only partly fill, and so on. Artesian wells are sunk or bored by means of instruments screwing on to the end of a set of iron rods each screwing into the end of the other as shown in fig.4. The first piece is generally a sort of gouge(a) two or three inches wide, and so made as to bring up a cylindrical piece of earth when forced in and screwed round, the weight of the rods after a few pieces are added is sufficient to force the gouge into the earth, except in stony places, when a sort of “pick” is used.

The official report of General Desvaux on the artesian borings executed in the Desert of Zahara of the province of Constantine, in 1856-7, states, “that a spring affording 4010 quarts of water per minute, was the result of one of the borings, and that others affording35, 120, and 4,300 quarts respectively were successively completed.” And he goes on to say: “When the shouts of the soldiers announced the gush, the Arabs sprang in crowds to the spot, laving themselves in the welcome abundance, into which mothers dipped their children; while the old Sheik fell upon his knees and wept, returning thanks to Allah and the French. At Oum Thiour a well sunk to the depth of 170 metres and yielding 180 quarts a minute was at once taken as the centre of a settlement by a portion of a previously nomadic tribe.... As soon as the water appeared they began the construction of a village, the plantation of 1,200 date trees, and entirely renounced their wandering existence.”

According to General Desvaux’s report, these artesian wells are likely to have a most important influence on Arab life, and greatly to subdue the roving propensities of many of the tribes.

MINES.

Mines are excavations made in the earth for the purpose of raising the various minerals which exist below its surface, such as coal, rock-salt, and the various ores from which metals are extracted (see “Smelting”). Mines consist of those which contain minerals that lie in strata parallel (or nearly so) to the surface of the earth, as coal, rock-salt, or iron-stone, and those containing the ores and minerals which are imbedded in seams or fissures of the primitive rocks, and are nearly perpendicular to the surface. Of the former kind, coal-mines form the chief examples. When indications of coal are discovered, a “boring” is commenced to ascertain its existence, and the depth at which it is placed below the surface. Each piece of earth raised by the boring-tools is placed one beside the other, in the exact order in which they are raised, so as to show the kind of earth being bored through, and the thickness of each strata between the surface of the earth and the seam of coal; and it sometimes happens that the boring is stopped on arriving at certain kinds of rock—the old red sandstone, for example—for it would be useless to continue boring beyond this, no coal ever existing below it.

When coal is found, and its quality and the thickness of the seam ascertained to be such as to warrant further expense, a shaft is dug down of some eight or ten feet diameter, cased with brickwork or wood to prevent the falling-in of its sides, and in some cases powerful machinery has to be erected to pump out the water which flows in. On reaching the coal, galleries—called “gates,” or “bords”—are dug in it in opposite directions, forming one long straight passage, and from this other smaller ones, called “headways,” are dug, at right angles, to the depth of about twenty-four feet, and from these other “gates” are carried parallel with the first, forming a series of roadways joined by short passages, and having squares of coal between them; the height of all these passages is determined by the thickness of the seam of coal, usually from three to ten feet. The great masses of coal forming the squares between these passages are gradually dug away (as far as can be done with safety) and the gates continued onwards, but before long the ventilation becomes impeded, and the air foul and dangerous from “fire-damp” (carburetted hydrogen) or “choke-damp” (carbonic acid), gases which are given off from the fissures in the coal. It is therefore necessary to produce a continuous current of fresh air in every part of the mine, which is done by sinking another shaft at the furthest part of the mine and keeping a large fire burning at its mouth, over which a tall shaft is generally erected, from which a column of light air ascends, drawing fresh air down the other shaft and through every part of the mine, to supply this “up-cast” shaft, as it is called. This supply of fresh air is economised and regulated by doors or valves, so placed that any part requiring extra ventilation can obtain it at any moment by shutting these doors and letting the whole current go through that particular part.

The removal of coal is effected partly by digging with the “pick,” and partly by blasting with gunpowder; a large square mass is cut all round, and a charge of powder fired behind it, so as to bring down at once sixty or eighty tons of coal, which is brought along the gates on “trams” to the bottom of the shaft, where “corves” or baskets filled with it are drawn up to the “pit’s-mouth” by steam machinery, one corve ascending full while another is descending empty.

The mines from which most minerals, such as sulphuret of lead (galena) or of copper, are drawn, belong to the second class, or those whose shafts “cut” the vein of mineral at a very acute angle. When the existence of the required mineral and its “dip” or inclination is ascertained, a shaft is sunk so as to cut its upper surface, and then carried through it, cross-cuts being formed on to the vein, and “levels” or galleries right and left in the direction of the vein. From these levels “winzes” or small shafts are cut at intervals from one level to that below it, thus leaving square portions of the mineral vein to be explored, which is done by digging away the roof or upper part, so that the rubbish and ore falls down, when it is sorted and carried away.

SHIPS.

The first part of a ship “laid down” is the “keel;” this is the projection which runs along the whole length and forms the lowest part of the ship. The ship is built in what is called the “building slip,” which slopes towards the water; in this “slip” a row of oaken blocks are placed at a few feet apart, and about three feet high, on which the keel is laid; these blocks are for the purpose of allowing the workmen to cross from side to side below the keel and to form a foundation for the ship to rest on. In fig.1, Ais this arrangement of blocks, Cthe keel, at the hinder part of which is the “dead wood,” or the timbers filling up the space between the keel and the curved bottom of the ship, which is more curved than the keel, and very much so towards the “stern” or hindermost part. Across the keel are laid the “floor-timbers” or “ribs,”B, which are curved timbers laid at right angles to the keel and passing outwards and upwards in the exact curve which the sides of the ship are to assume; these are too curved and too long to be of one piece, others, therefore, are added, and joined end to end with the first by wooden bolts or “dowels;” these curved timbers are cut to a pattern, chalked on the floor of the “mould-loft.” The ribs as they cross the keel are bound to it by a piece of strong timber running along inside of or above them, but parallel to, and exactly over it, which is called the “keelson,” and is bolted to the keel through the centre of each lower piece of the ribs or floor-timbers; it is shown atD, fig.1. In large ships there are three of these keelsons, running side by side, and forming a strong support to the masts, which rest upon them. At each end of the keel a bar of timber rises, the hindermost being called the “stern-post,” and that in front the “stem-post” (markedE andF in fig.1). Across the ribs on the outside and parallel to the keel, are laid the “planks,” which are boards of oak of from two to six inches thick, laid close together and touching at their edges; these are fastened to the ribs by plugs of oak, called “treenails,” going right through the planks and ribs, and wedged at each end (fig.2). In large ships, similar planks line the inner side of the ribs, and oblique or diagonal braces are also sometimes used, to strengthen the ship and keep it from curving or “arching” when in the water.

The masts of a “ship” are three in number, a “schooner” has two, and a “sloop” but one. These masts pass right down through the decks, and rest upon the keelson. In small vessels each division is made of one piece, but in larger ships they are made up of a central piece, with others fastened round it so as to enlarge and strengthen it. The first or lower division of the central mast is called the “main” mast, and that above it the “maintop” mast; the fore mast is divided into “fore” mast and “foretop” mast, and the after mast is called the “mizen” and “mizentop” mast, and the pieces above these the “foretop gallant” mast, “maintop gallant” mast, and “mizentop gallant” mast. These masts are made, and raised by cranes in a building called a mast-house, and placed in the right position in the ship floating beneath.

The outside of ships, as high as the “water-line,” is covered with a sheathing of copper to defend it from the action of the “worm” (Teredo navalis), which bores into and destroys the wood exposed to its ravages; the copper also presents a smooth surface to the water, and facilitates the motion of the vessel. At the stern of the ship is placed the “rudder,” a wooden construction turning like a door on fastenings, and which, by being moved on one side, presents a greater amount of resistance to the water, and consequently tends to turn the stern of the ship away from that side, thus altering its course. The decks of a ship are like the floors of a house, running across from side to side, and supported on strong beams bolted into the sides; they are slightly arched, to increase their strength, as they have in ships of war to support the weight of the guns, &c. A section of the decks and other parts of a ship is shown at fig.3. where the general figure and the different parts described may be seen; the section is through the middle, from side to side. Most ships of any considerable size carry several boats with them, either on deck or suspended between the masts, to serve as a means of escape in case of fire, or any other accident requiring the crew to leave the ship, also as a means of keeping up communication with the shore. Ships of war are named according to the number of guns they carry, as a seventy-four, a hundred-and-twenty-gun ship, &c.

When ships have to be repaired they are brought into the repairing dock, which has a pair of gates shutting it off from the river; when they are closed (at low water) the water is pumped out from the dock, and the repairs done; when finished the water is let in, and the ship floats out. In small vessels it is sometimes sufficient to haul them on shore at high tide, so that when the tide is down they may be left high and dry and repaired, and when the tide is at the highest, hauled off again. Steam ships are constructed to be propelled either by paddle wheels having flat boards fixed to their circumferences, which on being turned round, take a great hold in the water, and so cause the motion of the ship; or by the screw-propeller, which has been described.

The iron ships, which of late have almost superseded those of wood, are made of plates of wrought iron, rolled, while red hot, between rollers to the thickness required, which is generally half-an-inch; these plates have holes punched all round them by machinery, and are united by rivets placed in the holes red-hot and rivetted by heavy hammers. The most magnificent specimen of iron shipbuilding ever attempted is the Leviathan. This ship is 680 feet long, and is not made of one thickness or case of iron plates, but is upon a new principle called the “cellular,” consisting of an outer and inner casing of iron plates held together by partitions of iron so as to separate them into square compartments or “cells.” The objects gained by this arrangement are greater strength, and greater safety, for in case of injury to the outer portion, the water would enter only between the two in that compartment where the injury happened to be, and so fill only that small portion with water. The whole ship is also divided into compartments by means of double screens of iron, making it like a fire-proof box, and even if a fire should occur in one of these, the others would be preserved from its effects. The masts and yards of this great ship are also of hollow wrought iron plates rivetted together, and are both stronger and lighter than they would be of wood. A machine is placed at the lower part of each mast which by compressing, can crush it up, and cause it to break off and fall over the side of the vessel in case such a thing should be required, as in a very violent storm; and the standing rigging, which is of wire-rope, can be let loose in a few minutes so as to completely free the ship of the mast. This great ship is constructed to be propelled both by screw and paddles; the screw engines are four in number, and are each of 1,600 horse power, the paddle engines are also four, of 1000 horse power each, being 10,800 altogether. They will require about 180 tons of coal a day to work them; 12,000 tons of which are capable of being carried. The Leviathan will have six masts, and be able to spread 6,500 yards of sail, will accommodate 4000 passengers, and when ready to sail, with all on board, will weigh about 25,000 tons.

CANALS AND LOCKS.

Canals are artificial water-courses, either for the purpose of connecting rivers, or for forming water communication for the conveyance of goods. There are about 2200 miles of canal-way in England, which is still in complete requisition, and but little affected by the enormous goods traffic of the railways. Canals afford a means of slow but cheap conveyance for heavy or bulky goods, not requiring a rapid transit, for the more rapid the pace the greater the resistance of the water. The usual rate of transit is somewhere about two-and-a-half miles an hour, at which pace a horse can draw about four times as much on water as he can on a railway, and about thirty times as much as on a level turnpike road; but if a greater speed were to be obtained, it is found that the resistance of the water would impede it so much, that at the rate of five miles an hour a horse could draw no more than he could on a railway, and at ten miles only a quarter as much.

In constructing canals, it is important to have a good supply of water, and this is generally secured by turning all the springs and streams in its course into it, or deriving its source at its highest level from a large river. The same works have often to be constructed on the line of canals that are required on railways, such as bridges, cuttings, embankments, tunnels, &c., and besides these, contrivances peculiar to canals, called “locks,” which are now to be described.

Locks are barriers or doors constructed so that these artificial rivers may be carried over rising ground and through valleys, without the labour and expense of cutting through the hills and filling up the hollows as would be required without them. Railways can be constructed on ground which is not quite level without any embankment or cutting, as it is not absolutely required that railways shall be perfectly level. But water always will be level, unless it is constantly flowing, as is the case with streams, and these only sink a few inches in a mile, or else they become so rapid, that if attempted to be imitated in canals, they would be useless; for it would require too much power to draw any vessel up the canal against such a stream, and would moreover require more water to supply them than can commonly be obtained. The means therefore adopted to overcome this difficulty are gates, or in other words, a pair of “locks.” The canal is constructed in such a manner that it shall be perfectly level for a certain distance, then sink down some ten or twelve feet at once, and again flow on a level and sink down. These sudden lowerings are effected as follows: two pairs of thick solid doors of wood are fitted to shut in the water, and another pair a short distance further on; behind these the bed of the canal is lowered the required distance. When a barge or other vessel has to pass down the canal the first pair of gates are opened, and the barge floated in between them and the second pair, the first pair are now closed, and the water beyond the second pair being lower, that between the gates in which the barge floats is let out by means of a valve worked by a rack and wheel; when this valve is raised the water flows out and sinks down to the level of the water beyond, carrying the barge with it; in a few minutes it is so low that, the second gates being opened, the barge is drawn out and continues on its way. But suppose the barge had to be brought up the canal, then it is floated into the space between the gates (as before), and the one behind the barge closed, the water beyond the gate in front being higher, is let into the space where the barge is by a valve, and this filling, lifts up the barge to the level of that in front, the front gates are then opened and the barge proceeds onwards. These gates are never made to shut level, but meet at an angle with the point towards the highest water, which is done that they may resist the great pressure which the water exerts, and for turning this to advantage, for this very pressure shuts the gates and keeps them close together. The canals which have much traffic on them have double locks, as in the engraving, that barges may go up and down at the same time without having to wait for each other. The barges and boats are generally “towed” or drawn by a horse attached to a rope, and walking on a “towing path” or road at the side of the canal.

THE END.