in a physical sense, the particular temperament or condition of the body.

CONSTRUCTOR, an appellation given to several muscles, on account of their drawing together or closing some of the orifices of the body.

The sense in which the term "Construction" is used in this place, is that in which it is employed in the supplement to Architecture; purporting, however, rather fabrication than conformation.

The object of construction is to adapt and combine fit materials in such a manner that they shall retain in use the forms and dispositions assigned to them by the constructor. If an upright wall be properly constructed upon a sufficient foundation, the combined mass will retain its position, and bear pressure acting in the direction of gravity, to any extent that the ground on which it stands and the component materials of the wall can sustain. But pressure acting laterally has a necessary tendency to overturn a wall, and therefore it will be the aim of the constructor to compel, as far as possible, all forces that can act upon an upright wall to act in the direction of gravity; or else to give it permanent means of resistance in the direction opposite to that in which a disturbing force may act. Thus when an arch is built to bear against an upright wall, the constructor applies a buttress or other counterfort in a direction opposed to the pressure of the arch. In like manner the inclined roof of a building, spanning from wall to wall, tends to thrust out the walls; and hence the constructor applies a tie to hold the opposite sides of the roof together at its base, where alone a tie can be fully efficient, and thus compels the roof to act upon the walls wholly in the direction of gravity; or where an efficient tie is inapplicable, he adds buttresses or counterforts to the walls, to enable them to resist the pressure outwards. A beam laid horizontally from wall to wall, as a girder to carry a floor and its load, may sag or bend downwards, and tend thereby to force out the walls; or the beam itself may break. Both these contingencies are obviated by trussing, which renders the beam stiff enough to place its load on the walls in the direction of gravity, and strong enough to carry it safely. Or if the beam be rigid in its nature, or uncertain in its structure, or both (as cast iron is), and will break without bending, the constructor, by the smith's art, will supply a check and ensure it against the possible contingency.

Stability is then the aim of the constructor; but perfect and enduring stability is not to be attained with materials which are subject to influences beyond the control of man, and all matter is subject to certain influences of that nature. The influences with which the constructor has mostly to contend are heat and humidity; the former of which produces movement of some kind, or to some extent, in all bodies; the latter, movement in many kinds of matter; whilst the two acting together tend to disintegration or to decay in all materials available for the purposes of construction. These pervading influences the constructor seeks to counteract, by the selection and disposition of his materials accordingly. From the tenacity of wrought iron, and its almost plastic character in the hands of the smith, the constructor will employ it to tie together other more bulky but less costly and more rigid materials; but on account of its exceeding susceptibility of heat, and its consequent expansion and contraction, he will use wrought iron in short lengths only, unless where protected from great alternations of heat and cold. The rapid decay, too, of wrought iron when exposed to humidity, and especially to alternations of wet and dry, will teach the constructor not to expect enduring stability in his works if he makes them dependent upon wrought iron. Cast iron is brittle, and may not be exposed with impunity to transverse strain, especially if such strain be attended by action tending to induce vibration: it expands and contracts under the influence of heat, but it resists compression also in every direction, and if used in small bodies, is valuable as a means of connecting other materials. Timber, being practically unchangeable in the direction of its length from the mere absorption of either heat or humidity, and at the same time practically both inextensible and incompressible in that direction, and being also readily wrought and easily combined alike with itself and with iron, is a valuable material in the hands of the constructor; but it shrinks and swells in the direction of its thickness, and, in consequence, is subject to rapid decay when exposed to alternations of moisture and dryness; and although in many varieties timber is perishable and unchangeable in form if it be kept either altogether free from moisture or always wholly wet, its quality of inextensibility is greatly diminished in value to the constructor on account of the comparatively slight resistance it offers to compressing power, and the comparative ease with which its fibrous structure is torn asunder. From this cause it cannot be grasped or otherwise held so that its power of resisting extension may be made available in any degree proportioned to its strength; whilst its quality of compressibility in the opposite direction is of less value to the constructor for many purposes which require that quality in the material; because it absorbs moisture by the ends of the fibre more readily, and with a far more mischievous effect, than it does in the direction in which it is compressible. Hence timber rots more rapidly by the ends than by the sides.

Stone and brick, the other main available materials in general construction, keep their places in combination by means of gravity. They may be merely packed together, but in general they are compacted by means of mortar; so that although the main constituent materials are wholly incompressible, masses of either, or of both combined in structures, are compressible until the mortar has indurated to the same condition of hardness.

That kind of stone is best fitted for the purposes of general construction which is least absorbent of moisture, and at the same time free to work. Absorbent stone exposed to the weather rapidly disintegrates; and for the most part non-absorbent stone is so hard that it cannot always be used with a due regard to economy. The constructor therefore, when he can command fitting stone of both qualities, exposes a face of harder stone to the weather, or to the action which the softer stone cannot resist, and forms the main body of the structure of the latter so protected. The hard and the soft should be made to bear alike, and should therefore be coursed and bonded together by the mason's art, whether the work be of stone wrought into blocks and gauged to thickness, or of rough dressed, or otherwise unshaped rubble compacted with mortar.

Brick, if good, is less absorbent of moisture than any stone of the same degree of hardness, and it is a better non-conductor of heat than stone. As the basis of a stable structure, brickwork is more to be relied upon than stone in the form of rubble, when the constituents bear the relation to one another last above referred to, the setting material being the same in both; because the brick by its shaped form seats itself truly, and produces by bonding a more perfectly combined mass; whilst the imperfectly-shaped and variously-sized stone as dressed rubble can neither bed nor bond truly; the inequalities of the form being to be compensated for with mortar, and the irregularity of size of the main constituent accounted for by the introduction of larger and smaller stones.

The most perfect stability is to be obtained nevertheless from truly wrought and accurately seated and bonded blocks of stone, mortar being used to no greater extent than may be necessary to exclude wind or water, to prevent the disintegrating action of both upon even the most durable stone. When water alone is to be dealt with, and especially when it is liable to act with force, mortar is necessary for securing to every block in the structure its own full weight and the aid of every other collateral and super-imposed stone in order to resist the loosening effect which water in powerful action is sure to produce.

In the application of construction to any particular object, the nature of the object will greatly affect the character of the constructions and the materials of which they are to be formed. The object of a breakwater is to check the run of the sea when it is acted upon with power. It may be that in some cases piles of timber driven into the bed of the sea, and made rigid by repetition or by combination, might have strength enough to withstand and to break the run of the sea; but there is a range between high and low tide throughout which the piles would be exposed alternately to opposite influences, either of which alone would do the timber little injury, but which acting in rapid succession will in a short time destroy what might otherwise have resisted the extremest force of the sea for ages. Timber falling in enduring usefulness, which it will the more rapidly if it be left in any degree dependent upon wrought iron; or if the depth and the run of the sea be too great for timber, stone is adopted. Large blocks of the densest stone may be tipped into the sea, and serve as a base to other blocks in succession until a mound, mole, or dike, be formed reaching above high-water level. This dike may have had such slopes given to it as experience had shown that heaped up blocks of rough-hewn stone would take on dry ground; but if, even while the work is yet in progress, a gale of wind occasion a great run of the sea, the half-raised breakwater will be converted into something little better than a shoal of rubble—it having been overlooked that the blocks of stone, when immersed in water, had lost so much of their weight that far less force than was necessary to move them in air tumbled them about in the water as small pebbles are rolled on the sea-shore. The stones were uncombined, and every surface stone was exposed in a half floating condition to the force of the sea, while its fellows below became exposed as those above them were rolled away. The construction wanted rigidity. The water must be excluded from among the stones, so that the whole shall form one mass, from the level at which the run of the sea in a storm can first be felt, up to and above the highest high-water level at the place. By such a process, labour and skill will perform effectually, with a comparatively small body of stone, what a huge mass of crude material unskilfully dealt with had failed to accomplish.

In a line of road, the object sought in constructing an embankment is a firm foundation to the roadway or the railway, by which the traffic may be carried on with perfect safety. The work that failed as a breakwater may form an enduring embankment on dry land. The loose blocks of stone which, when immersed in water, were too light to resist the force of the sea, will remain undisturbed by the trains rolling over them when heaped up in air, though unarranged by the art of the mason, if the stones be allowed to make such slopes as they will naturally form when dropped down from above as if dropped into the sea; or as a measure of dry sand tipped out upon an earthen floor will form its own slopes, so a rubble tipped embankment will take slopes and maintain them against any pressure from above. Such a construction is rigid in air, but subject to movement in water; the circumstances are different. But construction would nevertheless dictate the propriety of filling in the interstices between the rubble, to prevent water, as rain and snow, from passing through the embankment, and so to soften the ground beneath that the stones would sink into it, and thus produce movement in the otherwise rigid structure.

With these considerations would come also another. A small amount of skill in packing stone rubble will greatly reduce the quantity of material necessary to form such an embankment, by substituting less inclined slopes for the natural inclination of the dropped rubble. From merely packing dry rubble to laying it in courses—and from the construction of a heavy mound of rubble to the construction of a series of piers and arches—the steps are gradual; but safe structure and a stable foundation may be obtained by any of the means indicated. Circumstances must dictate the kind of construction proper in each particular case. The quiet traffic of a canal might seem to permit the use of constructions less carefully carried out than those which may be necessary to the carriage road, and more particularly to a railway; but while derangements may occur in the substructure of a railway without stopping its traffic, the canal must be so constructed as to prevent the possibility of defect, for the water is constantly and insidiously working to make defects, the existence of which may involve the safety of the surrounding country, as well as the interruption of its traffic. Hence canal works must be sound and secure constructions—there is no tampering with water—and the same rigorous attention to security should be given to railway constructions, which so often verge upon the dangerous. The railway or road embankment is more commonly made of clays and other plastic earths than of stone; and, as usually made, it is a formation rather than a construction. But an earthen embankment ought to be a construction in the sense in which a wall is a construction. It ought to be executed in layers, not tipped in heaps.

The lateral spread of an earthwork embankment from its crest to its base will depend mainly on the character of the soil. This is to be ascertained by experiments upon the inclination at which it will stand; but as there are soft and yielding places over which earthwork embankments have to be formed, it may be that an embankment will require a wider base because of the yieldingsness of the ground under it, than the soil of which the embankment is to be made would require for itself, even as a merely tipped deposit. On the other hand, it will be found in practice that the application of the principle of construction, as above stated, to the formation of earthwork embankments, gives the means of compelling the earth in any case to stand at much steeper slopes than it will without such disposition and working.

The converse of an embankment in a line of road of any kind, is a cutting, the matter cut out being the material of which the constructed embankment is formed. The sides of a cutting must be securely retained, and constructions of some kind may become necessary. They may be necessary from the peculiar character of the soil, or desirable for economy's sake, whether it be on account of the costliness of the site, or of the heavy earth-works involved to give the sides of a cutting such long slopes as the soil in the case may require. Constructions, that is to say, combinations of foreign materials disposed artificially, do become necessary both with or to embankments and in cuttings. Bridges over rivers or other water-courses, in the bottom of a valley so deep that the material from the nearest cuttings will not fill it, or fill it only at a cost greater than the cost of a bridge; or cuttings so deep, or in such a soil, that economy or stability dictate the employment of retaining constructions which shall be wholly independent of the soil in the work or in the slopes,—these involve considerations of the same nature and character as those which arise in the apparently complex design of Cologne Cathedral, or of the Abbey Church of Westminster.

Every piece of construction should be complete in itself, and independent as such of everything beyond it. A door or a gate serves its purpose by an application wholly foreign to itself; but it is a good and effective, or a bad and ineffective piece of construction, independently of the posts to which it may be hung.

Whilst the wheel of a wheel-barrow, comprising felligs, spokes, and axle-tree, is a piece of construction complete in itself, and independent as such of everything beyond it, an arch of masonry, however large it may be, is not necessarily a piece of construction complete in itself—it would fall to pieces without abutments. Thus, a bridge consisting of a series of arches, however extensive, may be but one piece of construction, no arch being complete in itself without the collateral arches in the series to serve as its abutments, and the whole series being dependent thereby upon the ultimate abutments of the bridge, without which the structure would not stand.

A bridge, of which the bridging way is formed upon arches of masonry, may be thus but one piece of construction; and in like manner, that paragon of constructive skill, the complete church, whether cathedral or otherwise, as built in the pointed style when that style was practised with perfect knowledge of and in full accordance with true constructive principles, is but one piece of construction. Like the long series of arches in a bridge, viaduct, or other such work, in which the piers are vertical supports to the bridging structure, and may be of no greater substance than is necessary to bear the weight coming directly by vertical pressure from the superincumbent structure and its possible load, but throwing all the pressure arising from weight acting laterally, or as thrust, upon terminal abutments—nothing may be omitted, as nothing can be removed from the structure of the pointed-arch cathedral, or other church built in that style,—the whole system of which is bridge-like in construction,—without leaving something unsupported or unrested that requires vertical support or lateral resistance. The western towers of a pointed cathedral form effective abutments to the long series of arches of the inner ranges over the piers which stand between the nave and the aisles on either side, whilst turrets or massive buttresses and deep porches upon the northern and southern transept fronts perform the same duty in respect of the arches of the transepts. The counteracting east end of the chancel forms a true constructive abutment to the arches of the chancel, whilst the tower, with, it may be, a spire upon it, at the intersection of the four grand compartments of the cross, gives, by its weight, abutal to them all. The want of this last-named grand and essential body in the system is but too strongly marked in many of the English cathedrals by the iron bars which have been applied to tie in the arches of the nave, transepts, and chancel, and to relieve the piers upon which the transept arches bear at a higher level, from the thrust to which—being without the weight of a tower upon them—they have continually yielded.

Transversely the weight and the thrust of the vaulted ceilings of the nave are brought up to, and thrown against, the piers of the clerestory, which stand upon the main piers or columns of the interior below, and are abutted by flying buttresses, which carry the thrust down to the pinnacle-weighted buttresses of the outer aisle walls which have already received the weight and thrust of the vaulted ceilings of the aisles themselves. Corbels in the walls, and spreading capitals upon shafts take the weight directly, and leave the walls and piers but little encumbered in the middle, so that the vertical structure is continued upwards without bearing upon the springing stones of the arches.

But it is not necessary that the arch employed should be the pointed arch to produce combinations as effective in construction as the most perfectly designed and extensively elaborated work of the kind referred to as models of constructive skill; the skill consists in a full and clear perception of the bearing and leaning of every part, and of the means necessary to support and counteract the bearings and the leanings within the reasonable limits of the work with reference to its object and purpose—to the end that the work may become complete in itself, and independent as a piece of construction of everything beyond it.

An application of the principles of construction exhibited in the most perfect works of constructive skill ever executed, as above indicated, may be made in the rougher operations of mere practical utility. It has been intimated that the sides of cuttings through certain earths in the for-

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1 This illustration is not intended to apply to the widely distended masses of the older bridges, by which each pier becomes sufficient to abut the arches springing from it; but which, in providing for a way over a river, chokes up the way by the river itself, or compels the river to turn round it down, or otherwise destroy its own banks.

2 In making reference to the noble works of construction above referred to, and in which the art of the mason is mainly employed, as works exhibiting construction most fully and most truly, the Hall must not be passed over without remark.

Of all the great halls of the class to which Westminster Hall belongs, this hall is itself the most effective as a work of construction; and its effect is wholly produced by the magnificent roof which covers it. This roof is a piece of carpentry admirably designed to resolve it into a compact body to act upon the walls in the direction of gravity alone. But the object was not wholly attained, and of this The expense of the first formation of a cutting under given circumstances is easily calculable, and so is the time within which the work may be effected. Experience has proved that there is for every soil a limit in depth beyond which it becomes more expedient to drift the required way, and construct a vaulted tunnel of sufficient dimensions, than to make an open cutting with the requisite slopes. Even when the first cost would not decide the question, the preference is nevertheless often given to the tunnel because of the greater security of constructed work.

A tunnel is expensive, not from the nature and extent of its constructions, but from the circumstances in which those constructions must be executed. The mere constructions are less than would be consumed by common retaining walls to the sides of a cutting not deeper than the height of an ordinary railway tunnel: the several parts of a tunnel derive support from each other, which is not the case with ordinary retaining walls, whose efficiency depends wholly upon the resistance which their own mass or weight and extent of base enable them to offer to the pressure of the body to be retained. If two opposite retaining walls be given sufficient means of assisting one another, they may be at once reduced to one-third of the bulk they would otherwise require, and would then be as safe as the sides of a constructed tunnel, the strength of which, supposing the work to be properly executed, is only limited by the power of the setting material employed in the work to resist compression.

Before proceeding to the consideration of the means of enabling opposite retaining walls to assist each other, it may be worth while to consider, whether retaining walls are generally constructed so as best to adapt their components to the duty to be performed.

No one would place a buttress intended to resist the thrust of an arch, within the springing walls, or under the arch whose thrust is to be resisted; yet in the construction of retaining walls, according to the common practice, the counterfort is placed on that side which receives the pressure, where its utility is very questionable, except to keep the retaining wall from falling back against its load, which, from the transverse section generally given to such walls, they would be apt to do, if not so propped up by their counterforts. Wharf and quay walls, and the revetment walls of military works, may require a face unbroken by projections; but this is not the case with retaining walls for roads and railways, where a long line of projecting buttresses would be objectionable, the counterforts becoming but tresses and merely changing places with the wall.

On account of the common practice of battering the faces of retaining walls in curved lines and of radiating the beds of the brickwork composing them from the centre of curvature in every part, the back of the wall must contain more setting material than the face, with the same quantity of solid brick; that is, if the work be bonded through. Counterforts must be built in the same courses, and consequently must have still thicker beds of compressible mortar than the wall; or the bond between the wall and its counterfort must be dropped, and the counterfort thus become utterly inefficient.

The retaining walls in the cutting upon the line of the extension of the London and North-Western railway, from Camden Town to Euston Square, are, according to the common practice, built wholly of brickwork in radiating courses and with counterforts following their own contour. In this case the centre of gravity of the wall falls wholly behind its base; and the counterforts not commencing until the wall has reached one-third its height, render it still more dependent for support upon the ground it is intended to retain. It is well known that these extensive walls, though furnished with all the collateral works necessary to protect them from exposure to undue influences, and although set nearly one-fourth of their height in the ground, failed to a considerable extent. A system of strutting with cast-iron beams, across from the opposite walls, to make each aid the other, was applied to meet the emergency; but this is limited to the upper parts of the walls.

Abutting struts from opposite walls, occurring at intervals only, leave the intermediate portions of the walls exposed to pressure from behind without support, unless these intermediate portions are so disposed as to communicate the pressure upon them to the struts. Hence a common retaining wall, abutted at intervals, would require these intervals to be more or less distant, in proportion to the strength of the wall between them. Instead, therefore, of a continuous wall on each side of the cutting, buttress walls should be placed at intervals, opposite to one another, and strutted apart at their toes by an inverted arch, and above, at a height sufficient for whatever traffic the cutting is to accommodate, by a built beam of brickwork, in vertical courses, supported on an arch, and prevented from rising under the pressure by an invert upon it. This built beam will then be, as it were, a piece of walling turned down on its vertical transverse section, and will resist any pressure brought upon it through the buttress walls, to the full extent of the power of such a wall built vertically, to bear weight laid upon its summit—the pressure would be applied in the line of the greatest power of resistance, and there would be no tendency to yield, except to a crushing force.

Let such transverse buttress walls, so strutted apart, with the road between them, be the springing walls of longitudinal counter-arched retaining walls, which, being built vertically and in horizontal courses, but arched in plan, against the ground to be retained, will carry all the force exerted against them to their springing walls, and the springing walls or buttresses will communicate, through the struts, the power of resistance of each side to the other, and thus insure the security of both.

This arrangement may be carried to any extent in height, by repeating the abutting beam or strut at such intervals as the thrust to be resisted and the strength of the buttress springing-walls may require.

To constructions thus arranged, any requisite power may be given, by altering the quantity of materials in each part: the length of the buttresses transversely of the cutting,—the number of struts to each pair of buttresses,—or the length of the compartments. The thickness of the buttresses should be in proportion to their height and length, and their length should be in proportion to the flatness and weight of the struts with their arches, and to the space in height between any two of them, as well as to the magnitude of the thrust brought to them by the counter-arched retaining walls. The inverted arch below and the built beam above must, of course, have sufficient substance to enable them to resist, without yielding in any direction, the pressure brought to them through the buttresses; and the retaining walls themselves must have substance given to them according to their height,—to the pressure they are liable to receive from behind,—to the length of the compartments—and the extent of their flexure;—subject, of course, as to all these, to the nature of the materials, workmanship, and mode of structure.

The positive strength which such constructions should possess depends much, of course, upon the nature of the soil, and its susceptibility of being affected by external influences; but it depends, even in a greater degree, upon the manner in which the constructions can be applied to the ground they are intended to retain. A very slight power will retain at rest a body which the exertion of great force could not stop if once in motion; and a half-brick counter-arch, set in close contact with undisturbed ground, would hold safely up what three times the substance would not stop if there were space and opportunity for motion between the ground and the brickwork. It is impossible, therefore, to state precisely what is the least strength which the retaining constructions must have; but there can be no question that too much strength is better than too little, and it is generally cheaper to pay in materials than in labour to save materials.

These diagrams represent a cutting 65 feet deep to the level of the rails. It is assumed, that the ground at the top may stand for the first 15 feet at less than 2 to 1, and that it may, therefore, be cheaper to run out to that depth with slopes, leaving 50 feet from the rails, or about 52 feet in all, to be retained. As the bricklayer may follow up the excavator with bay after bay, his work lying mostly on the side and out of the way of the excavator, the latter would run out the spoil without interruption, his work being benched onwards and shored as he proceeded. As every compartment, with its buttresses, invert, abutting beams and counter-arches is complete in itself, the ground being backed against the counter-arches as the work rises, the shoring would come out, and be sent on for use on the forward benches.

The invert may be turned upon footings in half-brick rings, to get the largest quantity of solid resisting matter in the curved line. At a height from the surface of the rails sufficient for headway—assumed at 14 feet 6 inches—a 14-inch bonded arch is turned from buttress to buttress, springing from skewbacks on corbelled courses. Upon the back of this arch the abutting beam is built of brick on end and edge, bonded as a wall, with beds vertical and widening over the haunches of the discharging arch and under the similar inverted arch turned upon it; so that although the beam be in the centre but 21 inches deep, it presents an abutment at each end of three times that depth. The object of the invert over the abutting beam is to stiffen it and to bring down and distribute the weight and pressure from the buttresses more effectually. The built beam, and its sustaining and stiffening arches, should be composed of particularly well-formed bricks of really good quality, set in Roman cement or other quick-setting mortar, that there may be no yielding to the pressure which must be immediately thrown upon this part of the construction.

Another built beam, of greater depth, because of the absence of any inverted arch to stiffen it, is thrown across over the back of a semi-circular arch, with its abutting ends extended in like manner.

To relieve the work from water, a drain being run along over the middle of the inverts, or side-drains being passed by ring culverts through the buttresses, drain-shafts are carried up at the backs of the buttresses against the springings of the counter-arches, to within a few feet of the surface. These shafts being steened with open joints at intervals to admit drainage water and communicating with the drains below, prevent the possibility of water lodging about the backs of the counter-arches, or even in the ground itself. The drain shafts should be semi-domed with bricks set dry and covered in, and the walls also backed up with good clean gravel, through which the surface water might percolate and pass freely down to the shafts.

The constructions are assumed to be of brickwork, for the obvious reason that the cases supposed being clay cuttings, brick is the material which would be most economical. But if masonry be cheaper, it may of course be used with the same effect. Where a cutting intersects loose beds of laminated stone, and particularly strata inclined to the horizon, so as to be unsafe with the ordinary slopes, such constructions are available; and in cases where the sides of the cutting will stand vertically or nearly so, as in chalk, it may be useful to apply similar constructions, though of slight character, to check the separation and fall of masses from the precipitous sides.

It is obvious, too, that these constructions present the means of security, when the stratum forming the base of any cutting is too weak to bear the weight of slopes, or of retained sides, without rising between them. Sheet-piling may be driven to any depth along the backs of the counter-arched walls so as to be retained at the head by the walls; and thus in effect the walls would be carried down to a safe depth, even through the weak stratum; whereas such piling at the toes of slopes is commonly found to be almost if not wholly useless, for the want of a stay to the head.

Embankments formed in the manner already described—that is, by a process of construction—may be, as previously stated, raised higher and with a relatively narrower base than if formed in the usual insatifiable way; but there is a limit which may not be passed in heaping up compressible or otherwise yielding materials, how skilfullysoever they may be disposed, and this will indicate the limit at which constructions proper may be introduced with economy, as in the converse case of retaining constructions to avoid insecure or expensively long slopes in cuttings.

The height to which the particular soil may be raised upon itself with safety and economy being determined, the greater height required is to be obtained, not by an endeavour to encase the bank by constructions extending laterally as retaining walls, but by bridge-like culverts built under the bank, or of so much of the bank as may be safely built of earth.

Such bridge-like culverts may be composed with great advantage, both in respect of economy and of strength, in the manner suggested by the present writer for the upper works of Westminster Bridge, in the practical treatise on Bridge Building, forming part of the Theory, Practice, and Architecture of Bridges, published by Mr Weale in 1842. This involves a system of groining whereby that important element in construction—rigidity—is obtained, whilst the composition is but a variety of that which is embodied in the structure of the nave and aisles of the model of construction before referred to. Such substracted works may be rough but must be sound as work, and being covered with fifteen or twenty feet in depth of earth, they would not feel the vibrations which act so mischievously upon the lofty and costly viaducts in which railway-makers have exhibited their skill and taste to the cows and the crows, and which react both upon the upper works of the railway and upon the rapidly passing train in the shock felt throughout when the moving load passes the line between earthwork and masonry.