01* CONSTRUCTION.] BUILDING 449 transepts, and cliauccl, 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 clere story, which stand upon the main piers or columns of the interior below, and are abutted by Hying buttresses, which carry the thriut 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 con structive skill ; the skill consists in a full and clear per ception 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. In making reference to the noble works of construc tion above referred to, in which the art of the mason is mainiy employed, as works exhibiting construction most fully and most truly, the hall must not be passed over without remark, and of all the great halls of the class to which Westminster Hall belongs, it 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 designer may not have felt quite certain of the results, so whilst erecting massive walls on which to place his elaborate combination of timber, he threw up against the lateral walls a series of flying but tresses to check any tendency of the roof to spread under its own weight in the absence of a thorough transverse tie ] for these buttresses are said to be independent of the walls, not being built into them. 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. The sides of cuttings through certain earths in the formation of lines of inland communi cation, whether carriage roads, railways, or canals, are sometimes required to be widened out to an inordinate extent because of the looseness or slipperiness of the soil, or must otherwise be retained or held upright by special constructions. 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. 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 com ponents 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, accord ing 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 unobjectionable, the counterforts becoming buttresses and merely changing places with the wall. On account of the common practice of battering the faces of retaining walls in curved lines, and Battering, 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 compres sible 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 Examples extension of the London and North- Western Railway, from a t Camden Camden Town to Euston Square, are, according to the Town< 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 Fig. 1. Transverse section of the Euston Incline retaining walls, one-half as executed with cast-iron struts to count erf orted and reclining walls, and the other half with the brick-built abutting beam to counter-arched retaining walls strutted at the toes of the springing walls by inverted arches. Plan of the above showing the part as executed above the iron struts, with the rails passing underneath, and the other part at the level of the rails, with the inverts in plan under them. 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 Strutting, cast-iron beams, across from the opposite walls, as shown in
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