Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/496

448 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 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 other timber 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 perdurable 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 incompressibility 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 or cement, so that although the main constituent materials are wholly incompressible, masses of either or of both combined in structures are compressible until the setting medium has indurated to a like 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. When, therefore, fitting stone of both qualities can be obtained, the harder stone can be exposed 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.

Good bricks are less iibsorbent of moisture than any stone of the same degree of hardness, and are better non conductors 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 having 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 bo necessary to exclude wind or water, to prevent the dis integrating 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.

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 in effective piece of construction, independently of the posts to which it may be hung. Whilst the wheel of a wheel barrow, comprising fellies, 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 com plete 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. 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 tend, in providing for a way over a river, to choke up the way by the river itself, or compel the river to throw it down, or otherwise destroy its own banks. A bridge, of which the 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 in full accordance with true constructive principles, is but one piece of construc tion. As in 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 unresisted 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 both sides, whilst turrets or massive buttresses and deep porches upon the northern and southern transept fronts perform the same services in respect of tlio arches of the transepts. The counteracting east end of the chancel forms a true con structive 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, abuttal 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, 