CON'STRI'CTION (Latin, construe, to heap up into one) the erection or disposition of several separate parts in such a manlier, as to fi win a perfi-et and compact whole.
A good knowledge of the principles of construction, forms an essential item in the qualification of an architect. The principles of construction arise out of and are entirely depen dent upon those of gravitation. 44 Gravity," says an excellent authority on this subject, " is the source of all the inventions, and ingenuity, called into action in the structure of architectural works. '1•e weight or downward temleney of their materials, is the cause of buildings holding together, or filling, or being thrust apart. Gravity, in its various dynamic modifications, is the sole acting power which operates in a building. All the mechanical perfections of scientific building result froth a clear knowledge of the operation of gravity, and from the ability to direct their course: all the mechanical defects of bniblings, result from an ignorance of the laws of gravity,and front inattention or inability to counter balance their effect. A judicious architect enslaves to his pur pose the active force of gravity, and compels it to exert, all its force in holding together more firmly his structure; an igno rant or careless architect or workman, allows that force to exert itself in wracking, straining, distorting, breaking, and destroying his work." The methods in which gravity acts upon materials, are by compression, by tension, and by cross-strain. The first of these modes of operation is the simplest and least destructive, unless exerted to too great an extent, and is that which forms the basis of the most sound (Nmstruction ; its tendency is to bring the particles of matter more closely together ; instances of its application occur in all simple constructions, such as upright piers, arches, &c. The second method, that of ten sion, has it directly opposite tendency to the last, and exerts its influence in disengaging the atoms from each other, it is of course not naturally favourable to construction, but the contrary, nevertheless it is made a very efficient and useful agent ; its influence is never exerted but upon materials which have a strong counteracting tendency, and it is made avail able to produce the first effect of gravity, or compression. Examples of its operation are to be met with in suspension bridges, and in the tie-beams and king or queen-posts of trusses. The third method by which materials are affected by gravity, is cross-strain, which is a combination of tine two last, as it is tension effected by pressure, and its result is to tear or wrench the particles of matter rounder; it is in prin ciple totally inimical to construction, and must be avoided or counteracted. Cross-strain occurs in unscientifically formed roofs, where struts rest 'Ton a tie-beam, also when any ver deal weight presses upon any horizontal beam, as in the ease of brest-summers; it happens likewise, when heavy um russed horizontal beams have too great a bearing, the effect in this case is termed sagging, and is counteracted by cambering or trus,ing the beam.
Analogous, and arising out of these operations of gravity, are the three great principles in construction—repose, equi poise, and tie. The first of these is the simplest, and is the prin
ciple most usually adopted in very ancient buildings; it is used where the materials are merely piled rip perpendicularly, so as to form piers or columns with cross-beams, architraves, or lintels, laid horizontally upon the piers or columns, pressing downwards merely with the gravity of these materials, without any thrust or other inclination to destroy the position of any part of the arrangement. " Buildings constructed on this principle, need only tenacity of material and unflinching foundations to he altogether perfect in construction ; but buildings of this kind, owing nothing to geometrical science, lead to an enormous consutnption ofmaterials; all the materials of the horizontal spanning masses, of even a small building, must be huge, and arc thence immensely expensi ve to procure, and to raise to their destined places ; if these spanning masses be either so long or so brittle as to y ield by their own weight, or by that which may be put upon them, the principle of simple repose becomes destroyed ; the horizontal masses sink, and the piers or sustaining masses are thrust outwardly." The disadvantages attending this mode of construction, led to the invention of others, yet at the same time they all aimed at attaining the same end, namely. simple repose through( (lit the materials and different members of a building. The prin ciple of equipoise in construction is this, that all tendencies to disturb or produce motion amongst the parts of a structure, should be counterbalanced by an equal and opposite tendency, and the most perfect exhibition of its powers is to be seen in the arch. This principle of building allows of the employment of the smallest materials, and ensures stability with the least possible quantity of smatter; it is therefore fiir prefi-rable to the first method or principle. The third principle, of tying, is of modern invention, and by it the quiescent state: of a structure is maintained, not by resisting the power as in the last case, by external opposition or abutments, but by con fining the power by internal restraint. The principle is embodied in the structure termed a truss.
The most perfect specimens of constructive science are to be found in the wonderful erections of the Gothic architects: "The mediteval Christian builders arrived to such a delicate and intimate acquaintance with architectural dynamics, that by the discovery of the way in which all the particles of their materials were affeeted by gravity, they were enabled, by merely subjecting them to the frangibility caused by com pression, so to economize them, and reduce their quantity, that many members of Gothic edifices, after five hundred years' devastation by time, are more sound than corresponding members of our modern builders, which have not subsisted fifty rears, and which contain five times their proportion of materials. So admirable in general is the skill displayed in the dynamic disposition of the material of a Gothic cathedral, so shrewdly are the forces of its gravitation reduced to simple compression, that the whole is like a wonderful piece of shoring, sublimely and permanently imitated in stone."