52
CONSTRUCTION OF THE BUILDING,
Its lightness ofproportion,
no evidence ofinstability.
General nature ofmaterials.
Quantities ofwrought iron;
Cast ironj
The plan dividedinto squares of24 feet.
Necessity of
realizing the de-tail of one 24-feetbay, in orderthereby to judgeof the whole area.
The descriptionof a 24-feet baycommenced.
The foundations.
public may pass freely from one lino of galleries to the other, bridges, at frequentintervals, span the 48 feet avenues, and at the same time divide them into courts,each of which has been so arranged as to present an “ ensemble” to the eye ofthe spectator looking down upon it from the galleries. The width of 48 feetwhich we have described as thus subdivided, and the second aisles, are roofedover at a height of 44 feet from the ground. The remaining portion of thebuilding in width consists of one story only, 24 feet high; in which, of course,there are no galleries. Ten double staircases, 8 feet wide, give access to thesegalleries.
The airy lightness of the whole structure, and its immense dimensions, are thefeatures which will no doubt first excite the wonder, and perhaps the timidity ofthe visitor; but when he learns how rigidly the strength of every portion hasbeen investigated, with what care the connection of every part has been made, andthat the whole of that which appears to him so complicated, is but the repetitionof a few simple elements, he will throw aside alarm, and rest upon the con-sciousness that those most competent to investigate questions of force to overturn,and strength to resist, have spared no pains to assure themselves of the perfectionof the parts, and the consequent stability of the whole.
The lightness of the proportions will at once assure the spectator of the natureof the material which forms the main supports of the building. While the verticalsupports consist entirely of cast-iron, the horizontal connections and girders areconstructed of both wrought and cast iron. Of wrought-iron it has been esti-mated that no less than 550 tons have been used, and of cast-iron 3,500 tons.The whole of the roof, above the highest tier of iron frame-work, consists of woodand glass, and the external enclosures and face-work are constructed almost entirelyof the same materials. It is estimated that 896,000 superficial feet of glass,weighing 400 tons, have been employed; whilst the quantity of wood used, in-cluding the whole of the flooring, has been no less than 600,000 cubic feet.
In designing the building, care has been taken so to arrange that theposition of every column shall occur at the points of intersection of lines, 24 feetapart, crossing one another at right angles, while in roofing and flooring thesquares, into which the whole plan has been thus allotted, have been subdividedinto others of 8 feet. This arrangement accounts for the beautiful regularity ofthe lines of the columns, &c., when viewed diagonally.
In order to afford some idea of the extent of mechanical difficulties involvedin the erection of such a building, and to furnish, as it were, a scale by which toestimate the nature of the work, we shall proceed, before entering upon thesubject of its general extent and arrangement, to describe the mode of construc-tion of one of the 24-feet bays or compartments, taken at random from the sideaisle adjoining the main avenue.
The exact situation of the four columns enclosing the space referred to havingfirst been determined, holes were dug to such a depth as to lay bare the gravel;which extends, with scarcely a fault, over the whole surface of the site, at anaverage depth of between 2 and 4 feet. The size of the holes dug out for thefoundations, and the quantity of concrete thrown into those holes in order to forma secure foundation for the superstructure, was determined by the estimatedweight of that superstructure; and it was so arranged that, allowing for everypossible contingency, under no circumstances should a pressure greater than2£ tons per foot superficial be brought to bear upon the foundation.