The roofs, of which there are five, one to each of the elevations, are constructed on the "ridge and furrow" principle, and glazed with sheets of glass of 49 inches in length. The construction will be at once understood by imagining a series of parallel rows of the letter V (thus, \/\/\/), extending in uninterrupted lines the whole length of the building. The apex of each ridge is formed by a wooden sash-bar with notches upon each side for holding the laths in which are fitted the edges of the glass. The bottom bar, or rafter, is hollowed at the top so as to form a gutter to carry off the water, which passes through transverse gutters into the iron columns, which are hollow, thus serving as water-pipes; in the base of the columns horizontal pipes are inserted, which convey the accumulated water into the sewers. The exhalations, from so large an extent of surface, from the plants, and from the breath of the innumerable visitors, rising and condensed against the glass, would descend from a flat roof in the form of a perpetual mist, but it is found that from glass pitched at a particular angle the moisture does not fall, but glides down its surface. The bottom bars are therefore grooved on the inside, thus forming interior gutters, by which the moisture also finds its way down the interior of the columns, through the drainage pipes, into the sewers. These grooved rafters, of which the total length is 205 miles, are formed by machinery, at a single operation.
The lower tier of the building is boarded, the walls of the upper portion being composed, like the roof, of glass. Ventilation is provided for by the basement portion being walled with iron plates, placed at an angle of 45 degrees, known as _luffer-boarding_, which admits the air freely, while it excludes the rain. A similar provision is made at the top of each tier of the building. These are so constructed that they can be closed at pleasure. In order to subdue the intense light in a building having such an extent of glass surface, the whole roof and the south side will be covered with canvas, which will also preclude the possibility of injury from hail, as well as render the edifice much cooler.
In the construction of the building care has been taken to give to each part the stiffest and strongest form possible in a given quantity of material. The columns are hollow, and the girders which unite them are trellis-formed. The utmost weight which any girder will ever be likely to sustain is seven and a half tons; and not one is used until after having been tested to the extent of 15 tons; while the breaking weight is calculated at 30 tons. At first sight, there would seem to be danger that a building presenting so great a surface to the action of the wind, would be liable to be blown down. But from the manner in which the columns are framed together they can not be overthrown except by breaking them. Experiments show that in order to break the 1060 columns on the ground floor, a force of 6360 tons must be exerted, at a height of 24 feet. The greatest force of the wind ever known is computed at 22 pounds to the superficial foot; assuming a possible force of 28 pounds, and suppose a hurricane of that momentum to strike at once the whole side of the building, the total force would be less than 1500 tons--not one-fourth of the capacity of the building to sustain, independent of the bracings, which add materially to its strength. So that, if any reliance at all can be placed upon theoretical engineering, there can be no doubt as to the safety of the building.