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.