The roof over the palm-house, Kew Gardens, is also worthy of description, on account of its exceedingly light and elegant appearance. The total length of this house is 362 feet 6 inches in the clear, the central portion measuring 137 feet 6 inches long, and 100 feet wide, by 63 feet high in the clear, exclusive of the lantern, which is 6 feet. The wings are each 112 feet 6 inches long, and 50 feet wide, and 27 feet high from the floor to the bottom of the lantern. The roof is in this case entirely of wrought-iron, the main ribs being formed of 9 inch deck-beam iron of circular section, hollow at the core, with four double flanges at right angles to each other. These ribs are obtained in lengths of about 12 feet, and are welded together to the required length, about 42 feet, and then bent upon a template to the requisite curvature.
The roof of the wings is of a single span, the rib being curved of a semicircular form, the extremities of which spring from the ground on either side of the plan, and foot into a solid block of granite, upon a concrete foundation. In the main or central building this semicircle is divided into two quadrants, which cover the aisles of this compartment, the lower ends being bedded upon granite, as above, and the upper ends into the tops of strong cast-iron columns which divide the width of this portion of the building into three aisles. From the top of the same columns springs a circular rib, similar to that in the wings, which is again surmounted by a lantern 6 feet in height. From the top of the columns brackets project, which on one side carry a gallery, running round the bottom of the upper and on the other side serve to assist in the strengthening of the ribs. The column heads are connected by a continuous curb of similar scantling to the ribs, and the whole of the ribs are braced together and strutted by wrought-iron tie-rods, passing through east-iron tubes, which act as purlins. These purlins are formed of a round bar 1-1- inch in thickness, welded in long lengths, and passing through the ribs, so as to form a continuous tension rod all round the house at each purlin, with means of strain ing them as tight as possible. This tension-bar is covered or enclosed in a tubular bar of wrought-iron, exactly fitting between the ribs, and acting as distance-pieces in opposition to the strain of the tension-rods : thus is the entire structure compacted together. The distance between the main ribs is
12 feet 6 inches, and between the purlins 9 or 10 feet. This is in every respect a very elegant roof, and astonishingly light for so great a span.
The iron roof erected over the railway station at Lime street Liverpool, is of great span, and of novel and ingenious construction. The area roofed over in one span extends from the titcade in Lime-street to the viaduct over which llothain - street passes, and ffom the inner faces of the receiving offices to about the middle of the old parcel office on the opposite side ; thus making the extreme length 374 feet and the breadth 153 feet 6 inches. The roof consists of a series of segmental principals, or girders, fixed at inter vals of 21 feet 6 inches from centre to centre : these are supported on one side upon the walls of the offices, as far as they extend, and thence to the viaduct, a distance of 60 feet 4 inches, upon a box-beam of •ronu-iron, whilst on the other side they rest on cast iron columns. The principals are trussed vertically by a series of radiating struts, which are made to act upon them by straining the tie-rods and diagonal braces ; they are trussed laterally by purlins placed over the radiating struts, and intermediately between them, as well as by diagonal bracing, extending from the bottom of the radiating struts to the top of the corresponding struts in the adjoining principal. These diagonal braces are connected with linking plates, by a bar of the same scantling, and also with the purlins already referred to. The curved ribs are thus firmly drawn together and attached to one another, and a rigid framework is formed, upon which the covering of corrugated iron and glass is laid.
Each principal girder is composed of a wrought-iron deck beam, 9 inches in depth, with a plate 10 inches wide and 1 inch thick, riveted on the top. The upper flange of the deck. beam is 41 inches wide and 1 inch thick ; the lower flange 3 inches wide and one thick : the web is about inch thick. This curved rib is formed of seven pieces connected with each other at the points where the radiating struts are attached, by means of plates riveted on both sides : these plates are 6 feet long, 7 inches broad, and inch thick. The beam is also strengthened at the haunches, for a distance of 27 feet from the springing, by plates 7 inches broad and inch thick, fastened by rivets.