The entire length of the Passengers' Shed (to which this roof belongs) is 212 feet, and 53 feet wide. There is room for about 24 carriages, which is divided into three lines of rails ; the outer lines being for the arrival and departure of trains, and the centre line for spare carriages. On each side of the shed is a platform raised nearly level with the floor of the carriages, 11 feet wide, and connected at the end; this is covered with asphaltc, and coped next the rails with stone. In the centre of the shed is a carriage-truck, that moves transversely to the rails, for the purpose of shifting carriages from one line to another, when any extra are required in a train. Between the rails of each line (170 feet in length) the ground is sunk between walls a sufficient depth to enablo workmen to get at the wider side of the carriages and engines when required.
The roof of Christ's Hospital is well worthy consideration. It covers an area 51 feet wide by 187 feet in length, and consists of principal trusses running across the building, with smaller longitudinal trusses between every two main trusses, to carry the rafters. In the main truss the tie-beam is sus pended from five points by king and queen-posts, which are however of iron, and bolted through the tie. Between th4 two innermost queen-posts, or suspenders, is a straining-beam, which abuts against their heads and is also bolted 'to the principals ; its length is 16 feet, and it is supported at two intermediate points by struts from the feet of the queen posts ; there is also another strut for the other side of the post to the head of the further or outermost one. The extre mities of the tie-beams and rafters rest in iron shoes, and partially upon cast-iron standards supported on stone corbels 10 feet below the ceiling line.
The five longitudinal trusses, to support the main rafter between the principal trusses, is extended from each queen post in one cross truss to the corresponding one in the next ; the distance between the main trusses, and therefore the length of the tie-beams, being 17 feet. These are simple king-post trusses without struts, and carry the main rafter on top of the king-post, which, in this case, is of timber, bolted to the tie-beam. Filling-in beams arc placed between these trusses to receive the ceiling-joists. The scantlings of this roof are as follow : Principal truss : Tic-beam, 14 inches by 14 inches.
Principal rafters outside the innermost queen-posts, 12 inches by 9 inches at lower end ; 10 inches by 9 inches at upper end.
Principals within and above the innermost queen-posts, 9 inches by 9 inches.
Straining-beam, 12 inches by 9 inches.
Struts, 6 inches by 6 inches, (longitudinal trusses.) Tie-beams, 12 inches by 7 inches.
King-post, 6 inches by (3 inches.
Struts, 6 inches by 6 inches.
Main rafters between trusses : At lower ends, 12 inches by 7 inches.
At upper end, above queen-posts, 9 inches by 7 inches. Common rafters, (longitudinal) 7 inches by 5 inches.
The roof over Exeter Hall is of simple construction, the span being not less than 76 feet, and the height of the roof from the underside of the tie-beam, 21 feet 6 inches ; the trusses are placed at alternate intervals of 2 feet 6 inches, and 9 feet.
Each truss has the tie-beam, which is scarfed, suspended in six points by queen-posts, the innermost pair being pro vided with a straining-beam, and with auxiliary rafters on the other side of them, each of which is supported by two struts resting on the foot of the queen-posts. The two outermost queen-posts on either side are hollow, and are suspended from the principal rafters, as are also the inner ones, by means of straps. The st•aining-beam, which is 22 feet long. is suspended from the apex of the principals by a king-post, from which struts are raised to shorten the bear ing of the principals. Two sloping timbers hut against the feet of the inner queen-posts, from the apex of which an iron suspender is bolted to the tie-beam. The ends of the tie-beams and principals rest in iron shoes. The scantlings are : Tie-beams, 141 inches by 71 inches.
Principals, S1 inches by 71 inches.
Auxiliary rafters, or under principals.] 4 inches by 71 inches. Inner queen-posts, SI inches by '71 inches.
Outer ditto, 10 inches by 41 inches.
Intermediate ditto, 12 inches by 41 inches.
King-post, 6 inches by 7- inches.
Straining-beam, 14 inches by 71 inches.
Braces, 71 inches by 71 inches.
Upper ditto, 6 inches by 7A- inches.
Purlins, 7A- inches by 4 inches.
Common rafters, 5 inches by 24- inches.
Ridge-piece, S inches by 31 inches.
Wall-plates, 131 inches by 61 inches.
Pole-plates, 12 inches by 4 inches.
The truss over the arched roof of St. George's Hall, Liverpool, which is 65 feet in width, is of very simple con struction. The tie-beam is sustained at five points by a king post and fun. queen-posts. The principals have auxiliaries under them, but do not meet at an apex, reaching only as far as the inner queen-posts, which are kept apart by a double straining-beam at top, and by another at their feet. Struts are placed between the head of the king-post, and feet of the queen-posts, and thence again to the head of the outer queen-posts.