CONSTRUCTION OF THE BUILDING. 79

the level of the lead flat, and on the tops of those columns timbers were laid,forming landing-stages or tram-ways, to receive the ribs when hoisted. It wasof course necessary to raise the ribs sufficiently high above the lead flat to enabletheir ends to descend upon the tram-ways. To effect this it was determined thattwo ribs should be placed on end, at a distance of 24 feet from each other, andframed together with purlins and diagonal ties, exactly as they would have to beframed in their finished state. Two complete sets of additional temporary tieswere further introduced, to provide for the strain to which the ribs would neces-sarily be exposed from their altered position in the act of hoisting. The feet ofthe ribs were securely joined to stout pieces of timber, to afford the means ofsafely attaching the cords by which they were to be raised. Thus framed together,the ribs were moved on rollers to the centre of the square formed by the inter-section of the nave and transept.

On the extra strong trusses which have been described as spanning the nave Provisions forat this point, two pairs of shear-legs were fixed at 24 feet from one another, andsecured by ropes connecting them with distant portions of the building. Thesehoisting shears consisted of two legs on each side of the transept, each leg beingformed of three stout scaffold-poles lashed together at the top, and footed onplanks laid across the lead flat. The heads of these shear-legs inclining slightlyforwards, had connected with them blocks and pulleys from which descendedropes, attached to the four ends of the two ribs. The hoisting ropes connectedwith the sets of pulleys passed down from the shears to leading-blocks, attachedto the four columns at the angles of the intersection of the nave and transept.

From these guide-blocks they were led off diagonally to four powerful crabs, soarranged that the gangs of men employed at each were placed opposite the end ofthe rib acted upon by the crab they worked ; and thus the foreman of each gangwas enabled so to regulate the exertions of his men as to make them correspondwith those of the remaining gangs, and to maintain the two ends on each side ina perfectly horizontal plane.

As the diameter of the semicircular ribs exceeded the width of the transept by Raising,their own thickness, it became necessary, in order that they might pass betweenthe trusses, to commence by raising two of their ends to a considerable heightfrom the ground; and to maintain their diameter at the same angle of inclinationuntil they- were hoisted above the columns into which they had to drop. Onraising them to a height of about 65 feet from the ground, the highest ends weredrawn in a horizontal direction, so as to hang over a portion of the lead flats, andthus room was left to allow the other ends to be lifted to a corresponding heighton the opposite side. The ribs were shifted slightly in a horizontal direction untilthe ends came over the columns; they were then lowered down upon rollersplaced upon the tram-ways above mentioned, and by means of these rollers theribs were moved along to the furthest end of the transept. The place in thecentre of the building occupied by the ribs thus hoisted was immediately takeny another pair, which were similarly connected, raised, and moved to within24 feet of the first pair.

When the whole of the ribs were thus elevated to their places, the spaces when raised, howbetween them were filled up with the necessary intermediate ribs and connections;and thus the whole roof was framed together complete.

The raising of the main ribs commenced on the 4th of December, and the Time occupied inwhole sixteen were fixed in one week. It occupied about an hour to raise a pair