Page:Walker (1888) The Severn Tunnel.djvu/147

84 is, centres made by bolting two or three thicknesses of elm planks together into an arch of 3 inches less radius than the interior of the brickwork of the tunnel. The centres would be set from 3 feet 6 inches to 4 feet apart, according to the weight they would have to carry; and the end ribs of each length should generally be thicker and stronger than the middle ribs, as they have to bear the weight of the crown-bars of the adjoining length. These end ribs are called ‘leading-ribs.’

When the centres have been set, one ‘lagging’ is placed on at each side. A lagging is an ordinary plank or batten, 6 or 7 inches wide, and 3 inches thick. Behind this the bricklayers lay over-handed the bricks of the arch of the tunnel, making good to the ground at the back of the arch as they come up; or if the ground is loose, to the polling-boards, which have to be left in. This system is continued on both sides at once, putting on one lagging at a time till the arch is completed, except about 18 inches in the crown. To complete this, which can only be done by one man, cross laggings are used, called ‘block laggings.’

These being very short, are only made of 1½-inch boards, resting upon the top laggings at each side; the bricklayer, placing one or two at a time, works himself backwards till at last he completes the length, or, at a junction between two lengths, comes into a small hole, just the size of his body, which is known as the ‘pigeon-hole.’ This pigeon-hole he