Page:Walker (1888) The Severn Tunnel.djvu/143

82 upon that; between these bars the roof is secured by polling-boards, the thickness of the boards varying from 1 to 3 inches, according to the weight of the ground to be carried.

When the two bars are placed as above, chambers are cut down to the floor of the heading at a slight angle each way from the length, and into these chambers are inserted props, which are tightly wedged to the bars; these props are known as ‘back-props.’

Having secured the two first bars in the manner mentioned, another chamber is cut following the section of the tunnel like a shelf on the one side of the heading, into which the third crown-bar is rolled, and then secured by back-props in the same manner as the first, the fourth, and other bars following; and when sufficient crown-bars are in this way inserted, secured by the back-props, and themselves holding the polling-boards in place, they are secured by stretchers or short timbers; between these a cutting is made, just inside the back-props, to the level fixed upon for the first sill. The sill-timbers are then brought in, in two pieces. They are large square timbers, from 12 to 15 inches square, scarfed in the middle, and when placed upon the ground in the space which has been taken out for them, are jointed together with strong iron straps, called ‘glands.’ Another row of props is then inserted under the crown-bars, one under each bar resting on the sill; and the same operation is continued till