86 Mr. Alexander B. Binnie [March 6, 



the walls of the shaft, was fixed a water-tight wrought-iron floor. As 

 the junction between the tunnel and shafts nearest to the river had to 

 be made under compressed air, provision was made for fixing tempo- 

 rary air-tight floors at a level of a few feet above the crown of the 

 tunnel. These air-tight floors were held down by wrought-iron girders 

 12 feet and 4 feet in depth secured to the sides of the shafts so as to 

 prevent the floors from being blown upwards under an air pressure 

 of 4000 tons. 



I think that I have now, in its main outlines, described the prin- 

 cipal features of the work, and must proceed to give some account of the 

 mode of its construction. In doing this, I shall have first to describe 

 the shield and then the mode of working it under compressed air. This 

 shield is a structure of steel, cylindrical in shape, 19 feet 6 inches in 

 length and 27 feet 8 inches outside diameter. It is stiflened by two cir- 

 cular partitions 3 feet apart, and its forward or working face, which 

 presses against the material to be excavated, is divided into twelve 

 pockets or cells, by three horizontal and three vertical partitions. It is 

 within these spaces, which are six feet in height, that the men work. 

 Between the two circular stiflening partitions are formed air-locks and 

 shoots for passing out the excavated material. Arranged round the 

 inner circumference of the shield and attached to it and the circular 

 partitions are disposed twenty-eight hydraulic rams 8 inches in 

 diameter, for the purpose of pressing or pushing the shield forward. 

 In the rear of the shield, or that portion of it which faces the 

 completed tunnel, is a space which is merely enclosed within the 

 outer skin of the shield ; this space is called the tail of the shield : it 

 always overlaps by 2 feet 6 inches, or one cast-iron ring, the last 

 completed portion of the tunnel, and within it are built up the various 

 rings of iron with which the tunnel is lined. Attached to the back 

 or rearward part of the two circular stiffening partitions and pro- 

 jecting into the tail of the shield are two hydraulic erectors for 

 placing the segments of the rings in position. There are two vertical 

 rams which cause a rackwork to move up or down in a vertical 

 direction. These racks gear into a pinion which carries an arm. 

 Consequently, the vertical motion of the rack causes the arm to move 

 through an arc of a little over 180°. This arm carries another ram 

 by which the arm can be lengthened or shortened as desired. In 

 working, the end of the arm can be attached to the lug or projection 

 cast on the centre of the inner side of each segment, where, by the 

 turning and lengthening motion of the arm, the segment can be 

 placed in any desired part of the ring. The shield, weighing about 

 250 tons, was built in an excavation at the top of shaft No. 4, and 

 when completed its ends were closed with timber to make it water- 

 tight and it was floated into shaft No. 4, which had been filled with 

 water. The water was then pumped out of the shaft, and as the water 

 fell the shield floating on its surface gradually descended until it 

 rested on the bottom. 



As above described it will be noticed that the twelve working 



