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THE MERSEY TUNNEL. 373 
was therefore difficult to see whether the lines were hanging free, they 
were tested electrically. A galvanometer and battery being included in 
circuit with the lines, the bobs hanging free in the air, one pole of the 
battery was put to earth, the other being connected with the plumb-line. 
If making earth by contact at any point the galvanometer was deflected, 
the wet condition of the shaft ensuring the making of good earth. If 
free there was no current shown. From each shaft is being driven a 
heading or drainage gallery, rising with gradients of 1 in 500 and 1 in 
900 towards the middle of the river, and connected, at intervals, with 
the main tunnel by bore-holes. The portion of this heading which is 
executed by hand is taken out 10 feet 4 inches in diameter, and lined 
with brickwork in cement 14 inches thick, thus leaving a net diameter of 
8 feet. Below the invert, and for the purpose of clearing the water from 
the brickwork during construction, a pipe-trench is cut in the rock to 
receive pipes 18 inches in diameter. An attempt has been made, with 
some success, to stop back a portion of the water by iron cribs and brick- 
work in the drainage headings. Cast-iron rings, of hollow box section, 
being some 18 inches on the bed and 6 inches deep, have been placed at 
intervals, one at each end of a section of brick lining, the rock being cut 
out sufficiently large in diameter to receive them. The ring or crib being 
placed in position, and standing vertical, was tightly wedged all round 
the outside edge between the crib and the rock with wood wedging, until 
this became so compact that a chisel would not enter it. The brickwork 
lining between the two cribs was then completed, and the whole made 
tight. The intention was then to seal up the two ends, and confine the 
water to that particular section, and so prevent its passage along between 
the brickwork and the rock on to the next section. Were the rock 
thoroughly impervious, the result would be perfect, but, in the case under 
notice, a considerable proportion of the water penetrated through the 
rock at the back of the cribs. Instead of using cribs, close building in 
brick is now being resorted to, the rock having been first carefully 
trimmed allround, If carefully done, this baffles the water to a large 
extent, and is far less costly than the method above described. The pipe- 
trench is made good with concrete put in place in bags before setting, 
and the invert is constructed with blocks of brickwork prepared on the 
surface. Altogether 930 lineal yards of this heading have been driven 
by hand, the average speed at each face being 11 yards per week. The 
cement was at first mixed in the proportions of three to one, but, upon 
testing the work with the head of water, it was not found to be thoroughly 
watertight, and the proportion has since been increased to two to one, 
with the most satisfactory results. The greatest care is taken to fill any 
cavities at the back of the brickwork with sandstone or broken bricks 
in cement of the same proportion. In the spring of the present year 
arrangements were matured for introducing into the Birkenhead heading 
the machine invented by Colonel Beaumont, R.E., and Captain English, 
R.E., which consists of a strong frame some 30 feet long, upon 
which is fixed an upper bed which carries the machinery. This upper 
bed can be moved forward by a screw-feed on the lower frame, the feed 
in the sandstone rock being 3 inch per revolution, and the speed of the 
bore-head about one and a half revolution per minute, being about one- 
third the speed at which it can be driven in chalk. When some 4 feet 
6 inches have been cut the action of the feed is reversed, and, the weight 
of the machine being taken by hydraulic jacks, the lower frame is moved 
