372 REPORT—1 883. 
line of the tunnel, no land being available, and each is therefore connected 
with the drainage heading by a cross cut forming at Liverpool nearly a 
right angle (97 deg.), and at Birkenhead an obtuse angle (133 deg.), 
with the heading itself. The centre line of the tunnel having been 
ranged across the river by means of a transit instrument, and permanently 
marked on either shore, the angles and lengths of the cross-cuts were 
carefully measured and the work transferred to below ground by means 
of plumb-lines hung in the pumping shafts. These shafts were so 
crowded that base-lines of 12 feet only could be obtained. The lines 
consisted of fine hard-drawn German silver wire, jj; in. in diameter. 
This was selected on account of its tensile strength, combined with free- 
dom from corrosion by water. There was a slow-motion screw at the 
point of suspension for adjusting the wires laterally, and thereby bringing 
them into exact position. The bobs at the ends of the plumb-lines were 
33 lbs. in weight, and were hung in buckets of water to steady them. 
The setting out by this method was checked twice over, with but very 
little variation, and it is anticipated that the lines will meet very closely. 
In the Birkenhead shaft, where there were many obstacles, and where it 
