66 TUB SEVEEN TUNNEL. 



Tho river heading was begun in December, 1874, and 

 passed again tiirongh the dipping beds into the Pennant. 

 The bed of coal shale was very heavy ground, wliich had to 

 bo double timbered and floored, which made the heading 

 small and low at that point. The Pennant was at first of a 

 red colour, and fnll of baohs, or open joints, from which 

 flowed a good deal of water. This water at first was of the 

 colour of weak porter, and had a very salt and bitter taste ; 

 but after a few days it ran clear. What was tapped first 

 had no doubt been lying there for ages undisturbed, the 

 colour showing probably the presence of iron. Some 

 copious springs were afterwards found in the Peimant ; so a 

 flood door was built in the Pennant, 340 yards from tho 

 shafii, to prevent the flooding of tho pumps in the event of a 

 breakdown, which it did on two or three occasions. This 

 door was very strong, and largo enough to let the trolleys 

 pass through ; it also had a 1'2-inch sluice valve. 



And now, seeing that this heading would have to be driven 

 two miles under the river from this one sliaft, it became of 

 great importance to adopt the most correct means possible 

 of getting the linos of tho Tunnel down the shaft and along 

 the heading. The usual way has been, to drop a plumbob 

 down each side of tho shaft ; but this was found to be not 

 nearly accurate enough for the purpose. 



The plumb lines could only bo got thirteen feet apart ; 

 and though tho bobs were large, and dropped into buckets of 

 water to steady them, they could not be made really steady, 

 for a pendulum two hundred feet long vibi-atcs slowly. It 

 has a beat of nearly eight seconds each way ; and wlien the 

 extent of the vibration is less than an eighth of sm inch, it 

 appears to the eye to bo steady; but when the cross hairs of 

 a transit below are fixed on the wires, first one and then tho 

 other apnears to leave the lino, and you may, as tlio writer 



