SUBTERRANEAN LINES. 
139 
They are one-third inch thick, and are provided throughout 
their whole length with a slit one inch wide, for the purpose of 
introducing the cable into the tube, without having to draw it 
through from one end. The tubes are cast with this slit at 
first. A bar of T-iron (fig. 80) enters the slit, and is fixed 
by three keys, one at each end of the tube, the other at the 
middle. The open space of the slit is then filled in with a 
cement made of iron filings, sal-ammoniac and sulphur, and 
this is thickly plastered on. 
The joining of two tubes is thus effected : The tubes are 
laid end to end in a straight line, and the flanges at their 
extremities are surrounded by a ring of india-rubber. When 
water or gas pipes are joined, these rings are entire ; but for 
telegraph lines, the ring must be cut, as it is not put in its 
place until after the cable has been laid. The two ends are 
cut so that their overlap has the same thickness as the band. 
D 
Fig. 80. 
On the india-rubber rings so formed, a wrought-iron hoop 
is fitted in such a manner as perfectly to clip the flanges, and 
the turned-up ends of the hoops are fitted with a bolt and a 
screw, by means of which the hoop may be tightened so as 
strongly to compress the india-rubber. The latter is pre¬ 
vented from being pressed outwards at p by placing there a 
small plate of copper, between the india-rubber and the iron 
collar. 
When the junction has been effected, the opening in the 
flanges, where they are traversed by the slit, must be stopped 
up. For this purpose a small piece of cast iron, T (fig. 82), 
of the exact shape of the flange and width of the slit is intro¬ 
duced. The T-bars are so cut at their ends as to permit the 
introduction of this additional piece. 
At points every 216 yards apart in the subterranean line, 
there are tubes a yard long, provided with an enlargement in 
the form of a vertical cylinder, closed by a cover at the top 
(fig. 83). These arrangements allow an examination to be 
