shield for soft ground tunneling. In 1863 he took out a patent for
such a shield with a cylindrical cast iron lining for the completed
tunnel. Of course James Henry Greathead very materially improved the
shield, so much so indeed that the present system of tunneling by means
of circular shields is called the Greathead not the Barlow system.
Greathead and Barlow entered into a partnership in 1869. They
constructed the tunnel under the Tower of London 1,350 feet in length
and seven feet in diameter which penetrated compact clay and was
completed within a period of eleven months. This was a remarkable
record in tunnel building for the time and won for these eminent
engineers a world wide fame. From thenceforth their system came into
vogue in all soft soil and subaqueous tunneling. Except for the
development in steel apparatus and the introduction of electricity as
a motive agent, there has not been such a great improvement on the
Greathead shield as one would naturally expect in thirty years.

The method of excavating a tunnel depends altogether on the nature of
the obstruction to be removed for the passage. In the case of solid
rock the work is slow but simple; dry, hard, firm earth is much the
same as rock. The difficulties of tunneling lie in the soft ground,
subaqueous mud, silt, quicksand, or any treacherous soil of a shifting,
unsteady composition.

When the rock is to be removed it is customary to begin the work in
sections of which there may be seven or eight. First one section is
excavated, then another and so on to completion. The order of the
sections depends upon the kind of rock and upon the time allotted for
the job and several other circumstances known to the engineer. If the
first section attacked be at the top immediately beneath the arch of
the proposed tunnel, next to the overlying matter, it is called a