heading, but if the first cutting takes place at the bottom of the
rock to form the base of the tunnel it is called a drift.

Driving a heading is the most difficult operation of rock tunneling.
Sometimes a heading is driven a couple of thousand feet ahead of the
other sections. In soft rock it is often necessary to use timber props
as the work proceeds and follow up the excavating by lining roof and
sides with brick, stone or concrete.

The rock is dislodged by blasting, the holes being drilled with
compressed air, water force or electricity, and, as has been said,
powerful explosives are used, nitroglycerine or some nitro-compound
being the most common. Many charges can be electrically fired at the
same time. If the tunnel is to be long, shafts are sunk at intervals
in order to attack the work at several places at once. Sometimes these
shafts are lined and left open when the tunnel is completed for purposes
of ventilation.

In soft ground and subaqueous soil the "shield" is the chief apparatus
used in tunneling. The most up-to-date appliance of this kind was that
used in constructing the tunnels connecting New York City with New
Jersey under the Hudson River. It consisted of a cylindrical shell of
steel of the diameter of the excavation to be made. This was provided
with a cutting edge of cast steel made up of assembled segments. Within
the shell was arranged a vertical bulkhead provided with a number of
doors to permit the passage of workmen, tools and explosives. The shell
extended to the rear of the bulkhead forming what was known as the
"tail." The lining was erected within this tail and consisted of steel
plates lined with masonry. The whole arrangement was in effect a
gigantic circular biscuit cutter which was forced through the earth.