[Illustration: Figure 23.--VERTICAL SECTION THROUGH BRUNEL'S SHIELD.
The long lever, x, supported the wood centering for turning the
masonry arches of the lining. (LAW, _A Memoir of the Thames
Tunnel._)]
[Illustration: Figure 24.--THAMES TUNNEL. SECTION THROUGH riverbed and
tunnel following one of the break-throughs of the river. Inspection of
the damage with a diving bell. (BEAMISH, _A Memoir of the Life of Sir
Marc Isambard Brunel_.)]
The Tower Subway at first operated with cylindrical cars that nearly
filled the 7-foot bore; the cars were drawn by cables powered by small
steam engines in the shafts. This mode of power had previously been
used in passenger service only on the Greenwich Street elevated
railway in New York. Later the cars were abandoned as unprofitable and
the tunnel turned into a footway (fig. 32). This small tunnel, the
successful driving due entirely to Greathead's skill, was the
forerunner of the modern subaqueous tunnel. In it, two of the three
elements essential to such work thereafter were first applied: the
one-piece movable shield of circular section, and the segmental
cast-iron lining.
The documentation of this work is far thinner than for the Thames
Tunnel. The most accurate source of technical information is a brief
historical account in Copperthwaite's classic _Tunnel Shields and the
Use of Compressed Air in Subaqueous Works_, published in 1906.
Copperthwaite, a successful tunnel engineer, laments the fact that he
was able to turn up no drawing or original data on this first shield
of Greathead's, but he presents a sketch of it prepared in the
Greathead office in 1895, which is presumably a fair representation
(fig. 33). The Tower Subway model was built on the basis of this and
several woodcuts of the working area that appeared contemporaneously
in the illustrated press. In this and the adjacent model of Beach's
Broadway Subway, the tunnel axis has been placed on an angle to the
viewer, projecting the bore into the case so that the complete circle
of the working face is included for a more suggestive effect. This was
possible because of the short length of the work included.
Henry S. Drinker, also a tunnel engineer and author of the most
comprehensive work on tunneling ever published, treats rock tunneling
in exhaustive detail up to 1878. His notice of what he terms
"submarine tunneling" is extremely brief. He does, however, draw a
most interesting comparison between the first Thames Tunnel, built by
Brunel, and the second, built by Greathead 26 years later:
FIRST THAMES TUNNEL SECOND THAMES TUNNEL
(TOWER SUBWAY)
Brickwork lining, 38 feet Cast-iron lining of 8 feet
wide by 22-1/2 feet high. outside diameter.
120-ton cast-iron shield, 2-1/2-ton, wrought-iron shield,
accommodating 36 miners. accommodating at most 3 men.
Workings filled by irruption "Water encountered at almost
of river five times. any time could have been
gathered in a stable pail."
Eighteen years elapsed between Work completed in about
start and finish of work. eleven months.
Cost: $3,000,000. Cost: $100,000.
[Illustration: Figure 25.--TRANSVERSE SECTION THROUGH SHIELD, after
inundation. Such disasters, as well as the inconsistency of the
riverbed's composition, seriously disturbed the alignment of the
shield's individual sections. (LAW, _A Memoir of the Thames Tunnel_.)]
[Illustration: Figure 26.--LONGITUDINAL SECTION THROUGH THAMES TUNNEL
after sandbagging to close a break in the riverbed. The tunnel is
filled with silt and water. (LAW, _A Memoir of the Thames Tunnel_.)]
[Illustration: Figure 27.--INTERIOR OF THE THAMES TUNNEL shortly after
completion in 1843. (_Photo courtesy of New York Public Library
Picture Collection._)]
[Illustration: Figure 28.--THAMES TUNNEL in use by London Underground
railway. (_Illustrated London News_, 1869?)]
[Illustration: Figure 29.--PLACING A segment of cast-iron lining in
Greathead's Tower Subway, 1869. To the rear is the shield's diaphragm
or bulkhead. MHT model--1-1/2" scale. (Smithsonian photo 49260-B.)]