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.
[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_.)]