A number of contemporary popular accounts of the tunnel exist, but one of the most thorough and interesting expositions on a single tunnel work of any period is Henry Law's _A Memoir of the Thames Tunnel_, published in 1845-1846 by John Weale. Law, an eminent civil engineer, covers the work in incredible detail from its inception until the major suspension in late 1828 when slightly more than half completed. The most valuable aspect of his record is a series of plates of engineering drawings of the shield and its components, which, so far as is known, exist nowhere else. These formed the basis of the enlarged section of the shield, shown to the right of the model of the tunnel itself. A vertical section through the shield is reproduced here from Law for comparison with the model (figs. 21 and 23). [Illustration: Figure 17.--SOFT-GROUND TUNNELING. The support of walls and roof of mine shaft by simple timbering; 16th century. MHT model--3/4" scale. (Smithsonian photo 49260-J.)] [Illustration: Figure 18.--SOFT-GROUND TUNNELING. The model of a 16th century mine in the Museum of History and Technology was constructed from illustrations in such works as G. E. von Loehneyss' _Bericht vom Bergwerck_, 1690, as well as the better known ones from _De re Metallica_.] [Illustration: Figure 19.--THE SUCCESSIVE STAGES in the enlargement of a mid-19th century railroad tunnel, using the Austrian system of timbering. MHT model.] [Illustration: Figure 20.--M. I. BRUNEL'S THAMES TUNNEL, 1825-1843, the first driven beneath a body of water. MHT model--1/4" scale. (Smithsonian photo 49260-F.)] THE TOWER SUBWAY Various inventors attempted to improve upon the Brunel shield, aware of the fundamental soundness of the shield principle. Almost all bypassed the rectangular sectional construction used in the Thames Tunnel, and took as a starting point a sectional shield of circular cross section, advanced by Brunel in his original patent of 1818. James Henry Greathead (1844-1896), rightfully called the father of modern subaqueous tunneling, surmised in later years that Brunel had chosen a rectangular configuration for actual use, as one better adapted to the sectional type of shield. The English civil engineer, Peter W. Barlow, in 1864 and 1868 patented a circular shield, of one piece, which was the basis of one used by him in constructing a small subway of 1350 feet beneath the Thames in 1869, the first work to follow the lead of Brunel. Greathead, acting as Barlow's contractor, was the designer of the shield actually used in the work, but it was obviously inspired by Barlow's patents.