[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 Löhneyss' _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.)]
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.
The reduction of the multiplicity of parts in the Brunel shield to a single rigid unit was of immense advantage and an advance perhaps equal to the shield concept of tunneling itself. The Barlow-Greathead shield was like the cap of a telescope with a sharpened circular ring on the front to assist in penetrating the ground. The diaphragm functioned, as did Brunel's breasting boards, to resist the longitudinal earth pressure of the face, and the cylindrical portion behind the diaphragm bore the radial pressure of roof and walls. Here also for the first time, a permanent lining formed of cast-iron segments was used, a second major advancement in soft-ground tunneling practice. Not only could the segments be placed and bolted together far more rapidly than masonry lining could be laid up, but unlike the green masonry, they could immediately bear the full force of the shield-propelling screws.