[Illustration: FIG. 124.--Longitudinal Section of Brunel’s Shield, First Thames Tunnel.] The illustration, Fig. 124, is the section parallel to the vertical plane of the tunnel through the center of one of the frames, and it shows quite clearly the complicated details of the shield construction. Two features which are to be particularly noted are the suspended staging and centering for constructing the roof arch, and the top plate of the shield extending back and overlapping the roof masonry so as to close completely the roof of the excavation and prevent its falling. Notwithstanding its complicated construction and unwieldy weight of 120 tons, this shield worked successfully, and during several months the construction proceeded at the rate of 2 ft. every 24 hours. There were two irruptions of water and mud from the river during the work, but the apertures were effectually stopped by heaving bags of clay into the holes in the river bed, and covering them over with tarpaulin, with a layer of gravel over all. The tunnel was completed in 1843, at a cost of about $5600 per lineal yard, and 20 years from the time work was first commenced, including all delays. [Illustration: FIG. 125.--First Shield Invented by Barlow.] The next tunnel to be built by the shield system was the tunnel under London Tower constructed by Barlow and Greathead and begun in 1869. In 1863 Mr. Peter W. Barlow secured a patent in England for a system of tunnel construction comprising the use of a circular shield and a cylindrical cast-iron lining. The shield, as shown by Fig. 125, was simply an iron or steel plate cylinder. The cylinder plates were thinned down in front to form a cutting edge, and they extended far enough back at the rear to enable the advance ring of the cast-iron lining to be set up within the cylinder. In simplicity of form this shield was much superior to Brunel’s; but it seems very doubtful, since it had no diametrical bracing of any sort, whether it would ever have withstood the combined pressure of the screw-jacks and of the surrounding earth in actual operation without serious distortion, and, probably, total collapse. It should also be noted that Barlow’s shield made no provision for protecting the face of the excavation, although the inventor did state that if the soil made it necessary such a protection could be used. The patent provided for the injection of liquid cement behind the cast-iron lining to fill the annular space left by the advancing tail-plates of the shield. Although Barlow made vigorous efforts to get his shield used, it was not until 1868 that an opportunity presented itself. In the meantime the inventor had been studying how to improve his original device, and in 1868 he secured additional patents covering these improvements. Briefly described, they consisted in partly closing the shield with a diaphragm as shown by Fig. 126. The uninclosed portion of the shield is here shown at the center, but the patent specified that