_By permission of the_ _“Tramway and Railway World” Publishing Co., London_] of a running tunnel, 16 feet in diameter, on the above line. The Great Northern shield is much more powerful than any hitherto employed. Greater hydraulic force is applied, and the “jacks” are more numerous, and considerably larger. The shield used for the sixteen-foot tunnel may be taken as typical of others up-to-date. Its cylindrical skin is composed of half-inch steel plates riveted together at the bottom of the indispensable shaft, which may be, in the future, anything from 50 to 500 feet beneath the surface. In length, the shield from the rear to the cutting edge in front is 8 feet 9 inches, half of this being used by the excavators (as in Brunel’s Thames tunnel), the after part for the erectors of the metal segments of the tube. Round the shield front are mounted ten heavy cast-steel cutters, the pressure upon them being no less than two tons to the square inch, the hydraulic rams exerting this pressure direct upon the back of the cutters, and the purchase is taken off the edge of the nearest tunnel segment already in position. The excavated soil is taken away in trolleys, which, as in a mine, are drawn by ponies on a miniature track, and afterwards sent up to the surface by the nearest shaft. London clay is generally the kind of soil thus bored through in the metropolitan tubes. The Central, while sinking the shafts, met with it 29½ feet below the surface; but before this was reached, 12 feet of made ground, 18 inches of loam, and 16 feet of gravel, had to be pierced. The London clay ran almost without a break between the Bank and Shepherd’s Bush, the only hiatus being at a point between Red Lion Street and Berner’s Street, where the Woolwich and Reading strata cropped up, which proved to consist of hard, red, streaky clay, some beds of white sand, and, strangely enough, beds of hard limestone rock, whose presence had not been anticipated. Tube railways are carried out at considerably varying depths; the Central running in places 100 feet (_i.e._ the height of Westminster Abbey’s nave) below the road, and at the Bank only 65 feet. Some of the proposed tubes burrow much deeper; for instance those of Charing Cross and Hampstead Railway will be from 120 to 216 feet below the surface. Apparently, there is no reasonable limit to the depth at which engineers are prepared to lay their railway tubes. THE TUBE MOLE AT WORK