structing a short section of the projected Broadway underground railway in this city 1868-69. The two tunnels under the Thames River and parts of London for the new London underground electric railway shortly to be opened to the public were built by means of these shields ; also the great railway tunnel under the St. Clair River between Port Huron Mich. and Sarnia Canada. Referring now to our front page illustrations we give at Fig. 1 a perspective front end view of the shield. front end is provided with an inclined cutting edge with horizontal and vertical braces or stanchions back of which are strong vertical bulkheads or diaphragms in which are a series of doors. Arranged around with in the outer walls of the shield are a series of hy draulic jacks of great power all operated by a common pump but each jack provided with a stop cock so that all or any one or any desired number of the jacks may be worked at once as desired. A glance at the engravings will show the positions of these jacks. The shield thus constructed is placed in the heading of earth and the front end of the tunnel is covered by and inclosed within the rear part of the shield. The latter may be said to form a protecting cap or cover to the front end of the tunnel and prevents the caving in of the earth upon the workmen. When the shield the front end of the tunnel as shown in Figs. 1 2 3. in at each door and is broken off into blocks by the men as shown in our illustrations. The pressure on the jacks is now stopped and the pistons of the jacks pushed back within their cylinders which leaves a free space within the rear part or hood of the shield and in this space a new section of the tunnel is built up and joined to the front end of the tunnel. The new section now becomes the front end of the tunnel and against it the jacks are now pushed to send the shield again ahead. The extreme front end of the tun nel is thus always protected and covered by the shield. The resistance encountered by the shield a
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