In closing these remarks concerning the rear-end construction, the accompanying table, prepared by Mr. Raynald Légouez, will be of interest, as a general summary of principal dimensions of most of the important tunnel shields which have been built. The figures in this table have been converted from metric to English measure, and some slight variation from the exact dimensions necessarily exists. The different columns of the table show the diameter, total length, and the length of each of the three principal parts into which tunnel shields are ordinarily divided in construction as previously described: +---------------------+-----------------------------------+ | | LENGTH IN FEET. | | NAME OF SHIELD. +---------+-----+-----+------+------+ | |DIAMETER.|TAIL.|BODY.|FRONT.|TOTAL.| +---------------------+---------+-----+-----+------+------+ |Concorde Siphon | 6.75 | 2.51| 2.55| 1.16| 6.67| |Clichy Siphon | 8.39 | 2.51| 2.55| 1.16| 6.16| |Mersey | 9.97 | 5.61| 2.98| 2.98| 11.58| |East River | 10.99 | 3.51| 0.32| 3.67| 7.51| |City and South London| 10.99 | 2.65| 2.82| 1.01| 6.49| |Glasgow District | 12.07 | 2.65| 2.82| 1.01| 6.49| |Waterloo and City | 12.99 | 2.75| 2.98| 1.24| 6.98| |Glasgow Harbor | 17.25 | 2.75| 2.98| 1.08| 8.49| |Hudson River | 19.91 | 4.82| 2.98| 5.67| 10.49| |St. Clair River | 21.52 | 4.00| 2.98| 11.25| 15.25| |Clichy Tunnel |23.7-19.8| 4.00| 2.98| 6.88| 17.22| |Clichy Tunnel |23.8-19.4| 7.44|11.90| 4.46| 23.65| |Blackwall | 27.00 | 6.98| 5.90| 6.59| 19.48| |Waterloo Station | 24.86 | 3.34| 5.51| 1.14| 10.00| +---------------------+---------+-----+-----+------+------+ A shield of 60 or 100 tons weight can hardly be directed along the line of the proposed tunnel and also through curves and grades, especially when driven through loose or muddy soils. The tunnels of the New York and Hudson River Railroad under the Hudson, and the tunnel of the New York Rapid Transit Railway under the East River, show marked evidence of how troublesome this work is. To avoid these and other inconveniences encountered in every shield, the Author has designed a new shield which was briefly described at page 251. [Illustration: FIG. 136.--Elevation and Section of Hydraulic Jack, East River Gas Tunnel.]