The transition was complete. The work had been closely followed by the technical journals and the reports of its successful accomplishment thus were brought to the attention of the entire civil engineering profession. As the first major subaqueous tunnel completed in America and the first in the world of a size able to accommodate full-scale rail traffic, the St. Clair Tunnel served to dispel the doubts surrounding such work, and established the pattern for a mode of tunneling which has since changed only in matters of detail. Of the eight models, only this one was built under the positive guidance of original documents. In the possession of the Canadian National Railways are drawings not only of all elements of the shield and lining, but of much of the auxiliary apparatus used in construction. Such materials rarely survive, and do so in this case only because of the foresight of the railway which, to avoid paying a high profit margin to a private contractor as compensation for the risk and uncertainty involved, carried the contract itself and, therefore, preserved all original drawing records. While the engineering of tunnels has been comprehensively treated in this paper from the historical standpoint, it is well to still reflect that the advances made in tunneling have not perceptibly removed the elements of uncertainty but have only provided more positive and effective means of countering their forces. Still to be faced are the surprises of hidden streams, geologic faults, shifts of strata, unstable materials, and areas of extreme pressure and temperature. BIBLIOGRAPHY AGRICOLA, GEORGIUS. _De re Metallica._ [English transl. H. C. and L. H. Hoover (_The Mining Magazine_, London, 1912).] Basel: Froben, 1556. BEACH, ALFRED ELY. _The pneumatic dispatch._ New York: The American News Company, 1868. BEAMISH, RICHARD. _A memoir of the life of Sir Marc Isambard Brunel._ London: Longmans, Green, Longmans and Roberts, 1862. BURR, S. D. V. _Tunneling under the Hudson River._ New York: John Wiley and Sons, 1885. COPPERTHWAITE, WILLIAM CHARLES. _Tunnel shields and the use of compressed air in subaqueous works._ New York: D. Van Nostrand Company, 1906. DRINKER, HENRY STURGESS. _Tunneling, explosive compounds and rock drills._ New York: John Wiley and Sons, 1878. LATROBE, BENJAMIN H. Report on the Hoosac Tunnel (Baltimore, October 1, 1862). Pp. 125-139, app. 2, in _Report of the commissioners upon the Troy and Greenfield Railroad and Hoosac Tunnel_. Boston, 1863. LAW, HENRY. A memoir of the Thames Tunnel. _Weale's Quarterly Papers on Engineering_ (London, 1845-46), vol. 3, pp. 1-25 and vol. 5, pp. 1-86. The pneumatic tunnel under Broadway, N.Y. _Scientific American_ (March 5, 1870), pp. 154-156.