[5] Ideally, the pressure of air within the work area of a pneumatically driven tunnel should just balance the hydrostatic head of the water without, which is a function of its total height above the opening. If the air pressure is not high enough, water will, of course, enter, and if very low, there is danger of complete collapse of the unsupported ground areas. If too high, the air pressure will overcome that due to the water and the air will force its way out through the ground, through increasingly larger openings, until it all rushes out suddenly in a "blowout." The pressurized atmosphere gone, the water then is able to pour in through the same opening, flooding the workings. INDEX Agricola, Georgius, 215, 216 Barlow, Peter W., 221, 227 Beach, Alfred Ely, 224, 227-229, 231, 237 Brunel, Marc Isambard (the elder), 204, 205, 217, 218, 221, 224, 229, 231, 236 Burleigh, Charles, 212, 213 Burleigh Rock Drill Company, 212 Burr, S. D. V., 236 Cochrane, Sir Thomas, 231, 232 Copperthwaite, William Charles, 224 Doane, Thomas, 210, 212, 213, 215 Drinker, Henry S., 224, 237 Greathead, James Henry, 204, 218, 221, 224, 229, 231, 235-237 Gwynn, Stuart, 210 Haskin, DeWitt C., 204, 232, 234-236 Haupt, Herman, 204, 209, 210 Hobson, Joseph, 237 Latrobe, Benjamin H., 208, 209 Law, Henry, 218 Mowbray, George W., 213, 215 Nobel, Alfred B., 213 Putnam Machine Works, 212 Shanley, Walter, 212 Shanley Bros., 215 Sommeiller, Germain, 210 Storrow, Charles S., 210 Tweed, William Marcy (Boss), 229 Weale, John, 218 * * * * * Transcriber's Notes All obvious typographical errors corrected. Formatting inconsistancies and spelling were standardized. Paragraphs split by illustrations were rejoined. The text in the reproduced handbill for the Thames Tunnel was transcribed with a slight modification to the figure description portion. The Index was extracted from the full publication Index.