[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. * * * * * CONTRIBUTIONS FROM THE MUSEUM OF HISTORY AND TECHNOLOGY: PAPER 42 THE "PIONEER": LIGHT PASSENGER LOCOMOTIVE OF 1851 IN THE MUSEUM OF HISTORY AND TECHNOLOGY _John H. White_ THE CUMBERLAND VALLEY RAILROAD 244 SERVICE HISTORY OF THE "PIONEER" 249 MECHANICAL DESCRIPTION OF THE "PIONEER" 251 FOOTNOTES _John H. White_ The "PIONEER": LIGHT PASSENGER LOCOMOTIVE of 1851 _In the Museum of History and Technology_ [Illustration: Figure 1.--THE "PIONEER," BUILT IN 1851, shown here as renovated and exhibited in the Museum of History and Technology, 1964. In 1960 the locomotive was given to the Smithsonian Institution by the Pennsylvania Railroad through John S. Fair, Jr. (Smithsonian photo 63344B.)] _In the mid-nineteenth century there was a renewed interest in the light, single-axle locomotives which were proving so very successful for passenger traffic. These engines were built in limited number by nearly every well-known maker, and among the few remaining is the 6-wheel "Pioneer," on display in the Museum of History and Technology, Smithsonian Institution. This locomotive is a true representation of a light passenger locomotive of 1851 and a historic relic of the mid-nineteenth century._ THE AUTHOR: _John H. White is associate curator of transportation in the Smithsonian Institution's Museum of History and Technology._