Of the building of the Baltimore & Ohio, tunnel-construction formed an early part, several paths being found across the steep profiles of the Alleghanies. The Kingwood Tunnel, which B. H. Latrobe drove, was nearly a mile long and the chief of these bores. But when the Hoosac Tunnel was first proposed--piercing the rocky heart of one of the greatest of the Berkshires--the country stood aghast. Four miles and a half of tunnel! That seemed ridiculous away back in 1854, when the plan was first broached and folk were not slow to say what they thought of such an absurd plan. For twenty years it looked as though these scoffers were in the right--the work of digging that monumental tunnel was a fearful drain on the treasury of the commonwealth of Massachusetts, which was lending its aid to the project. But the tunnel-diggers finally conquered--they almost always do--and the Hoosac remains to-day the greatest of all mountain tunnels in America. The system of continuous tunnels, by which the Pennsylvania Railroad recently reached its terminal in New York, stretches from Bergen Hill in New Jersey to Sunnyside, Long Island, a distance of some ten miles. In fact the largest feature of recent tunnel-work in this country has been in connection with terminal and rapid-transit development in the larger cities. For a good many years New York and Baltimore, in particular, have been pierced with these sub-surface railroads; it is a construction feature that increases as our great cities themselves increase. No river is to-day too formidable to be conquered by these underground traffic routes. A river such as the Hudson or the Detroit may sometimes halt the bridge-builders; it has but slight terror for the tunnel engineers. The tunnel-work is apt to be a separate part of the work of building a railroad. It calls for its own talent, and that of an exceedingly expert sort. If the tunnel is more than a half or three-quarters of a mile long it will probably be dug from a shaft or shafts as well as from its portals. In this way the work will not only be greatly hastened but the shafts will continue in use after the work is completed as vents for the discharge of engine smoke and gases from the tube. The work must be under the constant and close supervision of resident engineers. The survey lines must be corrected daily, for the tunnel must not go astray. It must drive a true course from heading to heading. In the shafts plumb lines, with heavy bobs, to lessen vibration, will be hung. Sometimes these bobs are immersed in water or in molasses.