In long tunnels the ventilation is a difficult problem, although the use of compressed air drills has aided greatly in its solution. [Illustration: Plan of St. Gothard Spiral Tunnels.] Among the great tunnels which have been excavated, the St. Gothard is the most remarkable. It is 9¼ miles long, with a section 26¼ feet wide by 19-2/3 feet high. The work on this tunnel was continuous, and it required 9¼ years for its completion. The Mont Cenis tunnel, 8-1/3 miles in length, was completed in 12 years. [Illustration: Profile of the Same.] The Hoosac Tunnel, 4¾ miles in length, 26 feet wide and 21½ feet high, was not prosecuted continuously; it was completed in 1876. These tunnels are notable chiefly on account of their great length; there are others of more moderate extent which have peculiar features; one, illustrated on the preceding page, is unique. This tunnel is a portion of the St. Gothard Railway, and not very far distant from the great tunnel referred to above. In the descent of the mountain it was absolutely necessary to secure a longer distance than a straight line or an ordinary curve would give; the line was therefore doubly curved upon itself. It enters the mountain at a high elevation, describes a circle through the rock and, constantly descending, reappears under itself at the side; still descending, it enters the mountain at another point and continues in another circular tunnel until it finally emerges again, under itself, but at a comparatively short horizontal distance from its first entry, having gained the required descent by a continued grade through the tunnels. The profile above shows the descent, upon a greatly reduced scale, the heavy lines marking where the line is in the tunnel. [Illustration: Portal of a Finished Tunnel; showing Cameron's Cone, Colorado.] [Illustration: Portal of a Tunnel in Process of Construction.] The remarkable success achieved by engineers in securing suitable foundations at great depths is, of course, hardly known to the thousands who constantly see the structures supported on those foundations, but in any fair consideration of such engineering achievements this must not be omitted. The beautiful bridge built by Captain Eads over the Mississippi River at St. Louis, bold in its design and excellent in its execution, is an object of admiration to all who visit it, but the impression of its importance would be greatly magnified if the part below the surface of the water, which bears the massive towers, and which extends to a depth twice as great as the height of the pier above the water, could be visible. [Illustration: Railway Pass at Rocky Point in the Rocky Mountains.]