The same system was followed afterwards by the engineer of the Canadian Pacific Railway in placing the spans of a bridge over the St. Lawrence, in a very rapid current. It is now used in replacing old spans by new ones, as it interrupts traffic for the least possible time. The solution of the problems presented at Hawkesbury gave the second introduction of American engineers to bridge building outside of America. The first was in 1786, when an American carpenter or shipwright built a bridge over Charles River at Boston, 1470 feet long by forty-six feet wide. This bridge was of wood supported on piles. His work gained for him such renown that he was called to Ireland and built a similar bridge at Belfast. Tunnelling by compressed air is a horizontal application of compressed-air foundations. The earth is supported by an iron tube, which is added to in rings, which are pushed forward by hydraulic jacks. A tunnel is now being made under an arm of the sea between Boston and East Boston, some 1400 feet long and sixty-five feet below tide. The interior lining of iron tubing is not used. The tunnel is built of concrete, reinforced by steel rods. This will effect a considerable economy. Success in modern engineering means doing a thing in the most economical way consistent with safety. The Saint Clair tunnel, which carries the Grand Trunk Railway of Canada under the outlet of Lake Huron, is a successful example of such work. Had the North River tunnel, at New York, been designed on equally scientific principles, it would probably have been finished, which now seems problematical. The construction of rapid-transit railways in cities is another branch of engineering, covering structural, mechanical, and electrical engineering. Some of these railways are elevated, and are merely railway viaducts, but the favorite type now is that of subways. There are two kinds, those near the surface, like the District railways of London, the subways in Paris, Berlin, and Boston, and that now building in New York. The South London and Central London, and other London projects, are tubes sunk fifty to eighty feet below the surface and requiring elevators for access. These are made on a plan devised by Greathead, and consist of cast-iron tubes pushed forward by hydraulic rams, and having the space outside of the tube filled with liquid cement pumped into place.