The construction of the Boston subway was difficult on account of the small width of the streets, their great traffic, and the necessity of underpinning the foundations of buildings. All of this was successfully done without disturbing the traffic for a single day, and reflects great credit on the engineer. Owing to the great width of New York streets, the problem is simpler in that respect, but requires skill in design and organization to complete the work in a short time. Although many times as long as the Boston subway, it will be built in nearly the same time. The design, where in earth, may be compared to that of a steel office building twenty miles long, laid flat on one of its sides. The reduplication of parts saves time and labor, and is the key to the anticipated rapid progress. Near the surface this subway is built in open excavation, and tunnelling is confined to rock. The construction of power-houses for developing energy from coal and from falling water requires much structural besides electrical and mechanical engineering ability. The Niagara power-house is intended to develop 100,000 horse-power; that at the Sault Ste. Marie as much; that on the St. Lawrence, at Massena, 70,000 horse-power. These are huge works, requiring tunnels, rock-cut chambers, and masonry and concrete in walls and dams. They cover large extents of territory. The contrast in size of the coal-using power-houses is interesting. The new power-house now building by the Manhattan Elevated Railway, in New York, develops in the small space of 200 by 400 feet 100,000 horse-power, or as much power as that utilized at Niagara Falls. One of the most useful materials which modern engineers now make use of is concrete, which can be put into confined spaces and laid under water. It costs less than masonry, while as strong. This is the revival of the use of a material used by the Romans. The writer was once allowed to climb a ladder and look at the construction of a dome of the Pantheon, at Rome. He found it a monolithic mass of concrete, and hence without thrust. It is a better piece of engineering construction than the dome of St. Peter’s, built fifteen hundred years later. The dome of Columbia College Library, in New York, is built of concrete. Concrete is a mixture of broken stone or gravel, sand, and Portland cement. Its virtue depends upon the uniform good quality of the cement. The use of the rotary kiln, which exposes all the contained material to a uniform and constant intense heat, has revolutionized the manufacture of Portland cement. The engineer can now depend upon its uniformity of strength.