To meet this situation, three massive reinforced concrete caissons were built on the surface. They are practically large two-story houses, each being over one hundred feet in length, about fifty feet in height, and about forty-five feet in width at their widest point. The bottom edges were sharp, and, with the use of air pressure and great weights, the three structures were sunk in the ground to the same grade as the intercepting tunnels, and the tunnels were then driven into them.
Particular attention should be given to the Jersey City to Hoboken tube, in the lower part of the caisson in the foreground, in the accompanying [illustration], which curls around the Hoboken to Jersey City tube, and rises to the elevation of, and connects into, the New York to Hoboken tube, at the caisson in the background, at the left of the illustration. Very few of the people who travel through the tube are probably aware of such manipulation. At the same time, the arrangement absolutely avoids any grade crossing whatever, and without such an arrangement of tracks the road could not be operated with trains run so closely together as under the prevailing system.
In constructing the river tunnels the work was carried on simultaneously from opposite sides of the river, the tunnels meeting under the river, and it is interesting, if not remarkable, when one considers the difficulties under which the engineering work had to be carried on, to note that the tunnels met with practically absolute accuracy.
What Causes Floating Islands?
A floating island consists generally of a mass of earth held together by interlacing roots.
They occur on the Mississippi and other rivers, being portions of the banks detached by the force of the current and carried down the stream, often bearing trees. Sometimes such islands are large enough to serve as pasture grounds.
Artificial floating islands have been formed by placing lake mud on rafts of wicker-work covered with reeds. They were formerly used in the waters around Mexico, and may be seen in Persia, India, and on the borders of Tibet. On these the natives raise melons, cucumbers and other vegetables which need much water.