Page:Popular Science Monthly Volume 88.djvu/360

332 busy for a while, discharging a volume of water as great as the daily supply required for a city the size of Atlanta. Care had to be taken, meanwhile, not to pump out sand along with the water, or the adjacent buildings would have come tumbling down, just as in a certain engineer's vision of the most effective way of destroying the city of Boston:

"An enemy need not bother mustering battleships or waste his time bombarding from afar the intellectual Hub of this land of ours. In time of peace let him have his spies build a big pumping station right in the middle of that city, and at the proper time start drawing indiscriminately from the ground below the water saturating the subsoil. You know a large number of Boston's big buildings rest upon floating foundations. Pump out the water in the supporting quicksand, and down those structures would tumble into the yawning cavities so created. It would be far more effective in its demolition than the projectiles of a hostile fleet!"

Up near the north end of Manhattan Island, at Lexington Avenue and One hundred and Twenty-ninth Street, the subway diggers had to construct another stout waterproof floor when they encountered what evidently was once a swamp.

We mentioned, in passing. the razing of the old Astor House, which was built upon sand. The tunnel which comes up Vesey Street and cuts under the site of the old hotel curves around into Broadway through big cylinders of cast iron.

Underground swamps and watercourses, sand, quicksand, sand mixed with boulders (as in Brooklyn)—all these the diggers encounter and vanquish. But what the subway builders fear most is something different from all of these: a material known to the geologist as Manhattan Schist and to the rest of us as "rotten rock." No material is more treacherous than this, for along with layers of extreme hardness are pockets and seams of disintegrated stuff, some of it so soft that, after it has been exposed a little