of internal machines and several other details of less importance than
those mentioned, but of deep consequence in the aggregate.

Instead of being carried on thick walls spread over a considerable
area of ground, the sky-scrapers are carried wholly on steel columns.
This concentrates many hundred tons of load and develops pressure which
would crush the masonry and cause the structures to penetrate soft
earth almost as a stone sinks in water.

In the first place the weight of the proposed building and contents
is estimated, then the character of the soil determined to a depth of
one hundred feet if necessary. In New York the soil is treacherous and
difficult, there are underground rivers in places and large deposits
of sand so that to get down to rock bottom or pan is often a very hard
undertaking.

Generally speaking the excavations are made to about a depth of thirty
feet. A layer of concrete a foot or two thick is spread over the bottom
of the pit and on it are bedded rows of steel beams set close together.
Across the middle of these beams deep steel girders are placed on which
the columns are erected. The heavy weight is thus spread out by the
beams, girders and concrete so as to cause a reduced uniform pressure
on the soil. Cement is filled in between the beams and girders and
packed around them to seal them thoroughly against moisture; then clean
earth or sand is rammed in up to the column bases and covered with the
concrete of the cellar floor.

In some cases the foundation loads are so numerous that nothing short
of masonry piers on solid rock will safely sustain them. To accomplish
this very strong airtight steel or wooden boxes with flat tops and no