the subway passes close to the foundations of lofty buildings, where
the construction needed to insure the safety of both subway and
buildings was quite intricate. As the subway is close to the surface
along a considerable part of its route, its construction involved the
reconstruction of all the underground pipes and ducts in many places,
as well as the removal of projecting vaults and buildings, and, in
some cases, the underpinning of their walls. A description in detail
of the methods of construction followed all along the line would make
an interesting book of itself. Space will only permit, however, an
account of how some of the more serious difficulties were overcome.

On Fourth Avenue, north of Union Square to 33d Street, there were two
electric conduit railway tracks in the center of the roadway and a
horse car track near each curb part of the distance. The two electric
car tracks were used for traffic which could not be interrupted,
although the horse car tracks could be removed without inconvenience.
These conditions rendered it impracticable to disturb the center of
the roadway, while permitting excavation near the curb. Well-timbered
shafts about 8 x 10 feet, in plan, were sunk along one curb line and
tunnels driven from them toward the other side of the street, stopping
about 3-1/2 feet beyond its center line. A bed of concrete was laid on
the bottom of each tunnel, and, when it had set, a heavy vertical
trestle was built on it. In this way trestles were built half across
the street, strong enough to carry all the street cars and traffic on
that half of the roadway. Cableways to handle the dirt were erected
near the curb line, spanning a number of these trestles, and then the
earth between them was excavated from the curb to within a few feet of
the nearest electric car track. The horse car tracks were removed.
Between the electric tracks a trench was dug until its bottom was
level with the tops of the trestles, about three feet below the