is a system of pneumatic mail tubes for postal delivery. Of course,
absolutely no change in alignment could be permitted while these tubes
were in use carrying mail. It was necessary, therefore, to support
them very carefully. The slightest deviation in alignment would have
stopped the service.

[Illustration: TWO COLUMN BENT VIADUCT]

[Illustration: TRAVELER FOR ERECTING FORMS, CENTRAL PARK TUNNEL--(IN
THIS TUNNEL DUCTS ARE BUILT IN THE SIDEWALLS)]

[Sidenote: _Concrete-lined
Tunnel_]

Between 33d Street and 42d Street under Park Avenue, between 116th
Street and 120th Street under Broadway, between 157th Street and Fort
George under Broadway and Eleventh Avenue (the second longest
double-track rock tunnel in the United States, the Hoosac tunnel being
the only one of greater length), and between 104th Street and Broadway
under Central Park to Lenox Avenue, the road is in rock tunnel lined
with concrete. From 116th Street to 120th Street the tunnel is 37-1/2
feet wide, one of the widest concrete arches in the world. On the
section from Broadway and 103d Street to Lenox Avenue and 110th Street
under Central Park, a two-track subway was driven through micaceous
rock by taking out top headings and then two full-width benches. The
work was done from two shafts and one portal. All drilling for the
headings was done by an eight-hour night shift, using percussion
drills. The blasting was done early in the morning and the day gang
removed the spoil, which was hauled to the shafts and the portal in
cars drawn by mules. A large part of the rock was crushed for