assembled, across the railroad tracks until the center of the bottom
chord was vertically under its true position, the truss still lying flat
and about at right angles to the center line of the bridge. Chains were
made fast to the top chord at the middle panel of the truss, which was
then turned up to a vertical plane, raised to its permanent position,
and guyed. The gin pole was then shifted and the operation repeated with
the longer truss, after which, half of the floor beams and a part of the
top laterals were bolted in position and the guys were removed, the
bridge being thus erected without the use of falsework of any kind.
During the lifting there was no sag in either truss that could be
noticed by the eye. Fig. 1, Plate LV, shows the bridge erected, with
the exception of the tight timber fence.

Pier No. 72 is directly over the North River Tunnels. When it was turned
over by the New York Central Railroad Company, the contractor for the
construction of those tunnels tore down the shed and removed the deck
and such piles as were in the path of the tubes. This left standing the
four northernmost, the four southernmost and two centers rows of piles
for the entire length of the pier. An additional row of piles was then
driven on either side of the two center rows, and battered so that at
the elevation of the tunnels they would be close to the center rows
and leave as much clear space as possible. The pier, therefore, was
constructed of three independent lines of four-post bents, which,
however, rested on sills which were continuous throughout the width of
the pier, as shown by Figs. 2 and 3, Plate LV.

[Illustration:
Fig. 6.
DETAIL OF BENTS FOR MATERIAL TRESTLE.]