the pairs of bolt holes near the center of the plate, and was held in
position by the running nut at one end.

In the beginning it was expected that the natural shape of the rings
would not show more than 1 in. of shortening of the vertical diameter;
this was slightly exceeded, however, the average distortion throughout
the tunnels being 1-7/16 in. The erectors were attached to the shield
and in such a position that they were in the plane of the center of the
ring to be erected when the shove was made without lead and just far
enough to permit placing the segments. If the shield were shoved too
far, a rare occurrence, the erection was inconvenienced. In driving with
high vertical leads, which occurred more frequently, the disadvantage
of placing the erector on the shield was more apparent. Under such
conditions the plane of the erector's motion was acutely inclined to the
plane of the ring, and, after placing the lower portion of the ring, it
was usually necessary to shove the shield a few inches farther in order
to place the upper plates. The practical effect of this action is
referred to later.

[Illustration: FIG. 1.]

At first the erection of the iron in the river tunnels interfered
somewhat with the mucking operations, but the length of time required to
complete the latter was ample for the completion of the former; and the
starting of a shove was seldom postponed by reason of the non-completion
of a ring. After the removal of the bottom of the diaphragms, permitting
the muck cars to be run into the shield and beyond, the two operations
were carried on simultaneously without serious interference. The
installation of the belt conveyor for handling the soft ground spoil in
Tunnel _A_ was of special benefit in this respect.