re demolition. View of South side showing
chutes. Jan. 21, 09.]

[Illustration: PLATE LI, FIG. 4.--A 54. P.R.R. Tunnels, N.R. Div. Sect. Gy.
West & Oj. View across North River on line of Tunnels, looking from New
York to New Jersey. Feb. 9, 07.]

In the first few sections of wall, the form was filled to within 1 in. of
the top of the bridge seat and allowed to set for about 2 hours; it was
then finished to the proper elevation with a plaster of 1 part of cement to
1 part of sand. This did not prove satisfactory, as there were indications
of checking and cracking, and, later, the form was filled to the required
elevation and the surface floated. The form was allowed to remain in place
for from 18 to 24 hours, depending on the weather. In most cases,
immediately after the form had been moved, a scaffold was erected against
the face of the wall, and the face was wet and thoroughly rubbed, first
with a wooden float and then with a cement brick, until the surface was
smooth and uniform.

The section 1 ft. square at the top of the bridge seat, which was left out
in order to brace the bottom of the form for the upper face wall, was
filled in after the walls had been completed. The old concrete was very
thoroughly cleaned before the new concrete was placed on it, and a gutter
was formed at the rear connecting with the box drains back of the wall to
carry off rain-water coming down the face of the upper walls.

In hot weather the walls were thoroughly wetted down several times a day
for several days after the form had been removed.

_Upper Face and Retaining Wall._--In cases where the top of the retaining
wall was at a higher elevation than the mixer, it was necessary to raise
the concrete in a bucket with a derrick, and dump it into cars on the
trestle above the top of the coping. Concrete was deposited through chutes,
as in the lower face wall, continuously from the bottom of the face wall to
the top of the retaining wall. At the commencement of each section of the
retaining wall a layer of mortar was put on the rock. A 1:2:3 mixture of
concrete was used in the face wall, and a 1:3:6 mixture in the retaining
wall.

As the face walls were so thin, the number of batches of concrete per hour
was reduced, for the form filled so rapidly that the concrete, before it
set, exerted an excessive pressure against the form, and this tended to
make it bulge. The proper rate at which to place the concrete behind a form
50 ft. long, wi