1.8 cents for "top charges."

Unfortunately, it was necessary to place the greater part of the
concrete lining in the river tunnels during the summer months when the
temperature at the point of work frequently exceeded 85 deg.; and the
temperature of the concrete while setting was much higher. This abnormal
heat, due to chemical action in the cement, soon passed away, and, with
the approach of winter, the contraction of the concrete resulted in
transverse cracks. By the middle of the winter these had developed quite
uniformly at the ends of each 30-ft. section of concrete arch as placed,
and frequently finer cracks showed at about the center of each 30-ft.
section.

While the temperature of the concrete was falling, a like change was
taking place in the cast-iron lining, with resulting contraction. The
lining had been erected in compressed air, the temperature of which
averaged about 70 deg. in winter and higher in summer. Compressed air having
been taken off in the summer of 1908, the tunnels then acquired the
lower temperature of the surrounding earth, slowly falling until
mid-winter. The contraction of the concrete, firmly bedded around the
flanges of the iron, and showing cracks at fairly uniform intervals,
probably localized the small corresponding movements of the iron near
the concrete cracks, and resulted in a loosening of the caulking at
these points. With the advent of cold weather, damp spots appeared in
numerous places on the concrete, and small seepages showed through quite
regularly at the temperature cracks, in some cases developing
sufficiently to be called leaks. Only a few, however, were measurable in
amount.

Early in January small brass plugs were firmly set on opposite sides of
a large number of cracks, and caliper readings and air temperature
observations were taken regularly throughout the winter and spring. The
widths of the cracks and the amount of leakage at them increased with
each drop in temperature and decreased as the temperature rose again,
but until spring the width of the cracks did not return to the same
point with each return of temperature.

The leakage was similar in all four tunnels, but was largest in amount
in Tunnel _D_, where, at the beginning of February, the ordinary flow
was about 0.0097 cu. ft. per sec., equivalent to 0.00000347 cu. ft. per
sec. per lin. ft. of tunnel. Of this amount 0.0065 cu. ft. per sec.
could be accounted for at eight of the cracks showing measurabl