There is an open stand-pipe where the two pipes and grades join.

When this section of the line was laid, the last car of 16-in. pipe was
late in arriving and, as it was desirable to get water into the
reservoir as soon as possible, 500 ft. of 10-in. pipe were laid in the
lower part of the 16-in. line, near the reservoir, as indicated on Fig.
2, which shows the hydraulic grades and the pipe diameters of this
section of the line.

When the first two velocity measurements, of March 10th and 31st, 1908,
described below, were made (after the line had been put into service on
February 20th, 1908), the 500 ft. of 10-in. pipe were still in the
16-in. line, and the hydraulic grade was defined by the solid line,
_ABCDE_, Fig. 2.

When the third measurement, of May 12th, 1909, also described below, was
made, the 10-in. pipe had been replaced by 16-in. pipe, and the
hydraulic grade was defined by the solid line, _ABE_.


[Illustration: FIG. 2.]


The dotted line, _AFE_, is the approximate theoretical position which
the grade, _ABCDE_, should have assumed when the 500 ft. of 10-in. pipe
were taken out of the 16-in. line. On the contrary, it took the position
of the grade line, _ABE_.

During the interval between March, 1908, and May, 1909, the water came
to overflow from the stand-pipe at _B_, when the line was running under