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 pipe had been replaced by 16-in.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
full pressure, indicating an increase of capacity in the 10-in. pipe
greater than a corresponding increase in the 16-in. The alignment of the
10-in. line, vertically and horizontally, is more regular and uniform
than the 16-in. line. The latter has many abrupt curves and bends,
vertically and horizontally. It crosses nine sharp ridges and dips under
as many deep arroyos. This introduces a fixed element of frictional
resistance which does not decrease with the increasing smoothness of
the interior surface of wood pipe, and probably accounts for the higher
resistance of the 16-in. line.

From Fig. 2 it appears that, while the 10-in. line had an initial
coefficient of roughness slightly greater than 0.009 and now equal to
it, the 16-in. line had one equal at first but now slightly less than
0.01.

The line from Bonito Creek to Nogal Reservoir was to have a capacity of
5 sec-ft. Referring to the profile, it was determined that for the
hydraulic grade of 33-1/3 ft. per 1000 ft., a 10-in. pipe was necessary,
and that a 16-in. pipe was required for the grade of 3 ft. per 1000 ft.

_Test No. 1_.--On March 10th, 1908, a quantity of bran was poured into
the upper end of the 10-in. pipe at _A_ (Fig. 2), and