where R = the hydraulic mean depth, and S = surface slope--were tried in
detail; thus 100, 76, and 83 average values thereof respectively were
taken from 581, 313, and 363 detail values. The ratios of these three
velocities to the mean velocity were taken out, and compared in detail
with Bazin's and Cutter's coefficients. Other formulae were contrasted
also in slight detail. Kutter's alone seemed to be of general
applicability; when the surface slope measurement is good, and the
rugosity coefficient known for the site--both doubtful matters--it would
probably give results within 71/2 per cent. of error. Improvement in
formulae could at present be obtained only by increased complexity, and
the tentative research would be excessively laborious. Now the first two
ratios varied far less than the third; thus their use would probably
involve less error than the third, or approximation would be more likely
from direct velocity measurement than from any use of surface slope. The
connection between velocities was probably a closer one than between
velocity and slope; the former being perhaps only a geometric, and the
latter a physical one. The mean velocity past the central vertical was
recommended for use, as not being affected by wind; the reduction
coefficient could at present only be found by special experiment for
each site. Three current meters were tried for some time with a special
lift, contrived to grip the meter firmly parallel to the current axis,
so as to register only forward velocity, and with a nearly rigid gearing
wire. No useful general results were obtained. Ninety specimens of silt
were collected, but no connection could be traced between silt and
velocity; it seemed that the silt at any point varied greatly from
instant to instant, and that the quantity depended not on the mean
velocity, but probably on the silt in the supply water. Forty
measurements of the evaporation from the canal surface were made in a
floating pan, during twenty five months. The average daily evaporation
was only about 1/10 in. The smallness of this result seemed to be due to
the coldness of the water--only 63 deg. in May, with 165 deg. in the sun
and 105 deg. in shade. Lastly, it must suffice to say that great care
was taken to insure accuracy in both fieldwork and computation.
* * * * *