capillary attraction causes the water to rise up the rule above
the actual surface, and thus to show a still greater depth.
When using a hook-gauge the top of the weir, as well as the
notch, should be fixed level and a peg or stake fixed as far
back as possible on the upstream side of the weir, so that the
top of the peg is level with the top of the weir, instead of
with the notch, as is the case when a rule or gauge-slate is
used. The hook-gauge consists of a square rod of, say, lin
side, with a metal hook at the bottom, as shown in Fig. 15, and
is so proportioned that the distance from the top of the hook
to the top of the rod is equal to the difference in level of
the top of the weir and the sill of the notch. In using it the
rod of the hook-gauge is held against the side of the gauge-peg
and lowered into the water until the point of the hook is
submerged. The gauge is then gently raised until the point of
the hook breaks the surface of the water, when the distance
from the top of the gauge-peg to the top of the rod of the
hook-gauge will correspond with the depth of the water flowing
over the weir.




CHAPTER VII.

RAINFALL.


The next consideration is the amount of rain-water for which
provision should be made. This depends on two factors: first,