the water escaping from this pond.

It may be likened to the overflow pipe of a horse trough which is
being fed from a spring. To measure the flow of water from such a
spring, all that is necessary is to measure the water escaping through
the overflow when the water in the trough has attained a permanent
level.

[Illustration: Detail of home-made weir]

[Illustration: Cross-section of weir]

The diagrams show the cross-section and detail of a typical weir,
which can be put together in a few minutes with the aid of a saw and
hammer. The cross-section shows that the lower edge of the slot
through which the water of the temporary pond is made to escape, is
cut on a bevel, with its sharp edge upstream. The wing on each side of
the opening is for the purpose of preventing the stream from narrowing
as it flows through the opening, and thus upsetting the calculations.
This weir should be set directly across the flow of the stream,
perfectly level, and upright. It should be so imbedded in the banks,
and in the bottom of the stream, that no water can escape, except
through the opening cut for that purpose. It will require a little
experimenting with a rough model to determine just how wide and how
deep this opening should be. It should be large enough to prevent
water flowing over the top of the board; and it should be small
enough to cause a still-water pond to form for several feet behind the
weir. Keep in mind the idea of the overflowing water trough when
building your weir. The stream, running down from a higher level
behind, should be emptying into a still-water pond, which in turn
should be emptying itself through the aperture in the board at the
same rate as the stream is keeping the pond full.

Your weir should be fashioned with the idea of some permanency so that
a number of measurements may be taken, extending over a period of
time--thus enabling the prospector to make a reliable estimate not
only of the amount of water flowing at any one time, but of its
fluctuations.

Under expert supervision, this simple weir is an exact
contrivance--exact enough, in fact, for the finest calculations
required in engineering work. To find out how many cubic feet of water
the stream is delivering at any moment, all that is necessary is to
measure its depth where it flows through the opening. There are
instruments, like the hook-gauge, which are designed to measure this
depth with accuracy up to one-thousandth of an inch. An