stream carries in suspension, and the sand and gravel and larger
stones which it pushes along its bed. Especially in times of flood
one may note the muddy water, its silt being kept from settling by
the rolling, eddying currents; and often by placing his ear close
to the bottom of a boat one may hear the clatter of pebbles as
they are hurried along. In mountain torrents the rumble of
bowlders as they clash together may be heard some distance away.
The amount of the load which a stream can transport depends on its
velocity. A current of two thirds of a mile per hour can move fine
sand, while one of four miles per hour sweeps along pebbles as
large as hen's eggs. The transporting power of a stream varies as
the sixth power of its velocity. If its velocity is multiplied by
two, its transporting power is multiplied by the sixth power of
two: it can now move stones sixty-four times as large as it could
before.

Stones weigh from two to three times as much as water, and in
water lose the weight of the volume of water which they displace.
What proportion, then, of their weight in air do stones lose when
submerged?

MEASUREMENT OF STREAM LOADS. To obtain the total amount of waste
transported by a river is an important but difficult matter. The
amount of water discharged must first be found by multiplying the
number of square feet in the average cross section of the stream
by its velocity per second, giving the discharge per second in
cubic feet. The amount of silt to a cubic foot of water is found
by filtering samples of the water taken from different parts of
the stream and at different times in the year, and drying and
weighing the residues. The average amount of silt to the cubic