nto the main channel.

[Illustration: Fig. 15.]

When the tortuous flexures of a river are extremely great, as often
happens in alluvial plains, the aberration from the direct line of
descent may be restored by the river cutting through the isthmus which
separates two neighboring curves. Thus in the annexed diagram, the
extreme sinuosity of the river has caused it to return for a brief space
in a contrary direction to its main course, so that a peninsula is
formed, and the isthmus (at _a_) is consumed on both sides by currents
flowing in opposite directions. In this case an island is soon
formed,--on either side of which a portion of the stream usually
remains.

_Transporting power of water._--In regard to the transporting power of
water, we may often be surprised at the facility with which streams of a
small size, and descending a slight declivity, bear along coarse sand
and gravel; for we usually estimate the weight of rocks in air, and do
not reflect on their comparative buoyancy when submerged in a denser
fluid. The specific gravity of many rocks is not more than twice that of
water, and very rarely more than thrice, so that almost all the
fragments propelled by a stream have lost a third, and many of them a
half, of what we usually term their weight.

It has been proved by experiment, in contradiction to the theories of
the earlier writers on hydrostatics, to be a universal law, regulating
the motion of running water, that the velocity at the bottom of the
stream is everywhere less than in any part above it, and is greatest at
the surface. Also that the superficial particles in the middle of the
stream move swifter than those at the sides. This retardation of the
lowest and lateral currents is produced by friction; and when the
velocity is sufficiently great, the soil composing the sides and bottom
gives way. A velocity of three inches per second at the bottom is
ascertained to be sufficient to tear up fine clay,--six inches per
second, fine sand,--twelve inches per second, fine gravel,--and three
feet per second, stones of the size of an egg.[269]

When this mechanical power of running water is considered, we are
prepared for the transportation before alluded to of large quantities of
gravel, sand, and mud, by torrents which descend from mountainous
regions. But a question naturally arises, How the more tranquil rivers
of the valleys and plains, flowing on comparatively level ground, can
remove the prodigiou