This parallel to the formation of flints in the
chalk strata deserves attention.(2*)
63. The slowness with which powders subside, depends partly
on the specific gravity of the substance, and partly on the
magnitude of the particles themselves. Bodies, in falling through
a resisting medium, after a certain time acquire a uniform
velocity, which is called their terminal velocity, with which
they continue to descend: when the particles are very small, and
the medium dense, as water, this terminal velocity is soon
arrived at. Some of the finer powders even of emery require
several hours to subside through a few feet of water, and the mud
pumped up into our cisterns by some of the water companies is
suspended during a still longer time. These facts furnish us with
some idea of the great extent over which deposits of river mud
may be spread; for if the mud of any river whose waters enter the
Gulf Stream, sink through one foot in an hour, it might be
carried by that stream 1,500 miles before it had sunk to the
depth of 600 or 700 feet.
64. A number of small filaments of cotton project from even
the best spun thread, and when this thread is woven into muslin
they injure its appearance.
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