51. In some countries, where wood is scarce, the evaporation
of salt water is carried on by a large collection of ropes which
are stretched perpendicularly. In passing down the ropes, the
water deposits the sulphate of lime which it held in solution,
and gradually incrusts them, so that in the course of twenty
years, when they are nearly rotten, they are still sustained by
the surrounding incrustation, thus presenting the appearance of a
vast collection of small columns.
52. Amongst natural operations perpetually altering the
surface of our globe, there are some which it would be
advantageous to accelerate. The wearing down of the rocks which
impede the rapids of navigable rivers, is one of this class. A
very beautiful process for accomplishing this object has been
employed in America. A boat is placed at the bottom of the rapid,
and kept in its position by a long rope which is firmly fixed on
the bank of the river near the top. An axis, having a wheel
similar to the paddle-wheel of a steamboat fixed at each end of
it, is placed across the boat; so that the two wheels and their
connecting axis shall revolve rapidly, being driven by the force
of the passing current.
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