Barth, informed me, the Bornu name for a
most excellent African contrivance, used in some parts of the Sahara
desert, by means of which tent-ropes may be secured, or horses picketed
in sand of the driest description, as in that of a sand dune, whence a
tent-peg would be drawn out by a strain so slight as to be almost
imperceptible. I have made many experiments upon it, and find its
efficiency to be truly wonderful. The plan is to tie to the end of the
tent-rope, a small object of any description, by its middle, as a short
stick, a stone, a bundle of twigs, or a bag of sand; and to bury it from
1 to 2 feet in the loose sand. It will be found, if it has been buried 1
foot deep, that a strain equal to about 50 lbs. weight, is necessary to
draw it up; if 1 1/2 feet deep, that a much more considerable strain is
necessary; and that, if 2 feet deep, it is quite impossible for a single
man to pull it up. In the following theoretical case, the resistance
would be as the cube of the depth; but in sand or shingle, the increase
is less rapid. It varies under different circumstances; but it is no
exaggeration to estimate its increase as seldom less than as the square
of the depth. The theoretical case of which I spoke, is this:--Let x be
part of a layer of shingle of wide extent: the shingle is supposed to
consist of smooth hard spherical balls, all of the same size.
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