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164   Art of Travel.

tent-ropes ; and, in rocky ground, heavy piles of stones may be made to answer the same purpose. The tent-ropes may

also be knotted to a cloth, on which stones are afterwards piled.

" Dateraiin " is, as the late Dr. 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 I foot deep, that a strain equal to about 50 lbs. weight, is necessary to draw it up ; if 12 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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