The Transport of Sand by Wind

The subject of course is but a special case of the general problem of the transport of solid particles by fluids. The Reynolds' Numbers involved are of the same order, whether sand is transported by air or water, and the type of motion of the grains, and their resulting drag on the fluid, appear to be much the same, though too close a parallel should not be drawn since the density ratios of sand and fluid differ greatly in the two cases. But because of the greater simplicity of experiments with air, especially in pitot-tube velocity measurements, it is possible that new facts applicable to sand-water transportation may emerge from the study of sand-air transportation. There would appear to be a closer resemblance between the behaviour of sand in air and that of drifting snow. For instance the peculiarly solid formation of wind-slab snow which G. Seligman describes in his book, 'Snow Structure and Ski Fields,' seems to have a very exact counterpart in a certain type of sand deposit which can be reproduced experimentally. Here again much might be learnt about snow-drifting from experiments made with sand, which is a far more convenient medium. The particles dealt with are defined as hard grains small enough to be set in motion by the direct forward velocity of the fluid, but too massive to be seriously affected during their flight by the less violent velocities of the fluid's internal movements. Small particles of dust are seriously affected by these internal movements, but their behaviour is beyond the scope of this paper. As an example of the susceptibility of sand grains to wind motion, a fine grain of the size (0-25 mm. diam.) commonly found at the crests of dunes when shot into still air at a speed of 4 m./sec. (9 miles per hour), travels 20 cm. before its speed is reduced by 50 per cent. Wind-driven, the sand grains move in bounds (or in saltation 2), rising steeply into the air stream, and there being urged forward by the pressure of the wind upon them. By their weight they fall to the ground again, but with a high horizontal velocity component acquired from the air. On impact with

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