Page:Popular Science Monthly Volume 49.djvu/685

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storms appear to be about twenty-five miles an hour. Velocities two and a half times as higb as this are not unknown, and the eflBciency of such a current is two hundred and five times that of one having a velocity of twenty-five miles an hour. If the ve- locity is doubled it increases the transporting power sixty-four times. Should such storms then occur on twenty days of the year instead of on two, the total work would be six hundred and forty times more effective than in the first instance. Still another in- crease would result from the greater vertical dispersion of the lower greater load in a wind with high velocity. A predomi- nance of aerial transportation may, of course, also be due to local inefiiciency of the aqueous work. But this digression is not made for the purpose of proving the possibility of such a pre- dominance in any place. A proof of this would be superfluous, since the topography of sand-hill regions is conclusive evidence on this point. It is merely desired to emphasize the fact that atmospheric work is subject to very great range in its effective- ness, and that as a consequence the great range of the estimates made above does not impeach their trustworthiness.

Some other stray items of information, culled from the news- paper accounts, may be briefly stated in closing. The only state- ment from which the distance over which dust has been trans- ported can be definitely estimated is in a notice coming from the southern part of California. From the notice made it is certain that dust must have been carried twenty miles. Of course, this does not speak against the possibility of a transportation over twenty times that distance. In one case a slow settling of fine dust from the air is reported as actually observed, and this was also in California. The maximum wind velocities reported in connection with dust storms are few, running from 36 to 90 miles per hour, viz. : 36, 40, 45, 50-60, 90. Some instances of the erosive effects of blown sand are described. It is stated that winds will raise dust on sparsely covered, not cultivated land ; that orange trees in California are sometimes girdled near the ground by sand blasts ; that the glass in the windows of railroad coaches is etched by impinging sand ; and that paint on the coaches is worn off in the same way. The softer wood in telegraph poles is sometimes worn away by sand so much faster than the harder wood in the knots that the latter are left protruding far out. Finally, there are some accounts of the coarseness of the transported material. It is generally fine enough to be called dust, but sand is often mingled with the dust, and occasionally there is fine gravel. Two reports mention pebbles, and in one instance these are said to have been large enough to " knock a man senseless."

To the writer the facts here presented appear most interesting in their incompleteness. While there are tens of thousands of

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