experiments, it appears that two ounces of dust suspended in about four thousand cubic feet of air render it as thick as it must have been, at the least, in the storms described. This would make about two thousand tons to a cubic mile. It should be added that these experiments were made in a wind moving about eight miles an hour, and with dust quite fine enough to be suspended in such a wind. A considerably larger quantity would no doubt be required if the material were to be coarser, such as would be carried by a strong wind. The estimate is therefore believed to represent a minimum for such storms as are described above.
Another perhaps less reliable estimate may be made from accounts which describe the drifting sand, thus: "The sand drifts, as snow does, and has attained such a depth as to cause a fear that vegetation in the simoom's path will be greatly damaged."—"Drifts of sand one foot high were piled up in thirty minutes on a railroad track."—"Cuts [along a railroad] were filled with immense drifts, which averaged about two thirds sand and one third snow."—"At Cheyenne Wells, Colorado, thirteen cars of sand were taken from the depot platform" (after a storm).—"Tracks were obliterated [by drifting sand] and the whole landscape was changed."
Under such conditions it may be surmised that a drift of twenty-five tons of sand might be deposited during six hours from a current of air forty feet wide from the lowest ten feet, in the lee of some intercepting obstacle, as in a railroad cut. In fact, such instances are on record. The velocity so near the ground would not exceed fifty miles an hour. Twenty-five tons may therefore be carried by 633,600,000 cubic feet of air, which makes nearly six thousand tons to a cubic mile. It is by no means likely that all, or even the greater part, of the sand carried by the lowest ten feet of the atmosphere can be left in the drift, and the estimate may again be much too low.
Still another approximation can be made by experimenting on the effects of dust in the atmosphere on the respiratory mechanism of the human body. Such effects are referred to in the following paragraphs:
"The wind sweeps down from the deserts and brings with it sand in such quantities as to almost make breathing impossible."—"The sand was blown in stifling clouds about them."—"During the sand storm the air is so full of dust that it feels as if it were impossible to breathe."
By some simple experiments it has been ascertained that less than two grains of mineral dust suspended in a cubic foot of air interferes with inhalation in a normal way. This would make about twenty thousand tons of dust to the cubic mile. It is pos-