observation, for the quantity of dust settling on floors during such storms is about a fourteenth of an ounce of dust on a surface of a square yard in half a day. A maximum estimate made on the basis of the above newspaper accounts would be at least five pounds to a square yard of surface for a storm lasting twenty-four hours. If we then suppose that a house that is twenty-four feet wide and thirty-two feet long has open crevices, which average a sixteenth of an inch in width and have a running length in windows and doors of one hundred and fifty feet, the wind may be supposed to enter half of these crevices with a velocity of five miles per hour for the time the storm lasts, or for twenty-four hours. The dust may be supposed to settle on not less than eighty-five square yards of surface, including floor space and horizontal surfaces of furniture. The minimum estimate, based on these figures, gives us two hundred and twenty-five tons of dust to the cubic mile of air. The maximum estimate would be one hundred and twenty-six thousand tons.
In the following citations the optical aspects of the dust-laden air are again characterized in a definite way: "The air was so full of sand that it resembled a fog."—"A wind storm struck us, bringing a dense cloud of dust."—"The sky assumed a deep, tawny hue, and fifty yards was the limit of clear vision."—"The dust was so thick and heavy that a person could not see more than a block through it."—"The dust was so thick that it was impossible to see halfway across the street."—"I have seen the dust so fill the air [in a Western dust storm] as to make it difficult to see more than a few rods."—"At times it was impossible to see across the street on account of the flying sand."—"A strong wind was made thick and yellow by flying real estate."—"The wind filled the air with dust as far as the eye could see. It immediately became dark, and lamps had to be lighted."—"During the sand storm it was dark as night, and people ran into each other in their flight through the streets."—"The wind was accompanied by dense clouds of dust that obscured the sky until all was dark as midnight."
From the phrases used it is evident that the transparency of the atmosphere must have been considerably less than when the sun could be viewed through it, or when objects might be seen dimly at a distance of one or two miles, as in some instances previously mentioned. This difference in the two cases is, of course, due to the increased quantity of dust carried by the air. Such conditions as are described here may readily be produced experimentally on a small scale by throwing dust into the air on a windy day. If the quantity of the dust be known, it is necessary only to estimate the degree of opacity produced and the bulk of the air in which the material is dispersed. From a number of