A persistent veil of volcanic dust in the upper air is thought to exercise marked effects upon terrestrial temperatures, and prolonged periods of intense vulcanism have been regarded as the cause, or one of the causes, of the recurrent ice ages of which geology furnishes the record. This explanation of ice ages was advanced by P. and F. Sarasin, in 1901, and was first put upon a scientific basis by Dr. W. J. Humphreys in 1913; but the idea that volcanic dust might be the cause of cold seasons was suggested by Benjamin Franklin as early as 1784. Franklin’s speculations on this subject were prompted by the cold winter of 1783-1784, which followed the extraordinary fog of 1783, already mentioned. Humphreys has published a list of all the great volcanic outbreaks recorded since 1750, and has shown that each of them registered itself in the temperatures of the earth and also, since accurate measurements began to be made of solar radiation, in these instrumental records. Thus, the intensely cold winters of 1783-1785 followed the tremendous eruptions of Asama, Japan, and Skaptar Jökull, Iceland, in 1783; the famous “year without a summer” (1816) was the sequel of the gigantic outbreak of Tomboro, in the Sunda Islands, in 1815, which is said to have hurled thirty-six cubic miles of solid matter into the atmosphere; and definite periods of low temperatures and reduced sunshine were observed after the eruptions of Mont Pelée, in 1902, and Mount Katmai, Alaska, in 1912. The effect of a volcanic dust veil in lowering temperatures on earth is attributed chiefly to the fact that, while the fine grains of dust are able to reflect back into space the short waves of radiation coming from the sun, they do not bar the passage of the long heat waves radiated outward from the earth. According to Humphreys’s calculations, such a veil is about thirtyfold more effective in shutting solar radiation out than in keeping terrestrial radiation in. This process is just the reverse of the familiar effect of the greenhouse; where the glass lets in the short waves of solar radiation but does not readily let out the long waves of earth radiation. A small contingent of atmospheric dust consists of common salt (sodium chloride) due to the evaporation of spray from the ocean. This substance is frequently found in rain, as well as in samples of air, not only near the seashore, but even in the interior of continents and on high mountains. According to Du Bois the amount of sodium chloride annually deposited on the dunes of Holland is at least 6,000,000 kilograms (more than 6,600 tons). One of the striking phenomena of arid regions is the dust whirlwind; exemplified in the “devils” of India and South Africa, the “twisters” of Texas, etc. E. E. Free, in his treatise on “The Movement of Soil Material by the Wind” (U. S. Bureau of Soils, Bulletin 68), says of these whirls: