Professor W. H. Pickering made seven photographs of Mars on April 9, and within twenty-four hours made seven additional photographs of the same region. The second series of photographs showed an area of white extending from the polar snow cap far down toward the equator, covering a surface which he estimated to be as large as the United States. It afterwards slowly disappeared. How shall we account for this sudden apparition of a vast area of white which the photographs of twenty-four hours before did not reveal. A boy of ten, as well as the philosopher would simply say a snow-storm had taken place in Mars. Is it, then, unreasonable to picture whirling snowflakes, snow-drifts, and dazzling whiteness from the Sun's rays, and in the rapid melting of the snow, broad rivers and turbulent brooks with water areas frozen at night? But why should we be compelled to imagine as naked the surface through which these waters find their way? Soil there must be from the continual erosion of running water. The character of the rock exposures we cannot guess at, but a picture of bare rock and lifeless ground is unthinkable. Such wide-spread storms without an atmosphere could not occur. The seasonal appearance of these snows and their slow disappearance not only indicates an atmosphere, but an atmosphere disturbed by established currents which convey the moisture-laden air to regions of congelation. A number of observers who have detected clouds in Mars described them as being yellowish in color. What more probable than that these yellowish masses are simply dust-storms such as one may often see whirling along over our American deserts? When the gusts of wind are fitful like squalls at sea, the obscuration would be fitful, to clear up again. The vast areas of desert land in Mars renders this supposition very probable. Since the above was written, my attention has been called to an early "Bulletin of the Lowell Observatory," in which Mr. Lowell, in discussing the appearance of a certain large projection on the terminator of Mars, says: "Finally, its color leads me to believe it not a cloud of water-vapor, but a cloud of dust. Other phenomena of the planet bear out this supposition." XII NOTES ON IRRIGATION _Your theory of vegetation becomes more and more probable._ SCHIAPARELLI in a letter to Lowell.