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velocities and temperatures this difference in viscosity can perhaps be altogether disregarded. The relative power of the atmosphere to sustain a load of fine sediments would, therefore, appear to be no more than, say ⁠1/2000⁠ of that of river water. But to be certain that this estimate shall not be too high, let us make the fraction ⁠1/5⁠ of this value and call it ⁠1/10000⁠. This means that if a cubic foot of water, e. g., in the Mississippi, will hold in suspension 15.48 grams of solid particles^[1], then the atmosphere above it can hold in the same manner in a cubic foot ⁠1/10000⁠ of this quantity, or about .0015 gram. It will be remembered that this is true only for material of a certain coarseness. If it is too coarse, the atmosphere cannot hold it at all; while if it is very fine, considerably more can no doubt be sustained. In order to ascertain approximately the effect of the variation of the size of the particles on the quantity of materials which can be thus suspended in the air, and also to make sure that the above estimate of the total load of sediments which can be sustained is not too high, some simple experiments have been made. These consisted in introducing dust of varied degrees in coarseness into a receiver, and then keeping the air in the receiver in constant agitation at a velocity of about five miles per hour. A certain quantity of dust would in this manner be kept floating in the circulating air, and this quantity was found to vary with the nature of the material introduced. The results may be tabulated as follows:

This apparently amply justifies the above estimate as to the quantity of dust which can be sustained in a certain bulk of atmospheric air. It is not supposed that the table gives exact determinations for the different materials, for the conditions of