moisture; the planting of trees and the growth of crops, restricting the run-off; the roots of the new vegetable life drawing up the moisture from below the surface of the ground and transpiring it to the air through the leaves of plants; the enormous quantities of water vapor ejected into the air by the combustion necessarily incident to a considerable population,—all had combined to increase the rainfall and render the sub-arid plains more responsive to the efforts of the husbandman. No one can fail to regret that this theory is not founded upon fact. But a moment’s thought by the scientist will indicate to him that the volume of air is so great, and under the heat of the growing period its capacity for moisture so enormous, that the addition of vapor of water by the processes herein described, great though it be, is ineffectual to appreciably change the amount of the rainfall that nature beforehand had ordained should be precipitated. The size of continental areas, the height and the trend of mountain ranges, the proximity of large bodies of water, and the direction of the prevailing winds are the factors that determine the amount of the precipitation of a region. Against these the puny efforts of man, stupendous though we think them to be, are entirely unavailing. As an illustration: If the Rocky Mountains were as old as the Appalachian Chain, and if they were eroded down to the height of the latter system, the winds from the Pacific Ocean, when they are drawn inland by the cyclonic storms of the Rocky Mountain plateau, or of the Mississippi Valley, instead of depositing their moisture on the west slopes of the first range of mountains, would carry the water vapor of the Pacific clear to that place in the Mississippi Valley where it would meet the moisture drawn by the same storms from the Gulf of Mexico and the Atlantic Ocean. This will appear clear when one understands that cyclonic storms, such as are continually passing across our continent in periods of about three days each, may embrace in their eddy-like circulating systems areas one to three thousand miles in diameter, in which the winds from all directions spirally flow towards the center of the cyclonic system and the system itself is moving eastward. The water vapor exists as a separate atmosphere from oxygen and nitrogen. It is screened off from the interior of continents by mountain ranges because it is condensed and precipitated as rain or snow at only moderate elevations. The windward side of mountains may, therefore, receive torrential rains while their leeward sides are entirely without precipitation.