The settlement of these bottom lands will also influence the prosperity of many commercial centers, as trade statistics indicate the general abandonment of the plantations that followed the great floods of 1882, caused a marked diminution in the shipments by the lower river, as well as in the receipts from that section; and that the partial reclamation of the lands and restoration of agricultural pursuits has already influenced the receipt and distribution of commercial products.
The project to reclaim by irrigation large districts of the arid region of the west, if successfully accomplished, may also exert an influence in the political and commercial relations of the future that cannot now be foretold. Two-fifths of the territory of the United States has been classed as arid; not in the sense that there is no water, for the greatest rivers on the continent have their sources almost in the midst of the region; but rather that the water is not available for enriching the ground. The rainfall is generally not in the season when the crops would require it, or is too small and uncertain for the husbandman to depend upon it. The whole region is not of this character; many districts are susceptible of the highest cultivation as nature has left them, and others have been redeemed by the application of the water supply through the simpler devices customary in irrigated countries; until now nearly all the districts have been occupied that are susceptible of agricultural pursuits, either in the natural state or by irrigation, unless water is secured by means generally beyond the reach of the individual or combination of individuals who may use it. And yet, it is believed there are millions of acres of rich land that may be redeemed and converted to the support of a large population, by the application of capital in the construction of works of irrigation. The progress of the surveys of the region, therefore, that have been instituted by the general government, are watched with absorbing interest. The districts susceptible of such extensive improvement are only approximately known, and as it is only through these surveys their availability will be made manifest, the importance of the work can hardly be overestimated. The prosperity of several states will be largely influenced by the success of operations of this kind within their borders, and in turn their greater development and increased wealth, must react upon the older communities and benefit them, on the principle that the healthful growth of a single member is strength to all.
The science of geography, as taught in the present day, is more comprehensive than the brief descriptions and delineations of the areas of land and water that satisfied the early explorers. The great strides that have been made in scientific research during the past century have opened new fields, and men are no longer content to picture that only which they can see. The varied features of the earth's surface, transformations now in progress and those which may be deduced from the facts we can observe, have led to many theories of the construction of the earth, ancient forms upon the surface and possibilities, if not probabilities, in the future. To ascertain the form of the earth has alone been the cause of heroic labor, and yet we have hardly passed the point that we can give it in probable terms with the general dimensions. Observations warrant the assumption that, discarding the accidents of nature—even the highest mountains—the sphere is far from being perfect. That it is flattened at the poles is now accepted as the true condition, but we have reason to believe, too, that this is not the only departure from the perfect sphere. The more thorough the research and precise the observations, the more certain does it appear that the crust has a form as though there had been great waves of matter that had been solidified. To locate the depressions of these great waves and measure their depths, to point to the crests and measure their extent, is a problem for the future to solve. Their study is claimed to be within the legitimate sphere of geography; and not until they have been satisfactorily answered can we assert the geographer is even approaching the end of the facts his science has yet to utilize.
In pre-historic geography we have had two papers presented to the Society during the past year, relating to the orographic features of the earth's surface in times past compared with the localities as we may see them to-day. In the first instance the comparison is evolved from an effort to trace the origin and growth of the rivers of Pennsylvania; and the second, in a description of the famed district around Asheville, North Carolina. These have a substantial interest to us, treating as they do of localities so well known; and they illustrate, too, the resources of induction in bringing to our view the probable wonders of ancient geographic forms.
The constitution of the interior of the earth is a subject of great interest in the science of geography, as many of the visible forms upon the crust have been wrought by the power of the agencies within it. The discussion has been warm in the past, and doubtless will be resumed with unabated interest as we find new phenomena for the argument. The apparent lull that has followed the promulgation of the theory, three years ago, that under the crust we should find a fluid, or semi-fluid, surrounding a solid nucleus, may not be of long duration. This hypothesis probably comes nearer to satisfying the conditions imposed by the physicist and geologist, than those which have preceded it, and may be accepted for the present; unless the processes of nature by which it is conceived this state of the interior of the earth has been produced, shall be demonstrated to have continued for sufficient time to have caused a condition of equilibrium and possible solidification of the whole sphere; when we might expect it to be repudiated by those who oppose the theory of isostacy, but commended by the physicists as supporting their claim that the earth must be substantially a solid even now. If we accept Mr. Frederick Wright's suggestion, isostacy may have an important bearing on the cause of the ice sheets that covered such great areas; a suggestion that opens to the vision of the imagination an orography beside which the grandest landscape we may see to-day would pale into insignificance. This is believed to be a new application of the isostatic theory, and may be a possible solution of a much vexed question when an initial cause for such great upheavals can be advanced that will not be inconsistent with other accepted conditions.
Theories are modified by new facts, and in any attempt to demonstrate the constitution of the interior of the earth, the increase of temperature with the depth is an important factor. The recent measures, therefore, in Germany, that indicate the figures generally accepted are not reliable, may be received with interest. The shaft was sunk especially for the purpose of observing temperatures at different depths, and every precaution that former experience had suggested seems to have been taken to secure accuracy. The greatest depth reached was about one mile. An elaborate discussion of the results fixes the increase of temperature at 1° F. for each 65 ft. increase of depth. This is about 15 ft. greater than the figures that have heretofore been given; a difference so large that we may question if they will be generally accepted until verified by further observations made with equally great care.
In conclusion permit me to note the fact that the United States was for the first time represented in the International Geodetic Association, at the meeting recently held in Paris; and also to record the successful conclusion of the fourth International Geographical Congress that assembled in Paris in August last. The reports from the Congress indicate a wide range of subjects discussed, and lead us to believe the interest in our science is progressive, and must receive the hearty appreciation of all who are inspired by the nobler instincts to develope the great sphere on which we live; that the riches, the beauties, and above all the grandeur of Nature, may be made manifest to ourselves and for our posterity.
REPORT—GEOGRAPHY OF THE AIR.
BY GEN. A. W. GREELY.