Far-reaching as are the economic aspects of the problem of transportation, this extraordinary revolution, the effects of which are visible on every side, has been brought about by the application of only a few types of mechanisms. The steam engine, the dynamo, and the gas engine are substantially responsible for the entire development in question. In the succeeding pages, which deal with the development of steamships, locomotives, automobiles, and flying machines, we have to do with the application of principles with which our previous studies have made us familiar; and in particular with the mechanisms that have engaged our attention in the preceding volume. Yet the application of these principles and the utilization of these mechanisms gave full opportunity for the exercise of inventive ingenuity, and the story of the development of steamships, locomotives, electric vehicles, automobiles, gyro cars, and flying machines, will be found to have elements of interest commensurate with the importance of these mechanisms themselves. Before we take up these stories in detail, however, we shall briefly review the story of geographical discovery and exploration in its scientific aspects. I THE CONQUEST OF THE ZONES The contrast between modern and ancient times is strikingly suggested by reflection on the limited range of geographical knowledge of those Oriental and Classical nations who dominated the scene at that remote period which we are accustomed to characterize as the dawn of history. The Egyptians, peopling the narrow valley of the Nile, scarcely had direct dealings with any people more remote than the Babylonians and Assyrians occupying the valley of the Euphrates. Babylonians and Assyrians in turn were in touch with no Eastern civilization more remote than that of Persia and India, and knew nothing of any Western world beyond the borders of Greece. Greeks and Romans, when in succession they came to dominate the world stage,--developing a civilization which even as viewed from our modern vantage-ground seems marvelous,--were still confined to narrow strips of territory about the shores of the Mediterranean, and had but the vaguest notions as to any other regions of the earth.