"With your telescopes you have no doubt noticed that there is a great bending sea connecting the oceans of the south with those of the north and running through the midst of the continents." "Quite so," said the astronomer who had spoken before, "we call it the Syrtis Major." "That long narrow sea," Aina went on, "forms a great channel through which the flood of waters caused by the melting of the southern polar snows flows swiftly toward the equator and then on toward the north until it reaches the sea basins which exist there. At that point it is rapidly turned into ice and snow, because, of course, while it is Summer in the southern hemisphere it is Winter in the northern. "The Syrtis Major (I am giving our name to the channel of communication in place of that by which the girl called it) is like a great safety valve, which, by permitting the waters to flow northward, saves the continents from inundation. "But when mid-Summer arrives, the snows around the pole, having been completely melted away, the flood ceases and the water begins to recede. At this time, but for a device which the Martians have employed, the canals connected with the oceans would run dry, and the vegetation left without moisture under the Summer sun, would quickly perish. "To prevent this they have built a series of enormous gates extending completely across the Syrtis Major at its narrowest point (latitude 25 degrees south). These gates are all controlled by machinery collected at a single point on the shore of the strait. As soon as the flood in the Syrtis Major begins to recede, the gates are closed, and, the water being thus restrained, the irrigating canals are kept full long enough to mature the harvests." "The clue! The clue at last!" exclaimed Mr. Edison. "That is the place where we shall nip them. If we can close those gates now at the moment of high tide we shall flood the country. Did you say," he continued, turning to Aina, "that the movement of the gates was all controlled from a single point?" "Yes," said the girl. "There is a great building (power house) full of tremendous machinery which I once entered when my father was taken there by his master, and where I saw one Martian, by turning a little handle, cause the great line of gates, stretching a hundred miles across the sea, to slowly shut in, edge to edge, until the flow of the water toward the north had been stopped." "How is the building protected?" "So completely," said Aina, "that my only fear is that you may not be able to reach it. On account of the danger from their enemies on Ceres, the Martians have fortified it strongly on all sides, and have even surrounded it and covered it overhead with a great electrical network, to touch which would be instant death." "Ah," said Mr. Edison, "they have got an electric shield, have they? Well, I think we shall be able to manage that."