Poised at the limit of clear visibility, the two men studied the incessant battle being waged beneath them. They saw not one, but fully a thousand of the globular craft high in the air and grouped in a great circle around an immense fortification upon the ground below. They saw no airships in the line of battle, but noticed that many such vessels were flying to and from the front, apparently carrying supplies. The fortress was an immense dome of some glassy, transparent material, partially covered with slag, through which they saw that the central space was occupied by orderly groups of barracks, and that round the circumference were arranged gigantic generators, projectors, and other machinery at whose purposes they could not even guess. From the base of the dome a twenty-mile-wide apron of the same glassy substance spread over the ground, and above this apron and around the dome were thrown the mighty defensive ray-screens, visible now and then in scintillating violet splendor as one of the copper-driven Kondalian projectors sought in vain for an opening. But the Earth-men saw with surprise that the main attack was not being directed at the dome; that only an occasional ray was thrown against it in order to make the defenders keep their screens up continuously. The edge of the apron was bearing the brunt of that vicious and never-ceasing attack, and most concerned the desperate defense. For miles beyond that edge, and as deep under it as frightful rays and enormous charges of explosive copper could penetrate, the ground was one seething, flaming volcano of molten and incandescent lava; lava constantly being volatilized by the unimaginable heat of those rays and being hurled for miles in all directions by the inconceivable power of those explosive copper projectiles--the heaviest projectiles that could be used without endangering the planet itself--being directed under the exposed edge of that unbreakable apron, which was in actuality anchored to the solid core of the planet itself; lava flowing into and filling up the vast craters caused by the explosions. The attack seemed fiercest at certain points, perhaps a quarter of a mile apart around the circle, and after a time the watchers perceived that at those points, under the edge of the apron, in that indescribable inferno of boiling lava, destructive rays, and disintegrating copper, there were enemy machines at work. These machines were strengthening the protecting apron and extending it, very slowly, but ever wider and ever deeper as the ground under it and before it was volatilized or hurled away by the awful forces of the Kondalian attack. So much destruction had already been wrought that the edge of the apron and its molten moat were already fully a mile below the normal level of that cratered, torn, and tortured plain.