On the back of this metal monster was a miniature replica of the strange mechanism upon the pinnacle. The little needle pointed up the canyon. A glance that way showed Dan that it pointed at the great device upon the mountain, which looked even more brilliant on this gloomy morning than in the uncertain radiance of the moon. The colossal ring was shrouded in a splendid mantle of purple flame; and the long, slender needle, which seemed to have swung on down to follow Mars below the horizon, still throbbed with scintillating white fire. For several minutes the two stood there, studying each other. A naked man, tense and bewildered in the presence of mysterious forces--and a grotesque machine, cased in gleaming white metal, whose parts seemed to duplicate most of the functions of a living creature. Then one of the writhing tentacles that shot from the "head" of the machine reached back under the metal case, and reappeared grasping what appeared to be a flat disk of emerald, two inches across and half an inch thick. This green disk it held up, with a flat side toward Dan. There was no sound, but a flash of green light came from it, cutting a wide swath into the jungle, and littering its path with smoking and flaming debris. * * * * * But Dan, expecting something of the kind, had flung himself sidewise into the shelter of the boulder beside which he had slept. Behind it, he gathered his feet under him, picked up a rock of convenient size for throwing, and waited, ready and alert. He heard the soft humming sound on the other side of the boulder. A glittering object flashed above him. Crashing through the brush the metal monster came to earth on the same side of the boulder with him. But the metal thing had not turned in its flight: consequently its rear end was toward Dan. As it began cumberously to turn about, he hurled his rock with an accuracy that came of a boyhood on the farm. Instinct had made him try for the little ring and needle on the back of the monster, apparently its most vulnerable part. Whether by luck or skill, the rock struck the gleaming ring, crushing it against the needle--and instant paralysis overtook the metal thing. Its tentacles and limbs became fixed and rigid, and it toppled over in the brush. Dan walked over to it, and examined it briefly. The green disk had fallen on the ground, and he picked it up. It was made of emerald crystal, it had a little knob of glistening metal set in one side. Rather afraid of it, Dan forebore to twist the knob. But he still clutched it in his hand a few moments later, when, partly for fear that others of its kind would come to succor the fallen monster, and partly to secure shelter from the threatening rain, he retired into the shadows of the tangled jungle.