My heart leaped suddenly, for I saw Elza's eyes widen, fear spring to her face; heard the sharp intake of her breath, and felt her hand grip my arm. "Jac! There's something wrong! See there? And you hear it?" From the instrument room I heard a vague drumming. A hiss, and then a drumming growing louder. It was not a new sound, for now I remembered I had been conscious of it for several moments past. Our encampment was awake to it! A confusion down there; people running about; a figure dashing wildly into the instrument room. And the aerials on the power house began to snap viciously. "Jac! What is it?" "I don't know. See there, Elza? The sub-ray lights!" The search-beams from our towers were inordinately active. Sweeping the empty snow-plain and the empty sky. Empty? To my fevered imagination they were peopled with enemies. And then one of the towers flashed on a sub-ray--the dull infra-red for envisaging the slow rays below the power of human sight. And another tower with its faint purple beam was using the ultra-violet. "That drumming, Elza! That's a microphone--the big one they just erected near the instrument room. There's something coming! That's the magnified sound of some distant rush of air. Very faint sound, but they must have heard it on the ear-phones long ago. That microphone must have just been connected--" Something coming? We could see nothing. "Let's go down, Jac! We must get back--" "I've got infra-red glasses--" I fumbled beneath my furs. But I did not have them. "Jac--" "Wait, Elza." My glasses would have been useless, for the sub and ultra beams from the towers were disclosing nothing. I could tell that by the hasty searching sweeps they made. And then from the big Wilton tower, the newly connected Zed-ray flashed on, I could hear the load of it in the deepened, throaty hum from the power house. Its dirty brown beam sprayed out over the plain; then swung to the sky, caught something, hung motionless, narrowed into great intensity. The powerful Zed-ray, capturing the visibility of dense solids only.[24] [Footnote 24: Similar doubtless to our present-day X-ray.] There was something up there in the sky! The Zed-ray met resistance; we could see the sparks, and hear the snap of them coming like a roar from the microphone above the drumming. Met the resistance and conquered it; gradually the snapping roar died away. "Jac! I see something! Something there--don't you see it?"