They were sheltered under the brow of the hill, but the blast from the valley beyond rocked them like an earthquake. They rushed to the top of the knoll. MacGregor was standing in the valley; he waved them a greeting and shouted something unintelligible. The gas had mushroomed into a cloud of steamy vapor. From above came snowflakes to whirl in the churning mass, then fall to the ground. A wind came howling about them to beat upon the cloud. It swirled slowly back and down the valley. The figure of MacGregor vanished in its smothering embrace. "Exit, MacGregor!" said Cyrus Thurston softly. He held tight to the struggling figure of Slim Riley. "He couldn't live a minute in that atmosphere of hydrogen," he explained. "They can--the devils!--but not a good egg like Mac. It's our job now--yours and mine." Slowly the gas retreated, lifted to permit their passage down the slope. * * * * * MacGregor was a good prophet. Thurston admitted that when, four days later, he stood on the roof of the Equitable Building in lower New York. The monsters had fed as predicted. Out in Wyoming a desolate area marked the place of their meal, where a great herd of cattle lay smothered and frozen. There were ranch houses, too, in the circle of destruction, their occupants frozen stiff as the carcasses that dotted the plains. The country had stood tense for the following blow. Only Thurston had lived in certainty of a few days reprieve. And now had come the fourth day. In Washington was Riley. Thurston had been in touch with him frequently. "Sure, it's a crazy machine," the pilot had told him, "and 'tis not much I think of it at all. Neither bullets nor guns, just this big glass contraption and speed. She's fast, man, she's fast ... but it's little hope I have." And Thurston, remembering the scientist's words, was heartless and sick with dreadful certainty. There were aircraft ready near New York; it was generally felt that here was the next objective. The enemy had looked it over carefully. And Washington, too, was guarded. The nation's capital must receive what little help the aircraft could afford. There were other cities waiting for destruction. If not this time--later! The horror hung over them all. * * * * * The fourth day! And Thurston was suddenly certain of the fate of New York. He hurried to a telephone. Of the Secretary of War he implored assistance. "Send your planes," he begged. "Here's where we will get it next. Send Riley. Let's make a last stand--win or lose." "I'll give you a squadron," was the concession. "What difference whether they die there or here...?" The voice was that of a weary man, weary and sleepless and hopeless. "Good-by Cy, old man!" The click of the receiver sounded in Thurston's ear. He returned to the roof for his vigil.