Then, of a sudden, a broad flash of lightning flared down into the cut and revealed them to each other. “Reddy!” cried the boy. “Reddy!” The sentry sprang toward him with uplifted club, his face distorted with rage. “Don’t you know me, Reddy?” cried Allan, springing back to avoid the blow. “Sure Oi knows y’!” shouted the madman, savagely, coming on. “An’ Oi’m a-goin’ t’ do fer y’, like Dan told me to. He told me y’re all in th’ plot ag’in me!” “It’s a lie, Reddy!” protested Allan, violently. “It’s a lie!” Reddy paused for an instant. “A loi, is it?” he repeated. “Wasn’t it you as told on me fer breakin’ that link motion?” “Yes,” admitted the boy; “but—” [Illustration: “HE STEPPED TO ONE SIDE, AND ... BROUGHT DOWN HIS CLUB UPON THE OTHER’S HEAD”] Reddy waited to hear no more. “Oi knowed it!” he yelled. “Oi knowed it! Oi’ll show you! Oi’ll show you, y’ dirty spy! Don’t try t’ run—it’s no use!” And he came charging down upon Allan, his club swinging savagely. But Allan was thinking not in the least of running. Instead, he stood his ground, his teeth clenched, his eyes alert, his club ready. He was not in the least excited; now, indeed, he found an instant in which to wonder at his calmness. Then Reddy was upon him and struck at him savagely. He stepped to one side, and, putting all his force into the blow,—oh, how he hated to do it!—brought down his club upon the other’s head. CHAPTER XVIII. THE SIGNAL IN THE NIGHT That blow had all the weight of Allan’s muscular young body behind it, for he had realized that this was no moment to hold his hand, however he might wish to do so, and Reddy tumbled in a limp heap upon the track. The tears were gushing from the boy’s eyes as he bent over the body and drew it to one side to the shelter of the rock. That he should have struck Reddy—perhaps even killed him! But he could not linger; with a last glance at the prostrate figure, he turned back to the task before him. Plainly he could not hope to cross the trestle with half a dozen men working on it—to try to do so would mean certain failure. Yet he must cross the ravine,—there was only one other way, and that not an easy one.