"That ought to stop the nonsense," Harry nodded. "But I don't imagine that any further efforts to destroy the wall will be made tonight, anyway." "We'll have the night patrol out _every_ night after this," Tom declared. "But I'm not so sure either, that another effort won't be made to-night, if we don't put a watch on to stop this wicked business. Harry, do you mind remaining out here while I run back and get the boat out?" "Why should I mind?" Hazelton wanted to know. "Well, I didn't know whether you would, or not---after seeing that imaginary something behind you." "Don't laugh at me! I may have had a start, but you ought to be the first to know, Tom, that I haven't frozen feet." "I do know it, Harry. You've been through too many perils to be suspected of cowardice. Well, then, I'll run back." Tom Reade had really intended to leave the flash lamp with his chum, but he forgot to do so, and, as he jogged steadily along over the wall he threw the light ahead of him. As he got nearer shore Tom increased his jog to a brisk run. Once, on the way, he passed the prowling negro without knowing it. That huge fellow, seeing the ray of light come steadily near him, hesitated for a few moments, then took to the water, swimming well out. After Reade had passed, the fellow swam in toward the wall. Up on the wall climbed the negro. For a few minutes he crouched there, shaking the water from his garments. Then, cautiously, he began to crawl forward. "Boss Reade, he done gone in," muttered the prowler. "Boss Hazelton, Ah reckon he's mah poultry!" Harry, keeping his lone vigil away out on the narrow retaining wall, was growing sleepy. He had nearly forgotten his scare. Indeed, he was inclined to look upon it as a trick of his own brain. CHAPTER II THE CALL OF ONE IN TROUBLE Once Tom Reade reached the solid land he let his long legs out into a brisk run. With his years of practice on the Gridley High School athletic team he was not one to lose his wind readily. So he made his way at the same speed all the way up to the camp. "Who dar?" called a negro watchman, as Tom raced up to the outskirts of the camp. "Reade, chief engineer," Tom called, then wheeled and made off to the right, where the more substantial barracks of the foremen stood. Superintendent Renshaw lived in a two-story barrack still farther to the right, as the guest of the young engineers. "_Quien vive_?" (who's there?) hailed another voice, between the two barracks buildings. "So, Nicolas, you rascal, you haven't gone to bed?" demanded Tom, halting. "What did I tell you about earlier hours?" Nicolas was the young Mexican servant whom Tom and Harry had brought back with them from Mexico. Readers of the previous volume know all about this faithful fellow. "You and Senor Hazelton, you waire not in bed," replied Nicolas stolidly.