"I don't believe you," came from Major Morris, promptly. "There is some mystery here, and as sure as you're born I'm going to find out what it is!" he went on. The major had scarcely finished when Ben's eyes fell to the floor, and he saw the outline of a trap-door under one of the regular's feet. One edge of the door was raised about half an inch above the floor proper, as if the door had been opened and not put back evenly into place. "Major, look at that trap-door!" he cried. "I'll wager they used it while we were coming up the outside stairs." "You must be right, captain. If you'll--" "We didn't use no trap-door," shouted the younger of the regulars, but he appeared much disconcerted over the discovery Ben had made. "Captain, I have them covered," came from Major Morris, as he brought out the two pistols with which he had wisely provided himself. "Perhaps you had better investigate." "I will," returned the young captain, and backed out of the room. The regulars wanted to stop him, but aiming his weapons at them the major told them to hold their peace. "If everything is all right, you won't be harmed," he said. "But it doesn't look right to me. You have no business here, for one thing." "And what business have you here?" demanded the older regular. And then he changed his manner. "We were captured in the fight of last week, and were just trying to get back to our lines again." "We'll talk about that when my friend the captain gets back, my man. If we are treating you unjustly, I'll apologize and do the handsome thing by you," he added. In the meantime Ben was making his way down to the bank of the stream, under the mill, with all possible speed. It was extremely dark, and he had to pick his way with caution for fear of tumbling into some ugly hollow. Below the mill was a fall of water, and here the stream ran between a series of sharp rocks. Ben had just gained the bank of the stream when a low moan reached his ears. At first he could not locate the sound, but presently discovered that it came from the vicinity of the rocks. Feeling his way along he managed, but not without great difficulty, to gain the top of the rocks. Here he saw the water foaming and boiling twenty feet below. "That woman must be down there," he muttered. Then he raised his voice. "Where are you?" "Down here, by the rocks!" came back faintly. "Help! please help me!" Locating the voice as well as he was able, the young captain began crawling down from one rock to another. This was difficult work, and he had to move with extreme care for fear of a tumble, which would land him directly into the boiling stream. At last, however, he found himself perched on a bit of a shelf, with the water less than two feet away.