For, drawn in some such manner as are the water-marks in paper, visible only when held up to the light, so on the plan, there was marked out a secret staircase, leading from the second to the third floor of the old mill! “By Jove!” cried Tom. “I have it. Now I see why that wall was so thick. The secret staircase is built in the thickness of the wall, and it doesn’t show from either inside or outside the mill. In fact it doesn’t show on the plans at first glance, and probably the old architect who drew them used a peculiar ink so that no one would know about the stairs. They must have been made so that anyone could get from the second to the third story of the mill in a hurry, and be hidden there when there was an attack by enemies. Well, I’ve made one discovery. Now to see how they use the stairs.” But this did not show on the plan, though Tom held it close to the light, hoping to discover lines so faint that a strong illumination was needed to bring them out. The secret staircase, plainly indicated now, seemed to begin abruptly on the second floor, at a point opposite the big stone grinders, and end on the third story. How to get to them was not shown, and, from what Tom remembered of the inside of the mill, he was sure that the interior wall at this point showed no break, and no signs of a secret door. “But that’s not saying it isn’t there,” he said aloud. “I’m going to have another look, now that I know where to search. I’ll have the boys out yet!” He was so excited that he could hardly get to sleep, and again he was making rapid plans for the rescue of his chums. “I’ll start the first thing in the morning,” he told himself. “Hurray! I’m on the right track now! And maybe I’ll be lucky enough to find the treasure, too!” By daylight he examined the plans again, holding the paper up to the sun. The secret staircase was shown more plainly, but there was nothing to indicate how it might be reached. “It’s just as if it was hidden between the thick walls,” murmured Tom. “But there must be some way of getting at it, and I’m going to find that way. Let’s see, now; what do I need?” He decided to take no weapon, for he did not believe the captors of his chums were desperate enough to fire. “But I’ll take one of our pocket electric flashlights,” he decided. “I may have to work in the dark if I’m looking for a secret passage.” What he would do when he got to the old mill, he never stopped to consider. “I can’t make any plans until I reach there,” he decided. “I’ll just have to size things up, and act on the spur of the moment. Jinks! If I only had one of the fellows with me it would be easy, but playing a lone hand isn’t what it’s cracked up to be. However, I’ll do my best.