All at once I could hear my heart beat. A black vertical line appeared in the centre of the plaque. That line got wider and wider. The two panels thus formed slid back to each side, leaving a gap some two feet six inches wide. I was on the right track. The mystery of the _Herrenhausen_ was to be solved at last! I had recovered control of myself, perfect control. I remember saying: "What a delightful way of studying history! I wonder what Monsieur Seignobos would think of it?" I passed through the opening, taking with me the stool which had been my table. On the inside the fire-back had two handles, one on each side. Very gently, but quite easily, I drew the panels together again, not absolutely touching, however, for fear of releasing some fatal spring. Do you remember the 24th August, my friend, in the village of Beaumont, in Belgium, when you and I went down into a cellar where the inhabitants said five Uhlans were hiding? You called me a rash fool and came behind, but I couldn't help smiling at the thought that those five fugitives were nothing compared with the darkness in which I was wrapped that night. When I had pulled the two panels to behind me I found myself in a little chamber, six feet wide, and six feet high. On each side of me were blank walls, but at the back was another bronze plaque with, as I expected, two more knobs to right and left. I put the hand of the first dial on the figure 18. The pointer of the second had just reached 15 when the noise of tearing wood, absolutely terrifying in that dead silence, froze me from head to foot. The lower half of the immense plaque, opening horizontally about three feet from the ground, had swung forward and smashed to matchwood the heavy stool I had placed against it as I came in. If I hadn't jumped back so smartly my feet would infallibly have been crushed. "Excellent!" I murmured. "So their secrets are man-traps, too!" I bent down and got into the second chamber, which was of exactly the same dimensions as the first. You can imagine that this time I took all precautions, standing carefully to the left as I put the pointer of the fifth dial on 21 and to the right as I turned that of the sixth to 4. I might have saved myself the trouble. The plaque parted in two vertically, like the first one, and the panels rolled aside on invisible hinges. Then I passed into the third and last chamber. It was of the same height, but twice as long and wide. The small arc of my electric torch gave a good light, but covered but a small area. At first I could only see what looked like white splashes on the floor.