At last, while working at one corner, he broke out a large piece of the pavement, eight or nine inches long, and found that it had covered a metal bar about an inch in diameter.  With his lantern he carefully examined this rod, and found that it was not iron, but appeared to be made of some sort of bronze.

“Now, what is this?” said Burke to himself.  “It’s either a hinge or a bolt.  It doesn’t look like a hinge, for it wouldn’t be any use for it to run so far into the rest of the pavement, and if it is a bolt, I don’t see how they got at it to move it.  I’ll see where it goes to.”  And he began to cut away more of the pavement toward the wall of the dome.  The pieces of stone came up without much trouble, and as far as he cut he found the metal rod.

“By George!” said he, “I believe it goes outside of the mound!  They worked it from outside!”

Putting the ladder in place, he ran up with his lantern and tools, and descended to the outside floor.  Then he examined the floor of the cave where the rod must run if it came outside the mound.  He found a line of flat stones, each about a foot square, extending from the mound toward the western side of the cave.

“Oh, ho!” he cried, and on his knees he went to work, soon forcing up one of these stones, and under it was the metal rod, lying in a groove considerably larger than itself.  Burke now followed the line of stones to the western side of the cave, where the roof was so low he could scarcely stand up under it.  To make sure, he took up another stone, and still found the rod.

“I see what this means,” said he.  “That bolt is worked from clean outside, and I’ve got to find the handle of it.  If I can’t do that, I’ll go back and cut through that bolt, if my chisel will do it.”

He now went back to a point on the line of stones about midway between the side of the cave and the mound, and then, walking forward as nearly as possible in a straight line, which would be at right angles with the metal rod, he proceeded until he had reached the entrance to the passageway which led to the outer caves, carefully counting his steps as he went.  Then he turned squarely about, entered the passage, and walked along it until he came to the door of the room which had once been occupied by Captain Horn.

“I’ll try it inside first,” said Burke to himself, “and then I’ll go outside.”

He walked through the rooms, turning to the right about ten feet when he came to the middle apartment,—­for the door here was not opposite to the others,—­but coming back again to his line of march as soon as he was on the other side.  He proceeded until he reached the large cave, open at the top, which was the last of these compartments.  This was an extensive cavern, the back part being, however, so much impeded by rocks that had fallen from the roof that it was difficult for him to make any progress, and the numbering of his steps depended very much upon calculation.  But when he reached the farthest wall, Burke believed that he had gone about as great a distance as he had stepped off in the cave of the lake.