. He knew that the
ladder had been left there, and, with a little trouble, he found it,
where Shirley had put it, behind some rocks on the floor of the cave. By
the aid of this he quickly descended into the mound, and then, moving
the foot of the ladder out of the way, he vigorously began to brush away
the dust from the stone pavement. When this was done, he held up the
lantern and carefully examined the central portion of the floor, and very
soon he discovered what he had come to look for. A space about three feet
square was marked off on the pavement of the mound by a very perceptible
crevice. The other stones of the pavement were placed rather irregularly,
but some of them had been cut to allow this single square stone to be set
in the centre.

"That's a trap-door," said Burke. "There can't be any doubt about that."
And immediately he set to work to get it open.

There was no ring, nor anything by which he could lift it; but if he
could get his heavy chisel under it, he was sure he could raise it until
he could get hold of it with his hands. So he began to drive his chisel
vigorously down into the cracks at various places. This was not difficult
to do, and, trying one side after another, he got the chisel down so far
that he could use it as a lever. But with all his strength he could not
raise the stone.

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 moun