you suppose that this dome was entirely covered by water when the lake
was full?"

"I think so," said he. "Judging from what I know of the depth of the
lake, I am almost sure of it."

"Ralph!" suddenly cried Mrs. Cliff, "don't try to do that. The thing may
break under you, and nobody knows what you would fall into. Come down."

But Ralph paid no attention to her words. He was half-way up the side of
the mound when she began to speak, and on its top when she had finished.

"Captain," he cried, "hand me up the lantern. I want to see if there is
a trap-door into this affair. Don't be afraid, Mrs. Cliff. It's as solid
as a rock."

The captain did not hand up the lantern, but holding it carefully in one
hand, he ascended the dome by means of the row of protuberances on the
other side, and crouched down beside Ralph on the top of it.

"Oh, ho!" said he, as he moved the lantern this way and that, "here is a
square slab fitted into the very top."

"Yes," said Ralph, "and it's got different mortar around the edges."

"That is not mortar," said the captain. "I believe it is some sort of
resin. Here, hold the lantern, and be careful of it." The captain took
his jack--knife out of his pocket, and with the large blade began to dig
into the substance which filled the joint around the slab, which was
about eighteen inches square. "It is resin," said he, "or something like