ou are."

They were about six hundred feet from the mouth of the cave, as nearly
as could be estimated.

"Before we return we might as well chart the cave, so we shall have some
idea of its crooks and turns. Have we anything to measure with? If not,
have you a cord, so we can get some idea of distances?"

George held out one of the spears, which was about five feet long.

"That will answer," said the Professor. "Let us call this five feet long
for the present. The first thing we must do is to establish a base line.
But what shall we do for something to mark our chart on? I haven't a bit
of paper."

Here was another difficulty. It would be impossible to make tracings on
their clothing. Harry's wit came to the rescue. "I have it. Why can't we
break off a piece of this chalk. Probably we can find some smooth piece
that will answer."

"That might be done," was George's answer, on reflection; "but what can
be used to mark on chalk?"

The Professor was now highly amused. "Can't you think of anything we
have here which will answer?"

"I haven't had a pencil, or anything except charcoal, since we touched
this wonder island."

"We have something here that is used among all civilized people the
world over for marking purposes." The boys opened their eyes in wonder.
"I have it here," said the Professor, looking at the lamp.

"The lamp?"

"No, not the lamp, but what is in the lamp."

"The oil? Where can we get anything to mark with in the oil?"

"We have been making a marking material all the time we have been in the
cave, and you are just as well acquainted with it as anything you know.
It is the soot from the burning oil."

The boys laughed, not at the information, but at their stupidity.

"Yes; the soot is carbon, and the best soot is made by imperfectly
burning oil, or fat, or any other fuel which has a large amount of
carbon."

The boys found several pieces of flat chalk, one of them a little over a
foot long. This was held above the flame of the candle until covered
with soot.

"The first thing to do is to establish a base line. This we will mark A
on the drawing. Now, starting from the point here where we lost the
light, you may measure along the line to the west, we will say, until
you get to the other end of the chamber. Twenty-five measures of the
spear? That makes the chamber 125 feet long, and it is about 90 feet the
other way. Let us roughly outline the floor plan. Now go out toward the
mouth of