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?"