"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 our cave, and measure off 50 feet. Stop there. You see, I have marked the line A and have laid down the slate slab so that this line is exactly on a line with the one you have measured along the chamber. The point B, 50 feet from here, which you have just measured, makes an angle, C. I will now take the slab to the point B, and you may measure off 50 feet more, and we will call that D. That gives us another angle line, E. You see, at every point we establish a new base line. C is the base line for the line E, and so on all through the cave."

[Ill.u.s.tration: _Fig. 32. The Cave, and how it was charted._]

All the measurements were made on the plan outlined, and scratched on the sooted slab.

"One thing more we must observe. As we are marking the successive points, you will notice that I took particular care to observe the side walls of the pa.s.sageway, so that I could roughly outline them, noting the distances from the various points, and in a crude way marking out the irregularities between the points."

The chart interested the boys so intensely that the disappearance of the light did not again recur to them until they emerged from the mouth of the cave, when it all came back again, with added wonder and speculation. How many more mysterious things would present themselves!

CHAPTER XIII

THE EXCITING HUNT IN THE FOREST

They returned to their home with conflicting emotions. It had been an eventful day. They had a glimpse of the outside world, and an experience below the surface of the earth. Both were unsatisfying. They could explain the one far away on the sea; but the other, close at hand, was a mystery.

"How do you account for the water being so far away from the mouth of the cave?" was George's inquiry after the evening meal.

"Haven't you noticed that we have had no rain for the past five weeks?"

Such was, indeed, the case. The boys had not been observant, like the Professor.

"Evidently the water is supplied from rains, and the floor of the cave permits more or less of the water to leak through, so that, in time, if we had no rains, the cave would be entirely free of water."

"But how about the animal in there; if it is a water animal, how did it get in; and if the water ever dries up, how can it live there?"

"There may not be any subterranean connection directly with the sea, and the animal has been trapped there; or it may be able to reach the sea in the cave at any time, by some underground channel."

"How far do you think the cave goes in?"

"That is one of the impossible things to say. It may extend for miles.

Like yourselves, I am curious to know all about it, as soon as we can make further investigations. In the meantime, don't forget about the flag."

"I had forgotten that. I am willing to start on it in the morning. How large should it be made?"

"We should make it a regulation flag, by all means."

"Let me see; how many stripes must we have?" said Harry.

"I know; thirteen."

"Yes; and they must be red and white."

"That is correct," responded the Professor; "but do you remember how many of each?"

The boys were silent.

"The top and the bottom stripes are red, and the intervening ones white.

What do these stripes represent?"

"The original States."

"I see you have a pretty good recollection. I remember a cla.s.s of over forty boys, on one occasion, which had only three boys who recalled that. Then we must have a field of another color, up in the corner."

"That is the blue field, with stars on it."

"Yes; but how many stars?"

"A star for each State."

"And how many States are there now?"

There was no response to this. How many boys or men, even, can tell offhand the number at the present time?