They had no time to think of these things when they were at work, but in the night their thoughts often wandered back again to their homes and friends. Could they be blamed for that homesick feeling which came over them?

"Now lead the way, George; we want to see some more of your mysteries.

Isn't it fortunate that the tide is out? It will give us a good chance to investigate."

The path which George had taken was farther to the right, but as he was in a hurry to get down as quickly as possible he followed a course, which was much steeper, with Harry and the Professor close on his heels.

When the bottom was reached there was no sign of a pocket, or a cave, or anything of that kind. George was very much annoyed. He could not be mistaken in the position, as it was directly to the right of Observation Hill, and not three hundred feet from the spot where Harry had landed on his first trip to the island.

"It seems to me, George, we are too low down. At high tide this place is all covered with water. It must he higher up in the cliff sides."

Harry scrambled up again part way, and shortly afterwards cried out: "I have found something here; come up at once."

He was distant not more than twenty feet above them, but so rugged were the cliffs that the opening was entirely hidden from below.

[Ill.u.s.tration: _Fig. 18. Cave Entrance._]

"This is an example of the corrosive effects of the sea, and of the elements in the water and in the rock. As these rocks are limestone formations, we may be able to see some beautiful decorations within, if the cave is of any extent."

"It is awful dark in there. I wish we hadn't forgotten the candles," he said, as he glanced at the Professor, who seemed to be quite absorbed in examining the rocks.

"Don't you think we had better go in?"

"I should like to do so, but we haven't any candles."

"I thought you came here for the purpose of examining the cave?"

"So we did."

"And came without making any preparations for it?" As he said this he drew out one of the small candles which they had been making and using for their evening work and recreation.

George and Harry were both very much ashamed of their carelessness. The Professor, on the other hand, did not make another remark on the subject. No doubt the silent rebuke was a lesson they would retain much better than if it should be more forcibly presented.

The boys, be it said to their credit, never resented any word or action on the part of the Professor. They had only love and veneration for him; and the Professor, by his constant att.i.tude toward them, showed that even these careless actions or any other examples of thoughtlessness on the part of the boys, were part of the training that would teach lessons of value.

Below the mouth of the cave were little streams of water which looked like springs, and the Professor was of the opinion that the floor or interior of the cave must be lower than the entrance.

"Why do you think so?" was the inquiry.

"The springs below seem to indicate that when the high tide fills the cave, the bottom of the cave leaks enough to let out the water. The height of the mouth from the normal level of the water is much above the usual high tide level of the water, and it is only when there is an abnormally high tide, as on the day that George saw it, when the cave could be filled with water."

"Who will go in first?" said George. "I have no objection to taking the lead; so here goes."

George moved forward cautiously, holding the candle, and Harry followed with another. The opening was fully ten feet high, and at least that much in width, but irregularly formed. They went in straight for twenty feet or more, when George announced that he had reached a wall. The Professor, who was in the rear, called out: "Look to the right, there is a turn here."

Such was the case, but the broken up character of the sides and floor prevented them from readily grasping the formation. After making a jog the cave again turned into the cliff, practically on a line with the opening section or mouth of the cave. It was dark at first, but now, for some peculiar reason, it grew lighter as they advanced, and finally George stopped.

"What makes that peculiar light?"

"It is not a light; it is merely white walls and ceilings."

"What causes it?"

"Carbonate of lime, or chalk, which is caused by lime water coming from above and trickling down through to openings or crevices, and leaving the deposits there. It is not an uncommon thing in caves, and I foreshadowed it in the cave when I stated that the rocks were of limestone formation. You will remember we made lime from this kind of rock."

A loud splash and a groan-like noise put further conversation at an end.

"What was that?" asked George, as he retreated. In doing so he tripped, and, in falling, the light he carried was extinguished as it flew from his hand.

Clearly there was water ahead. "Let us go forward, a little closer,"

said the Professor. "You might get your guns ready, in case of necessity."

Within thirty feet of them was the edge of water, and the light threw a beam beyond for a hundred feet or more.

"It seems as though we have reached the end of our explorations here."

As they looked, the water was agitated, and it was plain that some aquatic animal was within the cave.

To return was the only thing to do, and as they went back the sides of the cavern were examined, and the Professor took a number of samples, as he said: "Don't lose the candle until we get where we can see daylight.

Although we are not more than two hundred feet from the mouth of the cave, the remarkable bend or jog near the mouth of the cave makes it as dark as though we were in a thousand feet."

"What sort of animal do you suppose that was?"

"Possibly a sea lion, or a seal."

After the open air was reached the Professor said: "I do not see how this cave would account for the phenomena that George saw the other day."

"Why not?"

"The cave is too deep. It is not a true air pocket, and----"

"What is this? Here is another one, still larger, and lower down. Here, hold the candle." Harry was down in an instant.

"Probably this is what George saw."

True enough; it was an air pocket which extended in about fifty feet, and had no pa.s.sageway beyond.

CHAPTER VII

INVESTIGATING THE PROSPECTOR'S HOLE

The knowledge that some marine animal inhabited the cave was now a constant topic, particularly with George, who was determined, sooner or later, to find out something more about it. With this end in view he made secret preparations, particularly in constructing a lamp which would not be liable to overturn or be put out by wind or in falling.

Thus far the only light available was obtained from candles made from the fat of the animals, and it was not the kind of illuminating material they had been used to. When people knew nothing better than tallow candles, that light was satisfactory, but when petroleum was once used tallow candles were entirely unsuitable and too primitive.

The statement by the Professor that the hole into which George fell, some months before, contained asphaltum, hinted at a possible source of petroleum, and through the persistent efforts of George, the Professor agreed to accompany him to the place to make an investigation.