sick, and
for the first time he burst into tears. But George was a brave boy. He
knew that crying was useless, and felt a little ashamed of himself.

His reflections were not long, however. To his left he saw a peculiar
sight. At every inrushing wave there was a report like a cannon shot,
followed by a tremendous stream and spray of water, which was shot out
to sea high up above the waves.

This was an extraordinary sight to him, and unexplainable. The story was
related to the Professor that evening.

"That was an air pocket in the rocks."

"What is an air pocket?"

"From your description it is probably a large cave, so situated in the
wall of the cliff, that at a certain period the waves will entirely
close the mouth. When the wave dashes up against the cliff and closes
the mouth of the cave, the water tries to enter the cave. In doing so
air is compressed in the pocket, and when the wave again starts to go
out to sea, and the pressure is partly taken away, the compressed air
explodes, so to say, and shoots out the water into a spray, and also
causes the noise you heard."

"How much can air be compressed?"

"It is not known definitely how far. It has been compressed to less than
one-eight-hundredth of its bulk. It is the most elastic substance
known."

"Isn't water compressible?"

"No; if it had been compressible you would not have had that exhibition
at the air pocket."

CHAPTER V

AN EXCITING HUNT

"What is that rocking?" cried Harry, jumping out of his couch, one
night.

The Professor was awake and had noticed it.

"Probably an earthquake."

The rocking continued for several minutes, and then gradually subsided.
They boys were so excited that sleep was out of the question, for the
time, besides the shaking might again recur at any moment.

"Do you think there is any danger, Professor?"

"It is impossible to say what will happen when these symptoms in the
earth's crust take place."

"Are there not some instruments which indicate the extent and possible
dangers of the quakes?"

"There is an instrument called the seismograph, which records the
vibratory movements of the earth, and also locates the distances at
which the shocks are from the observer, but there is nothing to indicate
what the extent and probable dangers are."

"Is it true that the interior of the earth is in a liquid state?"

"Such has been the theory for many years; but it is now believed to be a
solid--a body with