g?"

"A yard and a half," said George readily, and immediately ducked to
escape a blow aimed in his direction by the owner of the feet in
question.

"Ten inches," replied John. "That is, my shoes are just exactly that
long, for I remember measuring them in the gymnasium just before I left
home. They're in the cave if you want them."

"Not now," said Grant. "It's too late to do anything to-day, anyway, and
it'll be dark in a little while. If your shoes are exactly ten inches
long though, we can measure with them and figure out the distance easy
enough."

"Are you sure that the shark rock the code speaks of is the one on the
end of the island here?" exclaimed Fred.

"Sho' it am," said Sam. "Dey nevah was two rocks lak dat one."

"I guess that's right," agreed Fred. "It must be the one."

"Certainly it is," said John. "We wouldn't have found two codes on this
island unless the spot they referred to was here too."

"Oh, that's the rock all right," said Grant confidently. "I wish we
could start right down there now, but I suppose it would be foolish."

"I think we've done enough for one day anyhow," said John. "As long as
we have solved the code we can't have much to complain of for one day's
work."

"You haven't told us how you did it yet," said George.

"Haven't you found out for yourself? My, but you're dull."

"Perhaps I am," admitted George. "I don't see it though."

"Nor I," added Fred. "Tell us how you did it."

"How many letters are there in the alphabet?" asked Grant.

"Twenty-six," said George.

"What's the first letter?"

"A."

"What's the second?"

"B."

"And the third?"

"C."

"What's the twenty-sixth?"

"Z."

"You know your alphabet anyway," laughed Grant. "Now this is how the
code works; _a_ is the first letter so we call it one, _b_ is
the second so we call that two, and so on all the way through. For
instance, the letter _s_ would be number nineteen, and _t_
would be twenty. Do you see the idea?"

"Yes, I see that," said George. "Explain the rest."

"Why, it's just this. Wherever number one came we put the letter
_a_. If number thirteen appeared we'd substitute the thirteenth
letter in the alphabet in its place."

"Which would be _m_," said George after a little calculation on his
fingers.

"That's right," exclaimed Grant. "Now do you see how it was done?"

"Of course. Isn't that simple?"

"It took us long enough to find it out though," said John.

"Well, I should