"No need, Mr. Hatfield. I'll put out the light and see that everything is ship-shape when I leave."

"Well, don't stay too long, Dan."

In departing, Mr. Hatfield and Brad lowered a canvas covering which served as a door across the cave entrance. Of no practical value in protecting the clubroom from intruders, it kept out wind and rain.

Left to himself, Dan devoted himself once more to the task at hand.

Writing out the letters of the alphabet in orderly rows, he gave each a number, thus:

A B C D E F G H I J K L M N 1 2 3 4 5 6 7 8 9 0 1 2 3 4

and on through the alphabet.

When this brought no solution he tried a second combination, starting with the number "2" instead of "1".

A B C D E F G H I J K L M N 2 3 4 5 6 7 8 9 0 1 2 3 4 5

For the next twenty minutes he worked on, trying many combinations. Each time, he started off with a higher number for the letter "A," working up as high as "7".

"It's no use," he decided at last. "I may as well go home."

On the pad before Dan was a string of unused alphabetical letters.

Absently, with no hope of striking upon anything that would work, he wrote in corresponding numbers, starting with "8."

A B C D E F G H I J K L 8 9 0 1 2 3 4 5 6 7 8 9 M N O P Q R S T U V W X Y Z 0 1 2 3 4 5 6 7 8 9 0 1 2 3

Referring to Jacques' coded message, he then began to transfer the numbers into counterpart letters.

"Let's see," he mused. "'0' could be either C, M or W; and '2' might be E, O or Y. Number 6 would have to be I or S. Number 1 could represent D, N or X. And 4 would have to be G-"

Dan went no further, for suddenly he saw that the puzzle actually was beginning to make sense. Excitedly he wrote in the first combination of letters:

020614 COMING

"Yipee! I've got it!" he muttered. "I've found the key!"

A strong gust of wind unexpectedly flapped the light canvas which hung across the cave doorway.

So engrossed was Dan that he did not notice. Nor did he see a shadowy figure crouching on the platform.

Then the light suddenly was extinguished. Startled, Dan jerked to attention.

As he groped for a match with which to relight the lamp, a deep voice entoned:

"_Take warning, Dan Carter! Remain away from Skeleton Island!_"

Dan felt the hair rise on his neck. Chills slithered down his spine. The mysterious voice, he realized, came from the cave entranceway.

Recovering from the first shock of surprise, the boy sprang to his feet, upsetting his chair.

In three long strides he reached the entrance and jerked aside the canvas flap.

No one was there. But disappearing down the steps Dan saw a lean, dark figure.

"Hey, you!" he shouted furiously.

The intruder only ran the faster, pulling a jacket high around his neck to shield his face.

Angered by the threat and determined to learn who had been spying upon him, Dan started down the steps in hot pursuit.

The intruder, a fleet runner, raced across the beach, heading for a clump of bushes along the highway. Dan pounded closer and closer at his heels.

Then, the one ahead unexpectedly tripped in the loose sand. He stumbled and fell. In a flash Dan was upon him, pulling the jacket away so that he could see the culprit's face.

"Ross!" he recognized him. "Ross Langdon! Well, of all the dirty, low tricks!"

"Take it easy, will you?" panted the Cub from the rival Den. "You're smashing my ribs!"

"Serves you right! So you've been spying on the Cave!"

"Aw, I wasn't spying," Ross protested. "Can't you take a joke?"

"How long were you hiding there behind the canvas flap?"

"Only a minute or two, Dan. Honest! I saw the light burning, so I thought I'd take a peek and see who was there."

"It was a lousy trick-especially that warning about going to Skeleton Island."

"Scared you, didn't I?" Ross chuckled, squirming to free himself from the other's tight grasp.

"You startled me. But I don't scare that easily."

"Like fun! You nearly jumped through the roof of the Cave! What were you working on so late, anyhow?"

"Oh, nothing."

"Nothing? You were so absorbed you didn't hear a thing until I waved the canvas flap to make the light go out. You were figuring out something with paper and pencil. Your income tax?"

"Just a little work for the Den," Dan replied vaguely.

"Keeping it to yourself, eh? If you'll climb off my mid-section, I'll amble back there with you. I've always been curious to see the set-up of your much advertised Cave."

Dan made no comment as he let Ross up. Both boys dusted their clothing free of sand.

"Well, let's go," Ross said impatiently. He started toward the Cave.

"I was just thinking it's time to go home," Dan said, following the other boy reluctantly. "It's getting late-"