“If it doesn’t mean anything, why did Gulliver hide it?” Bob asked.

“That’s exactly the point,” Jupiter said. “Why did he hide it? It looks as if he considered it important, somehow.”

Pete scratched his head. “Well, it certainly doesn’t say anything about any money.”

“This Spike Neely was in the prison hospital when he wrote it,” Bob said. “I think that letters from prisoners are always read by the authorities before they’re mailed. So Spike couldn’t say anything about any money without letting the prison authorities in on it.”

“Unless somehow he did it secretly,” Jupiter suggested.

“You mean a message in invisible ink, something like that?” Pete asked.

“It’s a possibility. I suggest we take this letter into Headquarters and analyse it.”

Jupiter went over to the iron grillwork that seemed to be leaning against the back of the printing press they had rebuilt some time ago. When moved aside, the grillwork revealed the opening of Tunnel Two, their main entrance into Headquarters. Tunnel Two was a length of large iron pipe about two feet in diameter, ridged the way pipes used in culverts are. It went, partly underground, beneath a pile of rather worthless junk until it came up underneath Headquarters, which was a mobile home trailer hidden from sight in the midst of the junk.

Jupiter went first, then Bob, then Pete, scrambling on hands and knees through Tunnel Two, which was padded with old rags so the corrugations in the pipe would not bruise their knees. They pushed up the trap-door at the other end and clambered out into the tiny office of Headquarters.

The three boys had built a tiny laboratory in the old trailer, complete with microscope and other necessary items. There was only room for one at a time in the lab, so Jupiter took the letter in while Pete and Bob watched from the narrow door. First Jupe put the letter under a microscope and went over it inch by inch.

“Nothing,” he said. “Now I’ll test for the most common kind of invisible ink.”

He reached for a jar of acid and poured some into a glass beaker. He held the letter above the beaker in the acid fumes, moving it back and forth. Nothing happened.

“As I expected,” he said. “Logic says that someone in a prison hospital wouldn’t be able to get hold of invisible ink, anyway. He just might be able to get a lemon, though, and lemon juice is a very simple kind of invisible ink. When you write with it, the writing can’t be seen, but if the paper is heated, the words written in lemon juice will appear. Let’s try that.”

He lit a small gas burner. Then, holding the letter by the corners, he moved it back and forth over the flame.

“Again, no results,” he said after a few moments. “Let me have the envelope to test.”

However, all tests on the envelope were also negative. Jupiter looked disappointed.

“It seems to be just an ordinary letter, after all,” he said. “Yet, after Gulliver received it, he hid it. Why did he do that?”

“Maybe he thought there was a clue in it, but he couldn’t find it,” Bob suggested. “Listen, suppose when he was in prison, this Spike Neely told him something about some hidden money, but not where it was. He could have said that because Gulliver was his friend, if anything ever happened to him he’d let Gulliver in on the secret.

“Then Gulliver gets this letter from the prison hospital. Spike is dying. Gulliver thinks Spike may have sent him a clue to where the money is, but he can’t find it, so he hides the letter, planning to study it some more.

“Some other criminals who knew Spike in prison learn somehow that he wrote to Gulliver. They suspect he told Gulliver the secret. So they come around to see Gulliver. Gulliver gets very frightened. He doesn’t go to the police because he doesn’t know anything he can tell them. But he’s afraid the crooks think he knows where the money is, and might even torture him to make him talk. So — he disappears. How does that sound?”

“Very well reasoned, Bob,” Jupiter said. “I think it may be what happened.

“However, we have studied the letter and can’t find any clue to a secret message. So I deduce that Spike Neely didn’t send any such message. He didn’t try because he knew the letter would be read first by the police.”

“Just the same, somebody thinks there’s a clue in that trunk,” Pete stated. “They want the trunk because of the clue they expect to find. If we don’t want trouble with some tough characters who will probably keep trying to get the trunk, we’d better get rid of it right away.”

“Pete has something there,” Bob said. “We can’t solve the mystery because we haven’t any clue. If we want to avoid trouble, we’d better get rid of the trunk. It doesn’t mean anything to us, after all.”

“Maximilian the Mystic wants us to sell it to him,” Pete put in. “I vote we put Socrates back in the trunk and let Mr. Maximilian have the whole shooting match. Get it off our hands. It’s too dangerous to keep around. How about it, Jupe?”

“Mmm.” Jupiter pinched his lip. “Zelda seemed to think we could help somehow, but it certainly doesn’t look like it. Two men follow us this morning when we left Zelda’s house and I don’t like that very much either.

“All right, we’ll telephone Mr. Maximilian, since he wants the trunk so badly. We’ll repack it and put Socrates back in. But we’ll have to warn him about other people wanting the trunk, so he’ll know. And I won’t charge him a hundred dollars — just the dollar I paid for it.”

“It would be awfully nice to have a hundred dollars.” Pete said.

“It wouldn’t be fair, if the trunk is dangerous,” Jupiter said. “I’ll call him in a minute. First I want to photograph this letter in case I get any new ideas.”

Jupiter made several photographs of both the letter and the envelope. Then he phoned Maximilian the Mystic, who said he’d be right over for the trunk.

After that they went outside and slid the letter back behind the torn lining, repacking the trunk carefully. Finally Jupiter went to get Socrates from his room.

He reached his room just in time to find Aunt Mathilda staring with a look of horror at the skull on the bureau.

“Jupiter Jones!” she said. “That — that thing… ”

Speechless, she pointed at the skull.

“Yes, Aunt Mathilda?” Jupiter asked.

“That awful thing!” the large woman exploded. “You know what it just did? It said ‘boo!’ to me!”

“Socrates said ‘boo’ to you?” Jupiter asked.

“It certainly did! I just came in here to clean your room and I said to it, ‘You ugly thing, I don’t know where Jupiter got you, but I can tell you one thing. You’re not staying in my house and that’s final. I won’t have it!”

“And then — then —” her voice faltered again — “it said ‘boo!’ just as plainly as anything. ‘Boo!’ I heard it as clearly as I hear you.”

“It’s supposed to be a talking skull,” Jupiter said, suppressing a smile. “It used to belong to a magician. If it said ‘boo’ it was probably playing a joke on you.”

“A joke? Is that what you call a joke? Having a nasty old skull grin at a person and say ‘boo’? I don’t care if it’s a talking skull or a talking horse, I want it out of here immediately. And that’s final!”

“Very well, Aunt Mathilda,” Jupiter agreed. “I’ll get rid of it. I was already planning to.”

“Be sure you do.”

In a thoughtful mood, Jupiter made his way back to the salvage yard with Socrates and the ivory base. He told Pete and Bob what had happened to his aunt.

“It’s very puzzling,” he concluded. “I have to admit I’m utterly baffled. Why should Socrates say ‘boo’ to Aunt Mathilda?”

“Maybe he has a sense of humour,” Pete said. “Let’s get him packed.”

“After this new development,” Jupiter said, “maybe we ought to keep Socrates and the trunk for a while. Perhaps he’s ready to talk some more.”

“Oh, no!” Pete said, grabbing Socrates, wrapping him up, and stowing him in the old trunk. “Your aunt says you have to get rid of him, and we’d agreed to get rid of him. We also agreed to let Mr. Maximilian have him and we can’t go back on our word now. I’m not in any mood to hear talk coming from a skull. Some mysteries I don’t want to solve.”