"What is it?" asked Kingsland.

"That the prophecy tells only half the story. To press the nails they must be flexible, but they're firm and immovable."

"Well?"

"Well, it's evident that there is some catch or spring to be worked first."

"How do you make that out?"

"These five nails we hear so much about are really the key to the lock, but until the movable impediments--or, to give them their technical name, the 'tumblers'--are so arranged as to release the key, the lock cannot be opened."

"It's a rum sort of key, with no keyhole," said Kingsland.

"The key to open this lock is a mental one, rather than one of steel and iron. In other words, a puzzle lock like this always has certain movable parts, the movement of which const.i.tutes the enigma."

"Ever heard of any locks like this one?"

"Not exactly, but the Russians, Hindoos and the Chinese have their puzzle locks in the shape of birds or animals, and they're locked or unlocked by pressing certain parts of their bodies. You can depend on it, some spring must be worked first, which relieves the nails from their tension and permits one to work the combination."

"But no such catch or spring is visible."

"Of course not. It would be the most carefully concealed of all the mechanism; but some lucky fellow will stumble on it eventually, and if he has presence of mind enough to press the nails also-- Presto! your door will fly open."

"And what will he find?" asked Kent-Lauriston.

"From present appearances," replied Mr. Riddle, "a little pile of dust, which some centuries before was a letter----"

"I shouldn't be satisfied with anything less than a mouldering skeleton in chains," said Kingsland.

"Or a complicated astrological machine, such as one hears about in Bulwer's grewsome ghost story," added Kent-Lauriston.

"The inhabitants of this house are too unfeignedly easy-going and comfortable to admit of such a supposition," replied Kingsland, and turning to Kent-Lauriston, added: "What do you think is inside the Tower?"

"I don't know, and if I did, I shouldn't tell anyone."

"Why not?"

"Because if its contents are so unpleasant, that they had to shut it up for ever, it certainly wouldn't prove a fit subject for conversation."

"Well, anyhow," said the Lieutenant, "I trust the discoverer will be a short man, or he'll hit his head a nasty crack, when he tries to go in."

"Wrong again," said Mr. Riddle. "I think you'll admit that I'm medium height for a man; but if I stood with my back to the door, my head wouldn't hit the top of the arch."

"Nonsense. Let's see."

Riddle took up the position indicated, facing them.

"You're right!" e.j.a.c.u.l.a.t.ed the young officer.

"I'm amazed! I supposed it was much lower. What do you measure?"

"Five feet eight inches. But it is the extreme width of the portal which makes it deceptive; it lowers it. I think, if I stretched out my arms, straight from the shoulder, I should no more than touch the side--see----" and he made a great cross of himself, against the black oak.

"What are you fumbling at?" asked Kingsland sharply.

"My fingers hardly touch--it's a stretch. Ah! now they do."

"You look ghastly in the moonlight; put your arms down and come away."

"I'm very comfortable here, barring my back; those silver nails are rather sharp," and he put his hands behind him.

"Come away," said Kingsland, nervously, seeing something in his face he did not like. "You look as if you'd been walled up a few months ago, by some inquisition, and we'd just unearthed you in your niche."

"By heavens! some of these nails are loose!" cried Riddle.

"Nonsense!" retorted Kingsland. "You've thought so much about it, you'd imagine anything. They're as firm as--well, nails. I tried them myself.

That door won't be opened in our lifetime, unless----" but the Lieutenant never finished his sentence, for he had paused suddenly, in open-mouthed astonishment. Without warning, and without a sound, the portal, closed for centuries, swung slowly inward, carrying Riddle with it; who, catching in vain at the sides of the door in an attempt to save himself, fell heavily backwards down three steps into the secret chamber.

Seeing that he did not immediately rise, but turned over partially on his side, Kingsland recollecting himself, sprang forward to his aid, crying:

"Have you hurt yourself?"

"No, no," he replied, waving him off, and slowly rising from the floor, covered with dust.

"By Jove!" exclaimed the Lieutenant. "How did you ever do it?"

"Don't know, I'm sure," replied Riddle, emerging from the portal, and vigorously brushing himself. "As I told you, the nails, or some of them, felt loose--I pushed them, and the next thing I knew the door revolved and I was on the floor."

"You're a genius!" exclaimed Kingsland. "But," peering down into the darkness of the tower, "where's Darcy's letter?"

"We need a little light on the subject," said Mr. Riddle. Stepping to the fireplace, he lighted an old wrought-iron sconce, full of candles, which stood on the broad mantelshelf, and approached the secret door.

In the light of the candles, all could see that, except for the little s.p.a.ce into which he had fallen, the whole interior of the tower was filled by a narrow stone staircase, which, in its ascent, half turned upon itself. Of the missing doc.u.ment, however, there was not a trace.

The stillness in the great hall was oppressive. Even their own footsteps on the stones seemed, to the hearers, preternaturally loud.

Mr. Riddle raised the sconce above his head, and there burst on a sudden a shimmering flash of a thousand prismatic colours from the head of the staircase. He fell back a step, as did the others, and Kingsland murmured in awe-struck tones:--

"What's that?"

Riddle again raised the sconce, and again the burst of light from the head of the stairs overwhelmed him, but this time he stood his ground.

"What is it?" asked Kent-Lauriston.

"I don't know."

"Let us examine."

"As far as I can make out, it's a flexible curtain of chain mail--hung across the staircase."