"I wonder if Darcy'll ever get his letter?" asked Kent-Lauriston, glancing at Mr. Riddle. "Anyway, it's as safe behind that portal as if it was in the Bank of England. Safer, in fact, for he can't get it out if he wants to." "I don't think there's much chance of anyone's opening it," said Mr. Riddle. "Cleverer men than Colonel Darcy have tried to solve that problem in the last two centuries, and failed. I imagine, however, if it ever does come to be opened, that a certain theory will be proved correct." "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 constitutes 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."