see this business to an end.

It was, however, impossible to see anything without a light; it would be
necessary to fetch one of the rush candles from the table in the corridor.
It was a matter of half a minute for the boy to go and return; then he
edged himself through the opening, and was standing in a kind of vaulted
tunnel, directly behind the fireplace, the warmth of which he could feel
when he laid his hand on the bricks on that side. The tunnel, which
extended along the interior of the wall toward the left, was about six
feet in height by two and a half in width. Archibald could walk in it
quite easily.

But, in the first place, he scrutinized the mechanism of the revolving
mantelpiece. It was an extremely ingenious and yet simple device, and so
accurately fitted in all its parts that, after so many years, they still
worked together almost as smoothly as when new. After Archibald had poured
a little of his gun-oil into the joints of the hinges, and along the
grooves, he found that heavy stone structure would open and close as
noiselessly and easily as his own jaws. It could be opened from the inside
by using the silver rod in a hole corresponding to that on the outside;
and, having practised this opening and shutting until he was satisfied
that he was thoroughly master of the process, he put the rod in his
pocket, pulled the jamb gently together behind him, and, candle in hand,
set forth along the tunnel.

After walking ten paces, he came face-up against a wall lying at right
angles to the direction in which he had been moving. Peering cautiously
round the corner, he saw, at the end of a shallow embrasure, a ponderous
door of dark wood, braced with iron, standing partly open, with a key in
the keyhole, as if some one had just come out, and, in his haste, had