on had been run across the passage six feet
from the end, with a door cunningly concealed in it. It was lit within
by slits under the eaves. A few articles of furniture and a supply of
food and water were within, together with a number of books and papers.

"There's the advantage of being a builder," said Holmes, as we came
out. "He was able to fix up his own little hiding-place without any
confederate--save, of course, that precious housekeeper of his, whom I
should lose no time in adding to your bag, Lestrade."

"I'll take your advice. But how did you know of this place, Mr. Holmes?"

"I made up my mind that the fellow was in hiding in the house. When I
paced one corridor and found it six feet shorter than the corresponding
one below, it was pretty clear where he was. I thought he had not the
nerve to lie quiet before an alarm of fire. We could, of course, have
gone in and taken him, but it amused me to make him reveal himself.
Besides, I owed you a little mystification, Lestrade, for your chaff in
the morning."

"Well, sir, you certainly got equal with me on that. But how in the
world did you know that he was in the house at all?"

"The thumb-mark, Lestrade. You said it was final; and so it was, in a
very different sense. I knew it had not been there the day before. I pay
a good deal of attention to matters of detail, as you may have observed,
and I had examined the hall, and was sure that the wall was clear.
Therefore, it had been put on during the night."

"But how?"

"Very simply. When those packets were sealed up, Jonas Oldacre got
McFarlane to secure one of the seals by putting his thumb upon the soft
wax. It would be done so quickly and so naturally, that I daresay the
young man himself has no recollection of it. Very likely it just so
happened, and Oldacre had himself no notion of the use he would put it
to. Brooding over the case in that den of his, it suddenly struck him
what absolutely damning evidence he could make against McFarlane by
using that thumb-mark. It was the simplest thing in the world for him to
take a wax impression from the seal, to moisten it in as much blood as
he could get from a pin-prick, and to put the mark upon the wall during
the night, either with his own hand or with that of his housekeeper.
If you examine among those documents which he took with him into
his retreat, I will lay you a wager that you find the seal with the
thumb-mark upon it."

"Wonderful!" said Lestrade. "W