in
Mr. Denson's pocket."

Plummer nodded sagaciously. "All in the plant," he said. "See--it's
brand new; clean as a new pin, and file marks still on it."

"Take us to this back door, Hutt," Hewitt pursued. "We'll try this key.
Is there a back staircase?"

There _was_ a small back staircase, leading to the coal-cellars, and
only used by servants. Down this we all went, and on a lower landing we
stopped before a small door. Hewitt slipped the key in the lock and
turned it. The door opened easily, and there before us was the little
courtyard which I think I have mentioned in one of my other
narratives--the courtyard with a narrow passage leading into the next
street.

Martin Hewitt seemed singularly excited. "See there," he said, "that is
how Denson left the building without passing the housekeeper's box! And
now I'm going to make another shot. See here. This key on Denson's bunch
attracted my attention because of its noticeable newness compared with
most of the others. _Most_ of the others, I say, because there is one
other just as bright--see! This small one. Now, Hutt, do you happen to
have a key like that also?"

Hutt turned the key over in his hand and glanced from it to his own
bunch. "Why, yes, sir!" he said presently. "Yes, sir! It's the same as
the key of the fire-hose cupboards!"

"Does that key fit them all? How many fire-hose cupboards are there?"

"Two on each floor, sir, one at each end, just against the mains. And
one key fits the lot."

"Show us the nearest to this door."

A short, narrow passage led to the main ground-floor corridor, where a
cupboard lettered "Fire Hose" stood next the main and its fittings. "We
have to keep the hose-cupboards locked," the housekeeper explained
apologetically, "'cause o' mischievous boys in the offices."

This key fitted as well as the other. A long coil of brown leather hose
hung within, and in a corner lay a piece of chamois leather evidently
used for polishing the brass fittings. This Hewitt pulled aside, and
there beneath it lay another and cleaner piece of chamois leather,
neatly folded and tied round with cord. Hewitt snatched it up. He
unfastened the cord; he unrolled the leather, which was sewn into a sort
of bag or satchel; and when at last he spread wide the mouth of this
satchel, light seemed to spring from out of it, for there lay a
glittering heap of brilliants!

"What!" cried Plummer, who first got his speech. "Diamonds! Samuel's
diamonds!"