"Assuming that such a piece of mechanism as you suggest had been
attached to the door," mused Gatton, "you would have noticed its
operation last night, unless one of you held the door open."

"Neither of us held the door!" I interrupted excitedly. "I remember
that we stood just outside looking in. I was behind the constable and
he was directing the rays of his lantern into the place."

"H'm," muttered Gatton. "Then it wasn't a contrivance for closing the
door; it was something else. Suppose we investigate the other door?"

We proceeded to the other door and I became aware of an intense
curiosity respecting what we should find, and of a conviction too that
there would be evidence here of another attachment. In this I was
quite correct. Some piece of mechanism had evidently been fastened to
this door also. Together we stood staring up at these tell-tale
screw-holes and then rather blankly we stared at one another.

"We only lack one thing," said Gatton; "the scheme upon which all
these contrivances and apparently isolated episodes were hung
together. Nothing, as we have already assumed, was accident, and
nothing coincidence. It was with some deliberate purpose that the
constable was instructed to walk through this garage, opening and
shutting the doors behind him."

"From whom did these instructions come?"

"That is one of the minor points which I have already cleared up," he
replied. "On my way here I called at the house agent's, as you know,