he window, the window!" cried Antony, pointing to it.

Bill turned back to the window, expecting it to say something. As it
said nothing, he looked at Antony again.

"He was opening the window!" cried Antony.

"Who?"

"Cayley, of course." Very gravely and slowly he expounded. "He came in
here in order to open the window. He shut the door so that I shouldn't
hear him open the window. He opened the window. I came in here and found
the window open. I said, 'This window is open. My amazing powers of
analysis tell me that the murderer must have escaped by this window.'
'Oh,' said Cayley, raising his eyebrows. 'Well,' said he, 'I suppose you
must be right.' Said I proudly, 'I am. For the window is open,' I said.
Oh, you incomparable ass!"

He understood now. It explained so much that had been puzzling him.

He tried to put himself in Cayley's place--Cayley, when Antony had first
discovered him, hammering at the door and crying, "Let me in!" Whatever
had happened inside the office, whoever had killed Robert, Cayley knew
all about it, and knew that Mark was not inside, and had not escaped by
the window. But it was necessary to Cayley's plans--to Mark's plans
if they were acting in concert--that he should be thought so to have
escaped. At some time, then, while he was hammering (the key in his
pocket) at the locked door, he must suddenly have remembered--with what
a shock!--that a mistake had been made. A window had not been left open!

Probably it would just have been a horrible doubt at first. Was the
office window open? Surely it was open! Was it?.... Would he have time
now to unlock the door, slip in, open the French windows and slip out
again? No. At any moment the servants might come. It was too risky.
Fatal, if he were discovered. But servants were stupid. He could get
the windows safely open while they were crowding round the body. They
wouldn't notice. He could do it somehow.

And then Antony's sudden appearance! Here was a complication. And Antony
suggesting that they should try the window! Why, the window was just
what he wanted to avoid. No wonder he had seemed dazed at first.

Ah, and here at last was the explanation why they had gone the longest
way round and yet run. It was Cayley's only chance of getting a start
on Antony, of getting to the windows first, of working them open somehow
before Antony caught him up. Even if that were impossible, he must get
there first, just to make sure. Perhaps they were o