letting the body be
found, he'd have put big stones in his pockets, and the only big stones
are near the water's edge, and they would have left marks, and they
haven't, and therefore he didn't, and oh, bother the pond; that can wait
till this afternoon. Bill, where does the secret passage begin?"

"Well, that's what we've got to find out, isn't it?"

"Yes. You see, my idea is this."

He explained his reasons for thinking that the secret of the passage was
concerned in some way with the secret of Robert's death, and went on:

"My theory is that Mark discovered the passage about a year ago the
time when he began to get keen on croquet. The passage came out into
the floor of the shed, and probably it was Cayley's idea to put a
croquet-box over the trap-door, so as to hide it more completely. You
know, when once you've discovered a secret yourself, it always seems
as if it must be so obvious to everybody else. I can imagine that Mark
loved having this little secret all to himself and to Cayley, of course,
but Cayley wouldn't count and they must have had great fun fixing it up,
and making it more difficult for other people to find out. Well then,
when Miss Norris was going to dress-up, Cayley gave it away. Probably
he told her that she could never get down to the bowling-green without
being discovered, and then perhaps showed that he knew there was one way
in which she could do it, and she wormed the secret out of him somehow."

"But this was two or three days before Robert turned up."

"Exactly. I am not suggesting that there was anything sinister about the
passage in the first place. It was just a little private bit of romance
and adventure for Mark, three days ago. He didn't even know that Robert
was coming. But somehow the passage has been used since, in connection
with Robert. Perhaps Mark escaped that way; perhaps he's hiding there
now. And if so, then the only person who could give him away was Miss
Norris. And she of course would only do it innocently not knowing that
the passage had anything to do with it."

"So it was safer to have her out of the way?"

"Yes."

"But, look here, Tony, why do you want to bother about this end of it?
We can always get in at the bowling-green end."

"I know, but if we do that we shall have to do it openly. It will mean
breaking open the box, and letting Cayley know that we've done it. You
see, Bill, if we don't find anything out for ourselves in the next day
or two, we've g