was good enough to show me. But I fear he has bilked me over the address.
He suspected me, that was plain, and left his umbrella by way of
experiment to see if I were watching him sharply enough to notice the
circumstance, and to avail myself of it to follow him. I was hasty and
fell into the trap. He cabbed it away from Luzatti's, and I cabbed it
after him. He has led me a pretty dance up and down London to-night, and
two cabbies have made quite a stroke of business out of us. In the end he
entered a house of which, of course, I have taken the address, but I
expect he doesn't live there. He is too smart a man to lead me to his den;
but the police can certainly find something of him at the house he went in
at--and, I expect, left by the back way. By the way, you never guessed
that simple little puzzle as to how I found that this _was_ a murder, did
you? You see it now, of course?"

"Something to do with that apple you stole, I suppose?"

"Something to do with it? I should think so, you worthy innocent. Just
ring your bell; we'll borrow Mrs. Clayton's sewing-machine oil again. On
the night we broke into Foggatt's room you saw the nutshells and the
bitten remains of an apple on the sideboard, and you remembered it; and
yet you couldn't see that in that piece of apple possibly lay an important
piece of evidence. Of course I never expected you to have arrived at any
conclusion, as I had, because I had ten minutes in which to examine that
apple, and to do what I did with it. But, at least, you should have seen
the possibility of evidence in it.

"First, now, the apple was white. A bitten apple, as you must have
observed, turns of a reddish brown color if left to stand long. Different
kinds of apples brown with different rapidities, and the browning always
begins at the core. This is one of the twenty thousand tiny things that
few people take the trouble to notice, but which it is useful for a man in
my position to know. A russet will brown quite quickly. The apple on the
sideboard was, as near as I could tell, a Newtown pippin or other apple of
that kind, which will brown at the core in from twenty minutes to half an
hour, and in other parts in a quarter of an hour more. When we saw it, it
was white, with barely a tinge of brown about the exposed core. Inference,
somebody had been eating it fifteen or twenty minutes before, perhaps a
little longer--an inference supported by the fact that it was only partly
eaten.

"I exam