t again. He made his escape, and for some reason thought that
he could do so more safely on foot than on the bicycle. He therefore
left his machine where it would not be discovered until he had got
safely away. So far we are within the bounds of possibility, are we
not?"

"Well, it is possible, no doubt," said I, with some reserve.

"We have to remember, Watson, that whatever occurred is certainly
something very extraordinary. Well, now, to continue our supposititious
case, the couple--not necessarily a guilty couple--realize after the
murderer is gone that they have placed themselves in a position in
which it may be difficult for them to prove that they did not
themselves either do the deed or connive at it. They rapidly and rather
clumsily met the situation. The mark was put by Barker's bloodstained
slipper upon the window-sill to suggest how the fugitive got away. They
obviously were the two who must have heard the sound of the gun; so
they gave the alarm exactly as they would have done, but a good half
hour after the event."

"And how do you propose to prove all this?"

"Well, if there were an outsider, he may be traced and taken. That
would be the most effective of all proofs. But if not--well, the
resources of science are far from being exhausted. I think that an
evening alone in that study would help me much."

"An evening alone!"

"I propose to go up there presently. I have arranged it with the
estimable Ames, who is by no means whole-hearted about Barker. I shall
sit in that room and see if its atmosphere brings me inspiration. I'm a
believer in the genius loci. You smile, Friend Watson. Well, we shall
see. By the way, you have that big umbrella of yours, have you not?"

"It is here."

"Well, I'll borrow that if I may."

"Certainly--but what a wretched weapon! If there is danger--"

"Nothing serious, my dear Watson, or I should certainly ask for your
assistance. But I'll take the umbrella. At present I am only awaiting
the return of our colleagues from Tunbridge Wells, where they are at
present engaged in trying for a likely owner to the bicycle."

It was nightfall before Inspector MacDonald and White Mason came back
from their expedition, and they arrived exultant, reporting a great
advance in our investigation.

"Man, I'll admeet that I had my doubts if there was ever an outsider,"
said MacDonald, "but that's all past now. We've had the bicycle
identified, and we have a description of our man;