He raised his head with unusual pleasure at the sight of the priest, and took him a few paces apart.  Meanwhile Merton was addressing the older detective respectfully indeed, but not without a certain boyish impatience.

“Well, Mr. Gilder, have you got much farther with the mystery?”

“There is no mystery,” replied Gilder, as he looked under dreamy eyelids at the rooks.

“Well, there is for me, at any rate,” said Merton, smiling.

“It is simple enough, my boy,” observed the senior investigator, stroking his grey, pointed beard.  “Three minutes after you’d gone for Mr. Royce’s parson the whole thing came out.  You know that pasty-faced servant in the black gloves who stopped the train?”

“I should know him anywhere.  Somehow he rather gave me the creeps.”

“Well,” drawled Gilder, “when the train had gone on again, that man had gone too.  Rather a cool criminal, don’t you think, to escape by the very train that went off for the police?”

“You’re pretty sure, I suppose,” remarked the young man, “that he really did kill his master?”

“Yes, my son, I’m pretty sure,” replied Gilder drily, “for the trifling reason that he has gone off with twenty thousand pounds in papers that were in his master’s desk.  No, the only thing worth calling a difficulty is how he killed him.  The skull seems broken as with some big weapon, but there’s no weapon at all lying about, and the murderer would have found it awkward to carry it away, unless the weapon was too small to be noticed.”

“Perhaps the weapon was too big to be noticed,” said the priest, with an odd little giggle.

Gilder looked round at this wild remark, and rather sternly asked Brown what he meant.

“Silly way of putting it, I know,” said Father Brown apologetically.  “Sounds like a fairy tale.  But poor Armstrong was killed with a giant’s club, a great green club, too big to be seen, and which we call the earth.  He was broken against this green bank we are standing on.”

“How do you mean?” asked the detective quickly.

Father Brown turned his moon face up to the narrow facade of the house and blinked hopelessly up.  Following his eyes, they saw that right at the top of this otherwise blind back quarter of the building, an attic window stood open.

“Don’t you see,” he explained, pointing a little awkwardly like a child, “he was thrown down from there?”

Gilder frowningly scrutinised the window, and then said:  “Well, it is certainly possible.  But I don’t see why you are so sure about it.”

Brown opened his grey eyes wide.  “Why,” he said, “there’s a bit of rope round the dead man’s leg.  Don’t you see that other bit of rope up there caught at the corner of the window?”

At that height the thing looked like the faintest particle of dust or hair, but the shrewd old investigator was satisfied.  “You’re quite right, sir,” he said to Father Brown; “that is certainly one to you.”