"Eaten him?" said the nightmare at his ear; and he sickened for an instant at the idea of rent, human remains absorbed and crushed into all that acephalous clockwork.

He recovered his mental health by an emphatic effort, and said to Flambeau, "Well, there it is. The poor fellow has evaporated like a cloud and left a red streak on the floor. The tale does not belong to this world."

"There is only one thing to be done," said Flambeau, "whether it belongs to this world or the other. I must go down and talk to my friend."

They descended, passing the man with the pail, who again asseverated that he had let no intruder pass, down to the commissionaire and the hovering chestnut man, who rigidly reasserted their own watchfulness. But when Angus looked round for his fourth confirmation he could not see it, and called out with some nervousness, "Where is the policeman?"

"I beg your pardon," said Father Brown; "that is my fault. I just sent him down the road to investigate something—that I just thought worth investigating."

"Well, we want him back pretty soon," said Angus abruptly, "for the wretched man upstairs has not only been murdered, but wiped out."

"How?" asked the priest.

"Father," said Flambeau, after a pause, "upon my soul I believe it is more in your department than mine. No friend or foe has entered the house, but Smythe is gone, as if stolen by the fairies. If that is not supernatural, I—"

As he spoke they were all checked by an unusual sight; the big blue policeman came round the corner of the crescent, running. He came straight up to Brown.

"You're right, sir," he panted, "they've just found poor Mr. Smythe's body in the canal down below."

Angus put his hand wildly to his head. "Did he run down and drown himself?" he asked.

"He never came down, I'll swear," said the constable, "and he wasn't drowned either, for he died of a great stab over the heart."

"And yet you saw no one enter?" said Flambeau in a grave voice.

"Let us walk down the road a little," said the priest.

As they reached the other end of the crescent he observed abruptly, "Stupid of me! I forgot to ask the policeman something. I wonder if they found a light brown sack."

"Why a light brown sack?" asked Angus, astonished.

"Because if it was any other coloured sack, the case must begin over again," said Father Brown; "but if it was a light brown sack, why, the case is finished."

"I am pleased to hear it," said Angus with hearty irony. "It hasn't begun, so far as I am concerned."

"You must tell us all about it," said Flambeau with a strange heavy simplicity, like a child.

Unconsciously they were walking with quickening steps down the long sweep of road on the other side of the high crescent, Father Brown leading briskly, though in silence. At last he said with an almost touching vagueness, "Well, I'm afraid you'll think it so prosy. We always begin at the abstract end of things, and you can't begin this story anywhere else.

"Have you ever noticed this—that people never answer what you say? They answer what you mean—or what they think you mean. Suppose one lady says to another in a country house, 'Is anybody staying with you?' the lady doesn't answer 'Yes; the butler, the three footmen, the parlourmaid, and so on,' though the parlourmaid may be in the room, or the butler behind her chair. She says 'There is nobody staying with us,' meaning nobody of the sort you mean. But suppose a doctor inquiring into an epidemic asks, 'Who is staying in the house?' then the lady will remember the butler, the parlourmaid, and the rest. All language is used like that; you never get a question answered literally, even when you get it answered truly. When those four quite honest men said that no man had gone into the Mansions, they did not really mean that no man had gone into them. They meant no man whom they could suspect of being your man. A man did go into the house, and did come out of it, but they never noticed him."

"An invisible man?" inquired Angus, raising his red eyebrows. "A mentally invisible man," said Father Brown.

A minute or two after he resumed in the same unassuming voice, like a man thinking his way. "Of course you can't think of such a man, until you do think of him. That's where his cleverness comes in. But I came to think of him through two or three little things in the tale Mr. Angus told us. First, there was the fact that this Welkin went for long walks. And then there was the vast lot of stamp paper on the window. And then, most of all, there were the two things the young lady said—things that couldn't be true. Don't get annoyed," he added hastily, noting a sudden movement of the Scotchman's head; "she thought they were true. A person can't be quite alone in a street a second before she receives a letter. She can't be quite alone in a street when she starts reading a letter just received. There must be somebody pretty near her; he must be mentally invisible."

"Why must there be somebody near her?" asked Angus.

"Because," said Father Brown, "barring carrier-pigeons, somebody must have brought her the letter."

"Do you really mean to say," asked Flambeau, with energy, "that Welkin carried his rival's letters to his lady?"

"Yes," said the priest. "Welkin carried his rival's letters to his lady. You see, he had to."

"Oh, I can't stand much more of this," exploded Flambeau. "Who is this fellow? What does he look like? What is the usual get-up of a mentally invisible man?"

"He is dressed rather handsomely in red, blue and gold," replied the priest promptly with precision, "and in this striking, and even showy, costume he entered Himylaya Mansions under eight human eyes; he killed Smythe in cold blood, and came down into the street again carrying the dead body in his arms—"

"Reverend sir," cried Angus, standing still, "are you raving mad, or am I?"

"You are not mad," said Brown, "only a little unobservant. You have not noticed such a man as this, for example."

He took three quick strides forward, and put his hand on the shoulder of an ordinary passing postman who had bustled by them unnoticed under the shade of the trees.

"Nobody ever notices postmen somehow," he said thoughtfully; "yet they have passions like other men, and even carry large bags where a small corpse can be stowed quite easily."

The postman, instead of turning naturally, had ducked and tumbled against the garden fence. He was a lean fair-bearded man of very ordinary appearance, but as he turned an alarmed face over his shoulder, all three men were fixed with an almost fiendish squint.

Flambeau went back to his sabres, purple rugs and Persian cat, having many things to attend to. John Turnbull Angus went back to the lady at the shop, with whom that imprudent young man contrives to be extremely comfortable. But Father Brown walked those snow-covered hills under the stars for many hours with a murderer, and what they said to each other will never be known.

A stormy evening of olive and silver was closing in, as Father Brown, wrapped in a grey Scotch plaid, came to the end of a grey Scotch valley and beheld the strange castle of Glengyle. It stopped one end of the glen or hollow like a blind alley; and it looked like the end of the world. Rising in steep roofs and spires of seagreen slate in the manner of the old French-Scotch chateaux, it reminded an Englishman of the sinister steeple-hats of witches in fairy tales; and the pine woods that rocked round the green turrets looked, by comparison, as black as numberless flocks of ravens. This note of a dreamy, almost a sleepy devilry, was no mere fancy from the landscape. For there did rest on the place one of those clouds of pride and madness and mysterious sorrow which lie more heavily on the noble houses of Scotland than on any other of the children of men. For Scotland has a double dose of the poison called heredity; the sense of blood in the aristocrat, and the sense of doom in the Calvinist.