As they threaded the steep side streets already powdered with silver, Angus finished his story; and by the time they reached the crescent with the towering flats, he had leisure to turn his attention to the four sentinels. The chestnut seller, both before and after receiving a sovereign, swore stubbornly that he had watched the door and seen no visitor enter. The policeman was even more emphatic. He said he had had experience of crooks of all kinds, in top hats and in rags; he wasn't so green as to expect suspicious characters to look suspicious; he looked out for anybody, and, so help him, there had been nobody. And when all three men gathered round the gilded commissionaire, who still stood smiling astride of the porch, the verdict was more final still.
"I've got a right to ask any man, duke or dustman, what he wants in these flats," said the genial and gold-laced giant, "and I'll swear there's been nobody to ask since this gentleman went away."
The unimportant Father Brown, who stood back, looking modestly at the pavement, here ventured to say meekly, "Has nobody been up and down stairs, then, since the snow began to fall? It began while we were all round at Flambeau's."
"Nobody's been in here, sir, you can take it from me," said the official, with beaming authority.
"Then I wonder what that is?" said the priest, and stared at the ground blankly like a fish.
The others all looked down also; and Flambeau used a fierce exclamation and a French gesture. For it was unquestionably true that down the middle of the entrance guarded by the man in gold lace, actually between the arrogant, stretched legs of that colossus, ran a stringy pattern of grey footprints stamped upon the white snow.
"God!" cried Angus involuntarily, "the Invisible Man!"
Without another word he turned and dashed up the stairs, with Flambeau following; but Father Brown still stood looking about him in the snow-clad street as if he had lost interest in his query.
Flambeau was plainly in a mood to break down the door with his big shoulders; but the Scotchman, with more reason, if less intuition, fumbled about on the frame of the door till he found the invisible button; and the door swung slowly open.
It showed substantially the same serried interior; the hall had grown darker, though it was still struck here and there with the last crimson shafts of sunset, and one or two of the headless machines had been moved from their places for this or that purpose, and stood here and there about the twilit place. The green and red of their coats were all darkened in the dusk; and their likeness to human shapes slightly increased by their very shapelessness. But in the middle of them all, exactly where the paper with the red ink had lain, there lay something that looked like red ink spilt out of its bottle. But it was not red ink.
With a French combination of reason and violence Flambeau simply said "Murder!" and, plunging into the flat, had explored, every corner and cupboard of it in five minutes. But if he expected to find a corpse he found none. Isidore Smythe was not in the place, either dead or alive. After the most tearing search the two men met each other in the outer hall, with streaming faces and staring eyes. "My friend," said Flambeau, talking French in his excitement, "not only is your murderer invisible, but he makes invisible also the murdered man."
Angus looked round at the dim room full of dummies, and in some Celtic corner of his Scotch soul a shudder started. One of the life-size dolls stood immediately overshadowing the blood stain, summoned, perhaps, by the slain man an instant before he fell. One of the high-shouldered hooks that served the thing for arms, was a little lifted, and Angus had suddenly the horrid fancy that poor Smythe's own iron child had struck him down. Matter had rebelled, and these machines had killed their master. But even so, what had they done with him?
"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