"She retraced her steps in equal secrecy and the door closed behind her again. I was about to climb the fence and follow, when I realized that the detective fever that had lured me into the adventure was rather undignified; and that in a more authoritative capacity I already held all the cards in my hand. I was just turning away when a new noise broke on the night. A window was thrown up in one of the upper floors, but just round the corner of the house so that I could not see it; and a voice of terrible distinctness was heard shouting across the dark garden to know where Lord Falconroy was, for he was missing from every room in the house. There was no mistaking that voice. I have heard it on many a political platform or meeting of directors; it was Ireton Todd himself. Some of the others seemed to have gone to the lower windows or on to the steps, and were calling up to him that Falconroy had gone for a stroll down to the Pilgrim's Pond an hour before, and could not be traced since. Then Todd cried 'Mighty Murder!' and shut down the window violently; and I could hear him plunging down the stairs inside. Repossessing myself of my former and wiser purpose, I whipped out of the way of the general search that must follow; and returned here not later than eight o'clock.
"I now ask you to recall that little Society paragraph which seemed to you so painfully lacking in interest. If the convict was not keeping the shot for Todd, as he evidently wasn't, it is most likely that he was keeping it for Lord Falconroy; and it looks as if he had delivered the goods. No more handy place to shoot a man than in the curious geological surroundings of that pool, where a body thrown down would sink through thick slime to a depth practically unknown. Let us suppose, then, that our friend with the cropped hair came to kill Falconroy and not Todd. But, as I have pointed out, there are many reasons why people in America might want to kill Todd. There is no reason why anybody in America should want to kill an English lord newly landed, except for the one reason mentioned in the pink paper—that the lord is paying his attentions to the millionaire's daughter. Our crop-haired friend, despite his ill-fitting clothes, must be an aspiring lover.
"I know the notion will seem to you jarring and even comic; but that's because you are English. It sounds to you like saying the Archbishop of Canterbury's daughter will be married in St George's, Hanover Square, to a crossing-sweeper on ticket-of-leave. You don't do justice to the climbing and aspiring power of our more remarkable citizens. You see a good-looking grey-haired man in evening-dress with a sort of authority about him, you know he is a pillar of the State, and you fancy he had a father. You are in error. You do not realize that a comparatively few years ago he may have been in a tenement or (quite likely) in a jail. You don't allow for our national buoyancy and uplift. Many of our most influential citizens have not only risen recently, but risen comparatively late in life. Todd's daughter was fully eighteen when her father first made his pile; so there isn't really anything impossible in her having a hanger-on in low life; or even in her hanging on to him, as I think she must be doing, to judge by the lantern business. If so, the hand that held the lantern may not be unconnected with the hand that held the gun. This case, sir, will make a noise."
"Well," said the priest patiently, "and what did you do next?"
"I reckon you'll be shocked," replied Greywood Usher, "as I know you don't cotton to the march of science in these matters. I am given a good deal of discretion here, and perhaps take a little more than I'm given; and I thought it was an excellent opportunity to test that Psychometric Machine I told you about. Now, in my opinion, that machine can't lie."
"No machine can lie," said Father Brown; "nor can it tell the truth."
"It did in this case, as I'll show you," went on Usher positively. "I sat the man in the ill-fitting clothes in a comfortable chair, and simply wrote words on a blackboard; and the machine simply recorded the variations of his pulse; and I simply observed his manner. The trick is to introduce some word connected with the supposed crime in a list of words connected with something quite different, yet a list in which it occurs quite naturally. Thus I wrote 'heron' and 'eagle' and 'owl', and when I wrote 'falcon' he was tremendously agitated; and when I began to make an 'r' at the end of the word, that machine just bounded. Who else in this republic has any reason to jump at the name of a newly-arrived Englishman like Falconroy except the man who's shot him? Isn't that better evidence than a lot of gabble from witnesses—if the evidence of a reliable machine?"
"You always forget," observed his companion, "that the reliable machine always has to be worked by an unreliable machine."
"Why, what do you mean?" asked the detective.
"I mean Man," said Father Brown, "the most unreliable machine I know of. I don't want to be rude; and I don't think you will consider Man to be an offensive or inaccurate description of yourself. You say you observed his manner; but how do you know you observed it right? You say the words have to come in a natural way; but how do you know that you did it naturally? How do you know, if you come to that, that he did not observe your manner? Who is to prove that you were not tremendously agitated? There was no machine tied on to your pulse."
"I tell you," cried the American in the utmost excitement, "I was as cool as a cucumber."
"Criminals also can be as cool as cucumbers," said Brown with a smile. "And almost as cool as you."
"Well, this one wasn't," said Usher, throwing the papers about. "Oh, you make me tired!"
"I'm sorry," said the other. "I only point out what seems a reasonable possibility. If you could tell by his manner when the word that might hang him had come, why shouldn't he tell from your manner that the word that might hang him was coming? I should ask for more than words myself before I hanged anybody."
Usher smote the table and rose in a sort of angry triumph.
"And that," he cried, "is just what I'm going to give you. I tried the machine first just in order to test the thing in other ways afterwards and the machine, sir, is right."
He paused a moment and resumed with less excitement. "I rather want to insist, if it comes to that, that so far I had very little to go on except the scientific experiment. There was really nothing against the man at all. His clothes were ill-fitting, as I've said, but they were rather better, if anything, than those of the submerged class to which he evidently belonged. Moreover, under all the stains of his plunging through ploughed fields or bursting through dusty hedges, the man was comparatively clean. This might mean, of course, that he had only just broken prison; but it reminded me more of the desperate decency of the comparatively respectable poor. His demeanour was, I am bound to confess, quite in accordance with theirs. He was silent and dignified as they are; he seemed to have a big, but buried, grievance, as they do. He professed total ignorance of the crime and the whole question; and showed nothing but a sullen impatience for something sensible that might come to take him out of his meaningless scrape. He asked me more than once if he could telephone for a lawyer who had helped him a long time ago in a trade dispute, and in every sense acted as you would expect an innocent man to act. There was nothing against him in the world except that little finger on the dial that pointed to the change of his pulse.
"Then, sir, the machine was on its trial; and the machine was right. By the time I came with him out of the private room into the vestibule where all sorts of other people were awaiting examination, I think he had already more or less made up his mind to clear things up by something like a confession.