“Excellent, Louis,” cried Carrados in great delight.  “We shall clear your man in a blaze of red and green lights yet and hang the glib, ‘greasy’ signalman from his own signal-post.”

“It is a significant fact, seriously?”

“It is absolutely convincing.”

“It may have been a slip, a mental lapse on Mead’s part which he discovered the moment it was too late, and then, being too cowardly to admit his fault, and having so much at stake, he took care to make detection impossible.  It may have been that, but my idea is rather that probably it was neither quite pure accident nor pure design.  I can imagine Mead meanly pluming himself over the fact that the life of this man who stands in his way, and whom he must cordially dislike, lies in his power.  I can imagine the idea becoming an obsession as he dwells on it.  A dozen times with his hand on the lever he lets his mind explore the possibilities of a moment’s defection.  Then one day he pulls the signal off in sheer bravado—­and hastily puts it at danger again.  He may have done it once or he may have done it oftener before he was caught in a fatal moment of irresolution.  The chances are about even that the engine-driver would be killed.  In any case he would be disgraced, for it is easier on the face of it to believe that a man might run past a danger signal in absentmindedness, without noticing it, than that a man should pull off a signal and replace it without being conscious of his actions.”

“The fireman was killed.  Does your theory involve the certainty of the fireman being killed, Louis?”

“No,” said Carlyle.  “The fireman is a difficulty, but looking at it from Mead’s point of view—­whether he has been guilty of an error or a crime—­it resolves itself into this:  First, the fireman may be killed.  Second, he may not notice the signal at all.  Third, in any case he will loyally corroborate his driver and the good old jury will discount that.”

Carrados smoked thoughtfully, his open, sightless eyes merely appearing to be set in a tranquil gaze across the room.

“It would not be an improbable explanation,” he said presently.  “Ninety-nine men out of a hundred would say:  ’People do not do these things.’  But you and I, who have in our different ways studied criminology, know that they sometimes do, or else there would be no curious crimes.  What have you done on that line?”

To anyone who could see, Mr. Carlyle’s expression conveyed an answer.

“You are behind the scenes, Max.  What was there for me to do?  Still I must do something for my money.  Well, I have had a very close inquiry made confidentially among the men.  There might be a whisper of one of them knowing more than had come out—­a man restrained by friendship, or enmity, or even grade jealousy.  Nothing came of that.  Then there was the remote chance that some private person had noticed the signal without attaching any importance to it then, one who would be able to identify it still by something associated with the time.  I went over the line myself.  Opposite the signal the line on one side is shut in by a high blank wall; on the other side are houses, but coming below the butt-end of a scullery the signal does not happen to be visible from any road or from any window.”