He returned to town by the next train, and drove to Paternoster Row. Here he had no difficulty in finding Herbert Potts. He was a man on the right side of thirty, with a dependable face, and a quiet, rather forceful manner. He seemed considerably annoyed that his excursion with Miss Penrose should have become known, fearing, as he said, that the girl would get talked about, and perhaps have to give evidence in court. But about the events on the night in question he corroborated her entirely. He also was positive the man in the boat was Austin. Though now employed in London, he was a Halford man and knew Austin’s appearance beyond possibility of mistake. The Inspector left him, feeling that in the face of these two witnesses he could no longer doubt Austin had been at the boathouse, and therefore had faked his alibi. But how? That was the question he must now set himself to solve. It seemed clear that Austin’s statement up to the time of his leaving the boat club pavilion, and after his arrival back there, was true. The testimony of the boatman Brocklehurst, Miss Drew, and Austin’s butler was overwhelming. The flaw therefore must lie in the evidence of what took place between those hours. Tanner went over this once again. It hinged, as he had recognised before, on the shoes. And firstly, had the prints at the Abbey been made by those shoes? He had thought so at the time, and on reconsidering the matter he felt more certain than ever that he was right. A very trifling dint in the edge of one of the soles, evidently caused by striking a sharp-edged stone, was reproduced exactly in the clay. It was unthinkable that another pair of precisely similar shoes should have a precisely similar dint in the exact same place. No, when or by whom worn, Austin’s shoes had made the tracks. So much was beyond question. Then with regard to the time at which the prints had been made. On this point the evidence of the butler corroborated Austin’s story. The butler had stated the shoes had been in Austin’s dressing-room in his, the butler’s, charge during the entire time from the Monday on which they were purchased till the Friday, with the single exception of this particular period on Wednesday evening. If this were true it followed that some person other than Austin wore the shoes, and made the tracks during this period. But was it true? Tanner recalled point by point his interview with the butler. Invariably he reached his conclusions quite as much from the manner and bearing of the persons he interrogated as from their statement. And in this case he was forced to admit the butler seemed to speak as a perfectly honest man. The Inspector felt he did not possess sufficient intelligence to make his story sound as convincing as it had, unless he himself believed it to be true. But might not the man have been mistaken?