Tanner did not think it could have been done. From London to Halford was thirty-five miles, and there and back made seventy. What speed could he reckon on? Considering how much of London would have to be traversed, and the amount of traffic to be expected on so important a road, Tanner felt sure not more than an average of thirty miles an hour at the outside. This would take two hours and twenty minutes at least, leaving from ten to twenty minutes. The motor never would have risked going up to Luce Manor, as it would have been heard—in fact, no motor did so. That meant that ten minutes must have been spent in going from the road to the boathouse, and another ten in returning. This even if it could be done at all, would leave no time in which to commit the murder, get out the boat and set the body and the oars adrift. Tanner considered it carefully, and at last came to the conclusion the thing would be utterly impossible. Indeed, he did not believe that an average of thirty miles an hour could be maintained. No, the alibi was complete. He felt he must unhesitatingly accept it. Inspector Tanner was a depressed man as he walked slowly back to New Scotland Yard. Up to the present he saw that he had been on the wrong track—that all his time and trouble had been lost. He was now as far off solving the mystery, as when he started the inquiry, indeed further, for the real scent must now be cooler. And Sergeant Longwell had been almost equally unsuccessful in his endeavour to trace the man who had made the fifth line of footprints on the river bank. With occasional assistance from Tanner the sergeant had made exhaustive inquiries in all the surrounding country, but without result. The only thing he had learnt which might have had a bearing on the matter was that a small, elderly man with a white goatee beard had taken the 5.47 a.m. train from St Albans to London, on the morning of the discovery of the crime. From Halford to St Albans was about fifteen miles, and Longwell’s theory was that this man—if he were the suspect—had walked during the night to St Albans, thinking that at a large station a considerable distance from Luce Manor he would be more likely to escape observation. But there was no real reason to connect this early traveller with the visitor to the boathouse. His boots had not been observed. But even if it had been proved that he was indeed the wanted man, the detectives were no further on. For the traveller had vanished into thin air at St Albans, and no trace of him could be found either in London or anywhere else.