As soon as the train stopped he darted down-stairs, and ran back towards the Third Avenue station; but he knew it was useless, as it proved. He saw nothing of Mr. Mitchel. Wilson was greatly disheartened, for he was most anxious to stand well with Mr. Barnes, his chief. Yet in revolving over the occurrences of the last half hour he could not see how he could have prevented the escape of his man, since it was evident that he had intentionally acted in a way to prevent pursuit. If one but knows or suspects that he is being shadowed, the Third Avenue elevated road, with its bridges at Thirty-fourth and Forty-second Streets, offers the most effectual means of eluding the most skilful detective. If Wilson had known anything whatever about the man who had escaped him, he might have been able to guess his destination, and so have caught up with him again by hurrying ahead and meeting him, as he had frequently done when following noted criminals with whose haunts he was acquainted. In this instance he was utterly in the dark, so could do nothing but swear. If he could not report where Mr. Mitchel had gone, at least he might discover at what time he returned to his hotel, and possibly Mr. Barnes might receive some valuable hint by the lapse of time. With this idea, Wilson returned to the Fifth Avenue Hotel and waited patiently. He telephoned to head-quarters only to hear that Mr. Barnes had gone back to Boston to bring Pettingill to New York. Seven o'clock arrived, and yet his vigil was unrewarded. It suddenly occurred to him that as he had seen Mr. Mitchel purchase tickets for the Casino, that might be a good place to watch, though of course there was no certainty that they were for that night. Upon this meagre hope he hastened up-town and stationed himself where he could keep an eye on all who entered. At ten minutes past eight he was about concluding that his task was useless when a cab stopped, and to his intense satisfaction he saw Mr. Mitchel alight, and then hand out a handsomely-dressed woman. Wilson had prepared himself for this possibility, by purchasing a ticket of admission, so that he followed the couple into the theatre, determined not to lose sight of his man again. The opera over, he found it easy to shadow the two, as the woman declined the proffered cab, perchance because the exhilarating, though cold night air made a walk home inviting. He was, however, somewhat amazed at last to see them enter the very apartment-house on Thirtieth Street to which he had traced Rose Mitchel in the morning. His mind was at once set at ease, for since both of his birds had flown to the same dove-cot, it seemed plain that they were connected. Evidently it was to this house that Mr. Mitchel had gone after eluding him in the morning. At least so argued the astute detective.