But as he emerged from the door a hansom passed him, and without thinking, he accepted the invitation of the driver to enter it.
Through the trap-door he told the man to drive to the Laburnum Road; and then as the vehicle moved along at a smart pace he gave himself up again to speculating in what way Kathleen might be in peril and from what motive Mme. Estelle had warned him.
He had come to no conclusion on this point when the hansom swung sharply round from the Finchley Road into Laburnum Road, which at that hour of the day was more quiet and deserted than ever.
Then a strange thing happened so suddenly that he had no time to ward off the danger in which he found himself.
Two steel arms, which had been so secreted in the upholstery of the cab as to be invisible, suddenly closed round his arms and body with a snap, and as the hansom was pulled up with a jerk he found himself a prisoner, so tightly squeezed by the encircling steel arms that he was unable to do more than wriggle in his seat.
In a moment the driver was off the dicky and had come round to the front of the cab. With a fascinated gaze Westerham watched him take a little phial from his pocket and saturate a handkerchief.
He divined the man's intention in a moment, and cried out an inquiry as to what he was about to do.
But the man made no answer, except to grin and climb on to the step of the cab.
A moment later he had clapped the handkerchief over Westerham's mouth and nose and held it there tightly for a few seconds.