Armitage was at once on the alert with all his faculties sharpened.

He turned and gradually slackened his pace, and the person behind him immediately did likewise.
The sensation of being followed is at first annoying; then a pleasant zest creeps into it, and in Armitage's case the reaction was immediate.
He was even amused to reflect that the shadow had chosen for his exploit what is probably the most conspicuous and the best-guarded spot in America.

It was not yet ten o'clock, but the streets were comparatively free of people.

He slackened his pace gradually, and threw open his overcoat, for the night was warm, to give an impression of ease, and when he had reached the somber facade of the Treasury Building he paused and studied it in the glare of the electric lights, as though he were a chance traveler taking a preliminary view of the sights of the capital.

A man still lingered behind him, drawing nearer now, at a moment when they had the sidewalk comparatively free to themselves.