“Now, Jimmy, get on the job,” instructed Mr. Birnes, as he stepped in.  “Keep that chap in sight and when he stops you stop.”

Mr. Wynne’s cab jogged along comfortably up the avenue, twisting and winding a path between the other vehicles, the while Mr. Birnes regarded it with thoughtful gaze.  Its number dangled on a white board in the rear; Mr. Birnes just happened to note it.

“Grand Central Station, I’ll bet a hat,” he mused.

But the closed cab didn’t turn into Forty-second Street; it went past, then on past Delmonico’s, past the Cathedral, past the Plaza, at Fifty-ninth Street, and still on uptown.  It was not hurrying—­ it merely moved steadily; but once free of the snarl which culminates at the Fifty-ninth Street entrance to Central Park, its speed was increased a little.  Past Sixty-fourth Street, Sixty-fifth, Sixty-sixth, and at Sixty-seventh it slowed up and halted at the sidewalk on the far side.

“Stop in front of a door, Jimmy,” directed the detective hastily.

Jimmy obeyed gracefully, and Mr. Birnes stepped out, hardly half a block behind the closed cab.  He went through an elaborate pretense of paying Jimmy, the while he regarded Mr. Wynne, who had also alighted and was paying the driver.  The small sole-leather grip was on the ground between his feet as he ransacked his pocketbook.  A settlement was reached, the cabby nodded, touched his horse with his whip and continued to jog on up Fifth Avenue.

“Now, he didn’t order that chap to come back or he wouldn’t have paid him,” the detective reasoned.  “Therefore he’s close to where he is going.”

But Mr. Wynne seemed in no hurry; instead he stood still for a minute gazing after the retreating vehicle, which fact made it necessary for Mr. Birnes to start a dispute with Jimmy as to just how much the fare should be.  They played the scene admirably; had Mr. Wynne been listening he might even have heard part of the vigorous argument.  Whether he listened or not he turned and gazed straight at Mr. Birnes until, finally, the detective recognized the necessity of getting out of sight.

With a final explosion he handed a bill to Jimmy and turned to go up the steps of the house.  He had no business there, but he must do something.

Jimmy turned the cab short and went rattling away down Fifth Avenue to await orders in the lee of a corner a block or so away.  And, meanwhile, as Mr. Wynne still stood on the corner, Mr. Birnes had to go on up the steps.  But as he placed his foot on the third step he knew—­though he had not looked, apparently, yet he knew—­that Mr. Wynne had raised his hand, and that in that hand was a small white envelope.  And further, he knew that Mr. Wynne was gazing directly at him.

Now that was odd.  Slowly it began to dawn upon the detective that Mr. Wynne was trying to attract his attention.  If he heeded the signal—­evidently it was intended as such—­it would be a confession that he was following Mr. Wynne, and realizing this he took two more steps up.  Mr. Wynne waved the envelope again, after which he folded it across twice and thrust it into a crevice of a water-plug beside him.  Then he turned east along Sixty-seventh Street and disappeared.