“I mean Number Seven.  We know where the murderer took a cab, now we’ll see where he left the hotel.”  And hurrying toward his dog, he called:  “Back, Caesar!”

Obediently the dog trotted back along the trail, recrossing the street where he had crossed it before, and presently reaching the point where he had first caught the scent.  Here he stopped, waiting for orders, eying M. Paul with almost speaking intelligence.

“A wonderful dog,” admired Gritz.  “What kind is he?”

“Belgian shepherd dog,” answered Coquenil.  “He cost me five hundred francs, and I wouldn’t sell him for—­well, I wouldn’t sell him.”  He bent over and fondled the panting animal.  “We wouldn’t sell our best friend, would we, Caesar?”

Evidently Caesar did not think this the moment for sentiment; he growled impatiently, straining toward the scent.

“He knows there’s work to be done and he’s right.”  Then quickly he gave the word again and once more Caesar was away, darting back along the sidewalk toward the Champs Elysees, moving nearer and nearer to the houses and presently stopping at a gateway, against which he pressed and whined.  It was a gateway in the wall surrounding the Ansonia Hotel.

“The man came out here,” declared Coquenil, and, unlatching the gate, he looked inside, the dog pushing after him.

“Down Caesar!” ordered M. Paul, and unwillingly the ardent creature crouched at his feet.

The wall surrounding the Ansonia was of polished granite about six feet high, and between this wall and the hotel itself was a space of equal width planted with slim fir trees that stood out in decorative dignity against the gray stone.

“This is what you call the alleyway?” questioned Coquenil.

“Exactly.”

From the pocket of his coat the detective drew a small electric lantern, the one that had served him so well earlier in the evening, and, touching a switch, he threw upon the ground a strong white ray; whereupon a confusion of footprints became visible, as if a number of persons had trod back and forth here.

“What does this mean?” he cried.

Papa Tignol explained shamefacedly:  “We did it looking for the pistol; it was Gibelin’s orders.”

“Bon Dieu! What a pity!  We can never get a clean print in this mess.  But wait!  How far along the alleyway did you look?”

“As far as that back wall.  Poor Gibelin!  He never thought of looking on the other side of it.  Eh, eh!”

Coquenil breathed more freely.  “We may be all right yet.  Ah, yes,” he cried, going quickly to this back wall where the alleyway turned to the right along the rear of the hotel.  Again he threw his white light before him and, with a start of satisfaction, pointed to the ground.  There, clearly marked, was a line of footprints, a single line, with no breaks or imperfections, the plain record on the rain-soaked earth that one person, evidently a man, had passed this way, going out.