And the dog answered with delighted leaps.

“What are you going to do with him?” asked the proprietor.

“Make a little experiment.  Do you mind waiting a couple of minutes?  It may give us a line on this visitor to Number Seven.”

“I’ll wait,” said Gritz.

“Come over here,” continued the other.  “I’ll show you a pistol connected with this case.  And I’ll show it to the dog.”

“For the scent?  You don’t think a dog can follow the scent from a pistol, do you?” asked the proprietor incredulously.

“I don’t know. This dog has done wonderful things.  He tracked a murderer once three miles across rough country near Liege and found him hidden in a barn.  But he had better conditions there.  We’ll see.”

They had entered the courtyard now and Coquenil led Caesar to the spot where the weapon lay still undisturbed.

“Cherche!” he ordered, and the dog nosed the pistol with concentrated effort.  Then silently, anxiously, one would say, he darted away, circling the courtyard back and forth, sniffing the ground as he went, pausing occasionally or retracing his steps and presently stopping before M. Paul with a little bark of disappointment.

“Nothing, eh?  Quite right.  Give me the pistol, Papa Tignol.  We’ll try outside.  There!” He pointed to the open door where the concierge was waiting.  “Now then, cherche!”

In an instant Caesar was out in the Rue Marboeuf, circling again and again in larger and larger arcs, as he had been taught, back and forth, until he had covered a certain length of street and sidewalk, every foot of the space between opposite walls, then moving on for another length and then for another, looking up at his master now and then for a word of encouragement.

[Illustration:  “‘Cherche!’ he ordered.”]

“It’s a hard test,” muttered Coquenil.  “Footprints and weapons have lain for hours in a drenching rain, but—­Ah!” Caesar had stopped with a little whine and was half crouching at the edge of the sidewalk, head low, eyes fiercely forward, body quivering with excitement.  “He’s found something!”

The dog turned with quick, joyous barks.

“He’s got the scent.  Now watch him,” and sharply he gave the word:  “Va!”

Straight across the pavement darted Caesar, then along the opposite sidewalk away from the Champs Elysees, running easily, nose down, past the Rue Francois Premier, past the Rue Clement-Marot, then out into the street again and stopping suddenly.

“He’s lost it,” mourned Papa Tignol.

“Lost it?  Of course he’s lost it,” triumphed the detective.  And turning to M. Gritz:  “There’s where your murderer picked up a cab.  It’s perfectly clear.  No one has touched that pistol since the man who used it threw it from the window of Number Seven.”

“You mean Number Six,” corrected Gritz.