One of the great lessons Coquenil had learned in his long experience with mysterious crimes was to be careful of hastily rejecting any evidence because it conflicted with some preconceived theory. It would have been easy now, for instance, to assume that this prim spinster was mistaken in declaring that she had seen the pistol thrown from the window of Number Seven. That, of course, seemed most unlikely, since the shooting was done in Number Six, yet how account for the woman's positiveness? She seemed a truthful, well-meaning person, and the murderer might have gone into Number Seven after committing the crime. It was evidently important to get as much light as possible on this point. Hence the need of M. Gritz.

M. Herman Gritz was a short, massive man with hard, puffy eyes and thin black hair, rather curly and oily, and a rapacious nose. He appeared (having been induced to come down by the commissary) in a richly embroidered blue-silk house garment, and his efforts at affability were obviously based on apprehension.

Coquenil began at once with questions about private room Number Seven. We had reserved this room and what had prevented the person from occupying it? M. Gritz replied that Number Seven had been engaged some days before by an old client, who, at the last moment, had sent a petit bleu to say that he had changed his plans and would not require the room. The petit bleu did not arrive until after the crime was discovered, so the room remained empty. More than that, the door was locked.

"Locked on the outside?"

"Yes."

"With the key in the lock?"

"Yes."

"Then anyone coming along the corridor might have turned the key and entered Number Seven?"

"It is possible," admitted M. Gritz, "but very improbable. The room was dark, and an ordinary person seeing a door locked and a room dark——"

"We are not talking about an ordinary person," retorted the detective, "we are talking about a murderer. Come, we must look into this," and he led the way down the corridor, nodding to the policeman outside Number Six and stopping at the next door, the last in the line, the door to Number Seven.

"You know I haven't been in there yet." He glanced toward the adjoining room of the tragedy, then, turning the key in Number Seven, he tried to open the door.

"Hello! It's locked on the inside, too!"

"Tiens! You're right," said Gritz, and he rumpled his scanty locks in perplexity.

"Some one has been inside, some one may be inside now."

The proprietor shook his head and, rather reluctantly, went on to explain that Number Seven was different from the other private rooms in this, that it had a separate exit with separate stairs leading to an alleyway between the hotel and a wall surrounding it. A few habitues knew of this exit and used it occasionally for greater privacy. The alleyway led to a gate in the wall opening on the Rue Marboeuf, so a particularly discreet couple, let us say, could drive up to this gate, pass through the alleyway, and then, by the private stairs, enter Number Seven without being seen by anyone, assuming, of course, that they had a key to the alleyway door. And they could leave the restaurant in the same unobserved manner.

As Coquenil listened, his mouth drew into an ominous thin line and his deep eyes burned angrily.

"M. Gritz," he said in a cold, cutting voice, "you are a man of intelligence, you must be. This crime was committed last night about nine o'clock; it's now half past three in the morning. Will you please tell me how it happens that this fact of vital importance has been concealed from the police for over six hours?"

"Why," stammered the other, "I—I don't know."

"Are you trying to shield some one? Who is this man that engaged Number Seven?"

Gritz shook his head unhappily. "I don't know his name."

"You don't know his name?" thundered Coquenil.

"We have to be discreet in these matters," reasoned the other. "We have many clients who do not give us their names, they have their own reasons for that; some of them are married, and, as a man of the world, I respect their reserve." M. Gritz prided himself on being a man of the world. He had started as a penniless Swiss waiter and had reached the magnificent point where broken-down aristocrats were willing to owe him money and sometimes borrow it—and he appreciated the honor.

"But what do you call him?" persisted Coquenil. "You must call him something."

"In speaking to him we call him 'monsieur'; in speaking of him we call him 'the tall blonde.'"

"The tall blonde!" repeated M. Paul.

"Exactly. He has been here several times with a woman he calls Anita. That's all I know about it. Anyway, what difference does it make since he didn't come to-night?"

"How do you know he didn't come? He had a key to the alleyway door, didn't he?"

"Yes, but I tell you he sent a petit bleu."

The detective shrugged his shoulders. "Some one has been here and locked this door on the inside. I want it opened."

"Just a moment," trembled Gritz. "I have a pass key to the alleyway door. We'll go around."

"Make haste, then," and they started briskly through the halls, the proprietor assuring M. Paul that only a single key was ever given out for the alleyway door and this to none but trusted clients, who returned it the same night.

"Only a single key to the alleyway door," reflected, Coquenil.

"Yes."

"And your 'tall blonde' has it now?"

"I suppose so."

They left the hotel by the main entrance, and were just going around into Rue Marboeuf when the concierge from across the way met them with word that CÆsar had arrived.

"CÆsar?" questioned Gritz.

"He's my dog. Ph-h-eet! Ph-h-eet! Ah, here he is!" and out of the shadows the splendid animal came bounding. At his master's call he had made a mighty plunge and broken away from Papa Tignol's hold.

"Good old fellow!" murmured M. Paul, holding the dog's eager head with his two hands. "I have work for you, sir, to-night. Ah, he knows! See his eyes! Look at that tail! We'll show 'em, eh, CÆsar?"

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 LiÉge 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 CÆsar 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 CÆsar 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.

"It's a hard test," muttered Coquenil. "Footprints and weapons have lain for hours in a drenching rain, but—Ah!" CÆsar 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 CÆsar, then along the opposite sidewalk away from the Champs ElysÉes, running easily, nose down, past the Rue FranÇois 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.

"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, CÆsar!"

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, CÆsar?"

Evidently CÆsar 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 CÆsar was away, darting back along the sidewalk toward the Champs ElysÉes, 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 CÆsar!" 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.

"I'll send the dog first," said M. Paul. "Here, CÆsar! Cherche!"

Once more the eager animal sprang forward, following slowly along the row of trees where the trail was confused, and then, at the corner, dashing ahead swiftly, only to stop again after a few yards and stand scratching uneasily at a closed door.

"That settles it," said Coquenil. "He has brought us to the alleyway door. Am I right?"

"Yes," nodded Gritz.

"The door that leads to Number Seven?"

"Yes."

"Open it," and, while the agitated proprietor searched for his pass key, the detective spoke to Tignol: "I want impressions of these footprints, the best you can take. Use glycerin with plaster of Paris for the molds. Take this one and these two and this and this. Understand?"

"Perfectly."

"Leave CÆsar here while you go for what you need. Down, CÆsar! Garde!"

The dog growled and went on guard forthwith.

"Now, we'll have a look inside."

The alleyway door stood open and, using his lantern with the utmost care, Coquenil went first, mounting the stairs slowly, followed by Gritz. At the top they came to a narrow landing and a closed door.

"This opens directly into Number Seven?" asked the detective.

"Yes."

"Is it usually locked or unlocked?"

"IT is always locked."

"Well, it's unlocked now," observed Coquenil, trying the knob. Then, flashing his lantern forward, he threw the door wide open. The room was empty.

"Let me turn up the electrics," said the proprietor, and he did so, showing furnishings like those in Number Six except that here the prevailing tint was pale blue while there it was pale yellow.

"I see nothing wrong," remarked M. Paul, glancing about sharply. "Do you?"

"Nothing."

"Except that this door into the corridor is bolted. It didn't bolt itself, did it?"

"No," sighed the other.

Coquenil thought a moment, then he produced the pistol found in the courtyard and examined it with extreme care, then he unlocked the corridor door and looked out. The policeman was still on guard before Number Six.

"I shall want to go in there shortly," said the detective. The policeman saluted wearily.

"Excuse me," ventured M. Gritz, "have you still much to do?"

"Yes," said the other dryly.

"It's nearly four and—I suppose you are used to this sort of thing, but I'm knocked out, I—I'd like to go to bed."

"By all means, my dear sir. I shall get on all right now if—oh, they tell me you make wonderful Turkish coffee here. Do you suppose I could have some?"

"Of course you can. I'll send it at once."

"You'll earn my lasting gratitude."

Gritz hesitated a moment and then, with an apprehensive look in his beady eyes, he said: "So you're going in there?" and he jerked his fat thumb toward the wall separating them from Number Six.

Coquenil nodded.

"To see if the ball from that," he looked with a shiver at the pistol, "fits in—in that?" Again he jerked his thumb toward the wall, beyond which the body lay.

"No, that is the doctor's business. Mine is more important. Good night!"

"Good night," answered Gritz and he waddled away down the corridor in his blue-silk garments, wagging his heavy head and muttering to himself: "More important than that! Mon Dieu!"