The Sarrions left their carriage at the corner of the Calle de la Merced, in the shadow of a tall house, for the sun was already strong at midday though the snow lay on the hills round Torre Garda.  They found the house closely barred.  The dust and the cobwebs were undisturbed on the huge windows.  The house was as empty as it had been these forty years.

Marcos tried the door, which resisted his strength like a wall.  It was a true monastic door with no crack through which even a fly could pass.

“That house stands empty,” said an old woman who passed by.  “It has stood empty since I was a girl.  It is accursed.  They killed the good fathers there.”

Sarrion thanked her and walked on.  Marcos was examining the dust on the road out of the corners of his eyes.

“Two carriages have stopped here,” he said, “at this small door which looks as if it belonged to the next house.”

“Ah!” answered Sarrion, “that is an old trick.  I have seen doors like that before.  There are several in the Calle San Gregorio.  Sitting on my balcony in the Casa Sarrion I have seen a man go into one house and look out of the window of the next a minute later.”

“Mon has not arrived,” said Marcos, with his eye on the road.  “He has the carriage of One-eyed Pedro whose near horse has a circular shoe.”

“But we must not wait for him.  The risk would be too great.  They may dispense with his presence.”

“No,” answered Marcos thoughtfully, looking at the smaller door which seemed to belong to the next house.  “We must not wait.”

As he spoke a carriage appeared at the farther end of the Calle de la Merced, which is a straight and narrow street.

“Here they come,” he added, and drew his father into a doorway across the street.

It was indeed the carriage of the man known as One-eyed Pedro, a victim to the dust of Aragon, and the near horse left a circular mark with its hind foot on the road.

Evasio Mon descended from the carriage and paid the man, giving, it would seem, a liberal “propina,” for the One-eyed Pedro expectorated on the coin before putting it into his pocket.

Mon tapped on the door with the stick he always carried.  It was instantly opened to give him admittance, and closed as quickly behind him.

“Ah!” whispered Sarrion, with a smile on his keen face.  “I have heard them knock like that on the doors in the Calle San Gregorio.  It is simple and yet distinctive.”

He turned and illustrated the knock on the balustrade of the stairs up which they had hastened.

“We will try it,” he added grimly, “on that door when Evasio has had time to go away from it.”

They waited a few minutes, and then went out again into the Calle de la Merced.  It was the luncheon hour, and they had the street to themselves.  They stood for a moment in the doorway through which Mon had passed.

“Listen,” said Marcos in a whisper.