And the carriage rolled across Piccadilly into Bond Street in the direction of Grosvenor Square and Lady Holbert’s.

The fog was settling down over London.  Moving objects were beginning to take on the loom of gigantic figures.  It was getting difficult to see.

It must have taken Hargrave half an hour to reach the club.  The first man he saw when he went in was Sir Henry, his hands in the pockets of his tweed coat and his figure blocking the passage.

“Hello, Hargrave!” he cried.  “What have you got in your room that old Ponsford won’t let me go up?”

“Not nine hundred horses!” replied the American.

The Baronet laughed.  Then he spoke in a lower voice: 

“It’s extraordinary lucky that I ran over to the Sorbonne.  Come along up to your room and I’ll tell you.  This place is filling up with a lot of thirsty swine.  We can’t talk in any public room of it.”

They went up the great stairway, lined with paintings of famous colonials celebrated in the English wars, and into the room.  Hargrave turned on the light and poked up the fire.  Sir Henry sat down by the table.  He took out his three newspapers and laid them down before him.

“My word, Hargrave,” he said, “old Arnold is a clever beggar!  He cleared the thing up clean as rain.”  The Baronet spread the newspapers out before him.

“We knew here at the Criminal Investigation Department that this thing was a cipher of some sort, because we knew about these horses.  We had caught up with this business of importing horses.  We knew the shipment was on the way as I explained to you.  But we didn’t know the port that it would come into.”

“Well,” said the American, “did you find out?”

“My word,” he cried, “old Arnold laughed in my face.  ’Ach, monsieur,’ he cried, mixing up several languages, `it is Heidel’s cipher!  It is explained in the seventeenth Criminal Archive at Gratz.  Attend and I will explain it, monsieur.  It is always written in two paragraphs.  The first paragraph contains the secret message, and the second paragraph contains the key to it.  Voila!  This message is in two paragraphs: 

“’"P.L.A. shipped nine hundred horses on freight steamer Don Carlos from N. Y.

“’"Have the bill of lading handed over to our agent to check up”

“’The hidden message is made up of certain words and capital letters contained in the first paragraph, while the presence of the letter t in the second paragraph indicates the words or capital letters that count in the first.  One has only to note the numerical position of the letter t in the second paragraph in order to know what capital letter or word counts in the first paragraph.’”

The Baronet took out a pencil and underscored the words in the second paragraph of the printed cipher:  “Have the bill of lading handed over to our agent to check up.”

“You will observe that the second, the eighth and the eleventh words in this paragraph begin with the letter t.  Therefore, the second, the eighth and the eleventh capital letters or words in the first paragraph make up the hidden message.”