“You are a very clever girl.  I’ve decided to turn this case over to you.  After all, your business is to decipher cipher, and you can’t do it without the book.”

They both laughed.

“I don’t see how you ever solved that,” he said, delighted to tease her.

“How insulting!—­when you know it is one of the oldest and most familiar of codes—­the 1-2-3 and a-b-c combination!”

“Rather rude of you to read it over my shoulder, Miss Erith.  It isn’t done—­”

“You meant to see if I could!  You know you did!”

“Did I?”

“Of course!  That old ‘Seal of Solomon’ cipher is perfectly transparent.”

“Really?  But how about this!”—­touching the sheets of the Lauffer letter—­“how are you going to read this sequence of Arabic numerals?”

“I haven’t the slightest idea,” said the girl, candidly.

“But you request the job of trying to find the key?” he suggested ironically.

“There is no key.  You know it.”

“I mean the code book.”

“I would like to try to find it.”

“How are you going to go about it?”

“I don’t know yet.”

Vaux smiled.  “All right; go ahead, my dear Miss Erith.  You’re officially detailed for this delightful job.  Do it your own way, but do it—­”

“Thank you so much!”

“—­In twenty-four hours,” he added grimly.  “Otherwise I’ll turn it over to the P.I.”

“Oh!  That is brutal of you!”

“Sorry.  But if you can’t get the code-book in twenty-four hours I’ll have to call in the Service that can.”

The girl bit her lip and held out her hand for the letter.

“I can’t let it go out of my office,” he remarked.  “You know that, Miss Erith.”

“I merely wish to copy it,” she said reproachfully.  Her eyes were hazel.

“I ought not to let you take a copy out of this office,” he muttered.

“But you will, won’t you?”

“All right.  Use that machine over there.  Hum—­hum!”

For twenty minutes the girl was busy typing before the copy was finally ready.  Then, comparing it and finding her copy accurate, she returned the original to Mr. Vaux, and rose with that disturbing grace peculiar to her every movement.

“Where may I telephone you when you’re not here?” she inquired diffidently, resting one slim, white hand on his desk.

“At the Racquet Club.  Are you going out?”

“Yes.”

“What!  You abandon me without my permission?”

She nodded with one of those winsome smiles which incline young men to revery.  Then she turned and walked toward the cloak room.

The D. C. was deeply in love with somebody else, yet he found it hard to concentrate his mind for a while, and he chewed his unlighted cigar into a pulp.  Alas!  Men are that way.  Not sometimes.  Always.

Finally he shoved aside the pile of letters which he had been trying to read, unhooked the telephone receiver, called a number, got it, and inquired for a gentleman named Cassidy.