"F'r the love o' God," growled Cassidy, "is that YOU, Mr. Vaux!" He lowered his arms sheepishly, reached out and switched on the ceiling light over the dining-room table. "Well, f'r--" he began; and, seeing Miss Erith, subsided. "What are you doing here?" demanded Vaux, disgusted with this glaring example of interference from another service. "What am I doing?" repeated Cassidy with a sarcastic glance at Miss Erith. "Faith, I'm pinching a German gentleman we've been watching these three months and more. Is that what you're up to, too?" "Herman Lauffer?" "That's the lad, sir. He's in the kitchen yonder, dressing f'r to take a little walk. I gotta get his coat and vest. And what are you doing here, sir?" "How did YOU get in?" asked Miss Erith, flushed with chagrin and disappointment. "With keys, ma'am." "Oh, Lord!" said Vaux, "we jimmied the door. What do you think of that, Cassidy?" "Did you so?" grinned Cassidy, now secure in his triumphant priority and inclined to become friendly. "I never dreamed that your division was watching Lauffer," continued Vaux, still red with vexation. "It's a wonder we didn't spoil the whole affair between us." "It is that!" agreed Cassidy with a wider grin. "And you can take it from me, Mr. Vaux, we never knew that the Postal Inspection was on to this fellow at all at all until you called me to stop outgoing letters." "What have you on him?" inquired Vaux. Cassidy laughed: "Oh, listen then! Would you believe this fellow was tryin' the old diagonal trick? Sure it was easy; I saw him mail a letter this afternoon and I got it. I'd been waiting three months for him to do something like that. But he's a fox--he is that, Mr. Vaux! Do you want to see the letter? I have it on me--" He fished it out of his inside pocket and spread it on the dining table under the light. "You know the game," he remarked, laying a thick forefinger on the diagonal line bisecting the page. "All I had to do was to test the letter by drawing that line across it from corner to corner. Read the words that the line cuts through. Can you beat it?" Vaux and Miss Erith bent over the letter, read the apparently innocent message it contained, then read the words through which the diagonal line had been drawn. Then Cassidy triumphantly read aloud the secret and treacherous information which the letter contained: "SEVEN UNITED STATES TRANSPORTS TO-DAY NEW YORK (BY THE) NORTHERN ROUTE. INFORM OUR U-BOATS. URGENCY REQUIRES INSTANT MEASURES. TEN MORE ARE TO SAIL FROM HERE NEXT WEEK." "The dirty Boche!" added Cassidy. "Dugan has left for Mexico to look up this brother of his and I'm lookin' up this snake, so I guess there's no harm done so far." "New York. "January 3rd. 1916. "My dear Brother: