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 catch-plate is not air-tight. But a fresh box of cartridges is well sealed, and its contents retain their lustre much longer."

He pointed to the first cartridge that had rolled out of the magazine.

"Observe that this one cartridge—the last to be inserted into the magazine—is a bit brighter than its fellows. The inf'rence is—you're an adept at inf'rences, y' know—that it is a newer cartridge, and was placed in the magazine rather recently."

He looked straight into Markham's eyes.

"It was placed there to take the place of the one which Captain Hagedorn is keeping."

Markham lifted his head jerkily, as if shaking himself out of an encroaching spell of hypnosis. He smiled, but with an effort.

"I still think your case against Mrs. Platz is your masterpiece."

"My picture of the Major is merely blocked in," answered Vance. "The revealin' touches are to come. But first, a brief catechism: . . . How did the Major know that brother Alvin would be home at twelve-thirty on the night of the thirteenth?—He heard Alvin invite Miss St. Clair to dinner—remember Miss Hoffman's story of his eavesdropping?—and he also heard her say she'd unfailingly leave at midnight. When I said yesterday, after we had left Miss St. Clair, that something she told us would help convict the guilty person, I referred to her statement that midnight was her invariable hour of departure. The Major therefore knew Alvin would be home about half past twelve, and he was pretty sure that no one else would be there. In any event,