There, concealed behind some other suits, was the complete outfit of a British cavalry captain.

"That's one on the Chief," he said to himself. "It must have been Hoff that Dean and Miss Strong saw. I wonder where he got it?"

With a grim smile of satisfaction he devoted himself to going carefully through all the pockets and over all the seams of the clothing in the closet. He even felt into the toe of the shoes and examined the soles.

There was nothing to be found anywhere, but he felt satisfied. The uniform in itself was to his mind damning proof of the young man's occupation.

No explanation that could be given by a young man of German name, even though he was American-born, or had an American birth certificate, could possibly account for his having a British uniform. It was prima facie evidence that Frederic Hoff was a spy. What puzzled Carter most was how Hoff managed to smuggle the uniform in and out of the apartment without being observed. For more than two weeks now every parcel that had arrived at the house of the Hoffs had been searched before it was delivered. The house had been constantly under the strictest surveillance. It was out of the question for him to have worn the uniform in or out as it could not be easily concealed under other clothing.

"There's somebody else in this place in league with the Hoffs," he muttered to himself. "I wonder who it can be."

He looked at his watch. The old servant had been out now nearly half an hour. She was likely to return at any moment. He must work quickly.

Swiftly he went through the dresser drawers but without satisfactory result. There was no time for him to do more. He hastened into the living room and summoned his aides.

"Find anything, Bob?" he asked.

"Not a thing."

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"Beat it up to the roof," he directed. "Have you those field glasses with you?"

"Sure," replied the operative, "and the handkerchiefs, too."

"All right. Get up there before she starts down. Begin putting up handkerchiefs and appear to be watching the river. That will mix her up so she will not know what to do. She will not dare to leave the roof while you are there. When we're through I'll send the elevator man up for you with the message that we have found the short circuit."

He turned to the other operative.

"Find anything, Williams?"

"Only this."

Carter's face brightened as his assistant held out to him two copies of an afternoon newspaper. In each of them a square was missing where something had been cut out.