The detective returned in ten minutes with a worried expression. "She wasn't in your office, Doctor," he reported. "Who? Oh, Thelma. Where was she?" "No one seems to know. She left yesterday afternoon and hasn't returned." "Oh, well, since I am out of the city, I expect she decided to take a vacation. Women are always undependable. Did you get hold of the rest?" "They'll be down at midnight, all but Davis. He'll come down in the morning." "Good enough! Now, Colonel, if you'll have the officers who are going out to-morrow assembled, we'll divide the territory and make our plans for the search." * * * * * A week later, the situation was unchanged. Secret service operatives and soldiers from the Proving Ground had covered, foot by foot, square miles of territory south of the Proving Ground, but without result. Not a single unexplainable thing had been found. Sensitive instruments sent down from the Bureau of Standards, instruments so sensitive that they would detect an electric light burning a mile away, had yielded no results. As a final measure, General Merton had ordered a dozen planes with steel-cylindered motors to the Proving Ground and they had repeatedly crisscrossed the suspected territory, but had acquired no static charge large enough to affect them. It was evident that Saranoff's device, if it existed, had been moved, or else was not in operation. Also, to Carnes' openly expressed and Dr. Bird's secret worry, Thelma Andrews had not returned to the Bureau of Standards. The Russian girl, formerly known as Feodrovna Androvitch, a tool and follower of Ivan Saranoff, had acted with Carnes and the doctor in their long drawn-out fight with the arch-communist often enough to be a marked woman. Urged by Carnes, Bolton, the head of the Secret Service, put a dozen of his best men on her trail, but they found nothing. She had disappeared as thoroughly as if the earth had opened and swallowed her up. At last, as the combing of the Aberdeen marshes yielded no results, Dr. Bird acceded to Carnes' request, and the detective left for Washington to take personal charge of the search. Dr. Bird sat alone in his quarters at the Officers' Club, futilely wracking his brains for a clue to his further procedure. The telephone rang loudly. With a grunt, he took down the receiver. A feminine voice spoke with a strong foreign accent. "I vant der Herr Doktor Vogel, plees!" "You want who? Oh, yes. Vogel--bird! This is Dr. Bird speaking." The voice instantly lost both its foreign accent and its guttural quality. "I thought so when you spoke, Doctor, but I wanted to make sure. This is Thelma Andrews." "Where the devil have you been? Half the Secret Service is looking for you, including Carnes, who deserted me and is in Washington." * * * * *