scrap of paper reading: 'I've kept my word. Von K.'"

Colonel Stopford looked at Dick fixedly. "Rennell, we may be fools,"
he said, "but we realize what we're up against. It's a big thing, and
we're going to need all our fighting grit to overcome it. You're one
of the four men we're depending on. We're counting on you because of
your record, and because of your degree in science at Heidelberg. The
President wishes you to take charge of the whole Eastern Intelligence
District, covering the entire south-eastern seaboard of the United
States. You are to have complete freedom of action, and all civil,
military, and naval officials have received instructions to co-operate
with you."

"There goes Mrs. Wansleigh's ball," thought Dick, but he said nothing.

* * * * *

"We're not the hunters, Dick Rennell," went on Colonel Stopford.
"We're hiding under cover, and I'm counting on you to turn the tables.
They even know my office is here. I had a long distance call from
Savannah this morning in mocking vein. They advised me to have the
White House watched to-night. I warned the President, and we've posted
guards all round it."

"They held the wire while you called up the President?" asked Dick.

"Damn it, no! They called me up from Scranton the instant he'd
finished speaking. They have the power of the devil, Rennell, with
that infernal invisibility invention of theirs. Rennell, we're
fighting unknown forces. Who this Invisible Emperor is, we don't even
know. But one thing we've found out. He has his headquarters somewhere
in your district. Somewhere along the south Atlantic seaboard. The
greater part of his activities emanate from there. But we're fighting
in the dark. The clue, the master clue that will enable us to locate
him--that's what we lack."

The sun had set, it was beginning to grow dark. Colonel Stopford
switched on the electric lamp beside his desk.

"What have you to say, Rennell?" he asked; and Dick was aware that the
two other men were regarding him attentively.

"It's evident," said Dick, "that Von Kettler possessed this means of
invisibility in his cell, and wasn't detected. He simply slipped out
when the guard came to fetch him."

"Invisibility? Yes! But invisible's not the same thing as
transparent," cried Stopford. "These folks have operated in broad
daylight. They're transparent, damn them! Not even a shadow! You know
what I mean, Rennell! What I'm thinking of! That