tead of allowing it to
spread about equally on all sides of the transmitting station. It
makes both for secrecy and economy, since nearly all the power used at
the sending apparatus is confined to an arc of about three degrees of
a circle. Directed to a given receiving station, receiving outfits to
one side or the other of that path are unable to listen in, and the
signal is markedly stronger in the chosen path. Exactly the same
process, of course, is used for radio directional signals, one of
which still buzzed monotonously in Bell's ears until he impatiently
turned it off. A plane in the path hears the signal. If it does not
hear the signal, it is demonstrably off the straight route.

* * * * *

Bell, then, was in a direct line from Rio to the source of a radio
direction signal. Fifty miles back, where the big amphibian had
crashed, he was in the same air line. To extend that line on into the
interior would give the destination of Ribiera, and the location of
the headquarters where direct communication with The Master was
maintained.

He worked busily. His maps were in separate sheets, and it took time
to check the line from Rio. When he had finished, he computed grimly.

"At a hundred miles in hour...." He was figuring the maximum distance
which could plausibly be accepted as a day's journeying by air. He
surveyed the maps again. "The plateau of Cuyaba, at a guess. Hm....
Fleets of aircraft could practise there and never be seen. An army
could be maneuvered without being reported. Certainly the headquarters
for the whole continent could be there. Striking distance of Rio,
Montevideo, Buenos Aires, La Paz, and Asuncion. Five republics."

* * * * *

Certainly, from his figures, it seemed plausible that somewhere up on
the Plateau of Cuyaba--where no rails run, no boats ply, and no
telegraph line penetrates; which juts out ultimately into that
unknown region where the Rio Zingu and the Tapajoz have their
origins--certainly it seemed plausible that there must lie the
headquarters of the whole ghastly conspiracy. There, it might be, the
deadly plants from which The Master's poison was brewed were grown.
There the deadly stuff was measured out and mixed with its temporary
antidote....

Paula came back, a young man with her. Her eyes were wide and staring,
as if she had looked upon something vastly worse than death.

"He--Ribiera," she gasped. "My uncle