undertake a difficult and dangerous mission to South America, and
Gates had been sent on from Washington to post him as to the details
of the case. The boys had waited at the San Francisco hotel three
days for the arrival of the Secret Service man, and waited
impatiently, as Sam Leroy, who was to be the third member of the
party, was anxious for the safety of his aeroplane, the Nelson, in
which the trip to "the roof of the world" was to be made.

The Nelson was lying, guarded night, and day, in a field just out of
the city, on the Pacific side, and Leroy was impatiently keeping his
eyes on the guards most of the time. There was a subconscious
notion in the minds of all the boys that there were enemies about,
and that the aeroplane would never be fully out of danger until she
was well over the ocean on her way south. Gates had arrived only
that morning, and now the lads were eager to be off.

A couple of hours before his appearance in the room that morning,
the Secret Service agent had left the boys in the lobby below to
arrange for the necessary papers and funds for the mission. Before
going out, however, he had been informed of the boys' suspicions,
and had made light of the idea that the aeroplane was in danger from
secret enemies, pointing to the fact that no one was supposed to
know anything about the proposed journey save the boys and himself
as conclusive evidence that the suspicion of constant surveillance
was not well founded.

Now, on his return, his cautious movements indicated that he, too,
was alarmed and on his guard. While Ned was wondering what it was
that had so changed Gates' point of view, there came a quick,
imperative knock on the door of the room, which was occupied by Ned