"Trouble!" Tom whispered in his chum's ear. "Most likely some of the
rascals that we drove out of camp have been trying to set back our work
with dynamite. If they have done so we'll teach 'em a lesson if we can
catch them!"

So the young engineers had started out over their narrow retaining wall.
We have seen how they had walked most of the distance when Harry had had
his sudden warning of the hostile arm uplifted over his head.

"What could it have been?" demanded Tom in a low voice, as he continued to
cast the light from his flash lamp out over the waters on either side of
the wall.

"It must have been my nervous imagination," admitted Harry. "Whew! But
it _did_ seem mighty real for the moment."

"Then you're inclined, now, to believe that it was purely imagination?"
pursued Tom.

"Ye---e---es, it must have been," assented Harry reluctantly.

Tom made some final casts with the light.

While they were conversing, well past the short radius of the flash lamp's
glare, a massive black head bobbed up and down with the waves. Out there
the huge negro who had swiftly vanished from the wall, and who had swum
under water for a long distance, was indolently treading water. Wholly at
home in the gulf, the man's black head blended with the darkness of the
water and the blackness of the night.