"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.
"Oh, then," suggested Reade, "we may as well go along on our way. Plainly there's nothing human around here to look at but ourselves."
So they started slowly forward over the wall. Leisurely the black man swam to the wall, taking up the dogged trail again in the darkness behind the pair of young engineers.
Several minutes more of cautious walking brought Tom Reade to a startled halt.
"Look there, Harry!" uttered Reade, stopping and throwing the light ahead.