Half an hour passed. Truth to tell, the young Army officer felt the monotony of his present position, confined to the boat and the pier. Passing the sentry at the gang-plank, who stood at present arms in salute, Hal Overton walked forward to the outer end of the pier. He had stood there some ten minutes, when, two or three miles up the river, he thought he saw a brief flash.
"That might have come from a search-light, swiftly operated," thought Lieutenant Hal, with a start. After a moment's reflection, he hurried on board the boat.
"Mate," he directed, "shove off and steam out just past the end of the pier."
"Why——" began the mate wonderingly.
"Obey the order, sir!"
As the boat moved slowly into open water Hal, standing by the search-light, gave another order:
"Switch on this light, mate."
"But your captain said——"
"Switch on the light, man! Be quick about it!"
As the light gleamed out Lieutenant Hal turned its broad flare up the river. Just on the edge of the beam he picked up a motor boat of considerable size. The other craft was some three miles up the river, headed due south across to the Mexican border.