a light supper and get down to the
river before it grows too dark."

Four weeks had passed since the events narrated in the last chapter,
and matters in and around the construction camp had once more quieted
down. Work was being pushed forward rapidly, and Dave and Roger were
making excellent progress in their chosen profession. They had made a
warm personal friend of Frank Andrews, as well as a friend of Mr.
Obray, and both of these individuals gave them many instructions
during off hours which proved highly beneficial.

No more had been heard from the Mexican raiders, and it was hoped
that those bandits had departed for some other locality along the Rio
Grande. The prisoners taken during the raid were still in jail,
awaiting trial.

Down along the stream over which the new Catalco bridge was being
constructed there was a favorite swimming place used by the civil
engineers and their assistants, the men and boys of the construction
gang using another spot farther down the stream.

"I'll beat you getting in, Dave!" cried Roger, as the pair neared the
bathing place that evening, and he started to take off some of his
clothes.

"Don't jump in too quickly, Roger," warned our hero. "Remember you
have just been eating and you are rather warm. Better take it easy on
the bank for a little while."

"I guess you're right," was the reply. "I don't want to get a cramp or
a chill, or anything like that."

To reach the swimming spot, the chums had to pass one end of the new
bridge. As they drew closer they saw somebody high up on the skeleton
structure gazing at them curiously.

"Hello! who's that up there?" remarked Dave.

"I'm sure I don't know," answered Roger. "I thought all our men were
back in camp."

As they came still closer the individual on the bridge turned to walk
toward them. Suddenly, however, he stopped short and tried his best to
hide himself behind some of the steel work.

"Say! that looks rather queer to me," remarked Dave. "He acts just as
if he didn't want us to see him."

"Just what I thought, Dave." The senator's son gave a sudden start.
"You don't suppose it's one of those Mexican raiders, do you?"

"I can't say anything about that. I'm going up there to find out who
he is. It seems to me he is acting very suspiciously. Maybe he's
trying to plant some more bombs."

Dave turned back to a point where he could get up on the bridge, and
his chum followed. From this point they could not see the pe