"My, Dave! but it's hot!" "I agree with you, Roger. This is the hottest day we've struck yet. And such a hard day as it's been too!" and our hero paused to wipe the perspiration from his brow. "What do you say if we take a swim this evening?" went on the senator's son. "A plunge into the river would feel good to me." "I'm with you, Roger. Let us eat 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."