"Simple enough, after all," Walter commented. "It's the simple things that puzzle one the most sometimes."
"Which reminds me of what Mr. Bruce hinted," Charley said, "that the solution of our mystery was in plain sight all the time, but hanged if I haven't puzzled over it till I made my head swim and can't make it out."
"Same here," Walter said. "I hope he is right and can make an end to this trouble, but I doubt it."
"Well, we will know in a few days. He promised to be out again within a week."
The lads turned back on their patrol and the conversation ceased.
The hours slipped slowly away while the four guards kept up their slow, weary, monotonous pacing back and forth. Three times Charley slipped in and administered doses of quinine. On the last trip he passed by the cook tent and, striking a match, glanced at the clock inside.
"It's just midnight," he said as he rejoined his chum. "That seems to be the favorite time for their devilment. I suppose we can look for trouble any minute now." He had hardly finished when there came the sharp crack of rifles from about the machine. "Good," Charley exclaimed, "they haven't all got silent guns this time. Those reports will give our men an idea where to shoot."
"Hadn't we better go out there?" Walter asked.
"No, we may have our hands full here," his chum replied. "Besides, the engineer will whistle if he wants us. Gee, look at that!"
The swinging platform of the machine was turning around and around at great speed and from it burst forth little jets of flame as the machine men answered the enemy's fire.