To the ordinary eye the map was quite confusing, for besides the multiplicity of typographical details there were numerous red and blue lines branching off from various points.
"What do they mean?" queried Don.
"The location of certain batteries and their range," explained the artillery officer. "Now, kindly step this way."
About fifty feet further on the three came to a halt before a rounded elevation, on a mound of earth.
"Entrez, Messieurs," said Lieutenant D'Arraing, with a smile—he pointed to a dark, gloomy-looking opening at the base,—"and I'll introduce you to one of our special favorites—'Le Grand Pere.' Presently it will be paying some attention to the wireless over yonder."
"Goodness gracious!—there's concealment for you!" cried Don.
Cautiously the boy stepped down into the entrance, in a moment or two finding himself face to face with the breech of a big gun. The weapon, its muzzle projecting through another opening at the opposite end of the mound, was well protected by a heavily-timbered roof covered with earth. Even in the underground retreat the polished surfaces of the steel monster caught and reflected every stray beam of light.
"'Le Grand Pere' has done his full share of service," declared the French officer, when all were standing inside.
Then, to show how easily the piece of mechanism could be operated, he raised, lowered and moved the muzzle from side to side by means of little wheels.
"It seems almost like perfection," commented the aviator's son, as he carefully examined the "elements," as the figures on the gun's-sighting apparatus are called. "And yet I suppose experts are continually trying to make improvements."