"What was that, fellows? Did you hear it?"
"Sounded to me like thunder," said Bart.
"With a sky like this?" replied Frank. "Never. Listen!"
Borne on the wind came a long, booming sound, growing longer and louder as they sped toward it, falling fitfully at times, only to swell into a mightier rumble like the roar of waves dashing against the coast.
They looked at each other with comprehension dawning in their eyes.
"It's thunder all right, Bart," said Frank, quietly. "It's the thunder of the guns! We are getting near the fighting front at last!"
CHAPTER XVI
THE AIRSHIP RAID
The signs multiplied now that they were approaching the battle lines. Apart from the ominous roar which had now become unceasing, war showed its grim face on every hand.