The monotonous boom of the distant guns, sounding like the roar of waves upon a beach, kept up unceasingly, but the boys had got so used to it that they scarcely noticed it.
But suddenly, among these bass notes came another sound, or series of sounds, sharp, shrill, metallic, which they had already learned to identify as the popping of anti-aircraft guns.
"That sounds as though they had sighted one of the Hun aeroplanes," commented Frank.
"More likely it's part of the practice," remarked Tom, carelessly.
"Look at those shrapnel puffs over there," cried Bart, pointing toward the sky.
High up in the air, following one another in quick succession, were light, bluish streaks, that after reaching an enormous height, suddenly burst in a cloud of white.
"They're certainly firing at something," remarked Billy, "but for the life of me I don't see what it is."
"I do," cried Bart. "Look! just at the edge of that fleecy cloud. It's so white you can hardly tell it from the cloud itself."
They strained their eyes in the direction where Bart was pointing. High up in the air, miles, it seemed, was a long, silver streak, shaped like an immense cigar. At that height it seemed almost to hang in the atmosphere, so gliding and imperceptible was its motion. And yet the boys knew that it was really shooting along with the speed of an express train.
"A Zeppelin!" they shouted, in chorus.