"I know!" said Henri, suddenly. "It's the guns we hear. The wind is changing and that is why it is coming to us now. There is a battle. In olden days we could see its smoke but now they fight without making smoke. And the noise, too, seems to come from the direction in which we are going."
Once he had named the cause, there was no mystery about the sound. It was less a sound, however, than a beating of the air. There were no sharp reports; it was a steady, ceaseless murmur. But even so, there was no mistaking it. For the first time they were within hearing distance of a battle.
"We will soon be on our way to Berlin, now," said Henri. "That must mean that we have turned—that the great battle has begun."
"It needn't mean that," said Frank. "It may be only artillery covering a rear guard action. I wish you'd remember, Harry, that a retreat may mean mighty hard fighting. Not a rout—a retreat. It isn't easy for an army to move backward. But it's been done by a good many armies that won later."
"Well, come on! We're not getting any nearer to the English by stopping here to talk."
"No. We'll be off again. That noise is getting nearer, Harry. Or louder, anyhow. Perhaps that only means that more guns are going into action."
Somehow the nearness of the battle stimulated them. They found themselves making better time, though they had certainly seemed to be riding as fast as they could before. And all the time the sound of the cannon in front of them grew louder, and the quality of the noise gradually changed. Soon loud explosions began to be distinguishable amid the general hum of battle, and, too, there was an overtone,—a sharper, less steady noise.
"Rifle fire, I think, too," said Frank. "It's lighter than the sound of the cannon, but it seems to be just about as steady. And to think that that's going on, all the way from here to the Swiss border nearly! They're fighting here and near Verdun, and in the Vosges mountains."
"Look over there," said Henri, suddenly. "Do you see? That looks like an omnibus!"
"It is—one of the sort they use in London!" said Frank, in surprise.