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.

The great, unwieldy vehicle came lumbering toward them. It rolled along the road, raising a tremendous cloud of dust, and they could see that behind it were many more. Just behind it, too, a man on a motorcycle came suddenly into view. He was mounted on a high-powered machine, and they could hear the roar of his motor as he came up to them.

"Halte!" he cried, in a broken French. "Arretez vous!"

They were off their machines in a moment, saluting, as he stopped his motor and put one foot on the ground to steady his machine. He was dressed in khaki, and both of them recognized his uniform as that of the British forces.

"We speak English," said Frank.

"The deuce you do! That's good! Well, tell me how to get to Guise. We've lost our blooming way, that's what we've done! And we've got supplies for the troops."

"You're going the wrong way--straight to Amiens," said Henri. "The road to Guise is back four miles, at least. Can you turn your 'buses here? We will guide you. We are going that way."

"You are, are you?" said the English officer. He laughed, curtly. "I doubt that, young fellow! I do, indeed! However, you can come along with us as far as that. Then I'll wash my hands of you. But I can tell you that if you go on much further, you'll get into some fighting that isn't meant for boys!"

They made no reply, for as they understood their errand, they were not supposed to tell every officer they met what they were doing, but were to answer questions only when it was plain that not to do so meant that they would be prevented from reaching their destination.

It was not the easiest of tasks to manage the reversing of the supply train of omnibuses, but the officer in charge was efficient, and it was managed. When the convoy had turned around, he rode up beside the boys.

"Seen any signs of Germans?" he asked.

"Only at a culvert a few miles back," said Frank. He described the fight there as best he could, and the officer looked a little worried.

"As far as that, eh?" he said. "We hadn't heard of their being in that quarter at all. H'm!"

Then he rode on ahead, to what had, until a few moments before, been the rear of his train.

"He's doing well enough, now that he knows his way," said Frank in an undertone to Henri. "But I think he was in a bad way. I've got an idea that the Germans are behind us. Do you know what I think? It's funny for a supply train like this to be here without any escort of troops, isn't it?"

"Yes. I thought of that, too."

"Well, I believe he was supposed to meet a guard, and missed it. Suppose he'd run into the Germans?"

"Yes, that would have been a nice mess! I suppose some English soldiers would have gone hungry to-night!"

The road was rising a little, enough for them to feel the added pull in propelling their wheels. And now, at the crest of the little rise, they saw that the officer had dismounted. He had unstrapped a box from his machine and was setting it up. In a few minutes, as they reached him, he had set up a tripod-like machine, not unlike a surveyor's instrument, and was flashing a small mirror.

"h.e.l.lo!" he said. "Field heliograph kit. Ever see it before?"

"No, sir, but I know about it," said Frank, while Henri looked on admiringly. "I know the Morse code, too."

"Do you? Good! Then watch those answering flashes. Check off the message for me."

Harry obeyed, having spotted in that moment the answer of a similar instrument on a hill perhaps five miles away. He read off the Morse signs carefully, and the officer nodded.

"And that's all right," he said, with a sigh of relief. "They'll have an escort here for us as quickly as it can ride over. I suppose you know I signalled for that?"

"Yes, sir."

The officer was plainly puzzled by Frank and Henri. He could not quite understand what they were doing in what was decidedly disputed ground.

But he had not the instinct that would have prompted a French, and more especially, a German officer, to question them and, if he was not fully satisfied, to put them under restraint.

"All right. We'll be getting on," he said. "Ride along, now. I'm going back. Don't get out of touch. And if I'm not around when we get to the road where we are to turn off for Guise, stop them. They know you're guiding us."

He went off, with a great sputtering of his engine, and Frank and Harry rode along quietly. But Frank felt a strange uneasiness.

"I feel as if there was something wrong around here," he said.

"What do you mean, Frank? Everything's quiet now. Even the firing is not as heavy as it was."

"I know, but just the same, that's how I feel. As if there was something in the air. What's this--a village we're coming to?"

"Yes, and the crossroads where the 'buses must turn, for Guise is just beyond here, too."

"Doesn't look much like war, does it?" said Frank. "Look at that church.

I suppose it's been there for centuries. But the clock looks new, doesn't it?"

"Yes, and it's stopped, too," said Henri, with a laugh. "I suppose they are so excited about the war that they've forgotten to wind it properly."

"The time of day doesn't matter much just now," said Frank. "I think--"

He stopped short, staring as if fascinated at the clock. Then with a cry to Henri to wait for him, he turned and pedalled furiously back in the direction the officer had taken.

"Who is the commander?" he called to the soldier driver of one of the 'buses.

"Capting 'Ardy," replied the man.

"Thanks," Frank called, and went on as fast as he could. He met Captain Hardy coming toward him. Swiftly he told him what he had seen, and Hardy, tugging at his revolver, sped on. Frank followed but was left far behind, naturally, by the speed of the motorcycle. When he reached the church he looked up at the clock again. Captain Hardy's motorcycle was lying in the street, and Henri was staring at the church door greatly puzzled.

"What is the matter?" cried Henri. "The officer came back, jumped off his machine and tore into the church as if his life depended on it. He was pulling out his pistol, too. What--"

The sharp bark of a revolver interrupted him. It spoke three times and there was a cry from above. They looked up, to see the figure of a man dropping from the opening of the clock. A moment later Captain Hardy came down, reloading his revolver.