Before they ceased speaking, the first shot was fired, then another and another, all of them trial shots, as one of the soldiers explained. Then another battery to the left, heretofore unnoticed, began to fire, while one posted higher up on the right, and two more beyond, chimed in. This was a new experience to the boys, as the Belgians, with whom they had formerly associated, were lacking in field pieces compared with those they now saw and heard. The challenge was promptly accepted by the Germans, and within fifteen minutes the whole crest to the east of them, probably two miles distant, seemed to be ablaze. It was singular, however, that none of the missiles fell near the troops where the boys were located. On a little elevation to the right of the kitchen vans the boys noticed a group of officers with field glasses, intently scanning the rear of their position. "The blooming blokes are shooting too far," remarked a soldier. "Why are they doing that?" asked Alfred, who overheard him. "Oh, they'll get the range in time," he replied. "There's the first airship, to give them a pointer," said the corporal, as an airplane appeared in sight. "Two of our machines are coming up," shouted Ralph. "Now there'll be a fight." Ralph was right. The two English machines steered straight for the oncoming Fokker, the latter veering to the left in time to prevent too close a meeting. Within a minute three more German machines came into sight, their appearance being answered by four of their own machines, which came up from the rear, and sailed straight across above them to the German positions on the crest of the hill. They were much higher than the German machines, the reason being that the enemy had a number of anti-airship guns to meet any flyers who dared to cross the line at a low altitude. It was, indeed, an interesting sight to observe the numerous airplanes, which were employed by the two forces. They seemed to be flying back and forth in the utmost confusion, and, frequently, puffs of smoke would indicate that they were firing at each other. The corporal with whom the boys talked seemed to be unusually well informed, and quite a number of the soldiers addressed him for information. "How do they recognize an enemy?" asked Ralph. "Oh, that's an easy matter," he replied. "I suppose the flyers in the air do it on the same principle that we are in the habit of recognizing automobiles. Why, I have a lad not ten years old who can tell the make of almost any auto the moment he sees it. Generally, if the make of the plane is similar to those attached to the aviator's side he is able to recognize it by the special mark it carries." "What mark do you mean?" asked Alfred. "Why, the English and the French mark is in the form of a circle, generally three, each circle being of a different color," was the answer. "There is one,--why, it's red, white and blue," said Ralph. "That's correct,--those are the colors of the Allies."