Among the many who answered the summons of Roque in the intelligence bureau, the young aviators were most interested in a score of blond, blue-eyed, well-set-up Saxons, renowned as Zeppelin navigators, who were destined to guide the "terrors of the air" in furtherance of another raiding plan taking form in the fertile brain of the eminent promoter of trouble for the enemy. While the boys had faith only in the heavier-than-air machines, they conceded that the risk taken by the Zeppelin crews entitled the latter to brush elbows with the crack flyers of the other kind of bird craft. It was also true that when a Zeppelin got anywhere it was a tremendous factor in war. And it was no question but that the Fatherland had gone Zeppelin mad. Woe betide the hostile airmen who dropped the bomb on the Zeppelin works at Friedrichshaven if Roque had the means of catching them. It was only another score that he had marked up against Ardelle, whom the master agent of the empire charged with planning this destructive performance. "Roque said he was going to show us where these gas cruisers grow," Henri advised Billy one evening, getting this news while his chum was engaged in an argument with a Zeppelin worker. "Something I've been wanting to see," exclaimed Billy. "I owe something to a Zeppelin, even if it is like a balloon." This last was a sort of side swipe at the man who had been on the other side of the argument. "There is one thing sure, these dirigibles can't camp out." This was Billy's first remark in Friedrichshaven. He was peering into a big steel-framed shed with a glass roof which housed one of these grim engines of the air--a great cylinder flanked by platforms. This newest of the huge airships was about the length of a first-class battleship, and the opinion of the young aviator that it could not drop anywhere and everywhere like the aëroplanes he drove was not a prejudiced one. When Henri had a look at the powerful motors he was impressed with their capacity to drink up petrol at a most appalling rate. "What's her top speed?" he asked one of the big fellows who had traveled over from Berlin with them. "Forty-five miles in the calm," was the reply. "Gee!" exclaimed Billy. "We could get a seaplane home for breakfast while they were waiting supper on you!" "Yet," claimed the Zeppelin expert, "it's the car they're all afraid of." "It certainly does look like a scaremark," admitted Henri, who remembered a certain evening on the Belgian coast, when he was one of the company aboard a stranded hydroplane dragged ashore by the swinging anchor of a Zeppelin, which loomed overhead like a cloud, and buzzed like a million bees. A gang of at least a hundred men swarmed about the shed when the order issued for a trial trip of the new super-Zeppelin, a sample of the fleet in course of building, and Roque carefully noted every detail of equipment.