from the headquarters of the Crown Prince. That accounted for the numerous marks of car-tires which Tom had discovered on the drive. This lonely house by the roadside on the way to Metz was a nest of spies. Perhaps Carl Potzfeldt might be the chief, through whom negotiations were conducted and lesser agents sent forth.
Jack had got no further in his deduction when he saw the tall man returning. He carried a bundle that was wrapped in a cloth, and depended from his hand by means of a heavy cord, or some sort of handle.
This he set down on the landing, while he passed further words with the captain; and now it was Potzfeldt who asked the questions, as though he wished to learn how things were going at the front.
Between queries and guttural replies the hidden air service boys heard a series of sounds that gave them sudden light. Jack's hand pressed on Tom's arm, as though in this manner he wished to call the attention of the other to the noise.
Many times both of them had listened to similar sounds while watching some pigeon on the barn roof dare a rival to combat, or when wooing his mate. And as they could easily trace this to the covered package which Carl
