And so the conspirators set to work. For a week they ransacked father’s library, reading up on aëronautics generally, studying every pamphlet and authority they could lay their hands on. There was one thing that especially confused them. Each man supported a “totally different theory,” as Ernie plaintively complained. It was extremely trying, especially as dear father had worked almost entirely in his head, leaving very few directions or specifications to guide them to the right trail. At last Geof declared that he thought they would never get anywhere through books; that their one hope lay in practical experiment. Ernie quite agreed with him, and after that they spent hours in the airship, mastering as they supposed the intricate details of motor, steering apparatus, and machinery. Geoffrey even discovered what he considered a slight error in the automatic system of shifting weights. Finally, last Saturday morning, behind closed doors, the motor was taken out and started up. It ran like a dream. They came to the bold conclusion that nothing remained to hinder an experimental ascension! [Illustration: And so the conspirators set to work] All this time it must be understood mother and I had not the faintest suspicion as to what was going on. We knew that there was “a secret” in the workshop, “a beautiful surprise for the family.” Just how great a surprise, however, we neither of us dreamed. Yesterday afternoon was the time set for the ascension. How the two children managed alone to raise the heavy machine from the workshop floor to the roof by means of the trap-door and pulleys father had used in the previous experiment will always remain a mystery. But they did! At last it stood among the chimney-pots, with rakish sails and scarred sides, looking for all the world like “a tipsy eagle-bird,” as Ernie enthusiastically declared. Even by this time neither of the little idiots seems to have had the least realisation of _what_ it was they were attempting. On the contrary, they were quite wild with the frolic and excitement of the thing. Geof straightened out the sails, and opened the manœuvre valve. _Tick-tock_, sounded the motor. The framework quivered response. “Hold on there,” shouted Geoffrey, as he ran to attach the short length of anchor line to a hook in the stone coping at the front of the roof. “Don’t get in yet, Ernie. The place in the middle belongs to me. I’ve got to manage the steering gear.” “All right,” Ernie answered, climbing over the side, nevertheless. “I’m just looking to see which of these levers starts her.”