Bird who's doing the thinking. But perhaps it would be silly in me sleeping out here tonight. I'd better be traipsing back to bed right now, because, you see, I'm only half dressed and it's chilly."

"Good. I'll see you to the house, because I've got to walk home, now that my wheel has gone up the flume," remarked Frank.

"What's the matter with you using mine?" demanded the other. "That plug you put in holds dandy, and there's nothing the matter with it right now. Same old place, under the side porch here. Guess the lamp is on the bum, but you hardly need that. If a cop holds you up, explain what happened."

"All right, I guess I might as well ride as walk. But I hope I get my wheel back. It's nearly new, you know, and cost a heap," Frank remarked, as he dove under the stoop, to presently appear dragging the other bicycle.

"Apply to Puss and Company for further information," called Andy, holding the door open a crack to shoot the words out and then closing it.

Frank, laughing at the obstinate ways of his chum, pushed the machine out to the road and was soon moving along. Evidently he lacked the same confidence in Andy's wheel that he felt in his own, for he made no attempt to speed as he went toward town and home.

Fortunately he met no policeman, who might ask impertinent questions as to just why he was riding after dark without a light. And reaching home he found his father sitting up in his office waiting to hear his report.

Dr. Bird was quite satisfied with what Frank had to say in regard to the condition of the sick girl. He knew that the boy was well up in medicine, even though he had never tried to push him in the least. Frank gave evidence of being what is known as a "natural born doctor," keenly alive to everything pertaining to surgery. More than once he had set broken limbs for dogs and cats and done it in a manner that aroused the warmest praise from his father, who, deep down in his heart, knew the boy had it in him to become a famous surgeon, if he kept along in this path when he came to take up his life pursuit.

Frank believed he ought to tell about the dastardly attempt to destroy the monoplane. And, of course, the good doctor, who always thought the best of people, was horrified to hear his story.

He shook his head sadly after Frank had finished.

"I don't know what people are coming to nowadays," he remarked. "No matter who did that mean act, it was wicked. Man or boy, he ought to be severely punished for it. The rights of property seems to be getting less respect every year. It puzzles me to lay the blame for this spirit at the right door. But things were not so in my young days, Frank. We live in fast times, my boy, fast times!"

Frank thought so himself, as he went off to his room. Imagine his father, some forty years ago, ever dreaming of building an air-ship and speeding through the upper currents, perhaps thousands of feet above the earth, at the rate of a mile a minute! And yet that was what he and Andy had been doing, thinking nothing of the feat, as they became accustomed to its performance.

Fast times, indeed!

Frank did not allow the startling incidents of the night to keep him awake. He knew just how to get a grip on himself and put all these things out of his mind, once his head touched the pillow.

Time enough in the morning to begin worrying about that lost wheel and trying to figure out who the firebug could have been.

At breakfast Frank had to go over the whole story again for the benefit of Janet, who had heard enough about it from the doctor before her brother came down to whet her appetite for more.

Frank could see that she shared the suspicions entertained by Andy. Janet knew Puss Carberry of old and despised him most heartily. At one time he had taken a great liking to Frank's pretty sister, but when she learned what his nature was Janet had cut him dead on the street. And from that day on she had believed Puss capable of almost anything.