A half hour elapsed, then another. At length Penny's patience was rewarded. The alley doors swung open and a heavy truck which was covered over with a canvas top, emerged. The driver wore a cap and his head was bent low. In the semi-darkness of the dimly lighted street Penny could not catch even a glimpse of his face.
"I must follow that truck!" she thought tensely. "If Dad were here he would do it I feel sure! It's the only chance to gain real evidence!"
She waited at the window only long enough to see that the car had turned down Center Avenue. Scribbling a brief message to her father explaining what she intended to do, she left the note where he would find it in the event he returned to the office that night. Then she raced to the street.
By the time she had her roadster started the covered truck had disappeared. However, turning down Center Avenue, Penny caught it at the first traffic light. Satisfied that she would have no trouble in keeping it in view, she slowed down, falling back to a distance which was not likely to arouse suspicion.
Penny had no idea where the chase would lead, although the truck seemed to be driving directly out of the city. From the slow rate of speed at which it traveled, she thought that it must be heavily loaded with cargo.
"If I only knew what was hidden under that canvas cover I might have the solution to the mystery," she reflected. "I think I have it anyway, but I must secure definite evidence."
Penny was fully aware that she had launched herself upon a dangerous enterprise. In some manner Joe Franey had lost track of her completely, and she could no longer count upon his protection. In an emergency she must depend entirely upon her own resources.
Before Penny had traveled many miles out of the city she began to grow alarmed because her gasoline gauge showed that she had scarcely a gallon left. Although she had her purse with her, it contained only a dollar. She could buy about five gallons of fuel, but should the truck lead her much farther into the country, she easily might find herself stranded.
Apparently, the driver ahead faced a similar need for gasoline. At the next filling station he turned in.
Penny determined upon a bold move. At the risk of detection, she too drove into the station.