"Ready?" sang out Jimsy.
"All ready," came the reply.
"Then fire!"
Simultaneously with Jimsy's order came a deafening report. In that confined space it sounded as if a huge cannon had been fired. Roy staggered back under the "kick" of the heavy charge.
"Once more," he announced.
Again a sonorous report sounded, but this time a section of the door was blown right out of the framework. The daylight streamed in through it.
"Now then for the test," cried Roy. "Come on, Jimsy."
The two boys placed their shoulders to the door. With a suddenness that was startling, it burst open, and they faced freedom. The lock had been fairly driven from its hold by the twice repeated charge of shot.
The young aviators were free once more. But it remained to be seen if the men who wished them harm had wrought their vengeance on the Golden Butterfly.