"It is too bad that the weather threatens so," remarked their captor, who was politeness itself, to his prisoners; "otherwise we should now be in the air on our way back to my camp. In three more trips we shall be able, however, to carry off the rest of the treasure. We were well repaid for keeping our eyes on you."
The boys answered something, they hardly knew what. Frank in his nervousness looked at his watch. The strain was becoming painful. At last, to their intense relief, they rose from supper and the little officer shut himself in his own cabin. Outside, the boys could hear the feet of the two armed sentries crunching on the snow.
"The outrush of gas will stupefy them," whispered Frank, "we shall have nothing to fear from them after the explosion takes place."
"When is it due?" gasped Billy, with a ghastly attempt at a smile.
"At any moment now. It is impossible to calculate the exact time. But within half an hour we should know our fate."
Silently the boys and the professor waited, although the scientist was so nervous that he strode up and down the cabin floor.
Suddenly the silence was shattered by a loud shout from the engine room.
"The gas! The gas! We are—"
The sentence was never finished.
There was a sudden convulsion of the entire fabric of the big dirigible—as if a giant hand from without were shaking her like a puppy shakes a rat.