the bag, not a trace could be seen. Even as the onlookers gazed
horror-stricken at the sudden blotting out of the dirigible before
their eyes the loud roar of the explosion of its superheated gas
reached their ears.

"Every pound of steam you've got, chief," sharply commanded Captain
Barrington, almost before the dirigible vanished, "we must save them
yet."

The old engineer dived into his engine room and the Southern Cross,
with her gauges registering every pound of steam her boilers could
carry, rushed through the water as she never had before in all her
plodding career.

"Heaven grant we may not be too late," breathed Captain Hazzard, as,
followed by the boys, he clambered out of the rigging. "If only they
can swim we may save them."

"Or perhaps they have on life-belts," suggested Billy.

"Neither will do them much good," put in a voice at his elbow grimly.
It was Professor Sandburr.

"Why?" demanded Frank, "we will be alongside in a few minutes now and
if they can only keep up we can save them."

"The peril of drowning is not so imminent as another grave danger they
face," spoke the professor.

"What's that?"