A minute’s observation convinced Frank, however, that if they waited for this to happen, the frail support which was buoying Casper Blue and his mate up would have gone under long long before they could get within touch.

Already the second man was shrieking for them to hurry, because he could feel himself slowly but surely sinking; and he let them know that he could not swim a stroke.

Plainly, then, they must do something to quicken things, if they meant to be of any service to the two rogues, thus brought to a sudden halt just when escape had seemed most bright.

Frank remembered his engine.  But would it work under such strange conditions as this?  He quickly saw that the rear propeller was half buried in the water; and if it turned at all would have to churn things just as though they were in truth a queerly fashioned boat, instead of an airship, intended to mount to lofty heights, and vie with the eagle in his circling above the clouds.

Quickly, then, he started to make the trial; and Andy, seeing his movement, comprehended what he must have in mind; for he swung out in such fashion as to preserve a balance, and thus help things along as far as lay in his power.

What a sensation of relief that hum of the faithful little Kincaid engine brought in its train, as it once more took up the burden of its busy song.  Why, it seemed to Andy as though he could almost shout in sudden relief, when he heard it first, and saw the water flying from the partly submerged propeller.

But Frank was wise enough not to turn on full speed, knowing what a terrific strain this condition of affairs must be upon the entire fabric, flimsy at best; and if anything gave way it was all over with them; for if a chain is only as strong as its weakest link, a heavier-than-air flying machine certainly comes under the same category.

“We’re going it, Frank!” shouted Andy, when he saw that their speed had increased several hundred per cent, and that they were now heading straight for the partly submerged air navigators.

It never occurred to Andy that either of the men might offer the slightest objection to being rescued.  Frank looked a little further ahead; but even he could hardly believe that Casper Blue would prefer to drown rather than be saved.

What Frank was really concerning himself about more than anything else was how he could stow away the two fellows, once they found a chance to climb aboard the hydroplane; and whether he could get enough impetus from the engine with such an unusual load, to rise from the water, once he elevated his planes.

“Hurry! oh!  Hurry!  I’m going down!” cried the larger man.

Casper never said a word.  Possibly, being a swimmer, he did not feel the same degree of terror that his companion experienced.  Then again, he may have been coolly figuring on how he might turn the rescue to his own advantage in some way; for he seemed to have that little black box slung over his back by means of the strap; and it was easy to understand that it must, as Andy had guessed, contain something of considerable more value than a mere camera.