Frank hastened to his side and saw, ranged side by side, a number of wooden boxes about a foot square labeled "Dangerous."
"I guess that's the stuff all right, Ben," he said, "bear a hand and we'll drag it out. Only be very careful of it. It is probably a high explosive if not handled delicately."
One by one the boxes were transferred until two dozen of them stood on the beach, set in soft sand. Then a sudden difficulty flashed into Frank's mind. Constantio had said "equal parts of sand, water and the powder," but he had not said how much these equal parts were to be. The only thing to do was to experiment.
Fortunately the massive steel cylinders, in which the gas was to be generated, were provided with gauges to register the pressure. One thousand pounds were marked as top measure, so Frank assumed that somewhere about 800 pounds would be enough.
The first mixture they tried only registered three hundred pounds, but by gradually increasing the amount of powder they at last hit upon the required strength, and were ready to start on the work of inflation.
They had six cylinders full of the gas. Not enough to fully inflate the bag, but enough, Frank calculated, to render it sufficiently buoyant to carry the reduced weight it would be called upon to convey now that the crew was gone.
The inflation nozzle was connected with cylinder after cylinder, till the bag became so buoyant that it was necessary to weight the machine down with heavy stones. At last the cylinders were emptied and the great bag, expanded by the warm sun, swelled up till it seemed it must burst. The expansion of gas by the sun was one of the things Frank had counted on when filling the bag, and he was glad to see his theories work out right. The treasure bags were hastily laden on to the craft and then the boys, standing on the lower framework, one on each side, while Ben stood in the pilot-house, started to kick off the weights that restrained the ship from rising.
They had not cast off more than half a dozen before the ship gave a mighty bound upward that threatened to throw them off her frames and before they could catch their breath they had shot up 1,200 feet or more. Hastily clambering aboard and laughing at the sudden jump, the boys got the engine going and shaped a course that would bring them over the spot where they had left the Bolo.
They held steadily on their course that day and the next. Early in the morning of the second they encountered a surprising incident. Frank, who was on lookout, hailed "Air-ship ahead."
And there, sure enough, heading northward, was a big red dirigible coming toward them like the wind.