Everyone was bustling about. The professor was busiest of all. He went from one machine to another; from this apparatus to that, testing here, turning wheels there, adjusting valves and seeing that all was in readiness for the generating of the powerful gas. As the airship was half round on the bottom and as it rested in a sort of semi-circular cradle; it brought the entrance some distance above the ground. To make it easier to get in and out while preparations for the trial were going on, Bill and Tom had made an improvised pair of steps, which were tied to the side of the ship with ropes. Up and down these the professor, the boys and Andy went, taking in tools and materials, and removing considerable refuse which had accumulated during the building of the craft. Finally all was in readiness for starting the making of the gas. The ship was not wholly complete and no supplies or provisions for the long voyage had been taken aboard. The _Flying Mermaid_ was about a ton lighter than it would be when fully fitted out, but to make up for this the professor had left in the ship a lot of tools and surplus machinery so that the craft held as much weight as it would under normal conditions. If the gas lifted it now it would at any other time. “Start the generator,” said Mr. Henderson, to Mark. “We’ll soon see whether we are going to succeed or fail.” The boy turned a number of levers and wheels. The machine which made the powerful vapor was soon in operation. The professor had already added enough of the secret compound to the tank containing the other ingredients, and the big pump was sucking in air to be transformed into the lifting gas. The boys and the professor were in the engine room. Andy Sudds, with Bill and Tom, had taken their places in the living room, to more evenly balance the ship, since the things in it were not yet all in their proper places. As for Washington he was busy running from the shed to the ship with various tools and bits of machinery the professor desired. The gas was being generated rapidly. Throughout the ship there resounded a hissing noise that told it was being forced through the pipe into the aluminum shell above the ship proper. “I wonder how soon it will begin to lift us,” said Mark. “It will take about half an hour,” replied Mr. Henderson. “You see we have first to fill the holder completely, since there is no gas in it. After this we will keep some on hand, so that it will only need the addition of a small quantity to enable the ship to rise.” He was busy watching the pointer on a dial which indicated the pressure of the gas, and the lifting force. The boys were kept busy making adjustments to the machinery and oiling bearings. Suddenly, throughout the length of the craft there was felt a curious trembling. It was as though the screw of a powerful steamer was revolving in the water. “What is it?” asked Jack.