In places the steep banks were clothed with green trees, pines and larches, and at the foot of the escarpments there shot upwards innumerable white fumaroles, the vapor escaping from the soil as from an enormous reservoir in which the water is kept in permanent ebullition by subterranean fire.
The cook might have seized the opportunity of securing an ample supply of trout, the only fish the Yellowstone Lake contains in myriads.
But the "Albatross" kept on at such a height that there was no chance of indulging in a catch which assuredly would have been miraculous.
In three quarters of an hour the lake was overpassed, and a little farther on the last was seen of the geyser region, which rivals the finest in Iceland.
Leaning over the rail, Uncle Prudent and Phil Evans watched the liquid columns which leaped up as though to furnish the aeronef with a new element.
There were the Fan, with the jets shot forth in rays, the Fortress, which seemed to be defended by waterspouts, the Faithful Friend, with her plume crowned with the rainbows, the Giant, spurting forth a vertical torrent twenty feet round and more than two hundred feet high.
Robur must evidently have been familiar with this incomparable spectacle, unique in the world, for he did not appear on deck.
Was it, then, for the sole pleasure of his guests that he had brought the aeronef above the national domain?
If so, he came not to receive their thanks.
He did not even trouble himself during the daring passage of the Rocky Mountains, which the "Albatross" approached at about seven o'clock.
By increasing the speed of her wings, as a bird rising in its flight, the "Albatross" would clear the highest ridges of the chain, and sink again over Oregon or Utah, But the maneuver was unnecessary.
The passes allowed the barrier to be crossed without ascending for the higher ridges.
There are many of these canyons, or steep valleys, more or less narrow, through which they could glide, such as Bridger Gap, through which runs the Pacific Railway into the Mormon territory, and others to the north and south of it.
It was through one of these that the "Albatross" headed, after slackening speed so as not to dash against the walls of the canyon.
The steersman, with a sureness of hand rendered more effective by the sensitiveness of the rudder, maneuvered his craft as if she were a crack racer in a Royal Victoria match.
It was really extraordinary.
In spite of all the jealousy of the two enemies of "lighter than air," they could not help being surprised at the perfection of this engine of aerial locomotion.
In less than two hours and a half they were through the Rockies, and the
"Albatross" resumed her former speed of sixty-two miles an hour.
She was steering southwest so as to cut across Utah diagonally as she neared the ground.
She had even dropped several hundred yards when the sound of a whistle attracted the attention of Uncle Prudent and Phil Evans.
It was a train on the Pacific Railway on the road to Salt Lake City.
And then, in obedience to an order secretly given, the
"Albatross" dropped still lower so as to chase the train, which was going at full speed.
She was immediately sighted.
A few heads showed themselves at the doors of the cars.
Then numerous passengers crowded the gangways.
Some did not hesitate to climb on the roof to get a better view of the flying machine.
Cheers came floating up through the air; but no Robur appeared in answer to them.
The "Albatross" continued her descent, slowing her suspensory screws and moderating her speed so as not to leave the train behind.
She flew about it like an enormous beetle or a gigantic bird of prey.
She headed off, to the right and left, and swept on in front, and hung behind, and proudly displayed her flag with the golden sun, to which the conductor of the train replied by waving the Stars and Stripes.
In vain the prisoners, in their desire to take advantage of the opportunity, endeavored to make themselves known to those below.
In vain the president of the Weldon Institute roared forth at the top of his voice,
"I am Uncle Prudent of Philadelphia!"
And the secretary followed suit with,
"I am Phil Evans, his colleague!"
Their shouts were lost in the thousand cheers with which the passengers greeted the aeronef.
Three or four of the crew of the "Albatross" had appeared on the deck, and one of them, like sailors when passing a ship less speedy than their own, held out a rope, an ironical way of offering to tow them.
And then the
"Albatross" resumed her original speed, and in half an hour the express was out of sight.
About one o'clock there appeared a vast disk, which reflected the solar rays as if it were an immense mirror.
"That ought to be the Mormon capital, Salt Lake City," said Uncle Prudent.
And so it was, and the disk was the roof of the Tabernacle, where ten thousand saints can worship at their ease.
This vast dome, like a convex mirror, threw off the rays of the sun in all directions.
It vanished like a shadow, and the "Albatross" sped on her way to the southwest with a speed that was not felt, because it surpassed that of the chasing wind.
Soon she was in Nevada over the silver regions, which the Sierra separates from the golden lands of California.
"We shall certainly reach San Francisco before night," said Phil Evans.
"And then?" asked Uncle Prudent.
It was six o'clock precisely when the Sierra Nevada was crossed by the same pass as that taken by the railway.