West of them there was a deep and narrow gorge between high mountains. Down through this with the speed of a race horse there came a tumbling funnel-shaped cloud.

So swift did it come and so unexpected was it, that the voyagers were wholly unprepared and taken by surprise.

But in that swift instant all realized what it meant.

A storm in the Andes is a terrible affair. Darting among the great peaks it sweeps with cyclonic fury through the valleys, carrying destruction in its path until its force is spent.

Such was now swooping down upon the air-ship like a mighty bird of prey.

There was no time to get out of the way to make any change in course. Barney and Pomp sprang into the forward cabin, shutting the door behind them.

Frank and the captain leaped into the pilot-house.

They were not a moment too soon.

The storm struck the air-ship like a thunderbolt. What saved it from being totally destroyed was a literal wonder.

It was utterly impossible for any of the voyagers to recall what transpired in that period while the air-ship was in the clutches of the hurricane.

Frank had turned the lever to send the air-ship above the storm. But he had not done this quickly enough.

The weight of the storm had kept it down. It was whirled about like a top, dashed hither and thither madly.

The voyagers were hurled about the interior like puppets. Nothing could be seen beyond the windows, nor could they even guess where they were being whirled.

How long this lasted they were never able to guess.

Frank gave up all as lost. He had not the slightest faith that the helices and masts would successfully resist the shock.

When the wind ceased for a moment its force, he expected to be dashed to atoms upon the earth below.

But the unexpected often happens.

The storm departed almost as suddenly as it came. The wind died out, the air-ship ceased whirling and the blackness disappeared.

The sun burst forth once more and all was again light.

Barney crawled out from under a heap of wreckage, and Pomp relaxed a grip on the skylight frame.

Frank crept out from under the electric keyboard, and Captain Beere appeared from the wine closet with a fragment of the door in his hand.

“Great whales!” he bellowed; “I’ve sailed the high seas in many a hot typhoon, but I never saw the equal of this one.”

“We seem to be afloat,” said Frank, with an attempt to straighten a crick in his neck.

Then he glanced out of the window and saw that the helices were revolving furiously.

The air-ship was speeding upward and frost was forming on the windows. But Frank quickly checked the helices.

The Spectre sank a mile or two very quickly and continued going down until the gauge recorded an altitude of two thousand feet. Then all went to the windows and gazed out.

The scene spread below was a startling one.

Instead of tropical forests or craggy heights there was visible nothing but a mighty expanse of water.

As far as the eye could reach in all directions this was all that could be seen.

Astounded, the voyagers regarded the scene for a time in silence.

Then Frank cried:

“On my word I believe we have been carried clean over the continent and out to sea.”

“Begorra, it’s the ocean,” averred Barney.

“Golly! we must hab trabeled pretty fast,” declared Pomp.

But Captain Nicodemus studied the scene below critically and then said decidedly:

“It is the big lake again.”

“What!” exclaimed Frank.

“It is the lake we’re looking for,” declared the mariner. “We have made it at last, thanks to the storm.”

Frank was astounded.

“I have never heard of any body of water as large as this in Central South America,” he said.

“Well, I can’t help that,” said the captain logically; “here it is.”

“You say this is the Transient Lake?”

“I do.”

With redoubled interest the aerial voyagers now gazed upon the great body of water below.

None were disposed to dispute the old captain, and Frank, scanning the horizon more intently, saw the faint haze of the shore.

“By Jove!” he exclaimed, “it is a big body of fresh water. However, here we are. Now to recover the treasure. What course shall we pursue, Captain Nicodemus?”

The old captain was much excited.

He walked up and down the deck, rubbing his hands and studying the horizon.

At length he said:

“Steer west, skipper—due west.”

Frank complied with this.

The air-ship sailed westward for several hours. And still the great expanse of water lay beneath. Frank reckoned it full two hundred miles from shore to shore.

At length the western shore became well defined.

And now as they approached it, a curious thing was observed. The waters of the lake seemed to have risen and flooded a part of the country beyond, so that the water was full of trees and tangled brush.

Nicodemus was surprised.

“This was not the case when we were here,” he declared; “it is curious. I can see nothing of the shore where we rested.”

“It is under water,” said Frank.

“Yes.”

“Well, can you guess at the locality?”

“Yes. Yonder peak was north by west. Here—here was about the spot where we camped. The gold mound should be three miles or more in that direction out in the lake.”

These facts established, the interest of all reached fever height. The air-ship was held stationary over that spot.

A discussion was now held as to the best course to be pursued.

“Of course we can do no better than to wait for the disappearance of the lake,” said Nicodemus. “That may happen at any time.”

“Is there any regularity to the event?” asked Frank.

“That I cannot say,” replied the captain. “We were here but a few weeks, and the lake disappeared and reappeared only once in that time.”

“I have to suggest, then,” said Frank, “that we sail over to that distant peak and wait there until some change comes.”

The peak in question overhung the lake, so that its disappearance could be instantly noted.

The plan was adopted.

Soon the Spectre was resting upon this mountain height. A good spot was found for the air-ship to rest.

Night now shut down rapidly. As all on board were intensely weary, they retired early to rest.

Barney was left on guard. The night was black as Erebus.

The Celt sat in the bow of the air-ship where he could easily reach the valve of the searchlight. There was no danger of his sleeping at his post.

As the hours wore on, Barney relaxed his vigilance somewhat. It was past the hour of midnight when an unlooked for thing occurred.

A curious sound came from the distance over the lake.

It was a strange sullen roar and a curious sound like the suction of water through an orifice.

“Begorra, that’s a queer sound,” he muttered. “Phwativer can it be?”

He listened again.

Then he acted upon impulse and turned on the full glare of the searchlight. The pathway of radiance shot out over the lake.

Barney had expected to see the shimmer of the waters.

But he was startled at the fact that they were not visible. The glare of light showed only sand and rocks and weeds.

The pathway of radiance extended for fully a mile. But nowhere was water visible.

“Tare an’ ’ounds!” gasped the Celt, “phwat the divil is the maning av that?”

For a moment he was stupefied.

Then the realization of a startling truth dawned upon him. He sprang to his feet.

“Mither presarve us!” he muttered, “the lake has gone an’ disappeared, as the captain said it would.”

He started for the cabin to call Frank and the captain. But before he reached the cabin stairs second thought impelled him to change his mind.

“Divil a bit,” he muttered; “there’s no nade av that. Shure, they’ll see the thing fer thimsilves to-morrow, an’ to chate thim av their sleep now wud be foolish enough.”

So he went back to his post.

He flashed the rays of the searchlight over the sandy bed of the Transient Lake. Satisfied that there was no error, and that the lake had really departed, he finally shut off the rays and abandoned himself again to his duty of pacing the deck.

The night wore on.

Usually Pomp would come on duty to relieve him before the morning hour. But this night Barney had agreed to keep the full watch, if Pomp would do the same the next night, giving each a chance for a full night’s sleep.

Toward morning Barney relaxed his vigilance and sat in the cabin doorway, communing with his thoughts. It was just daylight when he heard a movement below.

He saw Frank Reade, Jr., cross the cabin.

Instantly he cried:

“Och hone, Misther Frank, shure I’ve a surproise for ye. The lake has gone intoirely, sor!”

This assertion created a sensation.