it is needless to say that the three adventurers lost no time in climbing up the gang ladder.

they were not a moment too soon.

frank had just cleared the rail when men burst from a thicket and sent a hail storm of bullets upward. they rattled on the steel hull of the air-ship.

“give it to ’em!” shouted nicodemus, angrily. “they deserve it richly.”

barney and pomp grabbed rifles and rushed to the rail.

but frank cried:

“wait! do not take life unnecessarily! we are safe just now. let us have a talk with them.”

taking care not to expose himself, frank shouted over the rail:

“ahoy down there!”

the firing had ceased, and a voice—that of dooley—came up in reply:

“well, what do you want?”

“what do you mean by this unprovoked attack upon us?”

“you are our enemies!”

“how do you prove that?”

“have you not come here to cheat us out of the gold buried under the lake, and which justly belongs to us?”

“neither of us are able to secure it just now.”

“well, but we shall be when the lake recedes.”

“in that event, why have we not as much right to a share of it as you?”

“that is neither here nor there. we lay claim to the gold, and don’t propose to divide with anybody.”

“what a pig-headed fellow,” said frank, angrily. “i’ve a mind to teach him a lesson!”

“i wish you would,” said nicodemus, eagerly.

“look here, my friend,” shouted frank, “let’s have no nonsense about this. that gold does not belong to you more than it does to us. for that matter, if i should choose, i could prevent your securing one jot of it. it is better for you to talk reason at once!”

“the only reason i will talk with you,” said dooley, obstinately, “is that you go on about your business and leave us and our affairs alone!”

frank turned from the rail.

“there is no use to talk with that fellow,” he said.

“what shall we do?” asked nicodemus.

“i will think up a plan,” said the young inventor, “in the meanwhile our best plan is to sail over to the other side of the lake and wait for it to recede. when it does we must be first on the scene where the gold lies.”

“good!” cried nicodemus; “if they attack us there——”

“then it will be time enough to retaliate!” declared frank.

and so the matter was settled.

the spectre sailed away across the arm of the lake and landed upon the summit of a small hill. it would have required several days’ journey for the dooley gang to have made the circuit around by the shore.

so the aerial voyagers felt safe. but this new development had put a different face upon matters.

however, if the lake should again recede, the air-ship could easily cover the three miles to the mound of gold before the men on foot could.

frank nor nicodemus either cared for but a fair share of the treasure. but the unfair and unreasonable conduct of dooley impelled them to seize the whole of it if they could.

another day and night passed and yet the lake presented the same smiling and smooth appearance. but that afternoon a curious thing happened.

barney, who was on deck, noted a curious yellow cloud rising from the west.

it mounted upward rapidly and soon had assumed angry proportions. the sun was hazy and the landscape took on a sickly appearance.

“on my word!” cried frank, “it looks as if we were going to have another andean storm.”

“jericho!” exclaimed nicodemus, “don’t say that. we had better anchor the air-ship hereabouts if that is true.”

all watched the cloud until the whole western sky was obscured. then there was a distant rumble, lightning shot across the heavens.

the ground began to rock and pitch violently, trees and shrubs swayed and were bent to the ground, the air-ship was tossed about like a pebble, and no man could stay on his feet a moment.

“an earthquake!” gasped frank.

“bejabers, wud yez luk at the lake!” cried barney.

all gazed in that direction and were dumfounded at the fearful spectacle presented.

the lake was like one mighty whirlpool, and went swirling about like a vast maelstrom. its waters seemed to be receding every second.

then the white sands of the shore line began to widen. islands appeared in its surface.

the truth was plain.

the transient lake, impelled no doubt by the earthquake, was about to make one of its periodical disappearances.

round and round swirled the waters. the ground had now ceased shaking.

but the yellow cloud continued to vomit lightning until it had passed beyond the zenith.

the aerial voyagers now all pressed to the rail and watched the wonderful phenomenon of the lake’s disappearance.

and, indeed, such it was.

lower and lower sank the swirling waters. suddenly they ceased their commotion.

then they became like glass. in a few moments they were like a mirror.

but rapidly now they sank. from the distance there came a strange noise like water running through a funnel.

this was the sound barney had remarked a few nights before. ten—twenty—thirty minutes elapsed, and then, as far as the eye could reach, no water was to be seen.

the glistening sands and rocks were rapidly drying in the sunlight. the transient lake had once more gone out.

how long it would remain was uncertain. hours, perhaps, and maybe days. however, this was the time for action.

“now for the mound of gold!” cried frank. “we shall have to get our bearings from you, captain.”

“all right!” cried nicodemus, studying the shore a moment. “north by east, skipper!”

barney started the air-ship forward in the direction indicated. soon the lake bed was beneath them.

as the spectre sailed on over the sands, suddenly nicodemus clutched frank’s arm and said:

“there they are!”

a body of men were seen rushing rapidly across the lake bed. they were dooley and his gang.

they could not help but see the air-ship, and even at that distance fired a number of shots at it. but of course they were without effect.

meanwhile the spectre rapidly left them out of sight.

three miles was a distance easily covered by the air-ship, and suddenly nicodemus clutched frank’s arm and cried:

“there is the spot! between those round hills! see it?”

frank sprang to the keyboard and taking the lever from barney, steered the air-ship down between the little hills of wet sand.

a moment later the spectre rested on terra firma.

and there was the stone mound for a fact. it was just as the party had left it years before, so nicodemus declared.

the captain was so excited that he hardly knew what to do.

he finally managed to procure a shovel and with frank climbed over the air-ship’s rail.

in a few moments the two men were alongside the mound.

the captain hastily dropped his shovel and climbed to the top of the mound. he removed the stone cap and peered down into the place.

he remained in this position for a long time and frank had thought of speaking to him, when he suddenly arose.

his face, as he turned it toward frank, was white as chalk.

“by whales!” he said huskily, “it is not there, frank!”

“not there?”

“no.”

“what do you mean?”

“the gold—the gold is gone!”

frank was so overcome with surprise and dismay that for a moment he could say nothing.

then nicodemus climbed down and faced his companion. his expression of face was strange to see.

“are you sure it is not there?” asked frank.

the captain nodded.

“i could see every corner of the cavity,” he said. “it is not there.”

“what! who could have removed it?”

the captain gave a sudden start and smote his knee.

“why did i not think of it?” he exclaimed. “of course the ship’s crew did it while langley and i were on our way to the camp.”

frank’s face lit up.

“that is probably the truth,” he agreed. “why didn’t we think of it before? doubtless they removed the gold just before they were overtaken and drowned.”

“in that event——”

nicodemus paused. frank finished the sentence.

“what did they do with it?”

this was a question not easily answered. a close search of the vicinity failed to disclose any evidence of gold.

if it had been deposited just outside the mound and left there, no trace of it could be found.

on the other hand, it did not seem possible that the fated men could have carried such a quantity of the precious metal any distance away with them.

if they had, then it must be found with their bones somewhere in the vicinity.

there was a possibility that in the intervening years some other parties had visited the spot and carried off the gold. but this was only a faint chance.

however, one fact was patent.

the gold was gone.

what had become of it? certainly it could not have taken wings and flown away of itself. the two gold seekers were completely mystified, and knew not what to do.