The dismay of the boys may be imagined. They were several miles from the camp and below them was nothing but the desolate expanse of the snow wastes that lay at the foot of the barrier range.

"Shall we have to go down?" asked Billy.

"Nothing else to do," said Frank with a grave face, "there's something wrong with the engine and we can't repair it up here. If we were not in this rarified atmosphere we could fly on the cylinders that are firing all right, but this atmosphere would not support us."

"Do you think it is anything serious?" asked the professor.

"I can't tell yet," was the grave reply, "that explosion sounded like a back-fire and that may be all that's the matter. In such a case we can drain the crank case and put in fresh oil; for if it was really a back-fire it was most likely caused by 'flooding.'"

Ten minutes later they landed on the firm, hard snow and lost no time in getting things in shape to spend the night where they were; for it was unlikely that repairs could be effected in time for them to fly back to the camp before dark. The canvas curtains at the sides of the aeroplane's body were drawn up, forming a snug tent. The stove was set going and soup and canned meats and vegetables warmed and eaten by the light of a lantern.

In the meantime Frank had discovered that the breakdown had been caused by a defect in the ignition apparatus which it would take some time to repair. Both he and Harry went to work on it after supper, however, and by midnight they had it adjusted.

They were just preparing to turn in, the professor and Billy having wrapped themselves in their blankets some time before, when a sudden sound, breaking on the stillness of the Antarctic night, made them pause. Both boys strained their ears intently and the sound came once more.

This time there was no mistaking it.

It was the same sound to which Rastus had called Frank's attention the night they were on watch outside the hut.

Pulling the curtain open, the boys gazed out, determined to unravel the mystery once and for all. The night was perfectly still except for the buzzing noise, and a bright moon showed them the snow lying white and undisturbed about them.

The sound did not proceed from the ground, that was evident, but from the air. The atmosphere seemed filled with it.

"What can it be?" exclaimed Harry.

"Look--look there!" shouted Frank, at the same instant clutching his brother's arm in his excitement.

Both boys gazed upward and as they did so a dark, shadowy form pa.s.sed above them far overhead. For an instant a brilliant light gleamed from it and then it vanished, going steadily eastward with the strange thrumming sound growing fainter as it receded.

The boys looked at each other in amazement and the words of Captain Hazzard flashed across Frank's mind.

"WE HAVE SOME VERY UNDESIRABLE NEIGHBORS AT CLOSE QUARTERS," the captain had said. Undoubtedly he was right.

"What did you make it out for?" asked Harry at length.

"A dirigible and no small one," was the reply, "and you?"

"Same here. You can't mistake the sound of an airship's engine. The question is what is the explanation of it all?"

"Simple."

"Simple, well I--"

"That aeroplane is the one which was bought in Europe. It is specially provided with radiators which electrically heat its gas, allowing it to navigate in these regions without fear of the gas condensing and causing the ship to descend."

"Yes, but whose is it? What are they doing in it?"

"The first question is easy to answer. That ship is the ship of the rival expedition."

"The j.a.panese one, you mean?"

"That's it. It must have been the light of it that I saw during the winter. I suppose they were experimenting with it then."

"Experimenting--what for?"

"For the work they are using it on to-night."

"And that is?"

"To forestall us in the discovery of the Viking ship and the South Pole."

CHAPTER XXIV.

SWALLOWED BY A CREVa.s.sE.

The early morning following the discovery of the night trip of the dirigible saw the Golden Eagle rising into the chill air and winging her way to the camp. The boys, as soon as they descended, hastened to Captain Hazzard's hut and detailed their adventures. As may be supposed, while both the leader of the expedition and the captain of the Southern Cross were deeply interested in the account of the flaming mountain and the prehistoric seal-like creatures, they were more deeply concerned over the boys' sighting of the airship.

"It means we have earnest rivals to deal with," was Captain Hazzard's comment, "we must set about finding the Viking ship at once. The search will not take long, for if she is not somewhere near where I have calculated she ought to be it would be waste of time to seek her at all."

Full of excitement at the prospect of embarking on the search for the ship, before long the boys dispersed for breakfast only to gather later on in Captain Hazzard's hut. The officer informed them that they were to fly to the position he indicated the next day and inst.i.tute a thorough search for the lost craft. The Golden Eagle was to carry her wireless and a message was to be flashed to the camp's wireless receiving station if important discoveries were made.

In the event of treasure being found, the boys were to at once "wireless" full details and bearings of the find and a relay of men and apparatus for saving the treasure would be sent from the ship to their aid on the motor-sledge. In the event of their not discovering the Viking ship they were to spend not more than three days on the search, wirelessing the camp at the end of the third day for further instructions.

The rest of that day was spent in putting the Golden Eagle's wireless in working order and stretching the long "aerials" above her upper plane. The instruments were then tested till they were in tune for transmitting messages from a long distance. The apparatus, after a little adjustment, was found to work perfectly.

Captain Hazzard warned the boys that, in the event of the rival expedition discovering them, they were on no account to resort to violence but to "wireless" the camp at once and he would decide on the best course to pursue.

"But if they attack us?" urged Frank.

"In that case you will have to defend yourselves as effectively as possible till aid arrives," said the commander.

Early the next day, with a plentiful supply of cordite bombs and dynamite on board for blasting the Viking ship free of the ice casing which it was to be expected surrounded her, the Golden Eagle soared away from the camp.

The boys were off at last on the expedition they had longed for. The professor accompanied them with a formidable collection of nets and bottles and bags. He had had prepared a lot of other miscellaneous lumber which it had been explained to him he could not transport on an aeroplane and which he had therefore reluctantly left behind. The engine worked perfectly and Frank antic.i.p.ated no further trouble from it.

As they sped along Harry from time to time tested the wireless and sent short messages back to the camp. It worked perfectly and the spark was as strong as if only a few miles separated airship and camp.

Nor did there seem to be any weakening as the distance between the two grew greater.

They pa.s.sed high above snow-barrens and seal-rookeries and colonies of penguins, the inhabitants of which latter c.o.c.ked their heads up inquiringly at the big bird flying by far above them. Their course carried them to the eastward and as they advanced the character of the scenery changed. What were evidently bays opened up into the land and some of them seemed to run back for miles, cutting deep into the many ranges that supported the plateau of the interior on which they had found the volcano.

These bays or inlets were ice covered but it was easy to see that with the advance of summer they would be free of ice. At noon, Frank landed the aeroplane and made an observation. It showed him they were still some distance from the spot near which Captain Hazzard believed the Viking ship was imprisoned. After a hasty lunch, cooked on the stove, the aeroplane once more ascended and kept steadily on her course till nightfall.

As dark set in, the boys found themselves at a spot in which the water that lapped the foot of the great Barrier washed--or would when the ice left it--at the very bases of the mountains, which here were no more than mere hills. They were cut into in all directions by deep gulches into which during the summer it was evident the sea must penetrate.