no telling," prophesied the inventor.

For several days the ship moved ahead at moderate speed. The machinery,
excepting for some minor accidents, worked smoothly. The gas bag did not
leak, which was the accident most dreaded, and it was not necessary to
run the gas generator, which proved a saving of the valuable chemical
from which the lifting-vapor was produced.

Now and then, when in need of water, the craft was lowered to the earth
in a secluded spot near a stream or lake, and the tanks were filled for
drinking and washing purposes. But so far, from the time of the hasty
flight, no one on the earth had spoken to the voyagers. Nor, so far as
was known, had their presence been noted, though the black speck in the
sky might have furnished plenty of talk all over the country for those
who observed it. The weather was pleasant, but it was noticed that it
was constantly growing colder.

One morning Jack, who was the first up, stuck his head out of the cabin
door before he had finished dressing. He quickly popped back again.

"Whew!" he exclaimed. "Colder than Greenland!"

"What's that about Greenland?" asked the professor, who had just
awakened.

"It's awful cold outside," said Jack, shivering from the remembrance.

Without a word the professor, wrapping a dressing-gown about him,
hurried to the engine room, where several thermometers were kept. One
was outside, and could be read through a glass side.

"No wonder you felt cold," he said to Jack, when he returned. "It's ten
degrees below zero!"

The boys hurried to complete their dressing. The professor did likewise,
as he was anxious to take some observations.

"Get out the fur garments," he said. "We must take no more chances now.
It will become colder rapidly, and ordinary clothes will be of no
protection."

The boys and the professor donned heavy fur coats, with immense gloves
and caps that covered all of their faces but the eyes. Then they went
outside. Jack was the first to look over the side of the ship. As he did
so he uttered a cry of astonishment.

Down below, about three-quarters of a mile, was a great white, snowy
waste. Giant mountains of ice were heaped on every side. It was a cold,
frosty silent world that the _Monarch_ was flying over. They had reached
the frozen north! They were at the beginning of the entrance to the land
of the Pole!

CHAPTER X

LOST IN AN ICE CAVE

"I'm not surprised that the thermometer is down below zero,"