had discovered it."

The prisoner of the Terrorists made no reply, but turned away in
silence to watch the rapidly receding building, in the angle of which
the flames were still raging furiously. A few minutes later the
_Ariel_ had rejoined her consorts. Her captain at once went on board
the flagship to make his report and deliver up his prisoner to Natas,
who looked sharply at him and said--

"Professor, will you give me your word of honour to attempt no
communication with the earth while it may be found necessary to
detain you? If not, I shall be compelled to keep you in strict
confinement till it is beyond your power to do so."

"Sir, I give you my word that I will not do so," said the Professor,
who had now somewhat regained his composure.

"Very well," replied Natas. "Then on that condition you will be made
free of the vessel, and we will make you as comfortable as we can.
Captain Arnold, full speed to the south-westward, if you please."

CHAPTER XXVIII.

A SKIRMISH IN THE CLOUDS.

A few minutes after two on the following morning, that is to say on
the 28th, the electric signal leading from the conning-tower of the
_Ithuriel_ to the wall of Arnold's cabin, just above his berth,
sounded. As it was only permitted to be used on occasions of urgency,
he knew that his presence was immediately required forward for some
good reason, and so he turned out at once, threw a dressing-gown over
his sleeping suit, and within three minutes was standing in the
conning-tower beside Andrew Smith, whose watch it then happened to
be.

"Well, Smith, what's the matter?"

"Fleet of war-balloons coming up from the south'ard, sir. You can
just see 'em, sir, coming on in line under that long bank of cloud."

The captain of the _Ithuriel_ took the night-glasses, and looked
eagerly in the direction pointed out by his keen-eyed coxswain. As
soon as he picked them up he had no difficulty in making out twelve
small dark spots in line at regular intervals sharply defined against
a band of light that lay between the earth and a long dark bank of
clouds.

It was a division of the Tsar's aerial fleet, returning from some
work of death and destruction in the south to rejoin the main force
before Berlin. Arnold's course was decided on in an instant. He saw a
chance of turning the tables on his Majesty in a fashion that he
would find as unpleasant as it would be unexpected. He turned to his
coxswain and said--

"How is the wind