adder fell over the side.

"Come, Professor, there is no time to be lost. I shall give the order
to fire in one minute from now."

He took out his watch, and began to count the seconds. Ten, twenty,
thirty passed and the Professor stood irresolute. Two of the
_Ariel's_ guns pointed at the gables of the Arsenal, and two swept
the crowded space in front.

Konstantin Volnow knew enough to see clearly the frightful slaughter
and destruction that twenty seconds more would bring if he refused to
give himself up. As Mazanoff counted "forty" he threw up his hands
with a gesture of despair, and cried--

"Stop! I will come. The Tsar has as good servants as I am! Colonel,
tell his Majesty that I gave myself up to save the lives of better
men."

Then the Professor mounted the ladder amidst a murmur of relief and
applause from the crowd, and, gaining the deck of the _Ariel_, bowed
coldly to Mazanoff and said--

"I am your prisoner, sir!"

The captain of the _Ariel_ bowed in reply, and stamped thrice on the
deck. The fan-wheels whirled round, and the air-ship rapidly
ascended, at the same time moving diagonally across the quadrangle of
the Arsenal.

Scarcely had she reached the other side when there was a tremendous
explosion in the north-eastern angle of the building. A sheet of
flame shot up through the roof, the walls split asunder, and masses
of stone, wood, and iron went flying in all directions, leaving only
a fiercely burning mass of ruins where the gable had been.

The Professor turned ashy pale, staggered backwards with both his
hands clasped to his head, and gasped out brokenly as he stared at
the conflagration--

"God have mercy on me! My laboratory! My assistant--I told him"--

"What did you tell him, Professor?" said Mazanoff sternly, grasping
him suddenly by the arm.

"I told him not to open the other cylinder."

"And he has done so, and paid for his disobedience with his life,"
said Mazanoff calmly. "Console yourself, my dear sir! He has only
saved me the trouble of destroying your laboratory. I serve a sterner
and more powerful master than yours. He ordered me to make your
experiments impossible if it cost a thousand lives to do so, and I
would have done it if necessary. Rest content with the knowledge that
you have saved, not only the rest of the Arsenal, but also
Petersburg, by your surrender; for sooner than that secret had been
revealed, we should have laid the city in ruins to slay the man who