"That's about it," said the Admiral, "this craft can do her two miles a
minute, and still have a good bit in hand if it came to chasing
anything."

He pulled back a couple of levers as he spoke and gave a quarter turn to
the wheel. The great airship took a downward slide, swung round to the
right, and in a few moments she had dropped quietly to the turf of
Greenwich Park alongside the Observatory.

Lennard's calculations had already reached the Astronomer Royal, and he
and his chief assistant had had time to make a rapid run through them,
and they had found that his figures, and especially the inexplicable
change in the orbit, tallied almost exactly with observations of a
possibly new comet for the last two months or so.

They were not quite prepared for the coming of an Imperial--and
hostile--visitor in an airship, accompanied by the discoverer of the
comet, the millionaire who owned the great telescope, and an American
gentleman in the uniform of a British admiral; but those were
extraordinary times, and so extraordinary happenings might be expected.
The astronomer and his staff, being sober men of science, whose business
was with other worlds rather than this one, accepted the situation
calmly, gave their visitors lunch, talked about everything but the war,
and then they all spent a pleasant and instructive afternoon in a
journey through Space in search of the still invisible Celestial
Invader.

When they had finished, the two sets of calculations balanced
exactly--to the millionth of a degree and the thousandth of a second. At
ten seconds to twelve, midnight, May the first, the comet, if not
prevented by some tremendously powerful agency, would pierce the earth's
atmosphere, as Lennard had predicted.

"It is a marvellous piece of work, Mr Lennard, however good an
instrument you had. As an astronomer I congratulate you heartily, but as
citizens of the world I hope we shall be able to congratulate you still
more heartily on the results which you expect that big gun of yours to
bring about."

"I'm sure I hope so," said Lennard, toying rather absently with his
pencil.

"And if the cannon is not fired, and the Pittsburg one does not happen
to be exactly laid, for there is a very great difference in longitude,
what will be the probable results, Mr Astronomer?" asked the Tsar, upon
whom the lesson of the afternoon had by no means been lost.

"If the comet is what Mr Lennard expects it to be, your Majesty," was