idered either one or the other; but in this particular
case the professor was right in adopting the supposition of its being
parabolic.

Just as in a circle, it is necessary to know three points to determine
the circumference; so in ascertaining the elements of a comet, three
different positions must be observed before what astronomers call its
"ephemeris" can be established.

But Professor Rosette did not content himself with three positions;
taking advantage of every rift in the fog he made ten, twenty, thirty
observations both in right ascension and in declination, and succeeded
in working out with the most minute accuracy the five elements of the
comet which was evidently advancing with astounding rapidity towards the
earth.

These elements were:

1. The inclination of the plane of the cometary orbit to the plane of
the ecliptic, an angle which is generally considerable, but in this case
the planes were proved to coincide.

2. The position of the ascending node, or the point where the comet
crossed the terrestrial orbit.

These two elements being obtained, the position in space of the comet's
orbit was determined.

3. The direction of the axis major of the orbit, which was found by
calculating the longitude of the comet's perihelion.

4. The perihelion distance from the sun, which settled the precise form
of the parabola.

5. The motion of the comet, as being retrograde, or, unlike the planets,
from east to west.

Rosette thus found himself able to calculate the date at which the comet
would reach its perihelion, and, overjoyed at his discovery, without
thinking of calling it Palmyra or Rosette, after his own name, he
resolved that it should be known as Gallia.

His next business was to draw up a formal report. Not only did he at
once recognize that a collision with the earth was possible, but he soon
foresaw that it was inevitable, and that it must happen on the night of
the 31st of December; moreover, as the bodies were moving in opposite
directions, the shock could hardly fail to be violent.

To say that he was elated at the prospect was far below the truth; his
delight amounted almost to delirium. Anyone else would have hurried from
the solitude of Formentera in sheer fright; but, without communicating
a word of his startling discovery, he remained resolutely at his post.
From occasional newspapers which he had received, he had learnt that
fogs, dense as ever, continued to envelop both hemisphere