information, namely, the periodic time, will be necessary to complete
our knowledge of the movements of the system. Professor H. Newton,
of Yale, had shown that the choice of possible orbits for the
meteoric swarm is limited to five. There is, first, the great
ellipse in which we now know the meteors revolve once every thirty
three and one quarter years. There is next an orbit of a nearly
circular kind in which the periodic time would be a little more than
a year. There is a similar track in which the periodic time would be
a few days short of a year, while two other smaller orbits would also
be conceivable. Professor Newton had pointed out a test by which it
would be possible to select the true orbit, which we know must be one
or other of these five. The mathematical difficulties which attended
the application of this test were no doubt great, but they did not
baffle Professor Adams.

There is a continuous advance in the date of this meteoric shower.
The meteors now cross our track at the point occupied by the earth on
November 13th, but this point is gradually altering. The only
influence known to us which could account for the continuous change
in the plane of the meteor's orbit arises from the attraction of the
various planets. The problem to be solved may therefore be attacked
in this manner. A specified amount of change in the plane of the
orbit of the meteors is known to arise, and the changes which ought
to result from the attraction of the planets can be computed for each
of the five possible orbits, in one of which it is certain that the
meteors must revolve. Professor Adams undertook the work. Its
difficulty principally arises from the high eccentricity of the
largest of the orbits, which renders the more ordinary methods of
calculation inapplicable. After some months of arduous labour the
work was completed, and in April, 1867, Adams announced his solution