l explanation of the phenomena, recourse was had to a
method introduced by Erman of computing meteoric orbits. It was found,
however, that conspicuous recurrences every thirty-three or thirty-four
years could be explained on the supposition of five widely different
periods, combined with varying degrees of extension in the revolving
group. Professor Newton himself gave the preference to the shortest--of
354-1/2 days, but indicated the means of deciding with certainty upon
the true one. It was furnished by the advancing motion of the node, or
that day's delay of the November shower every seventy years, which the
old chronicles had supplied data for detecting. For this is a strictly
measurable effect of gravitational disturbance by the various planets,
the amount of which naturally depends upon the course pursued by the
disturbed bodies. Here the great mathematical resources of Professor
Adams were brought to bear. By laborious processes of calculation, he
ascertained that four out of Newton's five possible periods were
entirely incompatible with the observed nodal displacement, while for
the fifth--that of 33-1/4 years--a perfectly harmonious result was
obtained.[1207] This was the last link in the chain of evidence proving
that the November meteors--or "Leonids," as they had by that time come
to be called--revolve round the sun in a period of 33.27 years, in an
ellipse spanning the vast gulf between the orbits of the earth and
Uranus, the group being so extended as to occupy nearly three years in
defiling past the scene of terrestrial encounters. But before it was
completed in March, 1867, the subject had assumed a new aspect and
importance.

Professor Newton's prediction of a remarkable star-shower in November,
1866, was punctually fulfilled. This time, Europe served as the main
target of the celestial projectiles, and observers were numerous and
forewarned. The display, although, according to Mr. Baxendell's
memory,[1208] inferior to that of 1833, was of extraordinary
impressiveness. Dense crowds of meteors, equal in lustre to the
brightest stars, and some rivalling Venus at her best,[1209] darted from
east to west across the sky with enormous apparent velocities, and with
a certain determinateness of aim, as if let fly with a purpose, and at
some definite object.[1210] Nearly all left behind them trains of
emerald green or clear blue light, which occasionally lasted many
minutes, before they shrivelled and curled up