member of which pursues an orbit in accordance with the well-known
laws of Kepler. In order to understand the movements of these
objects, to account satisfactorily for their periodic recurrence, and
to predict the times of their appearance, it became necessary to
learn the size and the shape of the track which the swarm followed,
as well as the position which it occupied. Certain features of the
track could no doubt be readily assigned. The fact that the shower
recurs on one particular day of the year, viz., November 13th,
defines one point through which the orbit must pass. The position on
the heavens of the radiant point from which the meteors appear to
diverge, gives another element in the track. The sun must of course
be situated at the focus, so that only one further piece of
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
of the problem. He showed that if the meteors revolved in the
largest of the five orbits, with the periodic time of thirty three
and one quarter years, the perturbations of Jupiter would account for
a change to the extent of twenty minutes of arc in the point in which
the orbit crosses the earth's track. The attraction of Saturn would
augment this by seven minutes, and Uranus would add one minute more,
while the influence of the Earth and of the other planets would be
inappreciable. The accumulated effect is thus twenty-eight minutes,
which is practically coincident with the observed value as determined
by Professor Newton from an examination of all the showers of which
there is any historical record. Having thus showed that the great
orbit was a possible path for the meteors, Adams next proved that no
one of the other four orbits would be disturbed in the same manner.
Indeed, it appeared that not half the observed amount of change could
arise in any orbit except in that one with the long period. Thus was
brought to completion the interesting research which demonstrated the
true relation of the meteor swarm to the solar system.
Besides those memorable scientific labours with which his attention
was so largely engaged, Professor Adams found time for much other
study. He occasionally allowed himself to undertake as a relaxation
some pieces of numerical calculation, so tremendously long that we
can only look on them with astonishment. He has calculated certain
important mathematical constants accurately to more than two hundred
places of decimals. He was a diligent reader of works on history,
geology, and botany, and his arduous labours were often beguiled by
novels, of which, like many other great men, he was very fond. He
had also the taste of a collector, and he brought together about
eight hundred volumes of early printed works, many of considerable
rarity and value. As to his personal character, I may quote the
words of Dr. Glaisher when he says, "Strangers who first met him were
invariably struck by his simple and unaffected manner. He was a
delightful companion, always cheerful and genial, showing in society
but few traces of his really shy and retiring disposition. His
nature was sympathetic and generous, and in few men have the moral
and intellectual qualities been more perfectly balanced."
In 1863 he married the daughter of Haliday Bruce, Esq., of Dublin and
up to the close of his life he lived at the Cambridge Observatory,
pursuing his mathematical work and enjoying the society of his
friends.
He died, after a long illness, on 21st January, 1892, and was
interred in St. Giles's Cemetery, on the Huntingdon Road, Cambridge.