examined, it was found that the date of their occurrence undergoes a
gradual and continuous change, which Professor Newton fixed at one day
in seventy years. It follows as a necessary consequence that the point
where the path of the meteors crosses the earth's track is not fixed,
but that at each successive return they cross at a point about half a
degree further on in the direction in which the earth is travelling. It
follows that the orbit in which the meteors are revolving is undergoing
change; the path they follow in one revolution varies slightly from that
pursued in the next. As, however, these changes proceed in the same
direction, they may gradually attain considerable dimensions; and the
amount of change which is produced in the path of the meteors in the
lapse of centuries may be estimated by the two ellipses shown in Fig.
78. The continuous line represents the orbit in A.D. 126; the dotted
line represents it at present.

This unmistakable change in the orbit is one that astronomers attribute
to what we have already spoken of as perturbation. It is certain that
the elliptic motion of these bodies is due to the sun, and that if they
were only acted on by the sun the ellipse would remain absolutely
unaltered. We see, then, in this gradual change of the ellipse the
influence of the attractions of the planets. It was shown that if the
meteors moved in the large orbit, this shifting of the path must be due
to the attraction of the planets Jupiter, Saturn, Uranus, and the Earth;
while if the meteors followed one of the smaller orbits, the planets
that would be near enough and massive enough to act sensibly on them
would be the Earth, Venus, and Jupiter. Here, then, we see how the
question may be answered by calculation. It is difficult, but it is
possible, to calculate what the attraction of the planets would be
capable of producing for each of the five different suppositions as to
the orbit. This is what Adams did. He found that if the meteors moved in
the great orbit, then the attraction of Jupiter would account for
two-thirds of the observed change, while the remaining third was due to
the influence of Saturn, supplemented by a small addition on account of
Uranus. In this way the calculation showed that the large orbit was a
possible one. Professor Adams also computed the amount of displacement
in the path that could be produced if the meteors revolved in any of the
four smaller ellipses. This investigation was on