spectra of the same general character, others with broad absorption
bands, or numerous dark lines like our sun, or a few absorption lines
only--all this shows us the universe as teeming with bodies in various
stages of evolution. We shall have a few more words to say on this
matter when we come to consider the astronomical significance of heat;
but we have reached a point where man's intellect can hardly keep pace
with the development of our instrumental resources, and where our
imagination stands bewildered when we endeavour to systematise the
knowledge we have gained. That great caution will have to be exercised
in the interpretation of the observed phenomena is evident from the
recent experience of Professor Rowland, of Baltimore, from which we
learn that spectral lines are not only widened by increased pressure of
the light-giving vapour, but that they may be bodily shifted thereby.
Dr. Zeeman's discovery, that a line from a source placed in a strong
magnetic field may be both widened, broadened, and doubled, will also
increase our difficulties in the interpretation of these obscure
phenomena.

CHAPTER XXIV.

THE PRECESSION AND NUTATION OF THE EARTH'S AXIS.

The Pole is not a Fixed Point--Its Effect on the Apparent Places of
the Stars--The Illustration of the Peg-Top--The Disturbing Force
which acts on the Earth--Attraction of the Sun on a Globe--The
Protuberance at the Equator--The Attraction of the Protuberance by
the Sun and by the Moon produces Precession--The Efficiency of the
Precessional Agent varies inversely as the Cube of the
Distance--The Relative Efficiency of the Sun and the Moon--How the
Pole of the Earth's Axis revolves round the Pole of the
Ecliptic--Variation of Latitude.

The position of the pole of the heavens is most conveniently indicated
by the bright star known as the Pole Star, which lies in its immediate
vicinity. Around this pole the whole heavens appear to rotate once in a
sidereal day; and we have hitherto always referred to the pole as though
it were a fixed point in the heavens. This language is sufficiently
correct when we embrace only a moderate period of time in our review. It
is no doubt true that the pole lies near the Pole Star at the present
time. It did so during the lives of the last generation, and it will do
so during the lives of the next generation. All this time, however, the
pole is steadily moving in the heavens, s