ent
of the most moderate measuring capacity.

[Sidenote: Tentative explanations.]

What, then, was the cause of this quite unforeseen behaviour on the part
of the star? The first thought of the observers was that something might
be wrong with their instrument, and it was carefully examined, but without
result. The next was that the apparent movement was in the plumb-line, the
line of reference. If the whole earth, instead of carrying its axis round
the sun in a constant direction, were to be executing an oscillation, then
all our plumb-lines would oscillate, and when the direction of a star like
[gamma] Draconis was compared with that of the plumb-line it would seem to
vary, owing actually to the variation in the plumb-line. The earth might
have a motion of this kind in two ways, which it will be necessary for us
to distinguish, and the adopted names for them are "nutation of the axis"
and "variation of latitude" respectively. In the case of nutation the
North Pole remains in the same geographical position, but points to a
different part of the heavens. The "variation of latitude," on the other
hand, means that the North Pole wanders about on the earth itself. We
shall refer to the second phenomenon more particularly in the sixth
chapter.

[Sidenote: Nutation?]

[Sidenote: Anomalous refraction.]

But it was the first kind of change, the nutation, which Bradley
suspected; and very early in the series of observations he had already
begun to test this hypothesis. If it was not the star, but the earth and
the plumb-line, which were in motion, then other stars ought to be
affected. The telescope had been deliberately restricted in its position
to suit [gamma] Draconis; but since the stars circle round the Pole, if we
draw a narrow belt in the heavens with the Pole as centre, and including
[gamma] Draconis, the other stars included would make the same circuit,
preceding or following [gamma] Draconis by a constant interval. Most of
them would be too faint for observation with Bradley's telescope; but
there was one bright enough to be observed, which also came within its
limited range, and it was promptly put under _surveillance_ when a
nutation of the earth's axis was suspected. Careful watching showed that
it was not affected in the same way as [gamma] Draconis, and hence the
movement could not be in the plumb-line. Was there, then, after all, some
effect of the earth's atmosphere which had been overlooked? We have
alre