ly led him on to renewed exertions.

The results obtained from Bradley's observations may be put in the form
of a diagram thus:--

[Illustration: FIG. 7.]

It will be seen that the maxima and minima fall in the spring and autumn,
and this fact alone seemed to show that the effect could not be due to
temperature, for we should expect the greatest effect in that case in
winter and summer. It could not be due to the parallax of the stars for
which Bradley began his search, for stars in different quarters of the
heavens would then be differently affected, and this was not the case.
"There remains," concluded Mr. Chandler after full discussion, "the only
natural conclusion of an actual displacement of the zenith, in other
words, a change of latitude." And he concludes this paper with the
following fine passage:--

"So far, then, as the results of this incomparable series of
observations at Kew and Wansted, considered by themselves alone, can
now be stated, the period of the polar rotation at that epoch appears
to have been probably somewhat over a year, and certainly shorter by
about two months than it is at the present time. The range of the
variation was apparently in the neighbourhood of a second of arc, or
considerably larger than that shown by the best modern observations.

[Sidenote: Bradley's greatness.]

"Before taking leave of these observations for the present I cannot
forbear to speak of the profound impression which a study of them
leaves upon the mind, and the satisfaction which all astronomers must
feel in recognising that, besides its first fruits of the phenomena
of aberration and nutation, we now owe also our first knowledge of
the polar motion to this same immortal work of Bradley. Its
excellence, highly appreciated as it has been, has still been
hitherto obscured by the presence of this unsuspected phenomenon.
When divested of its effects, the wonderful accuracy of this work
must appear in a finer light, and our admiration must be raised to
higher pitch. Going back to it after one hundred and sixty years
seems indeed like advancing into an era of practical astronomy more
refined than that from which we pass. And this leads to a suggestion
worthy of serious practical consideration--whether we can do better
in the future study of the polar rotation, than again to avail
ourselves of Br