haze on its edge. Lastly, whatever animal remains the moon may
contain would probably be rather in the form of fossils than of
skeletons. The skeleton is of course intended as an image of death and
desolation. It is a matter of taste: but a skeleton, it seems to me,
speaks too recently of life to be as appallingly weird and desolate as a
blank stone or ice landscape, unshaded by atmosphere or by any trace of
animal or plant life, could be made.

[25] Five of Jupiter's revolutions occupy 21,663 days; two of Saturn's
revolutions occupy 21,526 days.

[26] _Excircularity_ is what is meant by this term. It is called
"excentricity" because the foci (not the centre) of an ellipse are
regarded as the representatives of the centre of a circle. Their
distance from the centre, compared with the radius of the unflattened
circle, is called the excentricity.

[27] A curve of the _n_th degree has 1/2_n_(_n_+3) arbitrary constants
in its equation, hence this number of points specifically determine it.
But special points, like focus or vertex, count as two ordinary ones.
Hence three points plus the focus act as five points, and determine a
conic or curve of the second degree. Three observations therefore fix an
orbit round the sun.

[28] Its name suggests a measure of the diameter of the sun's disk, and
this is one of its functions; but it can likewise measure planetary and
other disks; and in general behaves as the most elaborate and expensive
form of micrometer. The Koenigsberg instrument is shewn in fig. 92.

[29] It may be supposed that the terms "minute" and "second" have some
necessary connection with time, but they are mere abbreviations for
_partes minutae_ and _partes minutae secundae_, and consequently may be
applied to the subdivision of degrees just as properly as to the
subdivision of hours. A "second" of arc means the 3600th part of a
degree, just as a second of time means the 3600th part of an hour.

[30] A group of flying particles, each one invisible, obstructs light
singularly little, even when they are close together, as one can tell by
the transparency of showers and snowstorms. The opacity of haze may be
due not merely to dust particles, but to little eddies set up by
radiation above each particle, so that the air becomes turbulent and of
varying density. (See a similar suggestion by Mr. Poynting in _Nature_,
vol. 39, p. 323.)

[31] The moon ought to be watched during the next great shower, if the
line of fi