when very striking proofs of the universality of the law of
gravitation convinced the most sceptical, it must still be borne in
mind that, while gravitation is certainly the principal force
governing the motions of the heavenly bodies, there may yet be a
resisting medium in space, and there may be electric and magnetic
forces to deal with. There may, further, be cases where the effects of
luminous radiative repulsion become apparent, and also Crookes'
vacuum-effects described as "radiant matter." Nor is it quite certain
that Laplace's proofs of the instantaneous propagation of gravity are
final.

And in the future, as in the past, Tycho Brahe's dictum must be
maintained, that all theory shall be preceded by accurate
observations. It is the pride of astronomers that their science stands
above all others in the accuracy of the facts observed, as well as in
the rigid logic of the mathematics used for interpreting these facts.

It is interesting to trace historically the invention of those
instruments of precision which have led to this result, and, without
entering on the details required in a practical handbook, to note the
guiding principles of construction in different ages.

It is very probable that the Chaldeans may have made spheres, like the
armillary sphere, for representing the poles of the heavens; and with
rings to show the ecliptic and zodiac, as well as the equinoctial and
solstitial colures; but we have no record. We only know that the tower
of Belus, on an eminence, was their observatory. We have, however,
distinct records of two such spheres used by the Chinese about 2500
B.C. Gnomons, or some kind of sundial, were used by the Egyptians and
others; and many of the ancient nations measured the obliquity of the