ticular class of inquirers has, as it were, secreted
its own particular order of truths from the general mass of material
which observation accumulates; and all other classes of inquirers have
made use of these truths as fast as they were elaborated, with the
effect of enabling them the better to elaborate each its own order of
truths.

It was thus in sundry of the cases we have quoted as at variance with M.
Comte's doctrine. It was thus with the application of Huyghens's optical
discovery to astronomical observation by Galileo. It was thus with the
application of the isochronism of the pendulum to the making of
instruments for measuring intervals, astronomical and other. It was thus
when the discovery that the refraction and dispersion of light did not
follow the same law of variation, affected both astronomy and physiology
by giving us achromatic telescopes and microscopes. It was thus when
Bradley's discovery of the aberration of light enabled him to make the
first step towards ascertaining the motions of the stars. It was thus
when Cavendish's torsion-balance experiment determined the specific
gravity of the earth, and so gave a datum for calculating the specific
gravities of the sun and planets. It was thus when tables of
atmospheric refraction enabled observers to write down the real places
of the heavenly bodies instead of their apparent places. It was thus
when the discovery of the different expansibilities of metals by heat,
gave us the means of correcting our chronometrical measurements of
astronomical periods. It was thus when the lines of the prismatic
spectrum were used to distinguish the heavenly bodies that are of like
nature with the sun from those which are not. It was thus when, as
recently, an electro-telegraphic instrument was invented for the more
accurate registration of meridional transits. It was thus when the
difference in the rates of a clock at the equator, and nearer the poles,
gave data for calculating the oblateness of the earth, and accounting
for the precession of the equinoxes. It was thus--but it is needless to
continue.

Here, within our own limited knowledge of its history, we have named ten
additional cases in which the single science of astronomy has owed its
advance to sciences coming _after_ it in M. Comte's series. Not only its
secondary steps, but its greatest revolutions have been thus determined.
Kepler could not have discovered his celebrated laws had it not been for
Tycho