And the advantage thus derived, as we have seen, is not confined
to the astronomer, who has often been able, by making fundamental
physical and chemical discoveries, to repay his debt to the physicist
and chemist for the apparatus and methods which he owes to them.

NEWTON AND EINSTEIN

Take, for another example, the greatest law of physics--Newton's
law of gravitation. Huge balls of lead, as used by Cavendish, produce
by their gravitational effect a minute rotation of a delicately
suspended bar, carrying smaller balls at its extremities. But no
such feeble means sufficed for Newton's purpose. To prove the law
of gravitation he had recourse to the tremendous pull on the moon
of the entire mass of the earth, and then extended his researches
to the mutual attractions of all the bodies of the solar system.
Later Herschel applied this law to the suns which constitute double
stars, and to-day Adams observes from Mount Wilson stars falling
with great velocity toward the centre of the galactic system under
the combined pull of the millions of objects that compose it. Thus
full advantage has been taken of the possibility of utilizing the
great masses of the heavenly bodies for the discovery and application
of a law of physics and its reciprocal use in explaining celestial
motions.

[Illustration: Fig. 38. The Cavendish experiment.

Two lead balls, each two inches in diameter, are attached to the
ends of a torsion rod six feet long, which is suspended by a fine
wire. The experiment consists in measuring the rotation of the
suspended system, caused by the gravitational attraction of two
lead spheres, each twelve inches in diameter, acting on the two
small lead balls.]

Or consider the Einstein theory of relativity, the truth or falsity
of which is no less fundamental to physics. Its inception sprang from
the Michelson-Morley experiment, made in a laboratory in Cleveland,
which showed that motion of the earth through the ether of space could
not be detected. All of the three chief tests of Einstein's general
theory are astronomical--because of the great masses required to
produce the minute effects predicted: the motion of the perihelion
of Mercury, the deflection of the light of a star by the attraction
of the sun, and the shift of the lines of the solar spectrum toward
the red--questions not yet completely answered.

But it is in the study of the constitution of matter and the evolution
of the elements, the deepest and m