gravitation."

The same subject was again proposed for a prize which was shared by
Lagrange [1] and Euler, neither finding a solution, while the latter
asserted the existence of a resisting medium in space.

Again, in 1774, the Academy submitted the same subject, a third time,
for the prize; and again Lagrange failed to detect a cause in
gravitation.

Laplace [2] now took the matter in hand. He tried the effect of a
non-instantaneous action of gravity, to no purpose. But in 1787 he
gave the true explanation. The principal effect of the sun on the
moon's orbit is to diminish the earth's influence, thus lengthening
the period to a new value generally taken as constant. But Laplace's
calculations showed the new value to depend upon the excentricity of
the earth's orbit, which, according; to theory, has a periodical
variation of enormous period, and has been continually diminishing for
thousands of years. Thus the solar influence has been diminishing, and
the moon's mean motion increased. Laplace computed the amount at 10"
in one century, agreeing with observation. (Later on Adams showed that
Laplace's calculation was wrong, and that the value he found was too
large; so, part of the acceleration is now attributed by some
astronomers to a lengthening of the day by tidal friction.)

Another contribution by Halley to the verification of Newton's law was
made when he went to St. Helena to catalogue the southern stars. He
measured the change in length of the second's pendulum in different
latitudes due to the changes in gravity foretold by Newton.