alling bodies, pendulums, projectiles, and the like. Among those
who occupied themselves with such labours may be mentioned Torricelli,
Castelli, Viviani, Borelli, Gassendi. Through the investigations of
these, and other Italian, French, and English natural philosophers, the
principles of Mechanics were solidly established, and a necessary
preparation made for their application in astronomy. By this time every
one had become ready to admit that the motion of the planetary bodies
would find an explanation on these principles.

[Sidenote: Application of Mechanics to the celestial motions.] The steps
thus far taken for an explanation of the movements of the planets in
curvilinear paths therefore consisted in the removal of the old
misconception that for a body to continue its motion forward in a
straight line a continued application of force is necessary, the first
law of motion disposing of that error. In the next place, it was
necessary that clear and distinct ideas should be held of the
combination or composition of forces, each continuing to exercise its
influence without deterioration or diminution by the other. The time had
now come for it to be shown that the perpetual movement of the planets
is a consequence of the first law of motion; their elliptic paths, such
as had been determined by Kepler, a consequence of the second. Several
persons almost simultaneously had been brought nearly to this conclusion
without being able to solve the problem completely. Thus Borelli, A.D.
1666, in treating of the motions of Jupiter's satellites, distinctly
shows how a circular motion may arise under the influence of a central
force; he even uses the illustration so frequently introduced of a stone
whirled round in a sling. In the same year a paper was presented to the
Royal Society by Mr. Hooke, "explicating the inflection of a direct
motion into a circular by a supervening attractive principle." Huygens
also, in his "Horologium Oscillatorium," had published some theorems on
circular motions, but no one as yet had been able to show how elliptical
orbits could, upon these principles, be accounted for, though very many
had become satisfied that the solution of this problem would before long
be given.

[Sidenote: Newton; publication of the "Principia."] In April, 1686, the
"Principia" of Newton was presented to the Royal Society. This immortal
work not only laid the foundation of Physical Astronomy, it also carried
the structure the