ness I will resolutely bid adieu to it eternally, except
what I do for my private satisfaction or leave to come out after me; for
I see a man must either resolve to put out nothing new, or to become a
slave to defend it." And again in a letter to Leibnitz: "I have been so
persecuted with discussions arising out of my theory of light that I
blamed my own imprudence for parting with so substantial a blessing as
my quiet to run after a shadow." This shows how much he cared for
contemporary fame.

So he locked up the first part of the _Principia_ in his desk, doubtless
intending it to be published after his death. But fortunately this was
not so to be.

In 1683, among the leading lights of the Royal Society, the same sort of
notions about gravity and the solar system began independently to be
bruited. The theory of gravitation seemed to be in the air, and Wren,
Hooke, and Halley had many a talk about it.

Hooke showed an experiment with a pendulum, which he likened to a planet
going round the sun. The analogy is more superficial than real. It does
not obey Kepler's laws; still it was a striking experiment. They had
guessed at a law of inverse squares, and their difficulty was to prove
what curve a body subject to it would describe. They knew it ought to be
an ellipse if it was to serve to explain the planetary motion, and Hooke
said he could prove that an ellipse it was; but he was nothing of a
mathematician, and the others scarcely believed him. Undoubtedly he had
shrewd inklings of the truth, though his guesses were based on little
else than a most sagacious intuition. He surmised also that gravity was
the force concerned, and asserted that the path of an ordinary
projectile was an ellipse, like the path of a planet--which is quite
right. In fact the beginnings of the discovery were beginning to dawn
upon him in the well-known way in which things do dawn upon ordinary men
of genius: and had Newton not lived we should doubtless, by the labours
of a long chain of distinguished men, beginning with Hooke, Wren, and
Halley, have been now in possession of all the truths revealed by the
_Principia_. We should never have had them stated in the same form, nor
proved with the same marvellous lucidity and simplicity, but the facts
themselves we should by this time have arrived at. Their developments
and completions, due to such men as Clairaut, Euler, D'Alembert,
Lagrange, Laplace, Airy, Leverrier, Adams, we should of course not h