discovery of facts; the facts were so, but how came they so?
What made the planets move in this particular way? Descartes's vortices
was an attempt, a poor and imperfect attempt, at an explanation. It had
been hailed and adopted throughout Europe for want of a better, but it
did not satisfy Newton. No, it proceeded on a wrong tack, and Kepler had
proceeded on a wrong tack in imagining spokes or rays sticking out from
the sun and driving the planets round like a piece of mechanism or mill
work. For, note that all these theories are based on a wrong idea--the
idea, viz., that some force is necessary to maintain a body in motion.
But this was contrary to the laws of motion as discovered by Galileo.
You know that during his last years of blind helplessness at Arcetri,
Galileo had pondered and written much on the laws of motion, the
foundation of mechanics. In his early youth, at Pisa, he had been
similarly occupied; he had discovered the pendulum, he had refuted the
Aristotelians by dropping weights from the leaning tower (which we must
rejoice that no earthquake has yet injured), and he had returned to
mechanics at intervals all his life; and now, when his eyes were useless
for astronomy, when the outer world has become to him only a prison to
be broken by death, he returns once more to the laws of motion, and
produces the most solid and substantial work of his life.

For this is Galileo's main glory--not his brilliant exposition of the
Copernican system, not his flashes of wit at the expense of a moribund
philosophy, not his experiments on floating bodies, not even his
telescope and astronomical discoveries--though these are the most taking
and dazzling at first sight. No; his main glory and title to immortality
consists in this, that he first laid the foundation of mechanics on a
firm and secure basis of experiment, reasoning, and observation. He
first discovered the true Laws of Motion.

I said little of this achievement in my lecture on him; for the work was
written towards the end of his life, and I had no time then. But I knew
I should have to return to it before we came to Newton, and here we are.

You may wonder how the work got published when so many of his
manuscripts were destroyed. Horrible to say, Galileo's own son destroyed
a great bundle of his father's manuscripts, thinking, no doubt, thereby
to save his own soul. This book on mechanics was not burnt, however. The
fact is it was rescued by one or other of