, perhaps, if he had been at Cambridge, but he was a shy and
solitary youth, and just as likely he might not. Up in Lincolnshire, in
the seventeenth century, who was there for him to consult?

True, he might have rushed into premature publication, after our
nineteenth century fashion, but that was not his method. Publication
never seemed to have occurred to him.

His reticence now is noteworthy, but later on it is perfectly
astonishing. He is so absorbed in making discoveries that he actually
has to be reminded to tell any one about them, and some one else always
has to see to the printing and publishing for him.

I have entered thus fully into what I conjecture to be the stages of
this early discovery of the law of gravitation, as applicable to the
heavenly bodies, because it is frequently and commonly misunderstood. It
is sometimes thought that he discovered the force of gravity; I hope I
have made it clear that he did no such thing. Every educated man long
before his time, if asked why bodies fell, would reply just as glibly as
they do now, "Because the earth attracts them," or "because of the force
of gravity."

His discovery was that the motions of the solar system were due to the
action of a central force, directed to the body at the centre of the
system, and varying inversely with the square of the distance from it.
This discovery was based upon Kepler's laws, and was clear and certain.
It might have been published had he so chosen.

But he did not like hypothetical and unknown forces; he tried to see
whether the known force of gravity would serve. This discovery at that
time he failed to make, owing to a wrong numerical datum. The size of
the earth he only knew from the common doctrine of sailors that 60 miles
make a degree; and that threw him out. Instead of falling 16 feet a
minute, as it ought under gravity, it only fell 13.9 feet, so he
abandoned the idea. We do not find that he returned to it for sixteen
years.

LECTURE VIII

NEWTON AND THE LAW OF GRAVITATION

We left Newton at the age of twenty-three on the verge of discovering
the mechanism of the solar system, deterred therefrom only by an error
in the then imagined size of the earth. He had proved from Kepler's laws
that a centripetal force directed to the sun, and varying as the inverse
square of the distance from that body, would account for the observed
planetary motions, and that a similar force directed to the earth would
account