published was the signal for an outburst of
criticism and sometimes of attack. I shall not go into these matters:
they are now trivial enough, but it is necessary to mention them,
because to Newton they evidently loomed large and terrible, and
occasioned him acute torment.

[Illustration: FIG. 68.--Newton when young. (_From an engraving by B.
Reading after Sir Peter Lely._)]

No sooner was the _Principia_ put than Hooke put in his claims for
priority. And indeed his claims were not altogether negligible; for
vague ideas of the same sort had been floating in his comprehensive
mind, and he doubtless felt indistinctly conscious of a great deal more
than he could really state or prove.

By indiscreet friends these two great men were set somewhat at
loggerheads, and worse might have happened had they not managed to come
to close quarters, and correspond privately in a quite friendly manner,
instead of acting through the mischievous medium of third parties. In
the next edition Newton liberally recognizes the claims of both Hooke
and Wren. However, he takes warning betimes of what he has to expect,
and writes to Halley that he will only publish the first two books,
those containing general theorems on motion. The third book--concerning
the system of the world, _i.e._ the application to the solar system--he
says "I now design to suppress. Philosophy is such an impertinently
litigious lady that a man had as good be engaged in law-suits as have to
do with her. I found it so formerly, and now I am no sooner come near
her again but she gives me warning. The two books without the third will
not so well bear the title 'Mathematical Principles of Natural
Philosophy,' and therefore I had altered it to this, 'On the Free Motion
of Two Bodies'; but on second thoughts I retain the former title: 'twill
help the sale of the book--which I ought not to diminish now 'tis
yours."

However, fortunately, Halley was able to prevail upon him to publish the
third book also. It is, indeed, the most interesting and popular of the
three, as it contains all the direct applications to astronomy of the
truths established in the other two.

Some years later, when his method of fluxions was published, another and
a worse controversy arose--this time with Leibnitz, who had also
independently invented the differential calculus. It was not so well
recognized then how frequently it happens that two men independently
and unknowingly work at the very same