Newton ( 1647-1727) achieved the final and complete triumph for which Copernicus, Kepler,
and Galileo had prepared the way. Starting from his three laws of motion--of which the first two
are due to Galileo--he proved that Kepler's three laws are equivalent to the proposition that
every planet, at every moment, has an acceleration towards the sun which varies inversely as the
square of the distance from the sun. He showed that accelerations towards the earth and the sun,
following the same formula, explain the moon's motion, and that the acceleration of falling
bodies on the earth's surface is again related to that of the moon according to the inverse square
law. He defined "force" as the cause of change of motion, i.e., of acceleration. He was thus able
to enunciate his law of universal gravitation: "Every body attracts every other with a force
directly proportional to the product of their masses and inversely proportional to the square of
the distance between them." From this formula he was able to deduce everything in planetary
theory: the motions of the planets and their satellites, the orbits of comets, the tides. It appeared
later that even the minute departures from elliptical orbits on the part of the planets were
deducible from Newton's law. The triumph was so complete that Newton was in danger of
becoming another Aristotle, and imposing an insuperable barrier to progress. In England, it was
not till a century after his death that men freed themselves from his authority sufficiently to do
important original work in the subjects of which he had treated.
The seventeenth century was remarkable, not only in astronomy and dynamics, but in many
other ways connected with science.
Take first the question of scientific instruments. * The compound microscope was invented just
before the seventeenth century, about 1590. The telescope was invented in 1608, by a
Dutchman named Lippershey, though it was Galileo who first made serious use of it for
scientific purposes. Galileo also invented the thermometer--at least, this seems most probable.
His pupil Torricelli invented the barometer. Guericke ( 1602-86) invented the air pump. Clocks,
though not new, were greatly improved in the seventeenth century, largely by the work of
Galileo. Owing to these inventions, scientific observa-
* On this subject, see the chapter "Scientific Instruments" in A History of Science,
Technology, and Philosophy in the Sixteenth and Seventeenth Centuries, by A. Wolf.