On January 7, 1610, Galileo, turning his telescope towards Jupiter,
was the first to see the beautiful system of that planet in which the
universe is epitomized. He had already studied the variegated surface
of the moon, and he had seen the spots upon the sun. A little later,
in spite of the feeble power of his instrument, he had discovered
that the sun rotates upon an axis, and something of the wonderful
nature of the planet Saturn had been revealed to him. The overwhelming
evidence thus afforded of the truth of the hypothesis of Copernicus
made him its chief exponent. The time had come for man to know, as
he had never known or even dreamed before, his true relation to the
universe of which he was so insignificant a part. In a single year
nearly all of these capital discoveries were made. It was truly an
era of intellectual expansion; never before and never since has man’s
intellectual horizon enlarged with such enormous rapidity. One needs
little imagination to share with this ardent philosopher the enthusiasm
of the moment when, because some, fearing the evidence of their senses,
refused to look through the slender tube, he wrote to Kepler: “Oh, my
dear Kepler, how I wish we could have one hearty laugh together!...
Why are you not here? What shouts of laughter we should have at this
glorious folly!”
Galileo died in 1642, and in the same year Newton was born. When
twenty-four years old he “began to think of gravity extending to the
orb of the moon,” and before the end of the century he had discovered
and established the great law of universal gravitation. Thus, at the
end of the seventeenth century, the foundations of modern physics were
in place. During the eighteenth century they were much built upon, but
it was the nineteenth that witnessed not only the greatest advance in
detail, but the most important generalizations made since the time of
Galileo and Newton.
In endeavoring to present to the intelligent but perhaps unscientific
reader a brief review of the accomplishments of that “wonderful
century” in the domain of physics, one must not attempt more than an
outline of greater events, and it will be convenient to arrange them
under the several principal subdivisions of the science, according to
the usually accepted classification.