before,
and there remained no astronomer in the field who is conspicuous in
the history of science as a champion of the Copernican doctrine. But in
truth it might be said that the theory no longer needed a champion. The
researches of Kepler and Galileo had produced a mass of evidence for the
Copernican theory which amounted to demonstration. A generation or two
might be required for this evidence to make itself everywhere known
among men of science, and of course the ecclesiastical authorities must
be expected to stand by their guns for a somewhat longer period. In
point of fact, the ecclesiastical ban was not technically removed by
the striking of the Copernican books from the list of the Index
Expurgatorius until the year 1822, almost two hundred years after the
date of Galileo's dialogue. But this, of course, is in no sense a guide
to the state of general opinion regarding the theory. We shall gain a
true gauge as to this if we assume that the greater number of important
thinkers had accepted the heliocentric doctrine before the middle of the
seventeenth century, and that before the close of that century the old
Ptolemaic idea had been quite abandoned. A wonderful revolution in
man's estimate of the universe had thus been effected within about two
centuries after the birth of Copernicus.

V. GALILEO AND THE NEW PHYSICS

After Galileo had felt the strong hand of the Inquisition, in 1632, he
was careful to confine his researches, or at least his publications, to
topics that seemed free from theological implications. In doing so he
reverted to the field of his earliest studies--namely, the field of
mechanics; and the Dialoghi delle Nuove Scienze, which he finished in
1636, and which was printed two years later, attained a celebrity no
less than that of the heretical dialogue that had preceded it. The
later work was free from all apparent heresies, yet perhaps it did
more towards the establishment of the Copernican doctrine, through
the teaching of correct mechanical principles, than the other work had
accomplished by a more direct method.

Galileo's astronomical discoveries were, as we have seen, in a sense
accidental; at least, they received their inception through the
inventive genius of another. His mechanical discoveries, on the other
hand, were the natural output of his own creative genius. At the very
beginning of his career, while yet a very young man, though a professor
of mathematics at Pisa, he had be