intained that these spheres were crystal; but Lactantius, and with him
various fathers of the Church, spoke of the heavenly vault as made of
ice. Now, the admission that comets could move beyond the moon was fatal
to this theory, for it sent them crashing through these spheres of
ice or crystal, and therefore through the whole sacred fabric of the
Ptolemaic theory.(120)

(120) For these features in cometary theory, see Pingre, vol. i, p. 89;
also Humboldt, Cosmos (English translation, London, 1868), vol. iii, p.
169.

Here we may pause for a moment to note one of the chief differences
between scientific and theological reasoning considered in themselves.
Kepler's main reasoning as to the existence of a law for cometary
movement was right; but his secondary reasoning, that comets move nearly
in straight lines, was wrong. His right reasoning was developed by
Gassendi in France, by Borelli in Italy, by Hevel and Doerfel in
Germany, by Eysat and Bernouilli in Switzerland, by Percy and--most
important of all, as regards mathematical demonstration--by Newton
in England. The general theory, which was true, they accepted and
developed; the secondary theory, which was found untrue, they rejected;
and, as a result, both of what they thus accepted and of what they
rejected, was evolved the basis of the whole modern cometary theory.

Very different was this from the theological method. As a rule, when
there arises a thinker as great in theology as Kepler in science, the
whole mass of his conclusions ripens into a dogma. His disciples labour
not to test it, but to establish it; and while, in the Catholic Church,
it becomes a dogma to be believed or disbelieved under the penalty of
damnation, it becomes in the Protestant Church the basis for one more
sect.

Various astronomers laboured to develop the truth discovered by Tycho
and strengthened by Kepler. Cassini seemed likely to win for Italy the
glory of completing the great structure; but he was sadly fettered by
Church influences, and was obliged to leave most of the work to others.
Early among these was Hevel. He gave reasons for believing that comets
move in parabolic curves toward the sun. Then came a man who developed
this truth further--Samuel Doerfel; and it is a pleasure to note that
he was a clergyman. The comet of 1680, which set Erni in Switzerland,
Mather in New England, and so many others in all parts of the world
at declaiming, set Doerfel at thinking. Und