352 
BIGELOW. 
dividual facts, laws, and theorems are sound, and have passed 
through the two antecedent stages of criticism. In illustra¬ 
tion of this principle, it may be profitable to call attention 
to one of the most important and interesting struggles for 
supremacy that has arisen in the history of the development 
of science. There may be some features of this topic which 
are not sufficiently familiar to those who have never made 
a special study of these subjects, to make it particularly 
easy to understand fully the merits of this controversy, but 
probably the main features can be readily apprehended. 
It may seem going very far apart to seek in the Ptolemaic 
system of astronomy, and the chemical theory of phlogiston, 
the remote beginnings of this modern strife, but that is ap¬ 
parently where it unconsciously began. There are three 
systems of astronomy which form the stages in the develop¬ 
ment of our modern science—the Ptolemaic, the Copernican, 
and the Newtonian. In the Ptolemaic system the stars and 
the planets were all regarded as having real motions about 
the earth, assumed ifself to be at rest; in the Copernican 
system the sun is the center of motion, while the earth and 
the other planets rotate on their axes and revolve about it; 
in the Newtonian system all celestial motions are due to the 
operation of the one law of universal gravitation. In addi¬ 
tion to the plausible account given of the observed motions 
of the stars and planets by the first system, its real strength 
consisted in the ancient difficulty of conceiving how the at¬ 
mosphere, how men, and all other things could remain on a 
rapidly rotating sphere without sliding off. The fact which 
really broke down the theory was the incontestible one ob¬ 
served by sailors, that the surface of the sea was not flat, but 
that their ships descended from view. The heliocentric 
theory of Copernicus not only showed that the celestial 
motions could be as well accounted for by it as by the Ptole¬ 
maic system, and more simply, but that it also contained 
greater possibilities of knowledge. The observations of 
Copernicus, Tycho, and Kepler, giving rise to Kepler’s three 
laws, culminated in the central law of gravitation, and New- 
