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inexplicable by the law of gravitation–they
now form one of its most striking proofs.”
Let us take one more discovery of Hal-
ley, furnishing directly a new triumph for
the theory. He noticed that Newton as-
cribed parabolic orbits to the comets which
he studied, so that they come from infinity,
sweep round the sun, and go off to infin-
ity for ever, after having been visible a few
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