An introductory text-book of logic - Mellone, Sydney - Rare Books at ...

OR SCIENTIFIC METHOD. 299
inversely as the squares of their distances from it, then
the following laws must hold : good (a) Their orbits
must be ellipses with the
"
"
centre of force in a focus ;
((3} the radius drawn from each moving body
to the
centre must describe equal areas in equal times ; (7) the
periodic times of their revolution vary
as the cubes of
their mean distances from the centre. These were the
same three laws which Kepler had shown, from Tycho s
observations, to be true of the motions of the planets
round the sun, and which other observations showed to
be true of the motions of satellites round their planets,
as was most evident in the case of Jupiter and Saturn.
Newton went further, and proved that if his law were
absolutely true, Kepler s could only be approximately so ;
for the attraction holds not only between the sun and the
planets, but between the planets themselves. Hence it
was impossible that their orbits should be perfectly ellip
tical; and the more accurate observation which afterwards
became possible, showed that just such
"
perturbations
take place as would be expected if Newton s law
"
were
true. And by rigorous deductions it has been shown
that his law is competent to account for the complex
motions actually observable in the solar system. They
are
"
"
accounted or
for," explained," in being proved to
be consequences of the law. It is this demonstration,
that the consequences of a law do actually agree with
facts, that forms for Science the verification of that law.
11. We now proceed to deal with the question,
How are hypotheses suggested ? There are two prin
cipal means by which facts may be made to suggest
a theory
(a) By the Method of Agreement ;
(b) By Analogy.
The account already given of the Method of Agreement