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

296
THE THEORY OF INDUCTION
by proving
plained
"
laws governing the motion of the : projectile these are,
it to be the result of two known
the first Law of Motion, that a body in motion continues
to move in a straight line with uniform velocity unless
acted on by some external body ; and the attraction
between the moving body and the earth, according to
the Law of Gravitation, that any two bodies attract each
other with a "force" varying (i) inversely as the square
of the distance between them, and (2) directly as the
product
of their masses. But the most fundamental
"
"
law by law
explanation of
is attained when a given
law can be shown to be a particular case of a more
general law. Newton s explanation of Kepler s Laws by
the Law of Gravitation affords an impressive instance of
this, and is also a perfect example of what we have
called the complete Inductive Method. The process by
which the first (and essential) part of Newton s great
generalisation was established may be analysed as
follows, according to the four stages mentioned earlier
in this section.
Newton s own genius, taking up facts of observation
and suggestions thrown out by previous investigators,
led him to formulate this law as an hypothesis : Any two
bodies attract one another with a force varying inversely
as the square of the distance between them. If this
hypothesis is true, the weight of an object (the pull
exerted upon it by the mass of the 1
earth ) should
decrease as its distance from the earth increases.
Within those short distances from the earth s surface
to which our observation extends, the intensity of gravity
does not appreciably diminish as we recede from the
1 The pull which the said object exerts upon the earth is of course
a real fact, but, in comparison with the earth s attraction, may be
reckoned as practically nothing.