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

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284
THE THEORY OF INDUCTION
"
more precisely quantitative in the middle of the
;
nineteenth century most of the phenomena of electricity
now
and electro-magnetism were known merely as facts ;
they can be for the most part exactly
calculated.
measured and
As soon as phenomena can be measured, there arises
the possibility of a more precise form of either of the two
primary
Methods. This is the Method of Concomitant
variations, the canon of which is thus stated by Mill :
Whatever phenomenon varies in any manner whenever
another phenomenon varies in some particular manner,
is either a cause or an effect of that phenomenon, or is
connected with it through some fact of causation.
When the variations are ascertained by experiment, this
may be regarded
as a modification of the Method of
Single Difference.
A simple but excellent example of this Method is given by
Mill (Logic, Bk. III. viii. 7), the experimental proof of the
First Law of Motion. This law states that all bodies in
motion continue to move in a straight line with uniform
velocity until acted on by some new force.
"
This asser
"
tion," says Mill, is in open opposition to first appearances ;
all terrestrial objects, when in motion, gradually abate their
velocity and at last stop. . . . Every moving body, however,
encounters various obstacles, as friction, the resistance of
the atmosphere, &c., which we know by daily experience to
be causes capable of destroying motion. It was suggested
that the whole of the retardation might be owing to these
causes. How was this inquired into? If the obstacles
could have been entirely removed, the case would have
been amenable to the Method of Difference. They could
not be removed, they could only be diminished, and the
case, therefore, admitted only of the Method of Concomitant
Variations. This accordingly being employed, it was found
that every diminution of the obstacles diminished the re
tardation of the motion ; and inasmuch as in this case the
total quantities both of the antecedent and consequent were