resence of _p_, differing otherwise from

B C
_q r_

in nothing except that A is also present. By the Canon of Difference,
therefore A is the cause of _p_. Or, again, when phenomena thus treated
are strictly quantitative, the method may be based on Prop. III. (b),
ch. xv. Sec. 7.

Of course, if A can be obtained apart from B C and directly experimented
with so as to produce _p_, so much the better; and this may often be
done; but the special value of the method of Residues appears, when some
complex phenomenon has been for the most part accounted for by known
causes, whilst there remains some excess, or shortcoming, or deviation
from the result which those causes alone would lead us to expect, and
this residuary fact has to be explained in relation to the whole. Here
the negative instance is constituted by deduction, showing what would
happen but for the interference of some unknown cause which is to be
investigated; and this prominence of the deductive process has led some
writers to class the method as deductive. But we have seen that all the
Canons involve deduction; and, considering how much in every experiment
is assumed as already known (what circumstances are 'material,' and when
conditions may be called 'the same'), the wonder is that no one has
insisted upon regarding every method as concerned with residues. In
fact, as scientific explanation progresses, the phenomena that may be
considered as residuary become more numerous and the importance of this
method increases.

Examples: The recorded dates of ancient eclipses having been found to
differ from those assigned by calculation, it appears that the average
length of a day has in the meanwhile increased. This is a residuary
phenomenon not accounted for by the causes formerly recognised as
determining the rotation of the earth on its axis; and it may be
explained by the consideration that the friction of the tides reduces
the rate of the earth's rotation, and thereby lengthens the day.
Astronomy abounds in examples of the method of Residues, of which the
discovery of Neptune is the most famous.

Capillarity seems to be a striking exception to the principle that water
(or any liquid) 'finds its level,' that being the condition of
equilibrium; yet capillarity proves to be only a refined case of
equilibrium when account is taken of the forces of adhesion exerted by
different kinds of bodies in contact.

"Many of the new elements of Chemistry," says Hersche