al crisis, must wait for a favourable opportunity. Again,
in making experiments we can vary the conditions of the phenomenon, so
as to observe its different behaviour in each case; whereas he who
depends solely on observation must trust the bounty of nature to supply
him with a suitable diversity of instances. It is a particular advantage
of experiment that a phenomenon may sometimes be 'isolated,' that is,
removed from the influence of all agents except that whose operation we
desire to observe, or except those whose operation is already known:
whereas a simple observer, who has no control over the conditions of the
subject he studies, can never be quite sure that its movements or
changes are not due to causes that have never been conspicuous enough to
draw his attention. Finally, experiment enables us to observe coolly and
circumspectly and to be precise as to what happens, the time of its
occurrence, the order of successive events, their duration, intensity
and extent.

But whether we proceed by observation or experiment, the utmost
attainable exactness of measurements and calculation is requisite; and
these presuppose some Unit, in multiples or divisions of which the
result may be expressed. This unit cannot be an abstract number as in
Arithmetic, but must be one something--an hour, or a yard, or a
pound--according to the nature of the phenomenon to be measured. But
what is an hour, or a yard or a pound? There must in each case be some
constant Standard of reference to give assurance that the unit may
always have the same value. "The English pound is defined by a certain
lump of platinum preserved at Westminster." The unit may be identical
with the standard or some division or multiple of it; and, in measuring
the same kind of phenomena, different units may be used for different
purposes as long as each bears a constant relation to the standard.
Thus, taking the rotation of the earth as the standard of Time, the
convenient unit for long periods is a year (which is a multiple); for
shorter periods, a day (which is identical); for shorter still, an hour
(which is a division), or a second, or a thousandth of a second. (See
Jevons' _Principles of Science_, ch. 14.)

Sec. 6. The principle of Causation is the _formal_ ground of Induction; and
the Inductive Canons derived from it are means of testing the formal
sufficiency of observations to justify the statement of a Law. If we can
observe the process of cause and effect