ion from an existing
sum of circumstances. But that is misleading in so far as it suggests
that the two instances must be separate sets of circumstances,
is shown by the fact that it misled himself when he spoke of the
application of the method in social investigations, such as the effect
of Protection on national wealth. "In order," he says, "to apply to
the case the most perfect of the methods of experimental inquiry, the
Method of Difference, we require to find two instances which tally in
every particular except the one which is the subject of inquiry.
We must have two nations alike in all natural advantages and
disadvantages; resembling each other in every quality physical and
moral; habits, usages, laws, and institutions, and differing only in
the circumstance that the one has a prohibitory tariff and the other
has not." It being impossible ever to find two such instances, he
concluded that the Method of Difference could not be applied in
social inquiries. But really it is not necessary in order to have two
instances that we should have two different nations: the same nation
before and after a new law or institution fulfils that requirement.
The real difficulty, as we shall see, is to satisfy the paramount
condition that the two instances shall differ in a single
circumstance. Every new enactment would be an experiment after the
Method of Difference, if all circumstances but it remained the same
till its results appeared. It is because this seldom or never occurs
that decisive observation is difficult or impossible, and the simple
method of difference has to be supplemented by other means.

To introduce or remove a circumstance singly is the typical
application of the principle; but it may be employed also to compare
the effects of different agents, each added alone to exactly similar
circumstances. A simple example is seen in Mr. Jamieson's agricultural
experiments to determine the effects of different manures, such as
coprolite and superphosphate, on the growth of crops. Care is taken
to have all the antecedent circumstances as exactly alike as possible,
except as regards the agency whose effects are to be observed. A field
is chosen of uniform soil and even exposure and divided into plots:
it is equally drained so as to have the same degree of moisture
throughout; the seed is carefully selected for the whole sowing.
Between the sowing and the maturing of the crop all parts of the
field are open to the same wea