oncomitance. It is often a matter of convenience whether we regard
concomitant phenomena as varying directly or inversely. It is usual to
say--'the greater the friction the less the speed'; but it is really
more intelligible to say--'the greater the friction the more rapidly
molar is converted into molecular motion.'

The Graphic Method exhibits Concomitant Variations to the eye, and is
extensively used in physical and statistical inquiries. Along a
horizontal line (the abscissa) is measured one of the conditions (or
agents) with which the inquiry is concerned, called the Variable; and
along perpendiculars (ordinates) is measured some phenomenon to be
compared with it, called the Variant.

Thus, the expansion of a liquid by heat may be represented by measuring
degrees of temperature along the horizontal, and the expansion of a
column of the liquids in units of length along the perpendicular.

[Illustration: FIG. 9.]

In the next diagram (Fig. 10), reduced from one given by Mr. C.H. Denyer
in an article on the Price of Tea (_Economic Journal_, No. 9), the
condition measured horizontally is Time; and, vertically, three variants
are measured simultaneously, so that their relations to one another from
time to time may be seen at a glance. From this it is evident that, as
the duty on tea falls, the price of tea falls, whilst the consumption of
tea rises; and, in spite of some irregularity of correspondence in the
courses of the three phenomena, their general causal connection can
hardly be mistaken. However, the causal connection may also be inferred
by general reasoning; the statistical Induction can be confirmed by a
Deduction; thus illustrating the combined method of proof to be
discussed in the next chapter. Without such confirmation the proof by
Concomitant Variations would not be complete; because, from the
complexity of the circumstances, social statistics can only yield
evidence according to the method of Agreement in Variations. For,
besides the agents that are measured, there may always be some other
important influence at work. During the last fifty years, for example,
crime has decreased whilst education has increased: true, but at the
same time wages have risen and many other things have happened.

[Illustration: FIG. 10.]

It will be noticed that in the diagram the three lines, especially those
of Price and Consumption (which may be considered _natural_ resultants,
in contrast with the arbitrary fixation