APPENDIX

299

in Fig. (i) to that represented by QM, involves an increase
of the quantity demanded. ON units were demanded at
the price PN, OM units are demanded at the price QM.
The increase in the quantity demanded, while the general
relation of the intensity of demand to the degree of plenty of
the supply remains unchanged, that is to say, while the
same demand curve continues to represent the condition of the
market, is what is called an extension of demand on p. 30.
A true change of demand occurs when the demand
curve has to be replaced by another. Thus, if the curve D4
in Fig. (i) be replaced by the broken-line curve on the
diagram, an increase of demand is indicated. The curve
Dd is moved upwards, with more or less of change of shape,
in taking the position of the broken line. Each point on the
broken line is above the point where its ordinate cuts Dd.
The point on the broken line which is as high above Ox as
P is, lies farther out than P from Oy. An increase of
demand, in the sense of the statement of p. 30, is what
corresponds to the replacement of a demand curve by one
lying above it, as in replacing the full-line curve Dd in
Fig. (i) by the broken-line curve, The inverse change will
represent a decrease of demand.
The fundamental contrast between the two kinds of
change, which are covered by the carelessly used phrases
about increase or decrease of demand, wiil be clear in the
light of their
diagrammatic
representations.
If we take
two points, P,Q,
near together on
a demand curve
Dd, as in Fig.
(ii), and draw
the ordinates
PN, QM, then
PN, QM represent
 the valuations,
 per unit,
of the commodity,
 whose
demand is in
question,accordingas
 thesupply

Fig. (un,