The stream takes its rise at some position on the channel (e.g., at a5a'5), the flow per unit of time at
that point being proportioned to the energy put forth in pumping from a certain source. As the
volume thus originated rolls down the channel, it continually increases by infiltration from the
neighbouring soil without any additional pumping, so that, the depth being preserved constant, the
volume is proportioned to the increasing breadth.84 Besides this increase due to its defluxion, the
volume may also in the course of its downward flow be increased by additional pumping from a
second source (e.g., a2a'2). This second increase corresponds to an increase in depth (not shown in
the figure); and this second contribution is augmented, like the first, by the infiltration which attends
defluxion. There may be as many sources as there are circles out by the descending stream. But there
need not be a source at each interval. The equidistant circles correspond to successive lines, not
always coincident with successive stages of production at each of which additional labour is
applied.85 The train of production thus represented terminates in a product ready for consumption—it
may be loaves or ribbons, wine or shoes—on the shore of a circumfluent sea of commodities. As in
the natural world rivers are replenished by the melting of the snow, which is formed on mountains
by the congelation of vapour, which is wafted up from the ocean, into which the rivers flow down,
so in the mundus economicus, by a compensation carried into more just detail, labour is restored and
recreated by a refreshing rain of commodities derived from that sea into which all finished
commodities are discharged. Volatile shoes and wine, and other commodities in due admixture up
to a certain value, find their way to each point upon the heights from which a source has been tapped,
the volume of this return corresponding to the volume of the original contribution,—not indeed the
same, but the same increased by a factor of accumulation, the ratio which the breadth of the stream
at the littoral bears to its breadth at the point of origin (e.g., a1a'1: a5a'5). The flight of the
commodities from the littoral to the heights need not be supposed to occupy an appreciable time.
The idea of a Flow which has been illustrated is primarily applicable to the case in which
materials and consumable commodities are used up once for all within a unit of time. But the case
of labour invested for longer periods is easily assimilated. Suppose that a plough lasts five years, and
that in each year of its existence it makes an equal addition to the consumable crop the year being
taken as the unit of time. Then, although the plough may have been made in a week or month, the
labour of its production is to be considered as invested in five unequal portions at unequal distances
in time from the epoch at which the invested labour meets with its return. The total labour of making
84. The broadening of the stream corresponds to the two consilient facts, that future pleasures are
discounted and that production is increased by “roundabout” methods. As to the first of these facts,
see in Marshall's Principles of Economics the passages which relate to discounting future pleasures,
and the remarks on those passages in the review of the second edition of the Principles in the
Economic Journal, Vol. I. (1891) p. 613. See also the admirably clear explanation and illustration
given by Professor Carver in his article on “Abstinence and the Theory of Interest,” Quarterly
Journal of Economics, Vol. VIII (1893) p. 48. As to both the first and second facts, see Bohm-
Bawerk's well-known expositions. But as to the congilience of the two facts see, rather, Professor
Marshall on the “fundamental symmetry” between the action of Supply and Demand (noticed in the
review referred to). See also Professor Carver s explanation of the double statement that interest is
payment for the sacrifice of abstinence, and that interest is paid because capital is productive (loc.
cit. p. 43).
85. Corresponding to the machines in the illustration given in the preceding paragraphs.