It was William Stanley Jevons
(1871: Ch. VII) who first conceived of capital as being characterized *as* time. More
specifically, capital is "the aggregate of those commodities which are required for
sustaining labourers of any kind or class engaged in work...The single and all-important
function of capital is to enable the laborer to await the result of any long-lasting work,
-- to put an interval between the beginning and the end of an enterprise" (Jevons,
1871: p.223). Thus, for Jevons, capital is merely a subsistence fund that goes into
maintaining the labor inputs working on the production of goods whose fruit will only
emerge in a later time.

Jevons (1871: p.229) distinguishes
between "the amount of capital invested" and the "amount of investment of
capital". The difference between these two concepts is subtly worded but nonetheless
substantial. The "amount of capital invested" is the "physical
quantity" of inputs into the production process while the "amount of investment
in capital" is that physical quantity of inputs multiplied by the *length* of
time in which each input has been in the production process.

To understand this, consider Figure 1 where we have plotted time against
increments of input additions. Consider a process of production which begins at time t_{0}
and ends at time t_{8}. This is what Ragnar Frisch (that lord of economic
nomenclature!) called a "flow-input, point-output" situation: where inputs are
added over time to a production process until a particular final time period, when all the
output is realized at once. The histogram in Figure 1 represents the "lumpy"
addition of inputs into this process over (discrete) time. The addition in any period is
represented by the lightly shaded rectangles. Thus, at t_{0}, we implement amount
I_{0} of inputs into the process, at time t_{1}, we add I_{1}
worth of inputs, and so on until the final time period t_{8} where we add I_{8}
worth of inputs and our production process ends. Time t_{8}, the final time
period, is when output is finally realized.

Fig. 1- Jevons's Triangle

If we were in continuous time, then our addition of inputs would be continuous until the final time period. This is represented heuristically in Figure 1 by the straight diagonal line stretching from the origin (not perfectly drawn). We begin at time t = 0 and end at time t = T, adding inputs continuously and, in the final time period, have I* worth of inputs "stored" in the production process.

What is the quantity of capital? According to Jevons, the "amount of
capital invested" is merely the sum of all the inputs during the production process -
or the sum of the heights of the *lightly*-shaded rectangles in our figure. In Figure
1, this adds up to I*, i.e. I* = I_{1} + I_{2} + .. + I_{8} in
discrete time. In general, for discrete time production process from an initial time
period t = 0 to a final time period t = T:

amount of capital invested = E/font>

^{T}_{t=0}I_{t}= I*

However, what Jevons called the "amount of investment of
capital" is the sum of the inputs multiplied by the length of time of production -
thus it is the *entire area* of the histogram, or the entire area of the triangle
formed by the production process. Thus:

amount of investment of capital = E/font>

^{T}_{t=0}(T-t)I_{t}

which is the area of the histogram in Figure 1.

What about in continuous time? In this case, the amount of capital
invested is simply I* = E/font> _{0}^{T} I(t) dt
where I(t) is the investment rate. For the amount of investment in capital, it is useful
to appeal to what Jevons called the "average time of investment" (Jevons, 1871: p.231) which is simple the average of
the production period, which can be defined in continuous time as q
= T/2 as shown in Figure 1. With the average time of investment so defined, then the
"amount of investment of capital" can be expressed as q
I* = TI*/2 - which is, as we see, the precisely the area of the triangle depicted in
Figure 1. This "amount of investment of capital" is, quite simply, the
measurement of the quantity of capital for Jevons - a concept quite similar to that
derived from the "average period of production" of Eugen von Böhm-Bawerk (1889).