Front Page Titles (by Subject) Expression for Amount of Investment. - The Theory of Political Economy
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Expression for Amount of Investment. - William Stanley Jevons, The Theory of Political Economy 
The Theory of Political Economy (London: Macmillan, 1888) 3rd ed.
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Expression for Amount of Investment.
To render our notions of the subject still more exact and general, let us resort once more to mathematical symbols.
Let Dp = amount of capital supposed to be invested in the time Dt; let t = time elapsing before its result is enjoyed, the enjoyment taking place in an interval of time Dt, which may be disregarded in comparison with t. Then t · Dp is the amount of investment; and if the investment is repeated, the sum of the quantities of the nature of t · Dp, or, in the customary mode of expression, St · Dp is the total amount of investment. But it will seldom be possible to assign each portion of result to an exactly corresponding portion of labour. Cotton goods are due to the aggregate industry of those who tilled the ground, grew the cotton, plucked, transported, cleaned, spun, wove, and dyed it; we cannot distinguish the moment when each labourer's work is separately repaid. To avoid this difficulty, we must fix on some moment of time when the whole transaction is closed, all labour upon the ground repaid, the mill and machinery worn out and sold, and the cotton goods consumed. Let t now denote the time elapsing from any moment up to this final moment of closing the accounts. Let Dp be as before an increment of capital invested, and let Dq be an increment of capital uninvested by the sale of the products and their enjoyment by the consumer. Thus it will be pretty obvious that the sum of the quantities t · Dp, less by the sum of the quantities t · Dq will be the total investment of capital, or expressed in symbols