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the price to be the same, so that the last pound she then buys hasthe same utility as the last pound she previously bought. She maybuy fifteen pounds. A fortnight later only five, all depending onher plans for using it. The whole yearly purchase may De 250 lbs.and we may write :
ut. (10th lb.) Jan. 1 = ut. (15th lb.) Jan. 15.
= ut. ( 5th lb.) Jan. 30.
-- etc.
= ut. (250th lb.) for whole year.
Thus : The marginal utility of h certain quantity of a commodityfor a given period (say a year) is defined to he the marginal utilityof that commodity on all occasions during that year at which it isbought or consumed, the sum of the individual purchases being thegiven yearly purchase and consumption.
§ 10 .
In the hypothetical case the marginal utility of 250 pounds peryear equalled the marginal utility of 10 cts. In the same mannerwe may practically estimate the marginal utility of 200 pounds bysupposing the price to be such that our housewife would buy 200pounds. Thus a number of alternative suppositions are made forthe same period. By means of these a utility curve can be con-structed, one of the coordinates of which is the yearly consumptionof sugar. To do this statistically is of course quite a different andmore difficult though by no means hopeless proceeding.
Curves of this nature are the only ones to be here considered.But it is clear that there also exist utility curves for each time ofpurchase.* These would differ both from the “ yearly ” curve aswell as from each other.
§ 11 .
To meet a possible objection it must be pointed out that the useof a “ yearly ” utility curve assumes no nice calculation on the partof the individual as to his future income and receipts. He may evenbe and generally is totally ignorant of the number of pounds ofbuttei he consumes per year. He creeps along from purchase topurchase and only at these individual acts does he estimate his needsand his abilities. Yet if he always completes his purchase with thesame estimate of marginal utility as measured against other com-
* They would be the curves of Fleeming Jenkin: Graphic Representation ofSupply and Demand. Grant’s Recess Studies, p. 151, Edinburgh, 1870,