Riches and poverty affect the estimate of sacrifice involved in the use of goods. The more highly the person is able to value his own time, because of his riches, income, etc., the more highly he values the earlier uses of duplicates to save his time, and the less highly he values the later uses of goods which tax his time. Each article in the scanty belongings of the poor is used a great deal ; each article in the abundant belongings of the rich is used very little, and the goods of the middle class occupy positions ranging between these ex tremes.
§ 4. Sacrifice involved in common use by several users. It often may happen that the further uses contained in goods may be taken by another person, possibly by several, each of whom may bring to the use his freshest powers. This may enhance the usefulness of the good, but again sacrifice may result from the inconvenience of common use, and sooner or later the outlay-value exceeds the gross income-value of the uses thus secured. A book stands many hours untouched on the shelves of the library; it has potential uses for some one during every minute of the twenty-four hours but they can be secured only with inconvenience. When, as often hap pens, two or more persons wish to use the book at the same hour, time and energy are wasted. The greatest net uses, therefore, are seen to be to the first user and in the first hour, for these uses cost the least time and trouble. If the mem bers of a family will take turns, one chair will serve for all of them ; but if all are to be able to sit down together, a chair must be provided for each. Often it will happen that only one chair is in use, the other nine chairs being valued only for their potential uses. I knew two young men who owned a dress-coat in partnership, and as they had different even ings free from business all went well until both were invited to a reception which both were very eager to attend. In tene ment houses there are sometimes let to lodgers beds or bunks that are never allowed to get cold, men working in the day occupying them by night, and men working by night sleeping in them by day.
§ 5. Gross and net uses. In cases of this kind each use of the chair, or the book, etc., is technically the same as every other use, but economically these uses must be looked upon as a series of unlike valuation-magnitudes. The value of each use is a net amount resulting from subtracting the outlay from the in come-value. Even if the
income-values remained un diminished by bringing in new uses, the outlay-values would be increasing, and the net use-value corre spondingly lessening. A point is reached where the net value of the use falls to zero. Thus in Figure 20 if on the line BB? be represented the height of successive gross uses and on the line CB? the height of the successive outlays, net uses are etc., until at there is no further net use.
§ 6. The doctrine of separable uses. We may think of the use of an object not as one undivided whole, but as a group of uses, separable each from the other. Likewise any one of these uses is separable from the use-bearer considered as a concrete object. Every economic good, the use of which can be spread throughout a period of time, may be looked upon as consisting of groups of uses. The simplest case of this results from mere physical divisibility. A basketful of peaches is eaten piece by piece; each peach may be counted as one use. In this case the use is consumptive: it is not sep arable from the using up of the use-bearer itself.
The truly separable use, however, is found in the case of a durable agent which continues through a period of time to render a series of durative uses. A very perfect example is a diamond necklace, the sparkle and charm of which is a use which is absolutely without detriment to the use-bearer (i.e., the necklace, tho perhaps not to the wearer). Many agents have this enduring quality more or less fully (see Chapter 11 on Consumption and Duration), and in all such cases the durative use may be treated practically and theoretically in economics as something separable from the use-bearer, in mat ters of valuation, of trade, and of price. The peaches may be gathered without harming the fitness of the trees to pro duce another crop. Shelter is furnished, once the house is built, without destroying the house more than disuse would, if it stood tenantless. The horse is the better if driven mod erately each day, and the carriage lasts for years. In every such case, the use is something different from the "using up" of a limited number of goods, for there is left in the use-bearer the power to go on yielding indefinitely some more of the same product or use if it is kept in repair.