Sec. 4] : UTILITY 45

whatever. Reference may be had to a specified group of
goods now existing, or to a specified group of goods in the
future, or to a specified flow of goods through a period of
time. For instance, the marginal desirability of coal to an
individual may be taken to refer to the particular stock of
coal in his bin at the present moment. If this stock consists
 of fifteen tons, its marginal desirability is the desirability
 of the fifteenth ton, or the difference to him between
the desirability of having fifteen and that of having fourteen
 tons. Or, reference may be had to an intended purchase
 of coal to be delivered in three months. If we consider
 a possible purchase of future coal to the extent of
fifteen tons, its marginal desirability then represents the
present desire for the fifteenth ton, in exactly the same
way as though reference were had to an existing stock.
Again, if a person is consuming in his household fifteen tons
of coal a year, its marginal desirability at any instant is
the desirability of the fifteenth ton, or the sacrifice which
would be occasioned were he to reduce his yearly consumption
 from fifteen tons to fourteen.
Again, the group of goods considered may consist of articles
 all of which are of the same kind, or of a heterogeneous
collection. In the preceding examples the goods were of
exactly the same kind. As an example of the marginal
desirability of a group consisting of diverse kinds, we
may cite the desirability of an additional monthly magazine
or newspaper. If a subscriber is already taking ten periodicals
 of different kinds, the desirability of a specified journal
additional to the existing assortment may be regarded as
the marginal desirability with reference to the entire group
of journals.
In the same way we may speak of the marginal desirability
 of a series of characteristics or features connected with
any article or articles of wealth. A person contemplating
the building of a house may have to decide how many windows
 he will putin. If he contemplates fifty windows, the