ent series of tools,
buildings, and other active capital goods shows forever the same
gradations of quality that are found in the case of land. There are
always to be found some instruments which are producing a large
amount--that is, they are adding a large amount to the product of the
labor and the further capital that are combined with them in
production. A given amount of labor and capital creates much more
wealth when working with a machine of the highest class than it would
if distributed in marginal positions; and this is equivalent to saying
that such an instrument is itself highly productive. Other instruments
are to be found which are creating less, and there is never wanting a
grade of no-rent instruments which are adding nothing to the marginal
product of the other agents. It would be as well for the labor that
used them if it should drop them and add itself to the force which is
working with good instruments. Any one manufactured instrument begins
its career as a maximum-rent instrument and ends it as a no-rent one.
The ship is at its best when it starts on its first voyage, and the
mill is at its best in the first year of its running. Each instrument
goes gradually downward in the scale till it reaches a stage in which
it really produces nothing, since it adds nothing to what would be
produced without it. The _permanent series_ of instruments never thus
deteriorates. All the depreciation of particular things is made good
by the repairing and the replenishing which go on. In the series as a
whole there are forever present grade number one, grade number two,
grade number three, etc., exactly as in the case of land. If we wish,
we can reckon the income that is to be gotten from each part of the
series according to the old-time formula that is familiarly used in
the case of land, "What labor and capital create by the use of this
piece of ground in excess of what they would create if they were
applied to the poorest land in use." For a grade of land read a grade
of the self-perpetuating series of artificial instruments, and it will
appear that each grade above the poorest yields, with the labor and
capital that are combined with it, a surplus above what this labor and
this capital could create if they were combined with the poorest grade
in the permanent series.

_Different Modes of Destroying and Replenishing Stocks of Capital
Goods of the Two General Classes._--The process of keeping up a stock
of tools of t