_True Capital rather than Capital Goods moved in Making such Tests of Productivity._--In applying these tests with scientific accuracy we should take away the true _capital_ used in connection with a rent-paying instrument and use it as marginal capital elsewhere, rather than take away the particular concrete thing in which that capital is now embodied. In the case of the ship the accurate test is made, not by taking stores, etc., bodily out of it and putting them into other ships, but by letting the stores first earn what they can where they are, converting the earnings into money, and, when the stores are completely used up, spending the money to procure marginal additions to the outfit provided for the other ships. _One Difference between Land and Artificial Capital Goods._--In the case of land a particular area is marginal or no-rent land, and, in a static state, it remains so. Any particular ship, wagon, engine, or other made tool begins its career as a rent payer and ends it as a no-rent instrument. If we watch the whole social stock of instruments of production, we shall see the no-rent points not fixed in location, but shifting from place to place. Now this machine, now another, and now still another reaches the unproductive state and is supplanted by instruments of similar kind that are new and efficient. _Original Elements in the Soil._--The real difference between the rent of a piece of land and that of a building, machine, vehicle, or any similar instrument arises from the fact that the land is not going to destruction and the artificial instrument is. There are elements in what is commonly called land that wear out as do the tools that are used in tilling it, but these elements are not land in the economic sense. Land, as Ricardo long ago said, consists in the "original and indestructible powers of the soil." He singles out certain constituent elements of every farm, forest, building site, or other piece of what is called land in ordinary usage, and gives to this new concept the name _land_ in an economic sense. These so-called "powers" are original elements because man does not make them; they are provided altogether by nature, and the only way in which man may be said to impart any productive power to them is by putting them into combinations in which they can produce. When men settle upon what has been vacant land, they bring the land into combination with labor, and when they break up the land for tillage and put buildings on it, they combine it with artificial capital. By means of these combinations land acquires productive power; but physically considered, it is altogether a natural product.