_The Mode of Calculating the Rent of Concrete Instruments._--Now the rent of such instruments of production, whether artificial or not, can be measured in exactly the same way in which the rent of land is measured. We saw that there are two margins of utilization of land, an extensive and an intensive one, and that the product of labor and capital at either of these margins may be used as a basis for computing the surpluses which constitute the rent of the land. The landlord gets from a good field what it produces minus what the labor and capital that are used on this field would produce if they were used on the poorest land in cultivation; or, what is the same thing, he gets from the field what it produces minus what this labor and capital would produce if they were set working somewhere on the intensive margin of cultivation. Take the men out of this field, add them in small detachments to the men who are already cultivating other fields, in order that such fields may be tilled a little more intensively, and measure the product which the laborers create when they are so placed. Withdraw also the capital from the field, add it, in small amounts, to the capital that is working elsewhere, and measure its specific product. The sum of these two specific products is the same amount that is arrived at by using the former standard. This labor and capital, formerly used on the good field, scattered as they now are among the users of other good land, will create the same amount that they would have created if they had been employed on the poorest land in cultivation. This amount is, as it were, what they produce by their own unaided power; and whatever is produced in excess of this amount when a good field comes to their assistance is the rent of that field, for it is the contribution which the field makes to the joint production. Total product of land, labor and auxiliary capital minus the product created by the labor and auxiliary capital when these agents are put in marginal positions equals the rent of the land.