_The Rent of an Instrument measured from the Intensive Margin._--We can measure the product of any instrument in this way. If it is a ship, it takes labor to sail it and requires a considerable amount of auxiliary capital. We must fill the bunkers with coal, stock the steward's department with provisions, furnish and light the staterooms and the saloons, and provide cordage and a wide variety of other ship stores. All this labor and all this capital we could take out of the ship and use elsewhere. We could convert them into marginal labor and capital. We could divide them among the owners of other ships where they would be used in a way that would make these other ships somewhat more efficient and cause each of them to earn a little more than it now earns. Whatever the labor and capital could, in this way, produce furnishes the basis for computing the rent of the ship. Subtract it from the total joint product of labor, capital, and ship, and you have what the vessel separately earns. _The Mode of Testing the Productive Power of a Ship._--Put the labor and capital into the ship and set it doing its proper work of carrying freight and passengers, and you cause a certain product to be created. The steamship company gets an aggregate amount for the service it renders by means of the labor, the auxiliary capital, and the ship. A certain smaller amount would be realized if the labor and the auxiliary capital were taken out of the ship, distributed, and used in the way we have just described. The difference between the two amounts is the rent of the ship, or its particular contribution to the general product. This gives us a formula for computing the rent, not only of land, but of buildings, tools, machines, vehicles, and every other concrete instrument of production. The formula, indeed, is so general that it enables us to compute the earnings of any agent whatsoever. _The rent of any such agent is what it adds to the marginal product of labor and capital used in connection with it._