this kind at any rate which does not greatly exceed the mere cost of loading and unloading it and be better off for so doing. If the entire difference between the cost of the stone at B and the cost of producing it at A from some other source is a very slight one, the amount of it still represents all that the ship can get for carrying the stone. The utmost that the traffic will bear is this difference in costs; and yet the business will be accepted, for the return exceeds the merely variable costs which it entails. The fixed charges, the interest on the cost of the vessel, and the outlay for maintaining it do not need to be paid in any part from the returns of this extra business. They are already provided for. If instead of returning from B with a hold quite empty, the vessel made both voyages with a hold only half full, the result would be similar. It would then be in a position to make a low bid for further freight in both directions. If this entails no cutting of the rates for carrying the original goods, the vessel can take further goods with advantage at any rate above the merely variable costs. _Production which is Advantageous though it does not repay all Costs._--There are two general conditions under which it is advantageous, both in making goods and in carrying them, to extend production, though the further returns which are in this way gained do not cover all costs. First, the producer must have an unused capacity for making or carrying goods. In such a case it is possible to make or carry an _increment_ of goods without entailing on himself an increment of cost that is proportionate to the amount carried. In his bookkeeping his original business is charged with costs amounting to a certain sum per unit of goods produced or carried. His further business is charged with a smaller outlay per unit.