Those enterprises which necessarily produce different products at the same time deserve special consideration.(639) Here we may speak of “_united_ costs of production,” and all that is needed is that the aggregate of these costs should be covered by the aggregate price of both products. This complicates to a certain extent the calculations which the seller must make to determine his minimum demand for each product. To ascertain this, he must subtract from the united costs of production the amount of value which he expects with certainty for the other product.(640) Section CVII. Equilibrium Of Prices. Goods whose cost of reproduction,(641) that is, the highest necessary cost of reproduction is the same, have uniformly the same value in exchange. Every deviation from this level immediately sets forces in motion which endeavor to restore the level, just as the water of the sea seeks its level, notwithstanding the mountains and abysses which the winds bring forth from its bosom.(642)(643) Section CVIII. Effect Of A Rise Of Price Much Above Cost. If the market price rises high above the cost of production, producers make a profit greater than the average profit made in the country. This induces them, by the appropriation of new land and the employment of new labor and capital, to increase their business. Other parties also engage in this profitable department of trade. This competition not only makes the means of production dearer, but must eventually, by increasing the demand, reduce the price of the product to the ordinary level of profit, that is to an equilibrium with other commodities.(644) Hence, in the beginning, every diminution of the cost of production(645) turns to the advantage of the producer; but afterwards and permanently to that of the consumers: an economic law exceedingly beneficent in its operations, and not unlike the action of positive legislation in the matter of patents. There is no greater stimulus to the making of improvements than the certainty of reward to the person who first introduces one. The moment, however, that the improvement is imitated by all producers, the advantage gained by it becomes the common good of the whole nation.(646) These are, as J. B. Say says, conquests made over the gratuitous productive force of nature. As a consequence, the value in use of a people’s resources increases; generally, also, their value in exchange, in so far as the production of the now cheaper goods increases in a degree greater than their cost of production has diminished.(647)