Again. "Those, therefore, who have the command of more food than they themselves can consume, are always willing to exchange the surplus, or, what is the same thing, the price of it, for gratifications of another kind. What is over and above satisfying the limited desire, is given for the amusement of those desires which cannot be satisfied, but seem to be altogether endless. The poor, in order to obtain food, exert themselves to gratify those fancies of the rich; and to obtain it more certainly, they vie with one another in the cheapness and perfection of their work. The number of workmen increases with the increasing quantity of food, or with the growing improvement and cultivation of the lands; and as the nature of their business admits of the utmost subdivisions of labours, the quantity of materials which they can work up increases in a much greater proportion than their numbers. Hence arises a demand for every sort of material which human invention can employ, either usefully or ornamentally, in building, dress, equipage, or household furniture; for the fossils and minerals contained in the bowels of the earth, the precious metals, and the precious stones." Adam Smith has justly observed, that it is extremely difficult to determine the rate of the profits of stock. "Profit is so fluctuating, that even in a particular trade, and much more in trades in general, it would be difficult to state the average rate of it. To judge of what it may have been formerly, or in remote periods of time, with any degree of precision, must be altogether impossible." Yet since it is evident that much will be given for the use of money, when much can be made by it, he suggests, that "the market rate of interest will lead us to form some notion of the rate of profits, and the history of the progress of interest afford us that of the progress of profits." Undoubtedly if the market rate of interest could be accurately known for any considerable period, we should have a tolerably correct criterion, by which to estimate the progress of profits.