Various minerals useful to man are also sources of rent. They differ from arable land in this important particular: they possess not, like the fertility of the soil, the advantage of being perpetually renewed by the spontaneous bounty of nature; but they have been stored away, by the same benignant agency, in the bowels of the earth, whence they are drawn at great cost, and always by a dimi-nution of their quantity. It depends upon the cost of working them, and their relative scarcity, whether they are able to yield rent; for when these minerals are very abundant near the surface of the earth, their price may not be more than sufficient to repay the labor and capital expended in bringing them to market.

One of the most valuable of these minerals is coal, as fuel, both for household purposes and for manufactures, especially for the fabrication of iron. It is to this mineral that England is mainly indebted for the great extent and excellence of her manufactures; for it constitutes the fuel of her steam-engines, which furnish her, at a moderate cost, with an unlimited motive power.

As population increases, the demand for coal has a correspondent increase, but the cost of extracting it from its native beds naturally augments as the mines grow deeper. This additional cost may, however, be counterbalanced by improved machinery and a larger application of capital. Should the increased demand for coal have more effect in raising its price than the counteractions mentioned have in lowering it, then all of its increased value, beyond the cost of bringing it to market, would be rent. In this way, coal-mines, exempt from neighboring competition, sometimes yield a high rent.

Next to coal, iron is the mineral most extensively used by man. It has, however, been furnished in such abundance by nature, that its ordinary price merely repays the labor and capital expended in converting its ore into metal, with a more or less liberal profit, without yielding any rent. Furnaces and forges for making iron may indeed be occasionally leased, but the consideration paid by the tenant, which is called a rent, like that paid for houses, is merely an interest on the capital expended on the buildings and machinery. Where, however, the machinery of these establishments is worked by running water, the gift of nature, this moving-power, like that of mills, may be an exception, and yield an annual rent.

Mines of the precious metals, when they can be monopolized, may yield a large rent, as the value of the products may greatly exceed the cost of obtaining them. Where the lands containing the precious metals are free to be worked by all, and when the yield is so great as it has been in California and Australia, the profits being the joint result of the bounty of nature and the industry of man, seem in part analogous to rent; and where spots of extraordinary richness are regarded by custom as the property of the occupants, they may be sold and transferred to others - in which case, the compensation received has all the features of rent.

The time may come when all the auriferous lands in California and Australia will be private property, and yield, according to their richness, an annual profit to the proprietor, whether he work them himself, or let them to a tenant.

Salt-mines and springs occasionally afford a rent. This mineral, so useful as a condiment to our food, and for preserving meat and fish from putrefaction, as well as for its extensive use in the laboratory and in manufactures, has been furnished by nature in such profusion, that its quantity, principally in a state of solution, would be sufficient, if separated from the water, to cover the whole surface of our globe with a layer of salt probably more than a hundred feet deep. But with all this abundance, there are many districts of country which obtain this commodity from a great distance. In the United States, the Atlantic portion is supplied principally from Liverpool and Turks Island. The Western States are supplied from the salt-works of New York, Virginia, and Pennsylvania.

When salt springs or mines are solitary, or nearly so, they may yield a very high rent. The proprietor, having a monopoly of this indispensable article, is able to obtain a liberal price for a large quantity in the sale of which he has no competitor. This was the case with Preston's salt-works on the Holstein, in Virginia. He found a vent for all that he made at a dollar a bushel; but an enterprising rival having purchased a piece of land in the vicinity, on which, as he rightly conjectured, salt water from the same spring could be found, sunk a well, and at a great depth obtained water still more strongly impregnated with salt than Preston's. To secure the monopoly entirely to himself, he then rented Preston's spring for $10,000 a year, and closed it up. In this way the public was deprived of the effect of competition, and of the full benefit of the bounty of nature.

On the Kanawha, in the same State, salt springs are found to an extent of several miles on the banks of the river, and there salt is produced to the amount of many millions of bushels in a year. The salt works at Salina, in the interior of New York, are still more productive. Such of these springs as have extraordinary richness, are rented according to the average value of their annual product beyond the cost of making the salt.

In time of war, salt is here extensively manufactured from sea water, and, to some extent, in time of peace. Should the springs which now furnish so large a part of the quantity consumed, become in process of time exhausted, or too weak to repay the cost of working them, the water of the ocean may become our only domestic resource.

Fisheries are also a source of rent. The shores of the Potomac, and some other of our principal rivers, abound in the spring with shad and herring, but there are particular spots on their banks which are alone adapted to hauling the seine - the only way in which those fish can be taken in large quantities. Sometimes the proprietor of a fishery prefers renting it to retaining it for his own use; and, since sometimes as many as 100,000 fish are taken at a single draught, these fisheries are very profitable, and their rent is proportionally high.

Wharves, quays, and the like facilities to commerce, are frequent subjects of rent.

In all these cases of mines, salt-works, fisheries, mills, and other productive establishments, as in the instance of cultivated land, whenever, from the relation between the supply and the demand, the value of the commodity produced exceeds the cost of producing it, such excess is rent, which is received either wholly by the proprietor, or partly by him and partly by his tenant, the temporary occupant.