The Mississippi valley contained over eight millions of inhabitants in 1840, having gained eighty per cent., during the last ten years. The present number cannot be less than ten millions. The last year, the Mississippi was navigated by four hundred and fifty steamboats, many of which are capable of carrying 2,500 bales of cotton, making an aggregate tonnage of ninety thousand. They cost above seven millions of dollars; and to navigate them, required nearly fifteen thousand persons--the estimated expense of their navigation is over thirteen millions of dollars. The increase since, may be calculated at fifty additional boats--which would make an advance in all these items in a ratio of ten per cent. Such statements as these, large as they seem, convey to the reader but a partial idea of the great valley, and of the wide extent of country upon which this city leans, and which guaranties her present and future prosperity. To form a full estimate, he must, besides all this, see her mountains of iron, and her inexhaustible veins of lead and copper ore, and almost boundless regions of coal. The first article mentioned (and the phrase in which it is expressed is no figure of speech) has been pronounced, by the most scientific assayer of France, to be superior to the best Swedish iron. These, and a thousand unenumerated products, beside the well known staples, constitute its wealth; all of which by a necessity of nature, must flow through our Crescent City, to find an outlet into the great world of commerce. With such resources nothing short of some dreadful convulsion of nature, or the more dreadful calamity of war, can prevent New Orleans from becoming, if not the first, next in commercial importance to the first city in the United States--perhaps, in the world. The flourishing towns upon the Mississippi and her tributaries, are merely the depositories for this great mart. In twenty years she must, according to her present increase, contain a population of three hundred thousand, with a trade proportionably extended.