According to calculation in the " Pittsburg Post," the iron manufacturers of this country have a sure demand before them of nearly $300,000,000 for fabrics to be turned out Irom their manufactories—a demand that will require all, and more than all their capacity to supply fast enough. The calculations upon which the statements are based, is the extent of railroad now in course of construction — With one hundred tons per mile, single track, it will require 1,300,000 tons of iron rail to complete the thirteen thousand miles of railroad, either in progress, or which will be in progress ere long, including the Pacific railroad. At $50 per ton this would require an outlay of $65,000,000 tor single tracks alone. But many of these roads will be double tracks, besides turnouts, &c. Then follows a vast outlay for cars, locomotives, and other iron works about such roads.