PITTSBURG.-The Pittsburg Gazette speaks of the rapid growth of manufactories in that city. Among other mills is mentioned a tack and brad factory, that turns out 4,000,000 to 5,000,000 per day. Two rolling mills of the largest size are building. There is also a cotton factory 150 feet long, 50 feet wide, and five stories high, to run 7,000 spindles, a corresponding number of looms, and to be propelled by steam. A smaller mill is nearly ready to run, of which no dimensions are given.