...ally known that the rebels now have five steam-rams nearly ready for use. Of these five, two are on the lower Mississippi, two are at Mobile, and one is at Norfolk. The last of the five, the one at Norfolk , is doubtless the most formidable, being the United States steam-frigate Merrimac, which has been so strengthened that, in the opinion of the rebels, it may be used as a ram. But we have not yet a si...