The air-line proj ect between New Yorkfcnd Boston is again agitated. It is ndw proposed to make use of the newly constructed Charles River Railroad, which will be met at a point called ScoWs Hill, in the town of Bellingham, by a direct line of road passing through Woonsocket R. I., thenceacross the State of Rhodelsland, and thence through Middletown and the State of Connecticut to New York City. ' The Pittsburgh and Steubenville Railroad is rapidly advancing to completion. This is Uie great connecting link between the Pennsylvania Central Railroad, and St. Louis road. The completion of these railroads opens a direct communication to the entire west. It is expected that the whole road irom Philadelphia to St. Louis, will be in successful opera-ft t by January 1851.