Extraordinary Despatch. The Steamboat New Philadelphia left Albany on Wednesday afternoon of last week, at 5, o'clock -- arrived here at about the same hour on Tuesday morning -- landed her passengers as usual, went round to the railway of the Dry Dock Company, in the East River, discharged the water from her boilers, was hauled up on the ways; and after the bottom was cleaned and graved, was relaunched; her boilers were refilled, her wood taken in; she was at her accustomed station at the foot of Courtlandt-Street on the same afternoon, at a quarter before 5: and at a quarter past 5 was again on her way to Albany. The New Philadelphia is a large steamboat, measuring probably 150 feet in length. This despatch evinces, very strongly, the utility and efficiency of the Dry Dock railways; and but that custom renders even wonders familiar, it might excite astonishment, that a steamboat could thus, within 36 hours, traverse a distinace of 320 miles and undergo besides, after being hauled up out of her element, a thorough cleaning on dry land. New York American.