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desirable to make this endwise velocity of the screw, and consequently of the vessel, as great as possible. To this end it is desirable to diminish the velocity of the water, or in engineering language, to diminish the "slip" of the screw. Increasing the diameter will accomplish this end, shortening the screw will diminish its friction upon the water, stnd decrease the weight, while increasing the number of threads to a certain extent, will equalize its action and prevent vibration. In addition to these considerations, the impracticability of allowing large and powerful engines to revolve rapidly, necessitates the existence either of a large pitch or the intervention of gearing. But a large pitch renders still more imperative the demawd for great diameter ; and thus we have two powerful inducements to obtain all possible latitude in this direction : first, the necessity for acting on a large quantity of water; and second, the desire to act on the water at as favorable an angle as possible. As it. would be evidently unsafe and impolitic to allow any portion of the screw to project below th^ keel, it has been considered practicable to obtain a diameter but little exceeding the loaded draught of the vessel.* The "Shepherd Iron Works," at Buffalo, were the first to construct, and were probably the first seriously to propose, a great increase of diameter by raising a considerable portion of the screw above the surface ; a practice now almost universal * The "Monumental City," with a draught of 12 feet, has a propeller with a diameter of 12 feet ; the "Albatross," draught 10' feet 6 inches, diameter of prop 10 feet ; the "Pioneer," draught 19 feet, prop 16 feet ; "John Hancock," draught 10 feet 6 inches, prop 8 feet 8 inches ; "Constitution," draught 11 feet, prop 10 feet 4 inches ; "Union," draught 11 feet, prop 10 feet ; " Rescue," (a tug-boat in New- York harbor,) draught 9 feet 9 inches, prop 8 feet 2 inches ; " Alleghany," (United States War Steamer,) draught 15 feet, prop 13 feet 6 inches ; " Sarah Sands," draught 15 feet, diameter of prop 14 fe^t. English screws, although geared up to such extent that the pitch but very slightly exceeds the diameter, retain about the same proportion to the draught, — the " Andes" and "Alps" (Cunard) have a draught of about 17 feet, and a diameter of prop 14 feet ; the " Santander," draught 9 feet, prop 8 feet ; and the *' Agamemnon," (line-of-battle ship,) draught 20 feet, diameter of propeller 18 feet. These are all cases selected at ran-dom, there being cases like the " Massachusetts," (auxiliary steam, U. S. Navy,) where the draught exceeds the diameter of propeller in the proportion of about 15 to 9 1/2