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ON ELEVATED SCREW-PROPELLERS. BT THOMAS D. STETSON, M. E. A SCREW revolved in water exhibits a tendency to move itself endwise in one direction, while it forces the water toward an opposite point. A screw working in a nut developes a similar tendency, but one is generally constrained, and the other allowed to yield. In the case of the screw-propeller, neither the screw nor the fluid is thus confined, and it becomes an object of great importance to decrease the movement of the water, and, consequently, increase that of the screw. The same amount of force, whatever it may be, is developed in each direction, since whatever energy is displayed by the screw in moving the vessel is derived from its hold on the water ; and as a given force exerted upon a large quantity will move it to a less distance than a smaller mass, it becomes desirable to work upon as large a quantity of water as possible. The only feasible method of compassing this, end is by enlarging the area of the screw ; and as the central portion or stock is but useless metal for propulsory purposes, the proper screw-propeller becomes a very broad thin thread, wound spirally about a small axis or shaft. We have now attained a general idea of the theory of screw propelling ; but the large amount of rubbing surface thus presented to the water, next demands attention, and we find by experiment that it is possible to make the screw too long. By successively cutting away the after extremity of the thread or blade, we discover that a very short screw, less in fact than one complete revolution, although somewhat less firm in its hold upon the fluid, and liable in consequence to a greater amount of "slip," is more efficient in consequence of its diminisfied friction. But a short screw with one thread is very irregular and "one-sided" in its action, and the discovery of this difficulty leads to the adoption of screws with two threads, the addition pillowing of a corresponding shortening of the screw, so that the amount of surface is not increased. The usual location of the screw-propeller is at the stern of the vessel, at which place there is always more or less " dead water," or water which is in motion following the vessel. The perpen-