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perpendicular extremity or stern-post is more immediately accompanied by this, and a two-bladed screw will consequently " thrust" more efficiently when its blades are in a perpendicular than when in a horizontal line. This variable condition of the water results in a series of jerks, which it is desirable to avoid by still further increasing the number of blades, and of course decreasing still further the length of the screw. But it is necessary to retain sufficient length to give strength to the centre ; and in addition to this consideration, a new difficulty is experienced as the number of blades increase. This is the resistance of the water to their cutting or forward edges, which is increased by each addition, and, aside from the various practical difficulties in the way of an indefinite increase, limits the number of blades on the ordinary screw-propeller to from two to four. The angle made by the acting surface, or the relation of pitch to diameter, is the next point to be considered, and this we find to depend, theoretically, on the amount of friction between water and metallic surfaces. To determine these points, broad, thin disks have been accurately mounted by experimenters on delicate shafts, and revolved with different degrees of rapidity, and under the pressure of different heads of water. The re- sults have proved the desirability of a large screw with a com- paratively small pitch, but revolving rapidly. The English in some degree attain this end by employing gear, but. the gearing itself is so far objectionable, as to induce American Engineers generally to forego its advantages, and construct slower screws with larger pitches, and connect the engines directly to the shaft. Avoiding entirely a host of important questions as irrelevant to our present subject, we shall assume that the foregoing positions are understood and assented to as self-evident, although every week brings forth ridiculous inventions, which prove that these simple laws have never yet been published, or at least sufficiently promulgated. Let us briefly recapitulate them. The screw tends to move; itself in one direction, and the Water which it acts on it another, and a portion of the power of the engine is consumed in producing each of these effects. It is de-