M. Gaget remarks that “it is very strange that Bushnell should have discovered and concealed with so much care the instrument of propulsion which Sauvage studied and introduced fifty years later.” The fact is, of course, that the principle of the screw-propeller was known in the seventeenth century and that in May, 1785, Joseph Bramah patented a screw-propeller, identical in general arrangement with those in use to-day. The first practical use of the screw was made by John Stevens, who in 1804 launched a steamboat eighteen feet long by fourteen feet beam with a direct acting high-pressure engine having a tubular boiler—and driving a screw with four blades. Although the principle of the screw for ship propulsion was thus recognised at this early period it was not till the thirties (of the nineteenth century) that the screw-propeller succeeded in attracting the attention of the engineering world. Professor Tuck in his boat (1884) placed the propeller directly beneath the centre of the hull, so that it should submerge on an even keel. Mr. Nordenfelt used vertical screws, which at first he fitted in side sponsons, but afterwards in the fore and aft line, and considered it absolutely essential that a diving boat should be kept horizontal when being submerged, as any inclination downwards with the impetus of a heavy boat would, he considered, almost to a certainty carry the boat below its safe depth, before it could be effectually counteracted by shifting weights. Such a theory was soon shown to be founded on a misapprehension. Some inventors (Waddington, Baker, &c.) have used four screws operating in pits equidistant from the centre of the boat, two on the upper part and two on the under part, but all such methods have been discarded in the newest designs. IMMERSION BY HORIZONTAL RUDDERS. The ordinary vertical rudder steers the ship either to port or starboard in the horizontal plane, and the horizontal rudder can be used similarly to control its position in the vertical plane. This method of steering a boat beneath the surface by the inclination of horizontal rudders is, of course, only applicable when the boat is moving. The position that the horizontal rudder or rudders should occupy is a question about which much has been written, and opinion appears to be still divided on the subject. Some hold that they should be placed at the stern, others that they should be placed on either side of the vessel, and these latter again differ as to whether they should be forward, amidship, or aft. In spite of all the arguments in favour of placing the rudders forward, Captain Hovgaard considers that this disposition can hardly be recommended except in very long boats where it may prove a necessity. The _Gustave Zédé_ has six diving rudders, two forward, two in the centre, and two aft; whilst in the _Narval_ class there are four rudders, two forward and two aft; the _Holland_ submarines have aft rudders only.