The behaviour in the sea of the boat when subjected to the influence of this rudder was somewhat unsatisfactory. For rather prolonged “dives,” such as would be necessary, for example, in forcing a blockade, the instability of the boat became very great, and the man at the helm was unduly fatigued without succeeding in retaining the mastery of his craft. On account of this instability no greater speed in submersions than 6 knots was attained. For this speed the inclination of the axis of the boat was from three to five degrees forward. The difference in the draught of water between the bow and the stern was 0·88 metre (about 34 inches) for an inclination of 3 degrees; for 5 degrees it reached 1·50 metres (nearly 5 feet). [Illustration: _Photo by_] [_M. Bar_ THE “GYMNOTE.” ] Another system of “dip” rudder, proposed in 1891, executed in 1893, and tested towards the end of 1894, consists in the employment of two horizontal floatboards, placed on each side of the boat at the height of the midship frame. Their use, combined with that of the stern rudder, has given better results. The boat is inclined less for the “dive,” dips more regularly, and lurches or “yaws” less. When navigating on the surface or when her hull is immersed to the water-line the _Gymnote_ carries a cupola, or movable shell with sidelights. This apparatus is composed essentially of a metal shell provided with sidelights all round its circumference. A cylinder of strong tarred canvas fixed on steel springs, which ensure rigidity, bind the shell itself to the upper part of the hull. Horizontal folds in front permit this canvas to double up regularly upon itself after the style of a Venetian lantern, and in such a way that when the doubling up is complete the height of the shell (or “casque”) comes exactly on a level with the upper platform of the boat. During navigation at the surface or on the water-line the whole structure can be raised or lowered by means of a vertical screw moved by a horizontal hand-wheel of which the movable nut constitutes the lower part of a vertical frame formed of metallic uprights, which go to rejoin the hull. When the system is at the end of an upward course the canvas is completely stretched and the observatory is at its maximum height; when, on the contrary, it is at the end of a downward run, the canvas is completely contracted and the “kiosk” has externally disappeared.