In the second, the weight of the boat remains constant, but the displacement is altered by the drawing in of “cylinders” or “drums;” thus she sinks until her displacement again equals her weight. The first inventor to employ the latter method was André Constantin, who built a vessel during the siege of Paris, which was furnished with pistons working in two cylinders; on these being drawn in from the interior the boat sank to the required depth. The actual trials were, however, not satisfactory. The _Nautilus_, of Messrs. Campbell & Ash, which underwent some trials in Tilbury Docks in 1888, depended also on the pulling in of cylinders (ten were employed, five on each side of the vessel), for her submersion; the results were equally discouraging, and some eminent men nearly lost their lives owing to the erratic behaviour of this craft. [Illustration: THE SUBMARINE OF ANDRÉ CONSTANTIN. (1874.) ] No serious ship-constructor would nowadays think of adopting this method of submersion, and we may therefore pass on to consider those which are brought to the submerged condition by the admission of water into special reservoirs or tanks. Submarine boats so far as their immersion is concerned may be divided into two classes. 1. Those which when submerged possess no floatability. 2. Those which in the same condition possess a small reserve buoyancy or floatability. Modern submarines almost without exception belong to the second division, as this class has been found to possess great advantages over the first. 1. SUBMARINES WITH NO FLOATABILITY WHEN SUBMERGED. [Illustration: “GOUBET II.” ] Boats belonging to this division possess when submerged a total weight equal to the weight of water displaced. During immersion it has been found necessary to make the weight of the vessel and its contents slightly exceed the weight of water displaced by the total volume of the vessel; this excess of weight causes a downward motion which rapidly accelerates unless checked, and care must be taken to regulate, either automatically or otherwise, the depth, lest the vessel sink to a depth where the pressure is greater than she can withstand. [Illustration: M. GOUBET ABOUT TO GO UNDER WATER IN HIS BOAT. ] Although M. Goubet is a believer in the “no-floatability” idea, it has, for some time past, been regarded with disfavour. Theoretically it is possible to navigate a submarine whose total weight equals the weight of water displaced so that she keeps at a given level without rising or sinking, but the system will not work satisfactorily when put to severe and prolonged tests. It is found to be impossible to obtain perfect equality between the two weights: submarine currents, variations of atmospheric pressure and temperature, and movements inside the boat all tending to disturb its equilibrium. 2. SUBMARINES POSSESSING FLOATABILITY WHEN SUBMERGED.