Theoretically this is an ideal method for submerged control, but in practice it works out badly, especially when a vessel has little stability, for the reason that there are so many disturbing influences to cause the vessel to take on dangerous angles in diving. If free surfaces exist in the water-ballast tanks, the slightest change from a level keel causes the water to flow to the lower end of the ballast tank. This is apt to augment the inclination still further, and cause the vessel to dive, or, _vice versa_, to broach. The density of the water also varies, especially where freshwater rivers empty into salt water. At times quite different densities are found at various depths. The fresh water and salt water, instead of rapidly mixing, seem to have a tendency to remain in strata which extend, in some cases, considerable distances off shore. Therefore it is practically impossible to secure and maintain a vessel in perfect equilibrium. The movement of the crew forward and aft, or the effect of the sea, which imparts a vertical motion to the water beneath the surface, all tend to destroy both trim and equilibrium to such an extent that many failures have resulted in vessels of this type. _Le Plongeur_ was no exception to this rule, because it was found impossible to control her depth when running submerged, and she would either dive into the bottom or broach to the surface. One report stated that even in depths of thirty feet she would make progress "by alternately striking the bottom and then rebound to the surface like an elastic india-rubber ball." One other novel feature introduced in _Le Plongeur_ was an "escape boat," which was carried on top of the main hull, to which it was secured by bolts. A double hatch connected the submarine and the escape boat together. In case the submarine became disabled or entangled in wreckage and could not be brought to the surface, the crew could enter through double hatches into the escape boat, secure the bottom hatch, and by turning the securing bolts from the interior release the escape boat and ascend to the surface. Mr. O. S. Halstead, of Newark, New Jersey, completed, in 1866, a submarine vessel on which the United States Government made a partial payment. This vessel is known as the _Intelligent Whale_, and is now installed as a permanent exhibit on the Green at the Brooklyn Navy Yard, New York. The vessel had a vertical and horizontal rudder at the stern for submerged control. According to official reports, she must have functioned fairly well when submerged. One of the features of this vessel consisted in its ability to be converted into a diving bell when resting on the bottom. A large trap-door was arranged in the bottom of the vessel. After filling the whole interior of the vessel with compressed air equal in pressure to the pressure of the water at the bottom of the vessel, the trap-door could be opened and the air pressure would keep the water from rising, the same as in a diving bell.