None of these boats used the principle which Mr. Nordenfelt applied to pull his boat down by mechanical means, while relying upon its always retained buoyancy for rising; so that if the mechanical apparatus failed the boat rose at once to the surface. Again, they did not have the tendency to steadiness given by the two forces of constant pulling down by the vertical screws, acting all the time, whether still or moving, against the pulling upwards caused by the buoyancy. Mr. Nordenfelt considered it most dangerous to rely upon a detachable weight in case of emergency, as the apparatus for detaching it would be always liable to fail. He confessed that he could not imagine how the longitudinal instability of a submerged boat could possibly have been satisfactorily controlled by any of the means applied to the previous boats. Even Goubet’s system of moving water or weights fore and aft inside the boat must act more slowly and cause more diving and oscillation than his rudders which always remained in the horizontal, and thus controlled the slightest tendency of the boat to get out of the longitudinally horizontal position. He considered it absolutely essential to keep the boat horizontal when moving, as he believed that any inclination downwards with the impetus of a heavy boat would almost to a certainty carry the boat below its safe depth before it could be effectually counteracted by shifting weights. The reason which led Mr. Nordenfelt to construct his submarine boats was the almost insuperable difficulty in carrying the Whitehead and Schwarzkopf fish torpedoes with any degree of certainty up to the short distance at which they could be considered infallibly effective. It seemed to him that a much greater chance would be given for carrying the torpedoes within striking distance, if, instead of trying to rush the distance by many boats, all the time exposed to the destructive fire from machine guns, he could carry the torpedo secretly up to this distance without the probability of being seen at all, and without any probability of being struck by the enemy’s shot even if seen.