The British were right in the secret note that they sent to the naval commanders that Fulton’s boat, even without the later improvements that he showed the British Government, could in the hands of the French have made an attack with very serious results upon an open roadstead such as the mouth of the Thames.
According to modern phraseology, Fulton’s British boat was a submersible rather than a submarine. The latter term defines a vessel that has powers of offense under water by torpedoes that in turn have means of locomotion. With such a torpedo neither Fulton nor the art was acquainted. His torpedoes or “bombs” were immobile affairs intended to be anchored, dragged by a boat or allowed to drift with the tide and to explode by concussion.
With the Nautilus it is true that he contemplated dragging a “bomb” beneath the bottom of a ship to be attacked, and in this respect the Nautilus possessed some feature of a true submarine. The plan that he proposed for the Nautilus presented many serious difficulties depending as it did on the fixing of a spike in the bottom of the other vessel. Fulton himself apparently reached the conclusion that this suggestion was impracticable, through actual experiments or further study. The boat that he proposed for the British Government had no such attachment, but instead was designed to carry “bombs” to be deposited secretly in an enemy harbor, and there to be anchored so as to remain beneath the surface when they would come in contact with the bottoms of passing vessels, or to be released in couples held by bridles and thus to be carried by tidal currents across the cables of anchored ships when the “bombs” would be drawn beneath the vessel and explode.
What Fulton called “bombs” are today known as mines. No means are shown in his plans by which these mines could be placed or released while his boat was submerged. The capability to submerge and to move beneath the surface was expected to permit the boat to work into a harbor unperceived, and there to lie in wait beneath the surface until night presented the opportunity to rise unseen, when the mines would be placed or set free. The successful experiment with the Dorothea showed that his mines could be completely effective and that, therefore, his submersible mine layer, as perhaps she can be correctly described, could have been developed into a very effective engine of war.