While Fulton was studying and experimenting with mechanical propulsion of boats on the surface of the water, it was but natural that he should take under consideration the possibility of constructing a boat that could be sunk and raised at will and move under water. This basal principle was far from being novel. From the earliest times man has not been content to remain only a land animal. As far back as records go, he has had the ambition to emulate the birds, and certainly during the Roman period he began to think of sharing with fishes the power to explore the depths of the sea. Perhaps William Bourne was the first writer on submarine vessel design. In his little quarto volume published in 1573, and entitled, “Inuentions or Deuises very necessary for all Generalles and Captaines, or Leaders of Men, as well by Sea as by Land,” he describes as the “18 Deuise,” “a Ship or a Boate that may goe vnder the water vnto the bottome, and so to come vp againe at your pleasure.” Recognizing that the variation in displacement of a vessel whose weight remains constant adds to or detracts from its buoyancy, he suggested a vessel with sides that could be distended or contracted at will by screws, thus permitting her to sink and rise. These distendable sides, he thought, might be made of leather. For ventilation when submerged, he would have a hollow mast, taking care that the depth of water in which the boat should plunge would never exceed the height of the mast. He did not propose any means of propulsion. Van Drebbel, a Dutch engineer, born in Holland in 1572, made actual application of Bourne’s ideas, and constructed a submersible boat in 1624. He tested it in 15 feet of water in the Thames at London, during one of which tests it is reported that he had King James I. as a passenger. Apparently he attempted propulsion by means of oars that passed through the boat’s sides, the apertures being covered by leather pockets attached to oars and boat. What plan he had for keeping the boat’s air respirable when submerged is not clear, though there are some fantastic but not authenticated claims that he used a chemical compound for refreshing it. If he really plunged, which is by no means certain, it was probably for only a few minutes at a time.