Drebbel, who was a really scientific man, may possibly have discovered this chemical secret. If so, he anticipated by more than 200 years a very important device now in use in all submarines, and in any case he was the originator of the idea. But his son-in-law, a German named Kuffler, who attempted after Drebbel’s death to exploit his submarine inventions, was a man of inferior ability, and either ignorant of the secret or incapable of utilising it. For another century and a half, submarine designers contented themselves with the small supply of air which was carried down at the time of submersion. Even the _Turtle_--Bushnell’s boat of 1776, which has been described as ‘the first submarine craft which really navigated under serious conditions’--was only built to hold one man with a sufficient supply of air for half an hour’s submersion. This was a bare minimum of habitability, and Fulton, twenty-five years later, found it necessary to equip his _Nautilus_ with a compressed air apparatus. Even with this, the crew of two could only be supplied for one hour. In 1827, the very able French designer, Castera, took out a patent for a submarine life-boat, to which air was to be supplied by a tube from the surface, protected by a float, from which the whole vessel was suspended. The danger here was from the possible entry of water through the funnel, and the boat, though planned with great ingenuity, was never actually tried. Bauer, in 1855, fitted his _Diable Marin_ with large water-tubes, running for thirty feet along the top of the boat and pierced with small holes from which, when desired, a continual rain could be made to fall. This shower-bath had a purifying effect on the vitiated air, but it had obvious disadvantages; and there is no record of its having been put into actual use before the unfortunate vessel sank, as before related. In the same year, a better principle was introduced by Babbage, an English inventor, who designed a naval diving-bell, fitted with three cylinders of compressed air. His method was followed by Bourgois and Brun, whose boats of 1863-5 carried steel reservoirs with compressed air, at a pressure of at least 15 atmospheres. The principle was now established, and was adopted in Holland and Lake boats, and in all subsequent types, with the addition of chemical treatment of the vitiated air.