Three years later, in 1861, Olivier Riou designed two boats, in both of which stability was to be preserved automatically by the device of a double hull. The two cylinders which composed it, one within the other, were not fixed immovably to one another, but were on rollers, so that if the outer hull rolled to the right the inner rolled to the left. By this counterbalancing effect, it was estimated that the stability of the vessel would be absolutely secured; but nothing is recorded of the trials of these boats. The celebrated French inventors, Bourgois and Brun, reintroduced the principle of water-tanks combined with a heavy iron ballast keel. But in 1881, the Rev. W. Garrett, the English designer of the Nordenfelt boats, invented a new automatic mechanism for ensuring stability. This consisted of two vertical rudders with a heavy pendulum weight so attached to them that, if the boat dipped out of the horizontal, the pendulum swung down and gave the rudders an opposite slant which raised the vessel again to a horizontal position. This arrangement, though perfect in theory, in practice developed fatal defects, and subsequent types have all returned to the use of water-tanks, made to compensate, by elaborate but trustworthy mechanism, for every loss or addition of weight. 3. _Habitability._--For the habitability of a submarine the prime necessity is a supply of air capable of supporting life during the period of submersion. The first actual constructor of a submarine, Cornelius van Drebbel, of Alkmaar, in Holland, was fully aware of this problem, and claimed to have solved it, not by mechanical but by chemical means. His improved boat, built in England about 1622, carried twelve rowers, besides passengers, among whom King James I. is said to have been included on one occasion, and was successfully navigated for several hours at a depth of ten to fifteen feet. ‘Drebbel conceived,’ says Robert Boyle, in 1662, ‘that ’tis not the whole body of the air, but a certain Quintessence (as Chymists speake) or spirituous part of it that makes it fit for respiration, which being spent, the grosser body or carcase (if I may so call it) of the Air, is unable to cherish the vital flame residing in the heart: so that (for aught I could gather) besides the Mechanical contrivance of his vessel he had a Chymical liquor, which he accounted the chief secret of his Submarine Navigation. For when from time to time, he perceived that the finer and purer part of the Air was consumed or over-clogged by the respiration and steames of those that went in his ship, he would, by unstopping a vessel full of the liquor, speedily restore to the troubled air such a proportion of vital parts as would make it again for a good while fit for Respiration.’