"I conceive that the boiler above described embraces the most eligible mode of applying the principle before mentioned, and that it is unnecessary to give descriptions of the variations in form and construction that may be adopted, especially as these forms may be diversified in many different modes." Boilers of the character of those described in the specification given above were used on the locomotive built by John Stevens in 1824-'25, and one of them remains in the collections of the Stevens Institute of Technology. The use of such a boiler 70 years ago is even more remarkable than the adoption of the screw-propeller, in such excellent proportions, 30 years before the labors of Smith and of Ericsson brought the screw into general use; and we have, in this strikingly original combination, as good evidence of the existence of unusual engineering talent in this great engineer as we found of his political and statesmanlike ability in his efforts to forward the introduction of railways. Colonel John Stevens designed a peculiar form of iron-clad in the year 1812, which has been since reproduced by no less distinguished and successful an engineer than the late John Elder, of Glasgow, Scotland. It consisted of a saucer-shaped hull, carrying a heavy battery, and plated with iron of ample thickness to resist the shot fired from the heaviest ordnance then known. This vessel was secured to a swivel, and was anchored in the channel to be defended. A set of screw-propellers, driven by steam-engines, and situated beneath the vessel, where they were safe against injury by shot, were so arranged as to permit the vessel to be rapidly revolved about its centre. As each gun was brought into line of fire, it was discharged, and was then reloaded before coming around again. This was probably the earliest embodiment of the now well-established "Monitor" principle. It was probably the first iron-clad ever designed. It has recently been again brought out and introduced into the Russian navy, and is there called the "Popoffka." The first of Stevens's boats performed so well, that he immediately built another one, using the same engine as before, but employing a larger boiler, and propelling the vessel by _twin screws_, the latter being another instance of his use of a device brought forward long afterward as new, and frequently adopted. This boat was sufficiently successful to prove the practicability of making steam-navigation a commercial success; and Stevens, assisted by his sons, built a boat which he named the "Ph[oe]nix," and made the first trial in 1807, but just too late to anticipate Fulton. This boat was driven by paddle-wheels. [Illustration: FIG. 86.--Stevens's Twin-Screw Steamer, 1805.]