of which these vessels are composed increasing in proportion to the diminution of capacity." The steamboat boiler of 1804 was built to bear a working pressure of over fifty pounds to the square
inch, at a time when the usual pressures were from four to seven pounds. It consists of two sets of tubes, closed at one end by solid plugs, and at their opposite extremities screwed into a stayed water and steam reservoir, which was strengthened by hoops. The whole of the lower portion was inclosed in a jacket of iron lined with non-conducting material. The fire was built at one end, in a furnace inclosed in this jacket. The furnace-gases passed among the tubes, down under the body of the boiler, up among the opposite set of tubes, and thence to the smoke-pipe. In another form, as applied to a locomotive in 1825, the tubes were set vertically in a double circle surrounding the fire.
The engine (Fig. 55) was a direct-acting, high-pressure condensing engine of ten inches diameter of cylinder, two feet stroke of piston, and drove a screw of four blades, and of a form which, even to-day, appears quite good. The hub and one blade of this screw are still preserved. The whole is a most remarkable piece of early engineering. The use of such a boiler seventy years ago is even more remarkable than the adoption of the screw-propeller in such excellent proportions thirty years before the labors of Smith and of Ericsson brought the screw into general use. We have, in this strikingly original combination, as good evidence of the existence of unusual engineering talent, in this fellow-countryman of ours, as we found of his political and statesman-like ability in those efforts to forward the introduction of railways already described.
Colonel 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.
This great engineer and inventor was therefore far in advance of