THE GROWTH OF THE STEAM-ENGINE. 



529 



Fig. 54. Stevens's "Sectional" Boiler, 

 1804. 



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 con- 

 sists 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 fur- 

 nace 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 cir- 

 cle 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 pre- 

 served. The whole is a most remarkable piece of early engineering. 

 The use of such a boiler seventy years ago is even more remark- 

 able than the adoption of the screw-propeller in such excellent propor- 

 tions thirty years before the labors of Smith and of Ericsson brought 

 the screw into general use. We have, in this strikingly original com- 

 bination, 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, Scot- 

 land. 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 

 sw r ivel, 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 



VOL. XII. 



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