Steam-Engine. 452 



tially in rotation by the engine itself, and which carries at its 

 summit a parallelogram formed of metallic plates, turning free- 

 ly one upon the other, in such a manner that this parallelogram 

 may open more or less in a horizontal direction, according as 

 the plates diverge more or less from the axis. This diverg- 

 ence is produced by the centrifugal force exerted by the axis 

 in turning, under the influence of the engine, with greater or less 

 rapidity 5 which causes the upper vertex of the parallelogram 

 to be depressed when the engine moves more rapidly, and ele- 

 evated when it moves more moderately. In order to give great- 

 er force to this ascending and descending motion, the extremi- 

 ties of the plates are loaded with spheres of solid metal, and 

 exert their power upon a lever, whose other branch communi- 

 cates with a plate placed transversely in the passage through 

 which the steam passes from the boiler to the cylinder ; so that, 

 when the engine works too slowly, the plate turns in such a 

 manner as to give a freer passage to the steam ; and, on the 

 other hand, if the engine moves too rapidly, the plate takes a 

 position more nearly in a transverse direction, and diminishes 

 the passage through which the steam has to pass. Thus the 

 engine is made to govern and regulate itself, in such a manner 

 as to preserve in its motions that uniformity which its purposes 

 require. There are many other details which would furnish 

 matter for curious speculation ; but these details, belonging to 

 the mechanism, must be omitted here, to make room for some 

 other particulars, no less important, relating to the principles 

 of the machine. 



525. The most essential of these is the determination of the 

 temperature, at which it is most advantageous to employ the 

 steam. In fact, the higher the temperature is, the greater will 

 be its elastic force, and consequently the greater will be its 

 effect upon the surface of the piston, the vacuum being always 

 on the other side. From the experiments of Southern, Clement, 

 Desormes, and Despretz, it has been found that the total quantity 

 of heat necessary to change the same mass from water to a state 

 of vapour, is very nearly if not quite the same for all tempera- 

 tures. According to this principle, then, it will not be necessary 

 to consume more fuel in order to form a given weight of steam 

 of a higher temperature and more elastic, than is required 

 Mech. 54 



