SECTION IV. 



OF THE MECHANICAL POWER OF STEAM, AND THE NATURE, GENERAL 

 PROPORTIONS, AND CLASSIFICATION OF STEAM ENGINES. 



289. The force of steam when confined, according to its density and tem- 

 perature, and the circumstances which affect its motion, having been considered, 

 our next object is to investigate the power of steam to produce useful effect, and 

 in this purpose I am desirous of proceeding with the simplicity and fulness 

 this important subject requires. 



FIG. 15. 



OF THE POWER OF STEAM, AND THE MODES OF 

 OBTAINING IT. 



290. The generation or production of steam, it has been 

 shown, takes place on the application of heat. Conceive a 

 cylindric vessel, A B, to be placed in a vertical position, with 

 a given depth of water in it ; and an air-tight piston on the 

 water balanced by a weight equal to its own weight and 

 friction. In this state let heat be applied to the base, A C, 

 then as the water becomes converted into steam, of slightly 

 greater force than the atmospheric pressure, the piston will rise 

 till the whole of the water be in the state of steam. It will be 

 remarked, that the generation of this steam of atmospheric 

 elastic force affords no power, the motion being barely pro- 

 duced ; it has simply balanced the column of atmospheric air, 

 and excluded it from a given 'height of the cylinder. 



291. By Condensation. But in this state of things if the steam be suddenly 

 condensed into water again, it is obvious that the piston will be impelled by a force 



