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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. 
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 
Fig. 15.