THE STEAM-ENGINE. 



421 



therefore, thus obtained, will be equivalent to double the former weight raised 

 half the former height. 



In like manner, if the piston were loaded with thirty pounds in addition to 

 the atmosphere, the whole pressure on the water being then three times the 

 pressure first supposed, the piston would be raised to somewhat more than one 

 third of its first height by the evaporation of the water. This would give a 

 mechanical force equivalent to three times the original weight raised a little 

 more than one third of the original height. 



In general, as the pressure on the piston is increased, the height to which 

 the piston would be raised by the evaporation of the water will be diminished 

 in a proportion somewhat less than the proportion in which the pressure on 

 the piston is increased. If the temperature at which the water is converted 

 into steam under these different pressures were the same, then the height to 

 which the piston would be raised by the evaporation of the water would be 

 diminished in precisely the same proportion as the pressure on the piston is 

 increased ; and, in that case, the whole mechanical force developed by the 

 evaporation of the water would remain exactly the same under whatever pres- 

 sure the water might be boiled. We shall explain hereafter the extent to 

 which the variation of temperature in the water and steam corresponding to 

 the variation of pressure modifies this law ; but, as the effect of the difference 

 of temperatures is not considerable, it will be convenient to register in the 

 memory the following important practical conclusion : — 



A cubic inch of water converted into steam will supply a mechanical force very 

 nearly equal to a ton weight raised afoot high; and this force will not be subject 

 to considerable variation, whatever be the temperature or pressure at which the 

 water may be evaporated. 



At the period to which we have now brought the history of the invention of 

 the steam-engine, Watt had directed his attention to the subject, and had obtain- 

 ed chiefly by his own experiments, a sufficient knowledge of the phenomena 

 which have been just explained, to enable him to arrive at the conclusion that 

 a very small proportion of the whole mechanical effect attending the evapora- 

 tion was really rendered available by the atmospheric engine ; and that, there- 

 fore, extensive and injurious sources of waste existed in its machinery. 



He perceived that the principal source of this wasteful expenditure of power 

 consisted in the quantity of steam which was condensed at each stroke of the 

 piston, in heating the cylinder previous to the ascent of the piston. Yet, as 

 it was evident that that ascent could not be accomplished in a cold cylinder, 

 it was apparent that this waste of power must be inevitable, unless some ex- 

 pedient could be devised, by which a vacuum could be maintained in the cylinder, 

 without cooling it. But, to produce such a vacuum, the steam must be con- 

 densed ; and, to condense the steam, its temperature must be lowered to such 

 a point that the vapor proceeding from it shall have no injurious pressure ; yet, if 

 condensed steam be contained in a cylinder at a high temperature, it will re- 

 turn to the temperature of the cylinder, recover its elasticity, and resist the 

 descent of the piston. 



Having reflected on these circumstances, it became apparent to Watt, that 

 a vice was inherent in the structure of the atmospheric engine, which rendered 

 a large waste of power inevitable ; this vice arising from the fact, that the 

 condensation of the steam was incompatible with the cendition of maintaining 

 the elevated temperature of the cylinder in which that condensation took place. 

 It followed, therefore, either that the steam must be imperfectly condensed, or 

 that the condensation could not take place in the cylinder. It was in 1765, 

 that, pondering on these circumstances, the happy idea occurred to him, that 

 the production of a vacuum could be equally effected, though the place where 



