SECT, iv.] POWER OF STEAM, &c. 151 



equal to the pressure of the atmosphere on the piston, and through a height equal 

 to that the piston had been raised by the generation of the steam. 



292. It thus appears that the power of steam of the elastic force of the 

 atmosphere is, when speadily condensed, directly as the space it occupies. That 

 is, multiply the area of the cylinder in inches by the pressure of the atmosphere 

 in pounds on an inch of area, and by the height in feet ; and the result, deducting 

 the friction, will be the quantity in pounds the steam would raise one foot in 

 height. 



293. The space occupied by steam of atmospheric elastic force may be in- 

 creased by raising its temperature above 212, the increase being equal to the 

 expansion of steam by the given change of temperature ; but a quantity of heat 

 nearly equivalent to the increase of volume will be absorbed, and hence the effect 

 of a given quantity of fuel would not be increased by the expedient. 



294. If the steam be slowly condensed, as it would be by applying external 

 cold, the effect would be much reduced, because the moving force at any period 

 of the stroke would be only the difference between the elastic force of the steam 

 and the atmospheric pressure ; and the most rapid condensation leaves a vapour 

 of some elastic force : but as it acts through the same space as the power of the 

 steam, it does not cause a sensible deviation from the ratio of the power, being 

 as the space the steam occupies, when the power is gained by rapid conden- 

 sation. 



295. By Generation. Conceive the same cylinder and apparatus to have 

 heat applied to its base, with only the difference of the piston being loaded with 

 a given pressure per inch of its area. The generation of the steam will raise the 

 loaded piston, but the height through which it will be raised will be less. The 

 steam being acting in opposition both to the pressure of the atmosphere and the 

 load on the piston, the space it will occupy will be in the inverse ratio of the 

 pressures which oppose it in the two cases, supposing the steam of atmospheric 

 elastic force to have been of the same temperature. Thus, if the load on the 

 piston be twice the atmospheric pressure, the piston will be raised only one-third 

 of the height ; but on rapid condensation it descends with three times the pressure, 

 and, therefore, whether the steam be generated of atmospheric elastic force, or of 

 a greater force, the power it affords by generation and condensation is the same 

 at the same temperature, and this power is directly as the elastic force of the 

 steam, multiplied by the space it occupies, when the motion of the piston is 

 rectilinear. 



296. But if, as in the last case, a loaded piston be raised, and then a valve 

 be opened which allows the steam to escape, the whole power gained will be equal 

 only to the weight raised descending from the height to which it was raised ; and 



