PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 479 



ence to page 25, the Naperian logarithm of four, will be found to be 1.386, 

 which is the increase of efficiency. The total efficiency of steam expended 

 during the stroke, therefore, which without expansion would have been one, 

 becomes 2.386 when expanded into four times its bulk, or in round num- 

 bers 2.4, as stated at page 12. 



And then after another example, he says: of all the expedients for 

 economizing fuel in steam engines, expandoR is that which has been at- 

 tended with the most success. 



[From Brand's Encyclopedia, article, "Steam."] 



" In the mechanical operation of steam, the pressure, density and tempera- 

 ture are supposed to remain the same during its action, and the mechanical 

 effect is produced by the continual increase of the quantity of steam pro- 

 duced by evaporation from the boiler. Thus the piston is moved by the 

 increased volume required, by the continual production of steam. It has 

 been proved that, by this process alone, the evaporation of a cubic inch 

 of water, whatever be the pressure under which it takes place, evolves 

 a mechanical force equivalent to one ton raised one foot high. But if 

 after this evaporation has been completed, the steam be separated from the 

 water which produced it, and the load of the piston be gradually dimin- 

 ished, the steam would expand by moving the piston in virtue of its excess 

 of pressure, and this expansion would continue until the pressure of the 

 steam shall be reduced to an equality with the load on the piston. All 

 mechanical effect, developed in this process, is due to the steam itself, 

 independent of any further evaporation. 



" To make this important quality of the expansive action of steam under- 

 stood, let us suppose the piston to be loaded to the pressure of four atmos- 

 pheres. If the water under the piston be evaporated under this pressure, it 

 will have a temperature of about 291 degrees, and by its evaporation it will 

 have raised the piston through a space of four feet; this will, therefore, be 

 the mechanical effect arising from tha immediate evaporation of the water. 

 But when the evaporation has been completed, and the piston with its load 

 of four atmospheres stands at four feet above the bottom of the cylinder, let 

 the pressure, equal to that of one atmosphere, be removed from the piston. 

 The remaining pressure of three atmospheres, being less than that of the 

 steam below the piston, the piston will be raised, and will continue to 

 rise until it has attained a height of five feet, and the steam thus expanded 

 will fall to a temperature of 275 degrees, and its pressure reduced 

 to that of three atmospheres, it will cease to rifie. By this process a 

 mechanical force, equal to the weight of three atmospheres raised one 

 foot, has been obtained, in addition to the effect obtained from the imme- 

 diate evaporation; but the expansive action does not stop here. Let the 

 piston be relieved of the pressure of another atmosphere; the superior 

 pressure of three atmospheres below the piston will cause it to rise to the 

 height of seven and a half feet, the temperature of the steam falling to 

 250 degrees, and its pressure reduced to two atmospheres. A further 

 mechanical effect, equivalent to the weight of two atmospheres raised two and 

 a half feet high, has thus been obtained, and it is evident that, by constantly 

 and gradually diminishing the load on the piston, an additional effect may 



