Prime Movers. 1 



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present in existence, bnt those forests also belonging to a 

 remote period, to which our coal-fields owe their origin. There 

 is no fuel which is not derived from one of the organic king- 

 doms — none, indeed, which has not had its origin in plants, at 

 least preparatory to its having entered into animal organiza- 

 tions. It is probable that, during combustion, the supporter 

 takes the place of combined caloric, which therefore is evolved. 

 Hence, when carbon becomes carbonic acid, heat is set free ; 

 but when carbonic acid is decomposed in any way, heat is 

 absorbed. 



We have reason to believe that the ancients knew more 

 about steam than is usually supposed ; and the mistake on this 

 point may have arisen from their terming steam "air." There 

 is no doubt that steam, and air expanded by heat, were used by 

 them for producing motive power. Hero, of Alexandria, who 

 flourished about B.C. 120, discusses the properties of air, as a 

 medium for communicating pressure and motion; and enters 

 into the nature of a vacuum — subjects which comprehend the 

 whole theory of the steam-engine. But, long as the properties 

 of steam have been known, and much as they have been studied, 

 the greatest actual efficiency of the steam-engine is still but 

 about one-sixth of its theoretical; that is, but one-sixth of what 

 its efficiency ought to be, taking into account the heat which is 

 expended in working it. Besides other sources of loss, steam 

 is wasted, in heating the cylinder, during the first part of the 

 stroke; winch is necessary, from the cooling of the cylinder, 

 during the expansion effected in the latter portion of the pre- 

 ceding stroke. And not only is the steam, which has been 

 condensed from this cause, only partially revived afterwards, 

 but its revival becomes, in some degree, mischievous ; since it 

 continues, while the exhaust steam is passing into the atmos- 

 phere, on the condenser ; and thus, by increasing the ( ' back 

 pressure," it lessens the power of the engine. The loss, from 

 this cause, particularly with high velocities, and great expan- 

 sion, is so serious, that it very much diminishes and sometimes 

 altogether destroys the advantage derived from using the steam 

 expansively. Some steam, also, is condensed behind the piston, 

 owing to the conversion of part of the heat into work, and the 

 consequent precipitation of water. This, in itself, is not a loss; 

 but, during the latter part of the stroke, it robs the cylinder of 

 heat, and the steam, thus condensed, and afterwards revived, 

 being formed too late, increases the back pressure. 



It was supposed that a large quantity of the heat, which is 

 carried off by the waste steam, might be retained; and the 

 Regenerative steam-engine was designed to effect this object. — A 

 furnace, placed under the cylinder, heated the steam to a tem- 

 perature higher than the boiling-point corresponding with its 



