HEAT. 65 



condenser to feed the boiler, regains a part, but a very small 

 part, of the heat. Employing the steam first for a high 

 pressure, and then before its rejection or condensation using 

 it for a low pressure cylinder, is a second mode ; a third is to 

 use the steam, after it has done its work on the piston, as a 

 source of heat or second furnace, to boil ether, or some liquid 

 which evaporates at a lower temperature than water. These 

 plans have certain advantages ; but the complexity of ap- 

 paratus, the danger from combustion of ether, and other 

 reasons, have hitherto precluded their general adoption. 

 Under the term * regenerating engine ' various ingenious com- 

 binations have lately been suggested, and some experimental 

 engines tried, with what success it is perhaps too early at 

 present to pronounce an opinion. The fundamental notion 

 on which this class of engine is based is that the vapour or 

 air, when it has performed a certain amount of work, as by 

 raising a piston, should, instead of being condensed or blown 

 off, be retained and again heated to its original high tem- 

 perature, and then used de novo \ or that it should impart its 

 heat to some other substance, and the latter in turn impart it 

 to the fresh vapour about to act. The latter plan has been 

 proposed by Mr. Ericsson : he passes the air which has done 

 its work through layers of wire gauze, which are heated by 

 the rejected air, and through which the next charge of air is 

 made to pass. M. Seguin and Mr. Siemens have constructed 

 machines upon the former principle, which are said to have 

 given good experimental results. There is, however, a theo- 

 retical difficulty in all these, not affecting their capability of 

 acting, but affecting the question of economy, which it does 

 not seem easy to escape from. Whether the heated air or 

 vapour be retained, or whether it yield its heat to a metallic 

 or other substance, this heat must exercise its usual repulsive 

 force, and this must re-act either against the returning piston 

 or against the incoming vapour, and require a greater pressure 

 in that to neutralise it. Vapour raising a piston and pro- 

 ducing mechanical force effects this with decreasing power in 

 proportion as the piston is moved. At a certain point the 

 piston is arrested, or the stroke, as it is termed, is completed, 

 but there is still compressed vapour in the cylinder capable 



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