244 Prof. Clausius on the Application of the Mechanical 



machines. This is the case, for instance, in machines with 

 heated air as at ])resent constructed ; for after every stroke, the 

 air which moved the piston in the driving cylinder is expelled 

 into the atmosphere, and in its place an equal quantity of air 

 from the same source is received into the feeding cylinder. 

 Similarly in steam-engines without condensers, steam is driven 

 from the cylinder into the atmosphere, and in its place fresh 

 water is pumped from a reservoir into the boiler. 



Further, a similar method is at least partially adopted even in 

 steam-engines provided with a condenser as usually constructed. 

 In them the water condensed from the steam is only partially 

 pumped back into the boiler, for being mixed with the cooling 

 water, a part of the latter also reaches the boiler. The remaining 

 part of the condensed water, together with the remaining part 

 of the cooling water, has to be got rid of. 



The first method has lately been employed in steam-engines 

 propelled by two vapours, e. g. those of water and aether. In 

 these machines the steam is condensed solely by contact with 

 metallic tubes filled with liquid sether, and the water thus pro- 

 duced is then completely pumped back into the boiler. In the 

 same manner the vapour of the sether in the metal tubes, which 

 are merely surrounded by cold water, is condensed and subse- 

 quently pumped back into the first space intended for the vapo- 

 rization of the aether. In order to maintain a uniform action, 

 therefore, only so much fresh water and aether is necessary as 

 will replace the leakage consequent upon imperfect construction. 



6. In a machine of this kind, where the same matter is con- 

 tinually re-employed, the several changes which this matter suf- 

 fers during a period must, as above stated, form a closed cycle, 

 or, according to the nomenclature in my former memoir, a cir- 

 cular process. 



On the contrary, machines in which a periodical reception and 

 expulsion of matter occurs are not necessarily subject to this 

 condition, though they may also fulfil it by expelling the matter 

 in the same condition in which it was received. This is the case 

 in steam-engines with condensers, where the water is ultimately 

 expelled from the condenser in the hquid state, and at the same 

 temperature as it had when removed from the condenser to the 

 boiler*. 



In other machines, the condition, when expelled, is different 

 from what it was when received. For example, heated-air 

 machines, even when provided with regenerators, expel the ait 



* The cooling water, which enters the condenser at a low and leaves it 

 at a high temperature, is not here taken into consideration, inasmuch as it 

 does not form a part of the matter manifesting the eflFect of the applied 

 heat, but merely constitutes a negative source of heat. 



