FOUR-WAY COCK. 



drawn off. Thus by turning the cock through a quarter of a 

 revolution towards the termination of each stroke, the operation 

 of the machine would be continued. 



34. It will be understood from all that has been stated that the 

 mechanical effect of the steam engine 



depends, other things being given, 

 upon the excess of the pressure of the 

 steam which impels the piston above 

 the reaction of the steam which escapes 

 at the end of the cylinder towards 

 which the piston is moving. To what- 

 ever extent, therefore, this reaction is 

 diminished, the efficacy of the engine 

 will be increased. 



Steam engines are resolved into two 

 distinct classes, according to the way in which the steam escaping 

 from the cylinder is disposed of, called non-condensing and con- 

 densing engines, or, more commonly, though less properly, high 

 pressure and low pressure engines. The objection to the latter 

 denomination being that, although non-condensing engines must 

 necessarily be worked with high pressure steam, condensing en- 

 gines need not be worked with low pressure steam, as will presently 

 appear. 



In the class of non-condensing or high pressure engines, the 

 exhaustion pipes of the cylinder open into the atmosphere ; in the 

 condensing or low pressure engines, they lead to an apparatus in 

 which the steam is condensed, the name given to the process of 

 reconverting it into water by exposure to cold. 



35. In non-condensing engines the exhausting pipe communicat- 

 ing with the external air, this air will, when the exhausting valve 

 is open, have a tendency to rush into the cylinder, while the steam 

 has, on the contrary, a tendency to rush out. ' If, in this case, the 

 pressure of the steam were not greater than that of the atmosphere, 

 its escape would be prevented by the counter pressure of the air, 

 and as the pressure of the steam is the measure of its reaction 

 against the piston, it follows that in this class of steam engine, 

 the reaction on the piston must always be somewhat greater than 

 the atmospheric pressure, which, as has been shown in vol. ii., p. 4, 

 amounts on an average to 15lbs. per square inch. 



Since, then, the piston of a non-condensing engine is subject, 

 necessarily and constantly, to a reaction exceeding 15lbs. per 

 square inch, the pressure of the steam by which it is impelled 

 must greatly exceed 151bs. per square inch. Thus a pressure of 

 SOlbs. per square inch would give an effective pressure much less 

 than 151bs. per square inch, because, besides the reaction of the 



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