THE STEAM ENGINE. 



steam, tlie impelling power is resisted by friction. A pressure of 

 45lbs. per square inch would give an effective force amounting to 

 less than 301bs. per square inch, and so on. 



Notwithstanding the disadvantage of this reaction on the piston, 

 and the consequent necessity of providing a boiler suitable to the 

 production of steam of this high pressure, non- condensing engines 

 are attended with several countervailing advantages which render 

 them not only preferable in certain cases to condensing engines, 

 but which render them efficient where the adoption of condensing 

 engines would be altogether impracticable. 



36. In condensing engines, the exhausting pipes which proceed 

 from the ends of the cylinder lead to a reservoir or vessel called a 

 condenser, in which the steam, being exposed to cold, is reduced 

 to water. Now, since a cubic foot of steam will, when re-converted 

 into liquid, form only about a cubic inch of water, it is plain that 

 by this process of condensation, efficiently conducted, the steam 

 escaping from the cylinder may be considered as passing into a 

 vacuum, and therefore not only is it not subject to the resistance 

 of the atmosphere, but to no resistance whatever, except what may 

 arise from the contracted dimensions of the exhausting pipe. The 

 conversion of the steam into water being, moreover, almost instan- 

 taneous, the reaction attending its escape, small as it is, is only 

 momentary, and affects the piston only at the commencement of 

 the stroke, throughout the remainder of which it will be subject 

 to no reaction whatever. 



Thus it appears, that, in condensing engines the pressure of the 

 steam which impels the piston instead of being subject, as in non- 

 condensing engines, to a reaction exceeding 15 Ibs. per square 

 inch, is subject to scarcely any reaction at all ; and consequently 

 its pressure, to be effective, need not exceed a few pounds, say 

 from 4 Ibs. to Ibs. per square inch. It is for this reason that 

 condensing engines have been commonly called low-pressure 

 engines. 



But although low-pressure steam may be used in this class of 

 engines, and in most cases is used, it is not thus used exclusively 

 or necessarily. Steam of any pressure, however high, may be 

 worked in them, and the condensing apparatus will still render 

 equal service. In certain applications of the engine, steam having 

 a pressure several times greater than that of the atmosphere is 

 worked with great advantage in engines constructed on this 

 principle. 



37. Since the condensing apparatus discharges such important 

 functions, it will be useful to show its structure and arrangement, 

 in connection with the piston and cylinder. 



A section of such an apparatus is shown in fig. 2o. A cistern, 

 30 



