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THE STEAM-ENGINE#

parts together in more substantial modes; hut all thoseessential forms and proportions which affect the perfor-mance of the machine, were so ascertained by the firstinventor, that no improvement has been since madein them, and every departure from those forms and pro-portions has impaired the performance in a greater or lessdegree.”

Thus have we taken a rapid survey of the history ofthe steam-engine. We have omitted the names of manyindividuals who have distinguished themselves by inge-nuity directed to this subject. We have omitted thelabours of Gebert, Alberti, Cardan, De Caus, Branca, Mor- 'land, Papin, Amonton, Leupold, Meyer, Bosfrand, Ges-sanne, and a hundred others who have, all in different de-grees, expended ingenuity upon the application of steam tothe production of mechanical power ; and these we have jomitted, not because we consider their labours eitherundeserving of notice or uninteresting to the generalreader, but because they have not contributed towardsthe production of the modern steam-engine, and becausean account of their works would rather serve to illustratethe possible varieties of the machine and the fertility ofthe human mind in mechanical devices, than either toconduct the reader along the stream of historical suc-cession, or render him better acquainted with the natureand mechanical peculiarities of the steam-engine itself.

Part II. —Description of the Modern Steam-Engine.

Of modern steam-engines there aretwo distinct species—the high-pressure and low-pressure engines. The for-mer is simple, light, and of few parts, generally used for