tit Pefcr'tpi'm and Effcn ♦^ 



VII. 



An Acctunt of the Principles and EffeEl of Steam Engines, which aB hy means of a Tiflm, 

 IVith Defcriptions of the Atmofpherical Engine of NewCOMEN and C AW LEY; the Engine 

 tf IVaTTi which works in Vacuo ; and a new Engine ty Mr. SaDLEK, in -which tht direSi 

 A3ion of Steam and the Prejfurt of the Attmfphere are tombined. 



s, 



^OME account of the original fteatn-engme of the Marquis of Worcefter^ and its fub- 

 fcquent improvements, has been given in a former paper in this work*. One of the chief 

 imperfe£l:ions of that engine was then (hewn to confift in the dircfl aftion of the ftsam, for 

 forcing, being neceflarily more than equal to the weight of the column of water required 

 to be raifed ; in confequence of which, it became in moft cafts an indifpenfable condition, 

 that the boiler and veffels fhould be very ftrong, as well as that a large quantity of fuel 

 fliould be confumed, to produce fteam fufficiently denfc. It is probable that thefe inconve- 

 tiiences may have early direcfted the thoughts of various ingenious men to the application of 

 a pifton, though the difficulties of the undertalcing feem to have retarded this purfuit for a 

 confiderable time. The firft fteana-engine, with a pifton, made in 1707 by Papin f , was 

 little calculated to remove thefe difficulties ; and it is to Newcomen and Cawley that we 

 are indebted for the application of a pifton with machinery, by which the indireft a£tioa 

 ©f fteam little ftronger than the atmofphere, or rather the dire6t adion of the atmofphere, is 

 made to aft with fafety and efFeiSl- againft the moft fevere piefTures. It appears that they had 

 brought their engine, about the year 1713 |, to a degree of perfection little different front 

 thofe which are to be feen at prefent. A panicular defcription of this engine, with draw- 

 ings of its parts, and a corifiderablc portion of the hiftory of its invention, are to be met with. 

 in Defaguliers's Courfe of Experimental PhilofophVy and it has alfo been defcribed in many 

 other booics. For thefe rcafojis, I fliall in this place give merely a verbal account of its 

 principles and mode of operation. 



Suppofe a very large fyringe to be fet upright, and a pifton or plug inferted at the upper 

 end, the ufual aperture being fuppofed to be at the lower extremity. If this laft aperture be 

 open, riie pifton will defcend by its own weight, neglefting the effeft of friiVion at its ciis- 

 cumference. But let it be imagined, that the pifton is fupported by a counter-weight at the 

 oppofite extremity, by a lever or by any other means. In this cafe the pifton will not defcend 

 wnlefs more weight be added to it. Among the various ways of applying, fuch a weight,, 

 there is one which eonfifts in exhaufting the air from the internal part of the cylinder 

 beneath the pifton. For, if this were done, it is evident that the whole prefTure of the atmo- 

 fphere, which amounts to about twelve pounds on every circular inch, will become adiv© 

 upon the upper furface. If the vacuum were to be produced by means of an air-pump, it may 

 eafily be allowed, that the labour of effecting it would be at leaft equal to that of any work 

 Whick might be performed by the fijbfequent defcent of the pifton.. We have feen that, in 

 Savery's engine, the operation of fteam is twofold j namely, by the direct preffurc from its 



* Philofophical Journal, I. 419. 

 \ Loco citato, p. 421. 



J Defaguliers in his Leftures, il. 467, fays that it had been in ufe near thirty jtm at the time he wrote» 

 Mtdthe imprimatur to his work bears date November 1.7, 1743. See aUb p. 531 of the fame volume. 



elafticityj 



