130 Defcrtpthn and E^eEt of the St tarn Engine of Watt and Bouiten. 



lightened D<n:iA'ity, render both unnecefiary. I fliall therefore coiitine m|felf to remark, that 

 the great features of improvement made in the engine of Newcamen by W.ut are, firft, that 

 the elafticity of the fteam itfelf is ufed as the adive power in his qnginci and fecondly, that • 

 tjefides various other judicious arrangements for the economy of heat, he condenfes the ; 

 fteam, not in the cylinder, but in a fcparate veflel. 



In the great cylinder or fyringe, concerning which we have fpoken, in defcribing the 

 aigine of Newcomcn, let.us fuppofe the upper part to be clofed, and the pifton-rod to Aide ■ 

 air-tight through a collar of leathers. In this fituation, it is evident, that the pifton might , 

 be deprefled by throwing the fteam npi^n its upper furface, through an aperture at the fuperior 

 e«d of the cylinder. But if we fuppofe the external air to have accefs to the lower furfacfi 

 of the pifton, we (hall find, that fleam no ftronger in its elafticity than to equal the weight of 

 the atrriofphere would not move the pifton at all j and confcquently that this new engine 

 would require much denfer fteam, and confume much more fuel tiian the old engine. The 

 remedy for this evil is to maintain a conftant vacuum beneath the pifton. If fuch a vacuum 

 were originally produced by fteam, it is certain that its permanency could not be depended 

 Oiijunkfs the engine contained a provifion for conftantly keeping it up. Mr. Watt's contriv- 

 ance in his fimpleft engine is as follows : The fteam is conveyed from the boiler to the upper 

 part of the cylinder through a pipe, which alfo communicates occafionally with the lower 

 part, and beyond that fpace with a veflel immerfed in a trough of water ; in which veftel the 

 condenfation is pe.rformed by an inje6led ftream of cold water. This water is drawn off^, not . 

 by an eduflion- pipe but by a pump, of which the ftroke is fufficiently capacious to leave 

 room for the elaftic fluid, feparated during the inje(Slion, to follow and be carried out with the 

 injeftion water. Suppofe now the pifton to be at its greateft elevation, and the communi- 

 cation from the boiler to the upper as well as to the lower parts of the cylinder to be opened. 

 The^ fteam will then pafs into the whole internal part of the engine, and will drive the air 

 downwards into the condenfer, and thence through the valves of the air-pump. In this 

 fituation, if the communication from the boiler to the lower part of the cylinder be ftopped, 

 and an injection be made into the condenfer, a vacuum will be produced in that veflel, and 

 the fteam contained in the lower part of the cylinder and communication pipe will expand 

 itfelf with wonderful rapidity towards the condenfer, fo that in a period of time too minute 

 to be appretiated, the whole of the fteam beneath the pifton will be pradically condenfed. 

 The fteam which continues to a6t .above the pifton will immediately deprefs it into the 

 vacuum beneath ; at the fame time that by conne<nion with the external apparatus the 

 pifton of the air-pump alfo defcends iu its barrel. When the ftroke is nearly com- 

 pleted downwards, the requifite part of the apparatus ftiuts the communication with 

 the boiler ; opens that between the upper and lower parts of the cylinder and condenfing 

 veftei ; and turns the injedion-cock. At this very inftant the pifton lofes its tendency to de- 

 fcend, bccaufe the fteam prefl"es equally on both furfaces, and continues its equality of 

 prefl'ure while the condenfation is performed. It therefore rifes j the injedioij is ftopped ; 

 and the air-pump making its ftroke fuflirs the injedion water and a confiderable part of 

 the elaftic fluid to pafs through its lower valve. The vacuum is thus kept up through the 

 whole. internal capacity of the engine. As foon as the pifton has reached the upper part of 

 itbc cylinder, the communication to the under part of the cylinder is ftopped, and that with 



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