Chap, i.] Engine. 395 



the pifton to be pulled up by the fuperior weight of 

 the pump rods ; and fo the engine is ready for another 

 ftroke. 



But the nature of Mr. Watt's improvement will be 

 perhaps better underilood from the following defcrip- 

 tion of it, as referred to a figure. The cylinder or 

 fteam vefTel A, of this engine (fig. 2.) is fhut at 

 bottom and opened at top as ufual, and is included in 

 an outer cylinder or cafe BB, of wood or metal, co- 

 vered with materials which tranfmit heat flowly. This \ 

 cafe is at a fmall diftance from the cylinder, and clofe 

 at both ends. The cover C has a hole in it, through 

 which the pifton rod E Hides ; and near the bottom is 

 another hole F, by which the fteam from the boiler 

 has always free entrance into this cafe or outer cy- 

 linder, and by the interftice GG between the two cy- 

 Jynders has accefs to the upper fide of the pifton HH. 

 To the bottom of the inner cylinder A is joined a pipe 

 I, with a cock or valve K, which is opened and fhut 

 when neceflary, and forms a pafiage to another vefiel 

 L called a condenfer y made of thin metal. This vefTel 

 is immerfed in a ciftern M full of cold water, and it 

 is contrived fo as to expofe a very great furface ex- 

 ternally to the water, and internally to the fteam. It 

 is alfo made air-tight, and has pumps N wrought by 

 the engine, which keep it always exhaufted of air an4 

 water. 



Both the cylinders A and BB being filled with 

 fteam, the paflage K is opened from the inner one to 

 the condenfer L, into which the fteam violently ruflies 

 by its elafticity, becaufe that veflel is exhaufted ; but 

 as foon as it enters it, coming into contact with the 

 cold matter of the condenfer, it is reduced to water, 

 and the vacuum ftill remaining, the fteam continues 

 to rufh in till the inner cylinder A below 'the pifton 



is 



