Chap, i.] Engine. 393 



water is kept always running upon the pifton, which 

 prevents the entry of the air; but this mode of fecur- 

 ing the pifton, though not injurious in the common 

 ones, would be highly prejudicial to the new engines. 

 Their pifton is therefore made more accurately; and 

 the outer cylinder, having a lid, covers it, the fteam 

 is introduced above the pifton j and when a vacuum 

 is produced under it, acts upon it by its elafticity, as 

 the atmofphere does upon common engines by its gra- 

 vity. This way of working effectually excludes the 

 air from the inner cylinder, and gives the advantage 

 of adding to the power, by increafing the elafticity of 

 the fteam. 



In Mr. Watt's engines, the cylinder, the great 

 beams, the pumps, &c. ftand in their ulual pofitions. 

 The cylinder is fmaller than ufual, in proportion to the 

 load, and is very accurately bored. 



In the molt complete engines it is furrounded, at a 

 fmall diftance, with another cylinder, furnifhed with 

 a bottom and a lid. The interftice between the cy- 

 linders communicates with the boilers by a large pipe, 

 open at both ends, fo that it is always filled with 

 fteam, and thereby maintains the inner cylinder always 

 of the fame heat with the fteam, and prevents any 

 condenfation within it, which would be more detri- 

 mental than an equal condenfation in the outer one. 

 The inner cylinder has a bottom and pifton as ufual i 

 and as it does not reach up quite to the lid of the outer 

 cylinder, the fteam in the interftice has always free 

 accefs to the upper fide of the pifton. The lid of 

 the outer cylinder has a hole in its middle ; and the 

 pifton rod, which is truly cylindrical, moves up and 

 down through that hole, which is kept fteam-tight by 

 a collar of oakum fcrewed down upon it. At the 

 bottom of the inner cylinder, there arc two regulating 



valves, 



