CHAPTER V.

Cylinder and Piston—Condenser — Air-Pump —Barometer and Steam-Gauge—Working Beam —Parallel Motion — Counter — Sun arid PlanetWheel—Fly and other Modes of regulating Ve-locity — Boiler •— Safety- Vlives — Furnace.

Having taken a brief review of the early historyand general principle of this stupendous machine,it may be advisable before we proceed to a de-scription of the principal engines now employed,to examine more minutely the separate parts andthe progressive improvements effected in each.

The cylinder and piston being those parts ofthe engine in which the effective force is moreimmediately produced, may first claim attention.

The piston of the atmospheric engine is gene-rally made of cast iron nearly fitting the inside ofthe cylinder, a circular ledge or rim being formedround it to receive the packing, without whichthe steam would find a passage through the inter-stices in the cylinder. Mr. Smeaton , who greatlyimproved the atmospheric engine, coated theunder side of the piston with elm or beechplanks about two inches and a quarter thick; thewooden bottom being screwed to the iron with adouble thickness of flannel and tar, to exclude the