152 CAUSE AND EFFECT OF SUPERHEATING.

CHAPTER IV.

SUPPLEMENTARY DETAILS OF MARINE BOILERS.

THE CAUSE AND EFFECT OF SUPERHEATING.To fairly understand any scientific question—whether relating to chemical compound ormechanical arrangement—a knowledge of theorigin of the subject investigated is essential.In the present instance an addition to themarine boiler is introduced : and in commonwith other descriptions, a brief notice of itsorigin will not be out of place. The naturalstate of steam is undoubtedly a pure gas, butthe mechanical contrivance for its productionbeing at fault, a compound is formed, com-posed of steam and water. Now the presenceof the latter greatly lowers the effect of theformer, and it is from this cause that thesteam is sometimes dried before it enters thecylinder. "With marine boilers this is espe-cially requisite, due to the steam spaces oftenbeing imperatively reduced in height above thewater, from which the vapour is generated.

The authorities on supeiheating do notagree on all points as to its value in a com-mercial sense. Some advocate it as the bestmeans to ensure the full effect of the steam.Others condemn it on account of the high tem-perature attained, injuring or defacing the sur-faces in working contact. Now if the latter evilcan be overcome, a great gain is accomplished,and repair thereby reduced. That, by drying

the steam, it is rendered more powerful thanwhen saturated, is an acknowledged fact, andperhaps no authority has proved this betterthan the eminent engineer, Air. John Penn,who stated in 1859, when addressing themembers of the “ Institution of Mechanical Engineers, ” On the Application of SuperheatedSteam in Marine Engines :—

“ The real source of advantage in employ-ing superheated steam appears to be in pre-venting the presence of any water in thecylinder of the engine, and ensuring that thecylinder shall never be occupied by anythingbut pure steam; making it a real steam engineinstead of one working with a mixture ofwater and steam. In all condensing enginesthe interior of the cylinder being open to thecondenser during half the time of each revolu-tion of the crank is in communication duringthat time with the low temperature of thecondenser, or about 110° when the vacuumis 131 lbs. per inch below the atmosphere or27 inches of mercury. There is consequentlya rapid radiation of heat from the sides andend of the cylinder, cooling down the wholemass of metal. The steam admitted into thecylinder in the next stroke, at a temperatureof 260° if at 20 lbs. per inch above the atmo-sphere, coming in contact with these cooled