CAUSE AND EFFECT OF SUPERHEATING. lfif)

stroke, and a partial re-evaporation at the end,on account of the metal of the cylinder beingcolder than the fresh high pressure steamentering from the boiler, hut hotter than theexpanded steam in the cylinder at the end ofthe stroke: since the whole metal of thecylinder could not change in temperaturetwice in each stroke (though the interior sur-face must do so), the temperature of the cylinderand piston must be an average of the tempera-ture of the whole of the steam coming in con-tact with them. He had tried a direct experi-ment suggested to him by Mr. Appold,namely,fixing a glass gauge tube in communication withthe interior of the cylinder, the outer end ofthe tube being closed. At the beginning of thestroke the interior of the glass became quitedull with moisture from condensation goingon in the cylinder; but towards the end of thestroke the moisture was entirely evaporatedand the glass became clear, showing that therewas perfectly dry steam in the cylinder by thattime. The cylinder was in fact a partialcondenser at the beginning of the stroke, anda boiler at the end of the stroke; and if itwere not for this boiling off of the condensedwater at the end of the stroke, the cylinderwould soon get very nearly to the temperatureof the steam.

“ In an expansion engine without a steamJacket he had found by a comparison of theactual indicator figures with the theoreticalfigures which ought to have been obtained ifuo condensation had taken place in the cylin-der, that the loss of power when cutting offthe steam at f- stroke amounted to a loss of11*7 per cent; at a stroke to a loss of 19'6per cent.; at stroke to a loss of 2 2'2 per

[ cent.; at ^ stroke to a loss of 44*5 percent.

“ But when the cylinder had a steam j’acketsupplied with steam direct from the boiler,he found the actual indicator figure almostexactly corresponded with the theoreticalfigure, except that at the end of the stroke itwas raised a little, about ^ lb. in pressureabove the theoretical line, in consequence ofthe superheating of the expanded steam fromthe higher temperature of the metal of thecylinder. With steam in the j’acket of thesame pressure as that in the boiler he did notthink there could he any condensation inthe cylinder; for all that was requisite toprevent this was to keep up the metal of thecylinder at the temperature of the enteringsteam, by supplying the heat abstracted byexposure to the cooler steam during expan-sion, and that lost by radiation, which wasvery small in a well lagged cylinder. Thepiston ought to have non-conducting surfacesor plates, and the cylinder ends should havesteam j'ackets.”

Mr. Penn also stated, when reference wasmade to the mixing the superheated and ordi-nary steam, that “ the superheating apparatusmust certainly be regarded as a portion of theheating surface, and some reduction mightconsequently be made in the boilers on thataccount, besides the reduction due to thesaving effected in the quantity of water to beevaporated in doing the same work. Inreference to the question of mixed steam, hehad not tried any experiment on the subject,and did not see that as regarded the final effectit mattered how the steam was heated, pro-vided the temperature of the whole steam that

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