154 CAUSE AND EFFECT OF SUPERHEATING.

lagged the same as the steam jacket, therewill be no more loss of heat by radiation fromthe outside.'

“ The mode of superheating the steam maybe varied in many ways, a general principleto be aimed at being to make use of the wasteImat for this purpose after leaving the boiler,so as to accomplish the superheating withoutany cost of fuel, and to place the apparatuswhere it will not be exposed to injury fromtoo great heat. The superheating apparatushas generally been placed in the smoke box oruptake flue in marine boilers, and has consistedof fagots of tubes or coils of pipe for thepurpose of obtaining the required extent ofheating surface within a limited space.”

Reference was next made by Mr. J. F.Spencer, the kind contributor of plates Nos. 3to 8, in this work, “ on the variation on thetemperature of steam in different parts of theboiler when superheated.”—“Fie rememberednoticing in the boilers of a large steamer,which had high steam domes with the uptakeflue passing up through so as to superheat thesteam at that part, that the temperature of thesteam in the top of the dome was as much as340°, whilst it was only 200° in the boiler justbelow the dome.”

Mr. E. A. Cowper next observed, “ That thepressure did not vary with the temperature;and whatever superheating took place, theeffect could only be an increase in the volumeof the steam and in its temperature, as itwould be impossible for any difference ofpressure to exist in the superheating appa-ratus, except indeed a slight diminution ofpressure that would arise from the resistanceof the small tubes to the passage of the steam.

The first effect of the superheating would bethe evaporation of all the moisture in thesteam, as steam always left the water in aboiler in a more or less wet or damp state,from the mixture of minute particles of waterwith it, even when there was no sensiblepriming; it would then become perfect ordry steam, but at first would not be raised atall in temperature; but when the superheat-ing was carried beyond that point, the tem-perature of the steam would be raised by allthe heat added, and its volume proportionatelyincreased, causing an increase in the totalquantity of steam supplied at the same pres-sure and from the same evaporation of water.Steam was expanded by increase of tempera-ture at pretty nearly the same rate as air andother gases; and since air at 32° was doubledin volume by an increase of temperature of480°, steam at 20 lbs. per inch or 260° wouldbe doubled in volume by 708° increase oftemperature (480° + 260 o -32°= 708°); and arise of 100° from 260° to 360° would conse-quently increase its volume l.-7th, causing anequal saving in consumption of fuel when thesuperheating was effected by using the wasteheat of the smoke box. As the specific heatof steam was only about 3-4ths that of air,steam would require only 3-4ths the quantityof heat to be supplied to it to produce thesame rise of temperature; and partly for thisreason steam was now used instead of air incaloric engines, since the same effect of ex-pansion was thereby obtained with so muchless supply of heat.

“ There was no doubt that in cylinders with-out steam jackets condensation of a portion ofthe steam took place at the beginning of the