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'Hie day pieces are previoufly baked juftto ignition, or to a red-heat barely vifible inthe day-time, and when applied to the gagethey juft reach the o. of the fcale. Every in-creafe of heat contracts the piece, which there-fore can be flid further into the gage. Theclay pieces will bear the moft fudden heatingand cooling without in the lead cracking orfplitting, and indeed their fize is fuch that intwo or three minutes that they have undergonethe utmoft effed! of any particular degree of fire,fo that no long continuance of the fame heat isneceflarv. To employ this pyrometer one ofthe pieces muft be put in or contiguous to thepart where the heat is to be afcertained, in-clofed if neceflary in a very fmall crucible. Ifthrown on burning coals in a high heat it be-comes fomewhat rough on the outfide andfirmly covered with a kind of black enamel,which poilibly may render it lefs accurate. Itfliould alfo be kept from the immediate atftionof any i'aline or other fluxes which would cor-rode its furface and impair its accuracy, but inother refpeds it requires but little precau-tion. It is a peculiar property, and generallyan advantage, of this pyrometer, that each clay-piece tells only the extreme!! heat to which ithas ever been expofed, and never returns toany inferior point of the fcale like all otherthermometers, fo that it may be gaged at anydiftance of time from the experiment. Even ifthe piece is broken or injured, if the compleatring of the cylinder remains, of whateverbreadth, it forms as accurate a meafure aswhen entire.
The degrees of this pyrometer only begin ata confiderably greater heat than the terminationof the mercurial thermometer, and hence inorder to render them at all comparable, it be-came neceflary to conned! the two by fomeintermediate fcale. This Mr. Wedgewood ef-fected by comparing the expanfion of a filvergage in the intermediate degrees •, and by expe-riments, which it is not neceflary here to detail,he was able to conned! the two fcales in a tole-rably fatisfadtory way, though the heats are foremote that very little advantage can ever bederived from this method. Mr. W. found thatthe mercurial range of temperature was fofmall compared with the pyrometrical, that aAngle degree of the clay pyrometer was equiva-lent to 130 degrees of Fahrenheit, and the o.of the pyrometer, which is a low red heat, hefound to fall at about 1077° Fahr. fuppofingthe mercurial fcale could be continued fo high,and with an uniform expanfion of the mercury.
Mr. W. has extended the fcale of his pyrome-ter to 240 degrees; this is entirely arbitrary,but it probably is more than amply fufficient toinclude all poflible produdfion of heat, fincethe greateft intenfity of furnaces has not beenfound to exceed about 180 of this fcale. Thefe240 degrees are eftimated from the above datato be equivalent to 31200 degrees of the mer-curial thermometer, and as the o. of the formeris equal to io77°Fahr. the extreme of the py-rometer fcale would correfpond with 32277 0Fahr. could this be continued as high. Flencetoo the whole diftance between freezing andboiling water, or from 32 0 to 212° Fahr. isequivalent to no more than about half a degreeof the pyrometer.
We fhall now give the pyrometrical corref-pondences with fome of the moft eafily ob-ferved ftages of the higher heats as obferved bythe inventor and others.
Red heat fully vifible in day-light oEnamelling heat for pottery 6
Brafs melts at 21
Swedifli copper melts at 27
Fine filver melts at 28
Fine gold melts at 32
Vitrefying (or higheft) heat of aplate-glafs furnace 124
Working heat of ditto 57
Welding heat of iron 120 to 132Greateft heat of a common fmith’sforge 125
Caft iron melts at 130
Bafalt melts at 100
Antient Etrufcan ware begins tocontrad! (and therefore wasbaked) at from 30 to 20
Ditto compleatly melts at 36
Queen’s pottery ware baked at 86Stone ware baked at 102
Worcefter china vitrefied at 94Derby ditto ditto at 112
Chinefe porcelain only foftens at 120Ditto finks down into femi-fufionat 156
True ftone Nankeen does not inthe leal! alter at 156
Pyrometer "pieces affume a porce!-*-lanous texture at no
Greateft heat of the bcft con-ftrudled air furnace probablydocs not exceed about 180
It remains to mention fome uncertaintiesthat Hill attend the ufe of this inftrument, and