THERMOMETER. 



turning with the mercury wlicn it defccnds, fliews diftinftly 

 and accurately how high tlie mercury has rifen, and con- 

 fequently wliat dec;ree of cold or heat has liappened. To 

 prevent the fpirit from cvaporatinjr, the tube at the end i is 

 clofely fealed. The daily rcftificatioii of this inftrument is 

 performed, by applying a fmall magnet to that part of the 

 tube againft which the index refts ; by the atlion of which 

 the included piece of Heel wire, and confequently the index, 

 is ealily brought down to Uie lurface of the mercury. When 

 this has been done, the inftrument is rettilied for the next 

 day's obfervation, witliout heating, cooHng, frparating, or 

 at all dillurbing the mercury, or moving the inftrument. 

 With a thermometer of this fort, Mr. Six obferved the 

 greateft heat and cold that happened every day and night 

 tliroughout the year 1781. But for the more particular de- 

 fcription of this inftrument, the illuftration of it by figures, 

 and an account of its advantages, the limits of this work 

 require our referring to Phil. Tranf. vol. Ixxii. part i. 

 p. 72, &c. 



A fimilar effeft to that produced by Six's thermometer is 

 obtained in Rutherford's arrangement of a pair of thermo- 

 meters, one with mercury, the other with fpirit of wine, 

 placed in a horizontal pofition ; one index being without the 

 furface of the mercury, the other within that of the fpirit : 

 the thermometers being in contrary direftions, both indices 

 may be brought back to their places, by merely raifiug the 

 end of the inilriuTient. (See Jig. 16.) Sclf-regiftcring ther- 

 mometers have alfo fometimes been conftrufted for keeping a 

 ftill more accurate account of all the variations of tempera- 

 ture that have occurred, by defcribing a line on a revolving 

 barrel, which ftiews the height for every inftant during the 

 whole time of their operation. 



M. de Luc has defcribed the beft method of conftruft- 

 ing a thermometer, fit for determining the temperature of 

 the air, in the menfuration of heights by the barometer. 

 He has alfo fhewn how to divide the fcale of a thermometer, 

 fo as to adapt it for aftronomical purpofes in the obferva- 

 tion of refraftion. See Recherches, &c. torn. ii. p. 35, &c. 

 p. 265, &c. 



Mr. Cavallo, in 1 78 1, propofed the conftruftion of a 

 thermometrical barometer, which, by means of boihng water, 

 might indicate the vai'ious gravity of the atmofphere, or the 

 height of the barometer. This thermometer, he fays, with 

 its apparatus, might be packed up into a fmall portable box, 

 and ferve for determining the heights of mountains, &c. with 

 greater facihty than with the common portable barometer. 

 The inftrument, in its prefent ftate, confifts of a cyhndrical 

 tin veffel, about two inches in diameter, and five inches high, 

 in which veflel the water is contained, which may be made 

 to boil by the flame of a large wax-candle. The thermo- 

 meter is faftened to the tin veft^el in fuch a manner, as that 

 its bulb may be about one inch above the bottom. The 

 fcale of this thermometer, which is of brals, exhibits on one 

 fide of the glafs tube a few degrees of Fahrenheit's fcale, 

 viz. from 200° to 216°. On the other fide of the tube are 

 marked the various barometrical heights, at which the boil- 

 ing water ftiews thofc particular degrees of heat which are 

 fet down in fir G. Shuckburgh's table. With this inftru- 

 ment the barometrical height is ftiewn within one-tenth of an 

 inch. The degrees of this thermometer are fomewhat 

 longer than one-ninth of an inch, and therefore may be 

 divided into many parts, efpecially by a Nonius. But the 

 greateft imperfeftion of the inftrument arifes from the fmall- 

 nefs of the tin veffel, which does not admit a fufficient 

 quantity of water ; but when the quantity of water is 

 fufficiently large, e.g. 10 or 12 ounces, and is kept boiling 



in a proper veffel, its degree of heat tnder the fame preffure 

 of the atmofphere is very fettled ; whereas when a thermo- 

 meter is kept in a fmall quantity of boiling water, the 

 quickfilver in its ftem does not ftand very fteady, fometimes 

 rifing or falling even half a degree. Mr. Cavallo propofes 

 a farther improvement of this inftrument in the Phil. Tranf. 

 vol. Ixxi. partii. p. 524. 



The ingenious Mr. Wedgwood, fo well known for his 

 various improvements in the different forts of pottery-ware, 

 has contrived to make a thermometer for meafuring the 

 higher degrees ot heat, by means of a diftinguilhing property 

 of argillaceous bodies, viz. the diminution of their bulk by 

 fire. This diminution commences in a low red heat, and 

 proceeds regularly, as the heat increafes, till the clay be- 

 comes vitrified. The total contraction of fome good clays 

 which he has examined in the ttrougeft of his own fires, is 

 confiderably more than one-fourth part in every dinieniion. 

 If, therefore, we can procure at all times a clay /nfficiently 

 apyrous or unvitrefcible, and always of the fame quality in 

 regard to contraftion by heat ; and if we can find means of 

 meafuring this contraftion with eafe and accuracy, we ftiall 

 be furniftied with a meafure of fire fufficient for every pur- 

 pofe of experiment or bufinefs. Some of the purelt Cornifti 

 porcelain clays (which, by the analyfis of Mr. Wedgwood, 

 appear to contain no calcareous earth nor gypfeous matter, 

 but to confift of pure argillaceous or alum earth, and 

 another indiffoluble earth, which he apprehends to be of the 

 filiceous kind, in the proportion of three parts of the former 

 to two parts of the latter) feem the beft adapted, both for 

 fupporting the intenfity, and meafuring the degrees of fire. 

 This material is prepared for ufe by waftiing it over, and 

 whilft in a diluted ftate palling it through a fine lawn : it is 

 then dried and put up in boxes. The dry clay is to be 

 foftened for ufe with about two-fifths its weight of water ; 

 and formed into fmall pieces, in little moulds of metal, ^'xrlhs 

 of an inch broad, with the fides exaftly parallel ; about 

 T^ths of an inch deep, and an inch long. The moulds are 

 to be oiled and warmed. Thefe pieces, when perfeftly dry, 

 are put into another iron mould or gage, confifting only of 

 a bottom, with two fides, -,*nths of an inch deep, to the 

 dimenfions of which fides the breadth of the pieces is to be 

 pared down. For meafuring the diminution which they are 

 to fuffer from the aftion of fire, another gage is made of two 

 pieces of brafs, twenty-four inches long, with the fides 

 exaflly ftraight, divided into inches and tenths, fixed five- 

 tenths of an inch afunder at one end, and three-tenths at the 

 other, upon a brafs plate ; fo that one of the thermometric 

 pieces, when pared down in the iron gage, will juft fit to the 

 wider end. If this piece be fuppofed to have diminiftied in 

 the tire one-fifth of its bulk, it will then pafs on to half the 

 length of the gage ; if diminiftied two-fifths, it will go on 

 to the narroweft end : and in any intermediate degree of 

 contraftion, if the piece be flid along till it refts againft; the 

 converging fides, the degree at which it ftops will be the 

 meafure of its contraction, and confequently of the degree 

 of heat it has undergone. The thermometric pieces may 

 be formed much more expeditioufly than in the fingle mould 

 by means of an inilrumcnt, confifting of a cylindrical iron 

 veffel, with holes in the bottom, of the form and dimenfions 



required The foft clay, put in the veffel, is forced by a 



prefs down through thele apertures, in long rods, which 

 may be cut while moift, or broken when dry, into pieces of 

 convenient lengths. After which, recourfe ftiould be had 

 to the paring gage for afcertaining and adjufting their breadth 

 when pcrfcftiy dry. 



Each divifion of the fcale, though fo large as a tenth of 



