THERMOMETER. 



inclien, anfwering to gr. 212 of Fahrenheit's: and thus 

 tliey may be eafily compared together. 



The fixed points of Fahrenheit's thermometer, which is 

 generally ufed in Great Britain, (as we have already ob- 

 ferved under ^fif/Tur/'a/ Thermometer,) are the congela- 

 tion produced by fal ammoniac and the heat of boiling 

 water. The interval between thefe points is divided into 

 212 equal parts ; the firft of thefe points is marked o, 32 

 degrees below the freezing point, and the other 212 ; the 

 diftance of courfe between the freezing and boiling points 

 being 180. The reafon why Fahrenheit fixed his fcalc fo far 

 below the water-freezing point was founded on an erro- 

 neous hypothefis relative to the real zero or point of abfolute 

 privation of heat ; neverthelefs it has this advantage, that 

 the diftinftion between the pofitive and negative terms, or 

 thofe which exprefs degrees above or below the zero, 

 much lefs frequently occurs in any experiments, and 

 fcarcely ever in the regifter of natural cold in temperate 

 climates, by which many accidental errors are avoided. 

 Reaumur, in his thermometer, the conftruftion of which he 

 pubhflied in 1 730, and which is generally ufed in France and 

 other parts of the continent, begins his fcale at an artificial 

 congelation of water in warm weather, which, as he ufes 

 large bulbs for his glaffes, gives the freezing point much 

 higher than it (hould be, and at boihng water he marks gr. 

 80, (the diftance between both points being 80,) which point 

 Dr. Marline apprehends to be more vague and uncertain 

 than his freezing point. The fpirit in the ihermometer, 

 he obferves, is abfolutely incapable of fuch a great heat 

 as Reaumur afcribed to it, and that not by a fmall or 

 trifling difference. He finds, that highly reftified fpirit of 

 wine cannot be heated beyond ^r. 175 in Fahrenheit's ther- 

 mometer, while boihng water raifes the quickfilver 37 de- 

 grees higher ; and common brandy was able to conceive a 

 heat no greater than about ^r. 190. So far, he concludes, 

 was Reaumur in the wrong, when he thought that all fpi- 

 rits, weak and ftrong, immerged in boihng water, received 

 a given degree of heat, and that equal to the heat of the 

 furrounding water. He fuppofes his ftandard heat could 

 take a heat only of about ^r. 180 ; lefs by 32 degrees than 

 what he reckoned. In order to determine the correfpond- 

 ence of his fcale with that of Fahrenheit, it is to be confi- 

 dered that his boiling-water heat is really only the boiling 

 heat of weakened fpirit of wine, coinciding nearly, as 

 Dr. Martine apprehends, with Fahrenheit's gr. 180. And 

 as his gr. loj is the conftant heat of the cave of the ob- 

 fervatory at Paris, or Fahrenheit's gr. 53, he thence finds 

 his freezing point, inftead of anfwering juft to gr. 32, to 

 be fomething above gr. 34. 



The thermometer of M. de I'lde, of which he prefented 

 an account to the Academy of Sciences at Peterfburg in 

 1733, has only one fixed point, which is the hsat of boiling 

 water, and, contraiy to the common order, the feveral de- 

 grees are marked downwards from this point or zero, ac- 

 cording to the condenfations of the contained quickfilver, and 

 confequently by numbers increafing as the heat decreafes to 

 150, the freezing point. In order to determine the extent 

 of the degrees of this fcale, M. de I'llle firft weighed the 

 empty tube, and then weighed it full of mercury ; and 

 the difference of thefe two weights gave him that of the 

 mercury. He then expofed the thermometer to the heat of 

 boiling water, and took care to preferve the mercury, 

 which this increafe of heat forced out of it ; this he accu- 

 rately weighed, and dedufting its weight from the total 

 weight of the mercury, he made the remainder, or that 

 which was left in the thermometer, equal to 10000 ; he 

 then found by calculation how many loooo parts of this 



refidue that forced out of tlie tube contained, and thefe 

 parts formed the divifion* of the fcale from the point, 

 determined by the condenfation of the mercury to the 

 fame, point at which it ftood before it was plunged in boil- 

 ing water, to the upper end of the tube ; and thefe divi- 

 fions formed the extent of the degrees of M. de I'lflc's 

 fcale. According to his llandards, the freezing point, 

 fays Dr. Martine, is near to his ^r. I JO, correfponding to 

 Fahrenheit's gr. 32, by which me:uis tlwy may be com- 

 pared ; but M. Ducreli fays, that this point ought to be 

 marked at leaft at gr. 1 54. 



M. Ducreft, in his fpirit thermometer, conftruftcd in 

 1740, made ufe of two fixed points; the firft, or o, indi- 

 cated the temperature of the earth, and was marked on his 

 fcale in the cave of the Royal Obfcrvatory at Paris ; and 

 the other was the heat of boiling water, which the fpirit in 

 his thermometer was made to endure, by leaving the upper 

 part of the tube full of air. He divided the interval be- 

 tween thefe points into 100 equal parts ; caUing the divi- 

 fions upwards degrees of heat, and thofe below o degrees of 

 cold. 



He afterwards regulated his thermometer by the degree 

 of cold indicated by melting ice, which he found to be 

 loj. In Celfius's, or the centigrade thermometer, the 

 freezing point, like that of Reaumur's, was o, the boiling 

 point at 100, and the diftance between both 100. See the 

 table at the clofe of the article. 



The Florentine thermometers made and ufed by the 

 members of the famous academy dd' Cimento, being fome 

 of the firft inftruments of the fort, were vaguely graduated, 

 fome of them having many more degrees than others ; but 

 thofe of their moft comjmon graduation were of two forts ; 

 in one fort the freezing point, determined by the degree at 

 which the fpirit ftood in the ordinai-y cold of ice or fnow 

 (probably in a thawing ftate), and coinciding with gr. 32 of 

 Fahrenheit, fell at gr. 20 ; and in the other fort at gr. 135: 

 and the natural heat of the vifcera of cows and deer, &c, 

 raifed the fpirit in the latter, or lefs fort, to about gr. 40, 

 coinciding with their fummer heat, and nearly with^gr. lo? 

 in Ftihrenheit's, and in their other long thermometer, 

 the fpirit, when expofed to the great midfummer heat in 

 their country, rofe to the point at which they marked ^r. 80, 

 The freezing point of one was 20, the boiling point 174, 

 and the diftance between both was 154: in the other the 

 freezing point was 13^, the boiling point 8l|, and the dif- 

 tance 68|. 



In the Parifian thermometer, or the ancient thermometer 

 of the Academy of Sciences, the freezing point was at 25, 

 the boiling point at 239, and the diftance between both 

 214. 



In the thermometer of the obfervatory at Paris, made of 

 fpirit of wine by M. de la Hire, the fpirit always ftands at 

 _fr. 48, in the cave of the obfervatory, correfponding to 

 gr. ^^ in Fahrenheit's; and his gr. 28 correfponded with 

 5 1 inches fix lines in Amontons's thermometer, and con- 

 fequently with the freezing point, or gr. 32 of Faliren- 

 heit's. This thermometer of De la Hire, which ftood in 

 the obfervatory of Paris above 60 years, feems to have 

 been graduated thus ; the freezing point 28, the boiling 

 point 1 99 J, and the diftance between both 171-I. In Amon- 

 tons's thermometer the freezing point was 51^, the boiling 

 point 73, and the diftance between them 2ii. 



In the thermometer of Poleni, made after the manner of 

 Amontons's, but with lefs mercury, 47 inches corre- 

 fponded, according to Dr. Marfine, with 5 1 in that of 

 Amontons's, and 53 with 59^. It was graduated thus ; 

 the freezing point at 47-nj> the boiling point at 62,%, and 

 382 th? 



