THERMO METER. 



l!,f ililluico btlwffii thcin i5,-o- J'> Crucquius's, the 

 tVi-c/ing point was 1070, the boiling point 15 10, and the 



ilitbncc 44c. r L D 1 c 



III the aiai.-nt ftandard thrrmometer of the Royal bo- 

 cielv, after which thermometers were for a long time con- 

 llruded in England, Dr. Martin<; found that gr. 34^, an- 

 fwered to gr. 64 in Fahrenheit's, and gr. o to 89 or 88. 

 From that point the numeration afccnded and dcfcendeil 

 tbuj; the freezing point was 73^, t!ie boiling point 141^, 

 and the diftance between them 2i5i. In iir liaac New- 

 ion's, the freezing point was o, the boiling point 34, and 

 the diftance 34. 



In the thermometers graduated for adjufting the degi-ees 

 of heat proper for exotic plant?, &c. in lloves and green 

 houfes, the middle temperature of the air is marked at 

 gr. o, and the degrees of heat and cold are numbered 

 both above and bdow. Many of thefe are made on no 

 regular and fixed principles. But in that formerly much 

 uled, called Fowler's regulator, the fpirit fell, in melting 

 fnow, to about gr. 34 under o ; and Dr. Martine found, 

 that his gr. 16 above o, coincided willi nearly gr. 64 of 

 Fahrenheit. His o feenis to have coincided with about 

 the 53d or 54th degree of Fahrenheit's, and from that 

 point the- numeration afcended and dcfcended thus ; the 

 freezing point 34, the boiling point 250J, and the diftance 

 between them 2844. 



Dr. Hales (Statical EiTays, vol. i. p. 5S.) in his thermo- 

 meter made witli fpirit of wine, and ufed in experiments 

 on vegetation, began his fcale with tho loweft degree of 

 freezing, or gr. 32 of Fahrenheit, and carried it up to 

 gr. too, which he marked where the fpirit llood when the 

 ball was heated in hot water, o!i which wax fwimming tirft 

 began to coagulate, and this point Dr. Martine found to 

 correfpond with gr, 142 of Fahrenheit, But by expe- 

 rience Hales's or. ico falls confiderably above our gr. 142. 

 According to others, his freezing point was o, his boiling 

 point was 163, and the diftance of courfe 163. 



In the Edinburgh thermometer, made with fpirit of 

 wine, and ufed in the meteorological obfervations pub- 

 li(hid in the Medical EtTays, the fcale is divided into 

 inches and tenths. In melting fnow the fpirit Hood at 

 X,',, and the heat of the human (kin raifed to 22 '5. Dr. 

 Martine found, that the heat of the perfon who graduated 

 it was _fr. 97 of Fahrenheit. It feems to have been gra- 

 duated thus; the freezing point 8;, the boiling point 47, 

 and the diftance between them 3Sj. 



As it is often of ufe to compare different thermometers, 

 ill order to judge of the refult of former obfervations, we 

 have annexed from Dr. Maitine's Effays, the table by which 

 lie compared fifteen different thermometers. See PlateXVl. 

 Pneumatics, Jig. 4. See alfo the table at the clofe of this 

 article. 



There is a thermometer which was formerly much ttfed 

 in London, called the thermometer of Lyons, becaufe M. 

 Criftin brought it there into ufe, which is made of mer- 

 cury : the freezing point is marked gr. o, and the inter- 

 val from that point to the heat of boiling water is divided 

 into 100 equal degrees. 



From the above abftraft of the hiftory of the conftruc- 

 lion of thermometers, it appears that freezing and boiling 

 water have furnifhed the diftinguithing points that have 

 been marked upon almoft all thermometers. The inferior 

 fixed point is that of freezing, which fome have determined 

 by llie freezing of water, and others by the melting of 

 iLc ; ajid though the difFcrer.ce between thefe two tempera- 

 tures is not commonly very confidcrable, yet it is not \n- 

 vai-iable. * • 



It is now well known, that all, or almoft all bodies, by 

 changing from a fluid to a folid ftate, or from the ftate of an 

 elaftic to that of an unelaftic fluid, generate heat ; and that 

 cold is produced by the contrary procefs. 



In order to obtain this fixed point or limit, melting ice, or 

 ice powdered and mixed with water, will produce the fanu' 

 temperature. And thoiigii there may be fome trifling dif- 

 ference between the temperature of ice difpofed to melt, ai.d 

 that of melted ice or the water produced by it ; this differ- 

 ence, however, has no fenfible cffefl on the thermometer ; 

 confequently, the temperature of water fucceflively produced 

 by ice, and accumulated in its intcrftices, or from powdered 

 ice mixed with the water which is produced by it in melting, 

 affords, as De Luc obfervcs, a fixed point, which is eafily 

 obtained, and which fhould be adopted in the conftrudlion of 

 all thermometers. 



The fuperior fixed point of almoft all thermometers, i» 

 the heat of boiling water ; but this point cannot be confi- 

 dered as fixed, unlefs the heat be produced by the fame de- 

 gree of boiling, and under tlic fame weight of the atmo- 

 ipherc. With regard to the firft circumftance, it is obferved, 

 that water, when it begins to boil, has not attained to its 

 greateft degree of heat, which is known by its bubbling or 

 foaming from the bottom of the veffel, and over the whole 

 furface of the water, with the greateft violence which it is 

 capable of acquiring ; and in this ftate the water difcovers 

 an augmentation of heat more than one degree above the heat 

 it had when it began to boil. The temperature of water 

 which boils with vehemence (hould, therefore, be the 

 ftandard of the fixed point of thermometers : neverthelefs 

 it is to be confidered farther, that this degree of heat with 

 which water violently boils, is invariably the fame, only un- 

 der a given prefTure of the atraofphere ; but if the preflure 

 be diminifned or increafed, the boihng heat is diminifhed or 

 increafed. It is well known that water, placed under the 

 exhaufted receiver of an air-pump, will be converted into 

 fteam with a degree of heat far inferior to that which is ne- 

 cefTary to its boiling in the open air ; and under the preflure 

 of its own vapour, confined in Papin's digefter, it is faid to 

 fuftain a degree of heat, without boiling, far exceeding that 

 which, in the open air, would convert it into fteam. Hence 

 it follows that, in climates where the preiTure of the at»io- 

 fphere is liable to confidcrab'.e change, the heat of boihng 

 water, in open air, will be different at different times. Con- 

 fequently thermometers, made in different ftates of the ba- 

 rometer, will difagrcc ; unlefs allowance has been made for 

 the effe<fl ot the variation of the barometer upon accurate 

 principles. That the heat of boihng was variable, accord- 

 ing to the prelTure of the atm.ofphere, feems to have been 

 known to Fahrenheit as early as the year 1724. See Phil. 

 Tranf. N° 385. 



Some time after this period, MefTrs. le Monnier and Caf- 

 fini (Mem. de I'Acad. des Sc. for 1740) made fome decilive 

 obfervations, in order to Ihew that this quantity was very 

 confiderable. 



M. de Luc, in 1762, made a much more complete feries 

 of experiments, which he has defcribed and reduced into a 

 fyflem in his Rccherches fur les Mod. de I'Atmofphere, 

 vol. !. p. 382, &c. vol. ii. p. 338, &c. and thefe have been 

 fince verified by fir George Shuckburgh, in 1775 and •77^- 

 See Phil. Tranf. vol. Ixix. part ii. p. 362, &c. 



M. de Luc fixes the boihng point of his thermometer 

 when the barometer is at 27 Paris, or 28.75 Englifli inches, 

 that being its mean height at Geneva. He divides the fun- 

 damental interval, i. e. the whole extent of the fcale, be- 

 tween melting ice and boihng v/ater, after the French man- 

 .>;er, into eighty equal pnrlj ; and by a great number of ex- 

 periments 



