THERMOMETER AND PYROMETER. 



ture, from the freezing to the boiling 

 point of quicksilver, in its not soiling 

 the containing tube, and in its receiv- 

 ing the impressions of heat and cold 

 more readily, while its density rendered 

 capillary tubes filled with it perfectly 

 visible; and thus the instrument be- 

 came more portable and delicate. We 

 may also remark, that at the period of 

 its invention, there was no other scale 

 in use that could pretend to vie with it 

 in accuracy ; and it still possesses the 

 peculiar advantages, that from the low- 

 ness of its 0, the observer is seldom 

 troubled with negative degrees, and 

 from the number of its divisions i^has 

 rarely, in ordinary operations, to use 

 fractions of a degree. 



We are indebted also to Fahrenheit for 

 ihe knowledge of the fluctuation of the 

 boiling point of water, according to the 

 difference of atmospheric pressure.* 

 Le Monnier, in 1739, confirmed this fact, 

 by noting the temperature of boiling 

 water on the top of Mount Canigou, 

 one of the Pyrennees; and in 1744 

 it was fully established by Martin 

 Folkes, who found that water boiled on 

 the summit of Pic du Midi 15 of 

 Fahrenheit's scale lower than at Bag- 

 neres ; and at the latter place 3| lower 

 than at Bordeaux ; while he proved 

 that elevation in the atmosphere had no 

 sensible influence on the stability of the 

 freezing point.t These facts led to 

 an important correction in fixing the 

 boiling point of water or other liquids. 



It would now be a waste of time to 

 describe minutely the various thermo- 

 meters which were in use in France and 

 England before the time of Fahrenheit. 

 They were all without fixed points in 

 the scale; and though they were vaunted 

 as constructed after the models in the 

 Royal Observatory at Paris, or in the 

 apartments of the Royal Society of Lon- 

 don, they gave most discordant results. 

 An analysis of the most noted of them 

 has been elaborately and ingeniously 

 attempted by Dr. Martinein his valuable 

 Essays, and the results presented in the 

 very convenient form of a tabular view. 

 We shall therefore pass at once to notice 

 some of the other more accurate ther- 

 mometers that have been employed in 

 different parts of Europe, although the 

 principle in them all is similar to what 

 has been already described. 



The thermometer with which the 



'* Phil. Trans, xxxiii. No. 381. 

 t Phil. Trans, vol. xliii. p. 32. 



Dutch philosopher Cruquius made the 

 observations published in the Philoso- 

 phical Transactions, (vol. xxxiii. No. 

 381,) \vas an air thermometer, on which 

 he states the freezing point of water to 

 be indicated by 1070, and boiling 

 water by 1 5 1 : the lowest known cold, 

 which seems to have been the begin- 

 ning of his scale, he gives = 1000. 



The objections to the thermometer of 

 Amontons are clearly stated by Reau- 

 mur, * who proposed to adopt the 

 freezing and boiling points of water as 

 fixed points in the scale, but employed 

 spirit as the thermometric fluid. He 

 unquestionably fell into error when he 

 stated that 1000 parts of strong spirit 

 dilated to 1087.5 parts in passing from 

 the freezing to the boiling point of 

 water; for how could strong spirit 

 sustain so high a temperature without 

 being partially converted into vapour ? 

 His proposal was to use spirit of just 

 such strength, that between these two 

 temperatures it should expand from 

 1000 to 1080; and, commencing his 

 scale or at the freezing point of 

 water, he made the boiling point 80. 

 The principle of this construction was 

 good ; but Dr. Martine has shown that 

 from the large size of the bulbs of his 

 thermometers, which were from 3 to 4 

 inches in diameter, and the short time 

 they were immersed in the freezing 

 mixture, they could not have acquired 

 an uniform temperature ; and accord- 

 ingly Martine found their freezing point 

 too high,t and the error in the boiling 

 point from the cause already alluded to, 

 must have been still greater. 



These errors might have been obviat- 

 ed by the use of quicksilver instead of 

 spirit. This was accordingly soon done ; 

 by whom first is uncertain, although 

 there is strong reason to believe by De 

 Luc ; and the mercurial thermometer, 

 with the at the freezing point of 

 water, and 80 as its boiling point, soon 

 became general in France, and well 

 known over Europe under the name of 

 Reaumur's Thermometer. The only 

 material objections to such a scale, 

 when the instrument is accurately 

 made, arise from the largeness of the 

 divisions rendering fractional parts of a 

 degree of frequent occurrence, and the 

 elevation of often introducing + and 

 degrees in a series of observations-, 

 even at common natural temperatures. 



The mercurial thermometer of Mons. 



* Mmoires de 1'Acad. des Sciences, 

 jr Martina's Essays, Edin. 1792, p. 23. 



