6 



THERMOMETER AND PYROMETER. 



them seem to have considered that the 

 varying expansions of the thermome- 

 tric liquids at different temperatures, 

 and the expansions of the glass of the 

 instrument, must have materially af- 

 fected every attempt to subdivide the 

 stem of the thermometer into fractional 

 parts of the whole bulk of the con- 

 tained liquid. 



One of these questions, however, 

 seems to have about that time engaged 

 the attention of philosophers, viz. whe- 

 ther equal increments of temperature 

 caused equal expansions of the thermo- 

 metric fluid. Dr. Brooke Taylor tried 

 the experiment with an oil thermometer, 

 by mixing definite portions of hot and 

 cold water, and measuring the tempe- 

 rature of the mixture. His conclusion 

 was in the affirmative, but the delicacy 

 of his instruments was unequal to the 

 solution of this nice problem, although 

 he has the merit of pointing out how the 

 problem is to be solved. 



The construction and uses of ther- 

 mometers early engaged the attention 

 of the French Acadtfmie des Sciences ; 

 and several -were constructed by Mr. 

 Hubin for that learned body ; but nei- 

 ther these, nor the thermometers placed 

 in the observatory of Paris by De La 

 Hire, appear to have been graduated 

 on any fixed principle. The Memoirs 

 of the Academy contain several descrip- 

 tions of thermometers, and an account 

 of many interesting observations, with 

 these instruments ; but the first altera- 

 tion in their construction deserving of 

 notice is the air thermometer of Geof- 

 froy, which from the short description 

 appears to be an improvement on that 

 of Boyle, inasmuch as it is not affected 

 by atmospheric pressure. He describes 

 the tube as without any opening, except 

 one, which descends almost Fig. 6. 

 to the bottom of the ball, 

 and there dips into a small 

 portion of coloured liquid.* 

 There is no figure given 

 in the original, and but a 

 very rude one in our Phi- 

 losophical Transactions^ 

 seemingly from the de- 

 scription. It is not stated 

 how the ball was joined to 

 the tube, but it was most 

 probably by cement, as re- 

 presented mfig. 6. 



M. Amontons clearly saw 



M* M Acad> tom ' xiii< P- 12 - Jt was read in 

 iviay, i /Uv. 



t Phil, Trans, vol. xxiii. p. 962. 



the importance of fixed points in the 

 thermometric scale, and proposed to ob- 

 tain them from the boiling J^Q. 7 

 point of water.* His 

 thermometer consisted of 

 a tube four feet in length, 

 ending below in a ball 

 bent upwards, as in fig. 7, 

 and open at the other ex- 

 tremity. The measure of 

 the temperature was the 

 elasticity of a given por- 

 tion of air included in the 

 ball, and subjected to a 

 pressure equal to two at- 

 mospheres, by adding to 

 the usual atmospheric 

 pressure that of a column of mercury of 

 28 French inches. Each half- inch of 

 his tube is therefore equal to one inch 

 under the usual pressure ; and hence at 

 a mean pressure of 28 French inches, 

 the volume of the compressed air is 

 really equal to 56 inches under the 

 usual pressure. 



In passing from the mean tempera- 

 ture of a Parisian spring to the heat of 

 boiling water, Mr. Amontons found that 

 these 56 inches were increased by one- 

 third, or 1 8 inches 8 lines, and therefore 

 he fixed the boiling point of his scale at 

 56 + 18,8 = 74 inches 8 lines. To mea- 

 sure this on Amontons's principle a tube 

 of 47 inches is quite sufficient ; for 74 

 inches 8 lines minus 28 inches, the at- 

 mospheric pressure which need not be 

 considered in the length of the tube, is 

 equal to 46 inches 8 lines ; and, indeed, 

 as in Amontons's process, the compres- 

 sion at high temperatures is rather more 

 than in the duplicate ratio of the air we 

 breathe, the mercury in boiling water 

 will not rise above 45 of his scale.f 

 There is a slight discrepancy between 

 the original account of Amontons's ther- 

 mometer and that given by Martine, 

 who states its boiling point at 73 inches, 

 and its freezing point at 51 1 inches; 

 but, according to the Academicians, the 

 latter will be at 52 inches and about 8 

 lines. The ingenious contrivance of 

 the double pressure enabled him to 

 apply the instrument to measure the 

 temperature of boiling water, by a tube 

 less than four feet in length.:}: 



Although the idea of Amontons was 

 a fine approximation to an universal 

 standard for a thermometric scale, the 

 instrument is liable to such objections 



* Mdmoires de 1'Acad. for 1702. 



f Me"moires de 1'Acad. des Sciences, tom. xv. 



% Mm. Acad. des Sciences, tom. xv. for 1702 



