THERMOMETER AND PYROMETER. 



that its principle seems scarcely ever to 

 have been put in practice, except by its 

 inventor and the Marchese Poleni.* It 

 is difficult to construct two instru- 

 ments which shall correspond, from the 

 varying expansibility of air according 

 to its moisture or dryness ; the indica- 

 tions are liable to be affected by the 

 fluctuations of atmospheric pressure ; it 

 is liable to be deranged by the escape of 

 a portion of the included air, when the 

 instrument is moved about ; it is, more- 

 over, too unwieldy, and very liable to be 

 broken. 



Much about the period when those 

 attempts to perfect the thermometer 

 were made in France, important im- 

 provements on it were effected in the 

 north of Germany and in Holland, by 

 the introduction of quicksilver as the 

 thermometric fluid. 



The objections we have stated to the 

 use of the spirit thermometers, and to 

 the oil thermometer of Newton, led the 

 way to the employment of quicksilver 

 in the construction of the instrument. 

 Dr. Halley alludes to several advantages 

 of quicksilver as a thermometric fluid, but 

 seems to have rejected it on the ground 

 of its slight expansion by heat,t although 

 this objection might have so easily been 

 obviated by increasing the disproportion 

 between the bulb and the diameter of 

 the tube. On this account the claim set 

 up for his title to priority of invention 

 may justly be denied. It is most pro- 

 bable that science is indebted for this 

 great improvement to Roemer, the cele- 

 brated astronomer of Dantzic, to whom 

 the invention is ascribed by Boerhaave, 

 as well as the first idea of the scale now 

 known as that of Fahrenheit. Boer- 

 haave further adds, that as early as 

 1709, Roemer observed with that in- 

 strument a natural cold so intense as to 

 sink the mercury to the beginning of 

 the scale.J Thermometers of this con- 

 struction began to be made by Daniel 

 Gabriel Fahrenheit, a native of Dantzic, 

 who afterwards lived at Amsterdam, in 

 so admirable a manner, that he has 

 generally been considered the original 

 inventor ; they were speedily spread over 

 the north of Europe under his name, 

 and still maintain their ground in se- 

 veral countries, especially in Britain. 



It has commonly been alleged, that 

 at the time when Roemer' s or Fahren- 



heit's scale was proposed, its zero was 

 derived from the artificial cold pro- 

 duced by a mixture of salt and snow, 

 then supposed to be the lowest possi- 

 ble reduction of temperature. This, 

 however, seems to be inaccurate : Boer- 

 haave * gives a different account of the 

 matter, which is repeated in the Philo- 

 sophical Transactions, f The zero 

 was fixed from '' the lowest cold ob- 

 served in Ysland" (Iceland); which 

 was supposed to be as low a tempera- 

 ture as was likely to become the object 

 of philosophic investigation : but when 

 artificial methods of reducing the tem- 

 perature of bodies much lower, and 

 occasional natural colds brought the 

 mercury below that point, a scale of 

 equal parts was extended below the ; 

 the ascending series of degrees being 

 distinguished by sign + or plus, and 

 the descending series by the sign or 

 minus. 



The principle which dictated the pe- 

 culiar division of the scale is as follows. 

 When the instrument stood at the 

 greatest cold of Iceland, or degree, 

 it was computed to contain 11,124 

 equal parts of quicksilver ; which, 

 when plunged in melting snow, ex- 

 panded to 11,156 parts; hence the in- 

 termediate space was divided into 32 

 equal portions, and 32 was taken 

 as the freezing point of water : when 

 the thermometer was plunged in 

 boiling water, the quicksilver was ex- 

 panded to 11,336 parts ; and therefore 

 212 was marked as the boiling point 

 of that fluid.;}; In practice, Fahren- 

 heit determined the divisions of his 

 scale from two fixed points, the freez- 

 ing and boiling of water : the theory of 

 the division, if we may so speak, was 

 derived from the lowest cold observed 

 in Iceland, and the expansions of a given 

 portion of mercury. 



The mercurial thermometer was used 

 by the Italian philosopher Renaldini 

 before the end of the seventeenth cen- 

 tury : and he proposed, in 1694, an in- 

 genious method of graduating it between 

 the freezing and boiling points of water, 

 by successive mixtures of determinate 

 weights of boiling and ice cold water. 



The great advantages of Fahrenheit's 

 thermometer over every other pre- 

 vious invention, consisted in its appli- 

 cability to a greater range of tempera- 



* Phil. Trans. No. 421. 



f Phil. Trans, vol. xvii. p. 652. 



I Boerhaavii Chemise, tool. i. p. 720, 



Chemiae, torn. i. p. 720. 



f Vol. xliv. p. 680. 



f For 11156 11124 = 32, and 11336 1124=2 12. 



