T ii r. 



348 



T II I 



urbitr.uy. and consequently no two 11 



> thermo- 



i;iarking the pla 

 '.ood ill the tutu 1 when tli< 

 immersed in snow, and the place at which it Mood at the 



1 



\vns divided into 00 parts. Subsequently 

 in this c 



scale- or oil 



.iking the spac ( the 



tube equal to n whole volume: thus, 



supposing the ball of the thermometer and part of the tube 

 to he divide. 1 into ten t! and to be 



wholly occupied by the oil when the instrument is plunged 

 in melted i 1C found that by the heat of the human 



body the oil expanded 25li such parts, and by that of boil- 

 ater. 7i~> parts; then, c.m-idciing the point at which 

 ;i of the column stood in the tube, when the latter 

 was placed in ice, as the zero of the scale, he divided the 

 interval between this point and that at which the top of 

 the column stood when the ball of the thermometer was 

 placed under the arm of n man, into 12 parts. Afterward- 

 by proportion he found that the distance from the ice-point 

 to that of boiling water was equal to 34 such parts (Phil. 

 Trans., vol. xxii.' : this method, being of difficult execu- 

 tion, was soon abandoned. 



The scale which has been in general use in this country 

 since the year 1724, is supposed to have been invented by 

 Fahrenheit. It is quite unknown on what ground he made 

 choice of the fixed points on his scale, qr, of the number of 

 graduations between them ; but it is thought that one of 

 the fixed points was that of boiling water, and that the 

 other, which is the zero of the scale, was that at which the 

 top of the eolumn stood when the instrument was t-\ 

 to an intense cold in Iceland, in 1709. The extent of the 

 scale between this last point and that of boiling water is 

 divided into 212 parts, and the point of freezing water is 

 at the thirty-second division from the zero point. See the 

 scale on the right of the tube in the above figure. 



M. Reaumur constructed a thermometer in which spirit 

 of wine was employed, and he formed a scale in a manner 

 nearly similar to that which had been put in practice by 

 Sir Isaac Newton. He computed the volume of the glass 

 ball, and graduated the tube so that the space be; 

 two divisions was equal to one-thousandth part of that 

 volume : he then found the zero of the scale by marking 

 the place where the top of the column stood when the 

 thermometer was placed in water just freezing : and after- 

 wards, plunging the instrument in boiling water, he ob- 

 served whether or not the spirit rose exactly eighty illu- 

 sions. If not, he strengthened or diluted the spirit til! it 

 did so; and the point at which the top of the spirit stood 

 became the point of boiling water. Of this instniment an 

 account was published in the ' M^moires' of the Academy 

 onces for 1730, but the construction has been long 

 abandoned; for, besides the difficulty of gi\ing a 

 proper degree of strength to the spirit, it is well known 

 that the latter cannot be made to take the temperature of 

 boiling water, so that the determination of the 

 point in the scale must be very erroneous. That which is 

 now called Reaumur's thermometer is an improvement mi 

 the former, by M. Deluc, who determined the points of 

 freezing and boiling water by experiment, and divided flu- 

 distance between them into eighty parts, the zero of the 

 scale being at the former point. See the scale on the left 

 of the tube in the above figure. 



A third scale, called ' Centigrade,' has been much in use 

 among the philosophers of the Continent within the la-t 

 fifty years: it was invented 1,\ Ccl-iu-. a Swede, and il 

 differs from that of Ri'anmur'or Pelvic, only in tli. 

 i-en the points of freezing and hoiling- 

 being divided into UK) parts. The length of each degree 

 in this thermometer, as well as in that of Reaumur, is 

 greater than in the scale of Fahrenheit ; and consequently 

 the indications of temperature, when the top of the 

 or raercur\ n the lines of division, are rather un- 



certain, from the difficulty of estimating them accurately 

 2j ' ""' temperatures required to be .: 



_ often below the point <>t . tin 



lenitive -igns is of more frequent occur- 

 :> these thermometers than with those of Fahren- 

 heit. 



The following formulae will serve to convert any given 



number of degrees on Fahrenheit's scale into the corre- 

 sponding number of d and the Centi- 

 grade scales, anil rirt i 



Let F, R, ami any corresponding numbers of 



degrees on the three scales respectively : then 



(F-32}g=R, and (F-W ^ = U: 



also, i- C = R, and 7 R = C. 



> 4 



N.B. When F is between zero and 32, the values of R 

 and C are negative, and express the required numl 

 degrees below yero on Reaumur's and the Centigrade 

 scale. Also, when F. R, or C express. n number 



of degrees below zero on its proper scale, it must be 

 sidered as negat 



The scale invented by l)e 1'Isle , g, in 



\~:\.l. being -till occasionally in vise, it n, 

 to mention that it is formed by making the 

 in each degree equal to one hundred-thousandth pail of 

 the whole volume of the mercury: the' xcro of the 

 is at the point of boiling-water, and between this point and 

 that of free/ing-wa'cr the space is divided into IfAJ ) 



It may be observed that the situation of the freezing- 

 point on the scales of thermometers can be determined 

 with great accuracy if the ball and part of the tube be 

 immersed in pounded ice : for it is known that water con- 

 taining ice and snow remains of the same temj 

 the ice is entirely dissolved. c% . .n nf call, 



the water being employed in promoting .lion. 



Hut the point of boiling wat.r i- I'.ir from b. i 

 cisely known, since it \aiies with the deu-it\ of the atmo- 

 sphere at the time of making the determination. Dis- 

 tilled water in an open vessel, and under a given pr. 

 of the atmosphere, boils at an invariable temperature 

 cept as far a-s the nature of the vessel may make - 

 difference ; for if the heat communicated to the wa: 

 increased, the only effect produced is that of driving off a 

 greater quantity of steam in a given time : in a vessel ex- 

 hausted of the air the water will boil at a temperature 

 expressed by :>H or 100 of Fahrenheit's scale, while in a 

 i ucted so as to prevent the steam from escaping 

 it will remain in a liquid state at a temperature e\p, 

 by nbove 400. In order therefore that the tcmpciaturcz 

 indicated In different instrument.- mav agree together, :t 

 is recommended that this point .should be found from water 

 boiling in the open air at a time, if possible, when the 

 height of the mercurial column in the barometer i- :) 

 inches, and when the temperature of the air is indicated 

 of Fahrenheit's scale. 



This effect of the pressure of the atmosphere on the 

 boiling of water was noticed by Fahrenheit in 1721, and 

 M. Deluc. in his ' Recherches sur les Modifications de 

 V Atmosphere,' has investigated a formula for determining 

 the height of the boiling-point above the freezing-po; 

 the scale in terms of the height of the mercury in the 

 barometer; but the English artist Bird was the first who 

 applied a correction on account of the state of the baro- 

 meter, for the purpose of fixing the point of boiling water 

 on the scales of thermometers. 



The Royal Society having, in 177C, appointed a com- 

 nutiee to consider the best means of adjusting the fixed 

 points of thermometers, the formula of Deluc wits verified 

 and reduced to English measures for the benefit of ai; 

 in the event of then being obliged to make the instru- 

 under different states of the atmosphere with respect 

 to density and temperature : and the following are some of 

 the corrections which are given by Sir George Shuckburgh 

 for determining the true place of the boiling-poii 

 water. The first column contains the height of the baro- 



t, ,.]. 

 26 

 27 

 28 

 29 

 30 

 81 



^o 



- :V27 



- :) is 



(I 



+ IMill 



meler in inches; and the second, the correction which is 

 to be applied with its proper sign to the number 212 on 

 Fahrenheit's scale, in order to give the correct number of 



