112 Graduating Thermometers. [Book II. 



ways from O; the boiling point in his fcalc is at 80* 

 above o. The fcale of Fahrenheit begins confiderably 

 below the fieezing point, at that period of cold which 

 is produced by furrounding the bulb of the thermo- 

 meter v/ith a mixture of fnow or pounded ice and lal 

 ammoniac or fea-fah: he divided his fcale into mi- 

 nuter portions than either Newton or Reaumur, on 

 which account it is well known that the boiling point 

 in Fahrenheit's thermometer is at 212. Sir Ifaac New- 

 ton's thermometer is, I believe, now quite obfolete : 

 Reaumur's is dill tiled by many of the French, and 

 other experimentalifts. The degree of heat, however, 

 when noted on either of theie inftruments, may eafily 

 be computed, by remembering that 34 of Newton's 

 aiifwer to So of Reaumur, and to 21 2 of Fahrenheit ? 

 and that the freezing point, which is the commence- 

 ment of both the other fcales, is in Fahrenheit's at 32 

 above o. / 



The graduating a mercurial, or Fahrenheit's thermo- 

 meter, cannot, from 'what has been obferved, be a dif- 

 ficult talk. The mercury muft be carefully purged 

 from air, as that, being a more elaftic fluid, would 

 .ic fome irregularity in the expanfions of the metal, 

 or would colled: in the upper part of the tube, which 

 ought to be the mod perfect vacuum that can be 

 formed. It is well known that what is called the Tor- 

 ricellian * vacuum is formed by filling a glafs tube with 

 mercury, and then inverting the tube in a veflel of the 

 lame fluid, and withdrawing it flowly till the mercury 

 fubfides, by which means all that part of the rube 

 which is above the mercury will be free from air; on 

 withdrawing the tube out of the mercury, it is obvious 

 that the orifice mult be flopped with the finger or 



* From Torricelli, the inventor. 



fome 



