100 EXAMPLES. 



REV. Mercury freezes at a temperature of 40 C. or F.,and boils 

 at a temperature of about 350 C. or 662" F. ; it is therefore necessary, 

 for very high or very low temperatures, to employ other substances. 



For very low temperatures, spirit of wine is used ; this liquid has 

 never congealed, although a temperature of 140" 1 C. has been ob- 

 served, which is the lowest temperature yet attained.* 



High temperatures are compared by observing the expansion of bars 

 o" metal or other solid substances, and instruments called pyrometers 

 have been constructed for this purpose. 



51. Comparison of the Scales of these Thermom- 

 eters. Any degrees of temperature by either thermometer 

 may be converted into the corresponding degrees of the 

 other thermometers ; for the space between the fixed points 

 in Fahrenheit's being 180, in the Centigrade 100, and in 

 Reaumur's 80, we have 180 Fahrenheit = 100 Centi- 

 grade = 80 Reaumur ; and therefore each of Fahrenheit's 

 degrees = f of one of Centigrade = f of one of Reaumur. 



Let F, C, and R be the numbers of degrees marking the 

 same temperature on the respective thermometers ; then 

 since the space between the boiling and freezing points 

 must in each case be divided in the same proportion by the 

 mark of any given temperature, we must have 

 F 32 J7_ R_ m 

 180 = 100 ~~ 80 ' 



F 32 _ C _ R 

 9 = 5 :: 4* 



REM. The various scales were formed in the early part of the 18th 

 century Fahrenheit's in 1714, at Dantzic ; Reaumur's in 1731 ; and 

 the Centigrade somewhat later.f 



EXAMPLES. 



1. What temperatures on the other two scales are equiva- 

 lent to the temperature 50 F. ? \ Ans. 10 C., or 8 R. 



* Maxwell on Heat. t Besant's Hydrostatics, p. 88. 



t It is usual, in stating temperatures, to indicate the scale referred to by the ini- 

 tials F., C., R. 



