HEAT 



141 



B-- 



until it has taken the temperature of the boiling water, and the 

 level of the mercury is then scratched on the tube. 



Choosing the Scale. Some value must now be given to these 

 two fixed points, and of course they can be called anything the 

 maker likes, but for the sake of comparing one man's observa- 

 tions and experiments with those of other people it is most 

 convenient to graduate all thermometers in the same way. The 

 thermometers of this country are divided up in two ways (1) the 

 Centigrade scale, (2) the Fahrenheit scale. 



The Centigrade Scale. Here the freezing point is called zero 

 or ?io degrees, written C. The boiling point is called one 

 hundred degrees Centigrade, and is written 100 C. The space 

 between these two limits is divided 



into 100 parts, and each division C F 



called a degree Centigrade. 



The Fahrenheit Scale. On ther- 

 mometers marked in this way the 

 freezing point is called thirty-two 

 degrees Fahrenheit, written 32 3 F. , 

 and the boiling point two hundred 

 and tivelve degrees Fahrenheit, 

 written 212 F. The space between 

 the two limits is divided into 180 

 parts and each division is called 

 a degree Fahrenheit. The reason 

 of this difference is interesting. 

 The physicist Fahrenheit, after 

 whom the thermometer is named, 

 got, as he thought, a very low 



temperature, by mixing common salt with the pounded ice when 

 measuring the lower fixed point, and he imagined that he had 

 got the lowest temperature which could be reached, and called 

 it zero. His conclusion was wrong, and the mistake has brought 

 about two ways of measuring temperatures. 



Conversion of Scales. It should be clear from what we have 

 said that the interval between the boiling and freezing points, 

 that is, the same temperature difference, is divided into 100 parts 

 on the Centigrade scale and 180 parts on the Fahrenheit, 

 and consequently 100 Centigrade degrees are equal to 180 

 Fahrenheit degrees, which is the same as saying one degree 

 Centigrade is equal to nine-fifths of a Fahrenheit degree, 



*) O 



FIG. 60. The Fahrenheit and 

 Centigrade Scales. 



