308 TEMPERATURE. 



480. Thermometric Scales. There are two 

 scales used in this country, the centigrade and 

 Fahrenheit's. For these scales, the fixed points, de- 

 termined as just explained, are marked as follows : 



Centigrade. Fahrenheit. 



Freezing point, 32 



Boiling point, 100 212 



The tube between these two points is divided 

 into 100 equal parts for the centigrade scale and 

 into 180 for Fahrenheit's. Hence a change of 

 temperature of 5 C. is equal to a change of 9 F., 

 or an interval of one centigrade degree is equal to 

 FIG. 239. an interval of -| of a Fahrenheit degree. 



481. Thermometric Readings. To change 

 the readings of a centigrade thermometer to those of 

 Fahrenheit's, or vice versa, is a little more complicated 

 than to determine the relation between the intervals of 

 temperature. This complication arises from the fact that 

 Fahrenheit's zero is not at the freezing point but 32 de- 

 grees below. To reduce Fahrenheit readings to centigrade 

 readings, subtract 32 from the number of Fahrenheit de- 

 grees and multiply the remainder by {. 



To reduce centigrade readings to Fahrenheit readings, 

 multiply the number of centigrade degrees by | and add 32. 



F. = | 0. + 32. 

 o 



(a.} Suppose that we desire to find the equivalent centigrade 

 reading for 50 F. Subtracting 32, we see that this temperature is 

 18 Fahrenheit degrees above the freezing point. But one Fahren- 

 heit degree being equal to of a centigrade degree, this temperature 



