74 



GENERAL SCIENCE 



212< 



100 



melt the glass, but at the same time keep the water 

 boiling. While sealing the top more mercury will flow 

 out because of additional expansion due 

 to the high temperature of the flame. 

 As the glass tube cools at the top after 

 having been sealed, the mercury will 

 contract somewhat. After it stops con- 

 tracting mark the top of the mercury 

 100 for the Centigrade thermometer 

 or 212 for the Fahrenheit. 



Remove the ther- 

 mometer from the 

 flask and cool it, 

 and then place it in 

 some finely cracked, 

 melting ice from 

 which the water can 

 escape. After the 

 mercury stops con- 

 tracting, mark it o 

 for Centigrade or 32 for Fahrenheit. 

 This will be the freezing point. 



To make a Centigrade thermom- 

 eter, divide the distance between 

 the two marks, o and 100, into 

 100 equal parts. To make a Fah- 

 renheit thermometer, divide the 

 space between 32 and 212 into 180 PRINCIPAL POINTS 

 equal parts. Now mark off spaces THE CENTIGRADE AND 

 below 32 the same size as those 

 above it. Do the same on the Cen- 

 tigrade scale. Five-ninths of a Centigrade degree is equal 

 to one degree of the Fahrenheit scale. Why? 



How TO LOCATE 



THE FREEZING 



POINT 



32 



40 







-17 



-40 c 



FAHRENHEIT THERMOM- 

 ETERS 



