Fire and Heat 87 



parts. On this scale it appeared that water freezes at 32 

 and boils at 212, at normal atmospheric pressure. Later it 

 was decided to take the melting of ice and the boiling of 

 water as the fixed points and to make the scale accordingly. 

 By this scale the bodily temperature is found to be about 

 98.4 F. 



The fixed points of the thermometer are readily determined. 

 The freezing point is found by packing a thermometer in pure 

 cracked ice or snow in a glass funnel. The melting ice or snow 

 soon causes the mercury thread to stand at a certain level, 

 where it remains until all the ice is melted. This level, which 

 is the point of freezing or melting, is marked on the glass tube. 

 The bulb is then immersed in boiling water. The mercury 

 soon registers a high stationary level, which is the boiling tem- 

 perature and is so marked. What is the freezing temperature 

 on the Fahrenheit scale? What is the boiling temperature? 

 How many degrees are there between the freezing and the boil- 

 ing temperature? 



The Centigrade scale is used practically throughout Europe 

 and South America, and is the scale employed in almost all 

 scientific work. On this scale the freezing point is designated 

 (zero), and the boiling point 100. The interval between 

 these fixed points is divided into one hundred equal parts. 

 Centigrade is thus seen to mean centi, one hundred, and gradus, 

 degrees, or one hundred degrees. Readings above and below 

 the fixed points are similarly determined by the same distance 

 for each degree. It is well to notice that when readings below 

 zero on either scale are given, they are written with the minus 

 or negative sign ( ) as, for example, 4 C. or 12 F. 



Comparison of the two scales. The Centigrade and Fahren- 

 heit scales (Fig. 24) are so widely used that one must be able 

 to compute promptly from the readings on one the equiva- 

 lents on the other. They may be represented as parallel stair- 

 ways, the ground floor of one marked C., and of the other 

 32 F. The top landing of one is marked 100 C., and of the 



