82 



THE SUN'S GIFT OF HEAT 



tigrade. In this scale the point to which the mercury 

 column sinks when submerged in melting ice is marked 0, 

 and the point to which it rises at sea level when immersed 

 in unconfined steam (the boiling point of water) is 100. A 

 degree Centigrade, then, is -^^ the distance the column 

 expands when heated from freezing to boiling. 



The common household thermometer 

 of this country and England is the 

 Fahrenheit thermometer. It is named 

 after its inventor, who about two hun- 

 dred years ago began the making of 

 thermometers. He found that by mix- 

 ing ice and water and salt he obtained 

 a temperature much lower than that of 

 freezing water. This temperature he 

 took as his zero point. In this scale the 

 point at which ice and snow melt is 

 marked 32, and the point at which 

 water boils at sea level is marked 212. 

 The distance between the boiling point 

 and freezing points is divided into 180 

 equal parts, or degrees. A degree 

 o- r, Fahrenheit, then, is -rJ-o- the distance the 



JbiGURE o7. L/ENTI- 



QRADE AND FAH- column expands when heated from freez- 



RENHEIT SCALES . i v i p i xi 



COMPARED i n g to boiling, instead of Tiro as in the 



Centigrade scale. (Figure 37.) 



There are a number of different designs of thermometers. 

 Some are for measuring very high, others for measuring very 

 low, temperatures. Thermometers are also constructed so 

 as to be self-recording. (Figure 38.) 



The Measurement of Heat. Experiment 29. In each of 

 two beakers or tin cups weigh out 100 g. of water. Carefully heat 



