130 ELEMENTARY SCIENCE 



lower temperature on mountains than it does at sea- 

 level; that its boiling-point depends upon the atmos- 

 pheric pressure. Evidently, it is desirable to mark ther- 

 mometers with reference to the boiling-point of water at 

 a fixed and known pressure. So the unfinished thermome- 

 ter is placed with its bulb and thread of mercury com- 

 pletely enveloped in steam which rises from perfectly 

 pure water, the atmospheric pressure being known to be 

 standard. The point at which the mercury stops rising is 

 marked; this is the boiling-point. 



The next thing to do is to graduate the thermometer; 

 that is, to mark it off into degrees. The number of degrees 

 to be marked off between the freezing-point and the boil- 

 ing-point depends upon what kind of thermometer it is 

 to be. If it is to be a Centigrade thermometer, there will 

 be just one hundred degrees between these two points, 

 the freezing-point being zero. (Centigrade means "hun- 

 dred degrees.") The degrees below freezing are indicated 

 by a minus-sign. 



But if it is to be a Fahrenheit thermometer, the freez- 

 ing-point will be marked 32 and the boiling-point 212, the 

 interval between these two points being marked off into 180 

 equal parts, each part representing one degree. Similar di- 

 visions are made both above the boiling-point and below the 

 freezing-point. The zero on the Fahrenheit thermometer 

 is the lowest temperature obtainable by using a mixture 

 of ice and salt, such as is used in freezing ice-cream. 



The thermometers in ordinary use in America are 

 Fahrenheit, but in nearly all scientific work and generally 

 in Europe the Centigrade is used. The Fahrenheit is used 

 by the United States Weather Bureau and by physicians. 



Fahrenheit is named from Gabriel Fahrenheit, a Ger- 



