THE EFFECTS OF HEAT. 



235 



division is similarly continued below the freezing point to 

 tlio place 0, called zero, and each division upward from that 

 is marked with the successive numbers 1, 2, 3, etc. The 

 freezing point will now be the 32d division, and the boiling 

 point will be the 212th division. These divisions are called 

 degrees, and the boiling point will therefore be 212°, and the 

 freezing temperature, 32°. Fig. 203 represents the usual form 

 of thermometer, with its graduated scale. 



Thermometers of this character are called Fahrenheit's, 

 from a Dutch philosophical instrument-maker who first intro- 

 duced this method of graduation in the year 1724. 

 "WTiat other 537. In addition to Fahrenheit's 



besi™B\lhrJn- theimometer, two others are ex- 

 heifsareused? tensivclj used, which are known 

 as Reaumur's, and the Centigrade thermom- 

 eter, or thermometer of Celsius. 



The only difference between these three 

 kinds of- thermometers is the difference in 

 graduating the interval between the freezing 

 and boiling points of water. Reaumur's is di- 

 vided into eighty degrees, the Centigrade into 

 one hundred, and Fahrenheit's into one hundred and eighty. 

 According to Reaumur, water freezes at 0°, and boils at 80°; 

 oceording to Centigrade, it freezes at 0°, and boils at 100° ; 

 and according to Fahrenheit, it freezes at 32°, and boils at 

 212°; the last, very singularly, commences counting, not 

 at the freezing pouit, but 32° below it. 



The difference between these 



203. 



What consti- 

 tutes the dif- 

 ference be- 

 tween the dif- 

 ferent varieties 

 of the ther- 

 mometer ? 



instruments can be easily seen 

 by reference to Fig. 204. 



In England, Holland, and the 

 United States, the thermometer ^^-a^ ,^^ ^ 

 most generally used is Fah- 

 renheit's. Reaumur's scale is used in Ger- 

 many, and the Centigrade in France, Sweden, 

 and some other parts of Europe. The scale 

 of the Centigrade is by far the simplest and 

 most rational method of graduation, and at the 

 present it is almost universally adopted for 

 scientific purposes. 



538. The thermometer was invented about 

 the year 1600; but, like many other inven- 

 tions, the merit of its discovery is not to bo 

 ascribed to one person, but to be distributed 



among many. 



