MEASURING OF HEAT 121 



Previous to his time temperature was a matter of the comparison 

 of one's personal sensations of heat. 



It was not until 1700 that mercury thermometers were invented. 



The instrument most widely used for scientific work is the Centi- 

 grade. This scale of reading was invented by Celsius, a Swedish 

 physicist, in 1742. The freezing point of water is designated as 

 zero, while the temperature of the steam rising from boiling water 

 is 100. The intermediate degrees are obtained by dividing the 

 space from to 100 into 100 equal parts, degrees of equal size 

 naturally being used for temperatures above 100 and below 0. 



The thermometer which we use largely in this country is the Fahr- 

 enheit, invented in 1714 by a German of the same name. This 

 thermometer differs from the Centigrade only in the manner of grad- 

 uation. The freezing point of water is 32 and the temperature 

 of the escaping steam 212, the intervening space being divided into 

 180 equal parts, and the points above 212 and below 32 into degrees 

 of the same size. The zero of this scale was found by noting the 

 temperature of a mixture of equal weights of ammonium chloride 

 and snow. Fahrenheit called this zero because he believed it to be 

 the lowest possible temperature, i.e., the entire absence of heat. 



While Fahrenheit and Centigrade are the principal thermometers 

 utilized to-day, the Reaumur scale is used to no small extent in 

 Germany and Russia, though now being superseded by Centigrade. 

 The freezing point is while the boiling point is 80, with 80 equal 

 spaces between these points. In the original Reaumur thermometer 

 alcohol was used instead of mercury. 



For extremely high and low temperatures a hydrogen gas ther- 

 mometer is used, somewhat similar in principle to Galileo's original 

 instrument for recording heat. 



Use of Thermometers. The measure of temperature is very 

 important. People for many ages have measured heat roughly. 

 Testing the heat of a flatiron with the moistened finger and testing 

 the heat of the oven with a piece of paper are familiar illustrations. 

 The thermometer on the oven door has taken the place of the 

 paper. This is known as a metallic thermometer. 

 v . Many thousands of lives are now saved that would be sacri- 

 ficed were the old unreliable method of taking temperature by 



