72 GENERAL SCIENCE 



52. Thermometers. The thermometer is an instru- 

 ment used to measure the degree to which a body is 

 heated, that is, to determine its temperature. We some- 

 times use our fingers or even our whole bodies to measure 

 temperature. We often put a finger into water or on ob- 

 jects to see if they are hot or cold. If their temperature 

 is higher than that of our finger we say they are hot. 

 If the temperature of the object is lower than that of our 

 finger we say it is cool or cold. When we go from one 

 room to another we say that the rooms are of the same 

 or of different temperature, according to our feeling. 

 Our sensation of temperature is largely determined by 

 the condition of our bodies as regards health, and by the 

 moisture conditions of the surface of our bodies and of 

 the air. For example, 70 F. in January would be a 

 hot day, while 70 F. in July would be a cool day. Moist 

 air in winter seems cold and dry air seems warmer. In 

 summer dry air is cool and moist air is hot to our feeling, 

 while the actual temperature on the day when the air 

 is moist may be the same as when it is dry. For the 

 above reasons our bodies do not make very good ther- 

 mometers. Neither can we determine very accurately 

 the temperature of water or any object with our fingers. 



Certain substances have been found to be very sensi- 

 tive to changes of temperature, and these have been used 

 to construct mechanical thermometers, of which there are 

 three principal types. Only two will be considered here. 

 They are the Fahrenheit and the Centigrade thermometers. 

 The Centigrade thermometer is the easier of the two to 

 use, as its scale is based upon the decimal system. 



53. How to make a Thermometer. Take a glass 

 tube about 1 2 inches long, with a very fine hole at one end 

 and extending through to a closed bulb at the other end. 



