44 HEAT 



9. Write to pupils in schools four or five hundred 

 miles to the east or west of you, and ask them to tell you 

 the position of the Great Dipper at eight o'clock. . 



18. Expansion Due to Heat. 



Whenever anything- is heated it becomes larger. 

 This increase in size is called expansion. It is an in- 

 crease in length, breadth, and thickness. In liquids, 

 such as water, we notice the increase in volume. The 

 increase of volume is also evident in the case of gases, 

 of which air is an example. In solids, however, it is the 

 increase in length only, which causes us to be watchful. 



The next time you are near a railroad track look 

 for the spaces between the ends of the rails. Are the 

 spaces large or small? Is the day hot or cold? If these 

 spaces were not left when the rails were laid in cold 

 weather, they would expand in hot weather and having 

 no space in which to expand, would bend sidewise, al- 

 lowing the car wheels to leave them. If you were lay- 

 ing rails on a very hot day would you put the ends close 

 together, or would you leave a space between them? 



A practical use of expansion due to heat is made in 

 putting iron tires upon wheels. The tires are quite hot 

 when they are placed around the wheel and, as they cool 

 off they contract; that is, become smaller and thus grip 

 the wheel very tightly. See Section 72. 



Experiment 24. Heat Causes Expansion. 



Apparatus: Lamp chimney, candle, two small 

 blocks of wood, iron wire No. 18, rule, 4-ounce bottle, 

 cork to fit bottle, glass tube one foot long, tumbler, 

 alcohol lamp or Bunsen burner. 



