90 



NATURE STUDY 



by a fine wire at the mouth of the balloon is lighted. In a town, 

 or in the country during the dry season, this should not be at- 

 tempted from the danger of fire. 



The hot air balloon showing a heated mass 

 of air rising to a great height illustrates vividly 

 how larger masses of air lying next the ground, 

 which is heated by the rays of the sun, will 

 become heated and rise. 



The cooler air comes rushing in as it pushes 

 the hot air up. The balloon may be made to 

 illustrate this last point also. In the school 

 room when the balloon is well filled with hot 

 air, quickly turn it upside down and observe 

 the sudden rush of hot air up out of its mouth 

 and the equally sudden crushing together of 

 the sides of the balloon made by the cooler air 

 rushing toward it. 



Currents in Water. 



Fill a glass flask about two-thirds full of 

 water into which there has been sprinkled 

 some fine chalk dust that will show any move- 

 ments in the water. Place the flame of a can- 

 dle or alcohol lamp under the flask. Immedi- 

 ately currents of water will start from the 

 bottom to the top, and down the sides to the 

 bottom. As these are caused in the same way in which the cur- 

 rents in the air take place, their explanation can easily be arrived 

 at. If a glass jar of water, the larger the better, has sprinkled 

 over its surface some fine dust of an aniline dye, the particles of 

 the dye as they sink and dissolve make delicate colored threads in 

 the water. By heating either the sides or bottom of the jar ever 

 so slightly currents are formed as shown in the swaying of the 

 colored threads. 



Fig. 58. Ctments 

 water. 



