2O8 METHODS IN TEACHING 



parts of this opening for the direction of the air currents. 

 Explain. Have the pupils draw a diagram of the room, 

 representing by arrows the direction of the air currents in 

 it, and indicating which are warm and which are cold. 



For showing currents in water, fill a florence flask or 

 other thin-sided glass vessel with water which has been 

 previously boiled to free it from air bubbles. Floating par- 

 ticles of sawdust or paper may be placed in it to show the 

 movements of the currents. Heat very gently, and note the 

 results. Or, with a glass tube which has one end drawn 

 to a fine capillary tube, let into the bottom of a thin glass 

 jar or flask some ink or colored water. (The vessel should, 

 of course, be filled with water before this is done.) Touch 

 the bottom of the jar with a heated rod or with the blaze 

 of a match, and a column of ink will quickly rise from the 

 bottom. This is a striking experiment, but its preparation 

 takes some time and patience. 



In discussions with pupils in the higher grades connect 

 the experiments on air currents with the larger movements 

 of air upon the surface of the earth. 

 Air Currents Areas of low pressure are heated areas in 

 Over 

 The Earth which air currents are moving upward 



from the surface of the earth; they are in- 

 dicated by the word " low " on weather charts. " Highs " 

 are areas in which cooled masses of air are sinking to the 

 surface of the earth. The equatorial calms are a heated 

 belt characterized by upward currents of air. The calms 

 of the " horse latitudes " are characterized by descending 

 currents. All upward movements of air (" lows ") tend to 

 produce rain storms; downward moving masses (" highs ") 

 generally indicate fair weather. 



