87 



Currents in Air and Water, 



Immediate application of the facts learned in the last lessons 

 on heat in its expansion of both liquids and gases may be made 

 in the study of currents formed in air and in water by heating 

 them. It will be clear to pupils that if a portion of air is 

 heated and by this means is expanded it will be lighter than 

 the surrounding cooler air in which it rests. The same, of course, is 

 true for water. Such a heated mass of air will begin to rise being 

 pushed up by the heavier air as would a cork in water by the 

 heavier water. 



'^ There would be, then, two movements in the air, 



the upward movement of the heated air, and the 

 I movementof the surrounding cooler air toward the 



space below the upward moving portion. These 

 would be the beginning movements but it is plain 

 the disturbances of air thus started would be ex- 

 tended ver}^ much. The movements of air are 

 ■^ termed currents of air. 



Since the transparency of air prevents our de- 

 tecting currents in it, some means must be sought to 

 make its movements visible. This can be done by 

 Fig. M. v-ui- blackening it by smoke. For such purpose a sub- 

 "^"candie flamff."' stauce that will bum continuously with smoke but 

 without flame is desirable. The small sticks sold by the Chinese 

 called by the children "punk" will answer well for this. Excellent 

 "smoke paper" can be made by soaking strips of blotting paper 

 or carpet paper in a solution of saltpetre and then drying them. 



With strips of lighted smoke paper have the pupils explore 

 the condition of the air about a candle-flame or about a lighted 

 lamp. Currents of air will be seen rising above the flame and 

 others approaching it from the sides. Care must be taken to have 

 the air as still as possible and it is to be remembered that the heat 



