CONVECTION 



97 



acquire a somewhat circular course and blow around the 

 storm center or low. In the northern hemisphere the direction 

 of the winds about a low is the reverse of the direction 

 in which the hands of a clock move, or counter clockwise. 

 South of the equator the winds blow clockwise about a low. 

 The periods of fair weather between two lows are called 

 anticyclones. On a weather map such periods are marked 

 highs. 



FIG. 34. Heating and ventilating by means of a hot-air furnace. (Tower, 



Smith and Turton.) 



87. Convection in Liquids. Though convection currents in 

 liquids may be less noticeable, they are nevertheless as inevi- 

 table when a change in density occurs. Proof of this may be 

 seen by filling a florence flask with water and gently heating it 

 after dropping into the flask a few grains of potassium per- 

 manganate or other coloring matter to make the currents 

 visible. Our systems of hot-water heating depend upon a 

 convection circuit in which the water is heated in a boiler and, 

 becoming less dense, rises in the pipes to the rooms that are 



7 



