156 GENERAL SCIENCE 



from the place where the barometer reads high. These 

 places in the air which cause a high and low reading of 

 the barometer move across the United States from west 

 to east. Rain or cloudy weather usually follows a low 

 pressure and clear weather follows a high pressure. The 

 air in a low pressure center is usually rising and cooling, 

 and so clouds are formed, while in a high pressure center 

 the air is falling and getting warmer, so instead of forming 

 clouds it takes up moisture and is a drying wind. 



A BIPLANE JUST LEAVING THE GROUND 



Aeronauts who travel in balloons, airships, or aero- 

 planes have to study air currents. The lack of a knowl- 

 edge of air currents has caused the death of many airmen. 

 The aeroplane passing from one current into another 

 may plunge to the ground, if the current just entered is 

 moving in the same direction as the machine. Balloon- 

 ists control their direction by ascending or descending 

 until they get into a current going in the direction 

 that they wish. The operators of airships have to be 

 careful about passing from one current into another or 

 their machine may take a sudden plunge to the earth. 



