120 The Realm of Nature CHAP. 



brium of pressure is restored. Moving air is known as 

 wind, and always blows from regions where the pressure 

 is higher to those where it is lower. The greater the differ- 

 ence of pressure, or rather the gradient^ that is difference 

 of pressure in a definite distance, the stronger is the wind. 

 In English-speaking countries gradient is measured by 

 the number of hundreths of an inch difference in the read- 

 ing of two barometers at a distance of 1 5 nautical miles 

 (17 miles). For example, if the barometer at one place 

 read 29. 14, and at another 34 miles away it read 29.00, the 

 difference is 14 hundredths of an inch in 34 miles, or 7 in 

 17, and the gradient is spoken of as 7. The same gradient 

 would result from a barometric difference of only 3.5 

 hundredths of an inch if the stations were only 8J miles 

 apart. The strength of wind is proportional to the gradient 

 as the following table shows : 



Gradient 0.5 3 7 15 



Velocity of wind) 8o 



in miles per hour J 



Wind Light breeze. Fresh breeze. Gale. Hurricane. 



Wind ceases to blow as soon as the difference of pres- 

 sure ceases to exist. While blowing, currents of air move 

 spirally from areas of high pressure to areas of low pressure, 

 as is explained by Ferrel's law, deviating toward the right 

 hand in the northern hemisphere and toward the left hand 

 in the southern ( 89). The strength of wind is measured 

 by anemometers ( 442), and is expressed either in terms 

 of its velocity or of the pressure it exerts. Wind is 

 named by the direction from which it blows, a wind blow- 

 ing from east to west being called an East wind. 



176. Circulation of the Atmosphere. In order to 

 understand the movements of the atmosphere as a whole, 

 it is convenient first to consider the Earth as smooth and 

 entirely surrounded by the hydrosphere. The air between 

 the tropics, and especially over the equator, is always being 

 heated by strong solar radiation, and it consequently expands 

 and rises, through the rest of the air, as oil would rise through 

 water. This region forms the furnace which furnishes motive 

 power for the whole system of circulation. The cooler and 



