BELTS OF PRESSURE 7 



2. Transitional belts of increasing pressure north and 

 south of the belt of low pressure, with a steep gradient, 

 giving rise to the trade winds, which blow strongly 

 into the Equatorial trough from the north-east on its 

 northern side, from the south-east on the southern. 

 This is a region of high temperature and great evapora- 

 tion. 



3. Belts of permanently high pressure on the pole- 

 ward side of the tropics, respectively north and south 

 of the trade-wind regions, from which the trade winds 

 blow. In these calms generally prevail, and they are 

 called by sailors the " Horse Latitudes." 



4. Transitional belts of diminishing pressure, respec- 

 tively north and south of the belts of high pressure in 

 the temperate zones, with south-easterly winds in the 

 northern hemisphere, and north-westerly winds in the 

 southern hemisphere, the winds blowing outwards from 

 the poleward sides of the high-pressure areas. 



5. Two polar belts of low pressure, towards which the 

 westerly winds blow, are not so clearly defined, nor 

 are they so fully investigated as to enable one to speak 

 definitely regarding them. The theoretical circulation 

 of air requires them, but it may be that yet another 

 area of high pressure exists at or near the poles them- 

 selves. 



The surface movements of air thus described require 

 for the completion of the circulation a series of wind 

 currents in the upper atmosphere practically opposite 

 in direction, and observations on clouds and by means 

 of balloons show that such currents exist, though our 

 knowledge of the subject is still imperfect. 



The simple conditions we have outlined do not 

 actually exist, though over the great oceans there is a 

 very near approach to them. The influence of land, 

 with its diurnal and seasonal heating and cooling, 



