Theory of the Winds. 19 



the curvature of its path, it moves from the centre until the 

 air pressure at the cyclonic centre falls sufficiently to constrain 

 it to circulate round the centre. In an anticyclone in the 

 Northern Hemisphere the direction of motion is in the di- 

 rection of the hands of a watch. Here the air moving round 

 the anticyclonic centre is deflected also to the right ; but as 

 this direction is towards the centre, the air is compressed 

 there and high pressures result. 



Gold * has carefully compared the actual wind velocities 

 at moderate heights, accompanying actual pressure gradients, 

 and has found that they agree as closely as can be expected 

 with those calculated according to the theories of Ley and 

 Loomis. 



It must be remembered that every wind must be accom- 

 panied by one moving in the opposite direction. Even when 

 a wind circulates round the earth's axis along some line of 

 latitude this is the case. Sometimes the compensating winds 

 move side by side, in other cases the return current may be 

 above or below the one that is being considered. On this 

 account any force or forces setting the air in motion must 

 •deal with the whole circuit, and in doing so may produce 

 variations of air pressure. 



Perhaps the most simple form of circulation is that of a 

 wind following a line of latitude. In fig. 2 ab a' f b" is a 

 section of an air current moving from west to east. The air 

 forming the ring is moving faster than the earth below, and 

 in attempting to turn to the right tends to move bodily to a 

 lower latitude. As a result the air in lower latitudes is 

 compressed, and the air in higher latitudes is expanded. 

 Eventually the difference of pressure between the two sides 

 prevents further displacement towards the equator. "W e thus 

 have a high pressure region set up on its equatorial side and 

 a low pressure region on its polar side. This is approximately 

 the condition which exists between latitudes 30° and 60° in 

 the Southern Hemisphere. As long as the westerly wind of 

 this ring is moving at the same angular velocity over its 

 whole sectional area it is quite stable ; for although the lower 

 portions of the atmosphere are more dense than the upper 

 portions, and therefore exert a greater pressure towards the 

 •equator, the lower air, on account of its greater density, 

 requires a greater force to compress it. When the general 

 circulation of the atmosphere is approximately parallel to the 

 lines of latitude, and blows from the west, the tendency to 



* Report to the Director of the Meteorological Office on the Calcu- 

 lation of Wind Velocity from Pressure Distribution. Meteorological 

 Office Report, No. 190, 1908, p. 13. 



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