18 Mr. R. M, Deeley on the 



then it finds itself in a region which is moving more rapidly 

 than itself, and it is deflected towards the west, a". When 

 the force acts from the west the movement of the air is more 

 rapid than that required by the latitude, and it is deflected 

 to the south, a n ; but if the force acts from the east, then 

 there is a deficiency of velocity for the latitude and it moves 

 north, a x . These movements have been plotted on fig. 3, and 

 it will be seen that in whatever direction in the Northern 

 Hemisphere the air is urged the deflexion is always to the 

 right. Similar reasoning will show that in the Southern 

 Hemisphere the deflexion is always to the left. If, as is 

 shown in fig. 4, in the Northern Hemisphere, the air is 



Fig;. 4. 



urged towards any particular centre, the wind deflexions 

 are such as to cause the air to pass on the right side of the 

 centre. In this way a rotation around the centre may be 

 set up, which in the Northern Hemisphere is in the opposite 

 direction to the hands of a watch. These tendencies of the 

 winds to change their directions under the influence of the 

 rotation of the earth are greatest in the polar areas. At 

 the equator a west wind will retain its direction unaltered, 

 but will tend to rise ; an easterly wind, however, will tend 

 to move either to the north or to the south. 



The deflecting force tending to alter the direction of the 

 wind being equally powerful for the same velocity and 

 latitude in whatever direction the wind is moving, as the 

 wind changes its direction, the deflecting forces, the di- 

 rections of which are shown by the arrows in figs. 3 and 4, 

 also change their direction so as to remain at all times at 

 right angles to the wind movement. The deflecting force, 

 therefore, does not accelerate the velocity of the wind, 

 although it changes its direction of flow. Any change in 

 the velocity of the wind must be due to some force acting 

 upon it, other than that due to the rotation of the earth. 



In a cyclone in the Northern Hemisphere the air circu- 

 lates in the opposite direction to the hands of a watch. 

 Owing to the tendency of the air on all sides of the cyclonic 

 centre to be deflected to the right by the earth's rotation and 



