16 Mr. R, M. Deeley on the 



Southern Hemisphere. From a theoretical point of view 

 this is unfortunate; for if we had as much information of the 

 Southern Hemisphere temperature gradients as we have of 

 those of the Northern Hemisphere, the more simple conditions 

 prevailing there would very probably enable the relationships 

 between temperature, pressure, and wind to be seen more 

 clearly. Over South America, for example, the average 

 monthly pressure gradients are constant in direction the year 

 through. 



Although the surface winds move from high pressure to 

 low pressure areas, in the vast majority of instances they do 

 not do so in a direct manner. In the case of cyclones in the 

 Northern Hemisphere the winds on the earth's surface move 

 inwards in rough spirals; the circulation round the centre of 

 lowest pressure being in the opposite direction to the move- 

 ment of the hands of a watch. In the Southern Hemisphere 

 the movement is in the opposite direction to that in the 

 Northern. Similarly, winds moving north from the equator 

 acquire a velocity component directed towards the east, whilst 

 those moving south to the equator acquire a velocity com- 

 ponent directed towards the west. On the other hand, 

 winds moving from the south to the equator have a velocity 

 component directed towards the west, whilst those moving 

 south from the equator have a velocity component directed 

 towards the east. These wind deviations were shown by 

 Hadley to be due to the effects of the earth's rotation on 

 its axis. 



The manner in which the rotation of the earth affects the 

 direction in which the winds blow will be seen from the 

 following considerations. 



If the earth did not rotate upon its axis it would be approxi- 

 mately spherical in form. Owing to its rotation, however, 

 the equatorial regions are caused to bulge outwards somewhat, 

 whilst the polar areas are flattened. Here the gravitational 

 forces tending to make the earth assume a spherical form 

 have superimposed upon them the centrifugal forces resulting 

 from its rotation. As far as the winds are concerned, this 

 flattening of the polar areas has scarcely any appreciable 

 effect ; but the rotation affects the winds very materially in 

 other ways. 



Fig. 2 is a section through the earth parallel with its 

 axis. A mass of air at the equator AA is carried from west 

 to east at a velocity which causes it to rotate round the axis 

 of the earth once in 24 hours, or at a velocity of about 

 1036 miles per hour. At the poles BB the earth and air 

 have no translational velocitv, and at intermediate latitudes 



