LXIV PRESIDENTIAL ADDRESS. 



column towards the lower column, and material is thereby re- 

 moved from the former, weight is taken off the base of this 

 column, and is added to the base of ifhe poleward column, and 

 thus equilibrium is disturbed through the lessening of pressure 

 at the equator, and the increase of the pressure on the poleward 

 side. There would then already be a tendency for the air at the 

 base of the poleward column to flow in towards the equator, in 

 order to equalize the pressure. Thus a steady transfer of air sets 

 m from the summit of the great protuberant air belt, " Der 

 Wulst," at the equator to the sub-tropical belts on either side of 

 it. This is the great first principle of atmospheric circulation. 

 We at once see the reason for the existence of a constant belt of 

 low pressure at the equator, and two constant belts of high pres- 

 sure m sub-tropical latitudes. These latitudes are respectively 

 37 deg. north in the Northern Hemisphere, and about 35 deg. 

 south in the Southern Hemisphere. Their establishment at these 

 particular latitudes is probably in part connected with the fact 

 that on the surface of the sphere the areas between the equator 

 and the poles balance themselves at 30 deg. north or south of 

 that equator. If the air masses on tlie equator side of 30 deg., 

 and that on its poleward side possessed exactly the same density, 

 they would balance each other exactly at 30 deg. north or south 

 of the equator. But on account of the greater cold and conse- 

 quent greater density of the poleward lying air, as compared with 

 the equatorial air, it would probably be necessary to take in more 

 territory than is represented by the strip extending from deg. to 

 30 deg., in order to secure a mass of air which would approxi- 

 mately equipoise the poleward lying air. Whether this is the 

 correct explanation or not, the fact remains that these two great 

 belts of high pressure exist at latitudes 37 deg. north, and 35 deg. 

 south. Were it not for the rotation of the earth this simple 

 cause of differential heating would set up a very simple circulation 

 of southerly winds blowing from the southern high pressure belt 

 to the equator, and northerly high level winds blowing from the 

 equator back to the top of the high pressure belt. The rotation 

 of the earth leads to deflection, as explained by Ferrell. For 

 example, in the Southern Hemisphere, the trade winds, instead of 

 blowing from due south, become deflected to the left, and so con- 

 stitute the south-east trade winds. Similarly, the air which has 

 soared to a high level over the equator, as it moves poleward, is 

 deflected to its left, that is, in an easterly direction, hence the 

 southern anti-trade wind blows from the north-west to the south- 

 east. So far, atmospheric circulation seems simple. Before we 

 pass on to the less understood portion, it may be added that the 

 differential heating of the equatorial as compared with the extra 

 tropical air-belts, is further emphasized by the fact that there is 

 more aqueous vapour present near the equator than in the extra 



