HOW RAIN 18 FORMED. 299 



is coining froiu ii latitude where the eastward movement in less than 

 at the equator, and its own movement eastward is therefore less than 

 that of the surface over which it blows. A person, therefore, standing' 

 on the earth, is carried eastward faster than the air is moving, and the 

 wind seems to blow against him from the northeast. Similarly, to the 

 south of the equator, the trade- wind, instead of blowing from the south, 

 comes from the southeast. 



Thus then we have in both hemispheres a system of westerly winds 

 in all higher latitudes than 40°, and a system of easterly winds — viz., 

 the trade-winds — between about 30° and the equator; and if the globe 

 were either all land or all water, these systems would prevail right 

 round the earth. 



Now, it is the pressure of these winds, under the influence of centrif- 

 ugal force, that causes the two zones of high barometer in latitudes .")0o 

 to 40°, and the very low pressure in higher latitudes. It is not difficult 

 to understand how this comes about. You are ])robably aware that the 

 earth is not an exact sphere, but what is termed an oblate spheroid — 

 that is, it is slightly flattened at the poles and protuberant at the equa- 

 tor, the difl'erence of the equatorial and polar diameters being about 26 

 miles. It has acquired this form in virtue of its rotation on its axis. If 

 you whirl a stone in a sling, the stone has a tendency to fly oif at a tan- 

 gent, and so long as it is retained in the sling that tendency is resisted 

 by the tension of the cord. In the same way, every object resting on 

 the earth, and the substance of the earth itself, has a tendency to fly 

 off at a tangent, in conse(iuence of its rotation on its axis, and this 

 tendency is resisted and overcome b^- gravity. Were the earth not re- 

 volving, its form under the influence of gravity alone would be a true 

 sphere. If it were to revolve more rapidly than at present, it would be 

 still more oblate, flatter at the poles, and more bulging in the tropical 

 zone; if less rapidly, the flattening and bulging would be less. 



This is precisely what happens with the west and east winds of which 

 we have spoken. West winds are revolving faster than the earth, and 

 tend to make the atmosphere more protuberant at the equator than the 

 solid earth ; hence they press towards the equator, to the right of their 

 path in the northern hemisphere, and the tendency increases rapidly in 

 high latitudes. Easterly winds, on the other hand, tend to render the 

 form of the atmosphere more nearly si)herical, and they, too, i)ress to 

 the right of their path in the northern hemisphere or towards the pole. 

 In the southern hemisphere, for the same reason, both press to the left. 

 Theresult of these two ])ressures in opposite directions is to produce the 

 two zones of high barometer in the kititudes in which we find them — 

 viz, between the easterly trade-winds and the westerly winds, which are 

 the anti-trades that have descended to the earth's surface. And the 

 low barometer of higher latitudes is produced in like manner by the 

 westerly winds pressing away from those regions. 



Thus then we find that all this system of winds, and the resulting 



