now RAIN IS FORMED. 297 



spken^ W<4'«^ tlie «^aitirs surracc uiiilbrnily land or uiiilonnly water, 

 there probably would be a system of tradtv winds all round the globe, 

 blowing from both hemispheres towards the equator; but even in that 

 case they Avould not extend much, if at all, beyond their i)resent limits. 

 In the first place, every great mass of land sets up an independent sys- 

 tem of air currents, since the land is hotter than the oceau in the sum- 

 mer and colder in the winter. In the summer, therefore, there is a 

 tendency to an indraught of air from the sea to the land in the lower 

 atmosphere, and an outflow above, and in the winter the opposite ; and 

 this tendency modifies or interrupts the system of the trades and anti- 

 trades. We have this tendency shown most distinctljMu the monsoons 

 of southeastern Asia, where, both in the India and China seas, a south- 

 west wind in the summer takes the place which in the absence of the 

 Asiatic continent would be held by a northeast trade-wind. And it is 

 only in the winter that a northeast wind blows, and this is then termed 

 the northeast monsoon. 



In the second place, as I have said, the system of trade-winds could 

 not in any case extend far beyond their present limits in latitude, owing 

 to the fact that the earth is a si>here and not a cylinder. Let us fix our 

 attention for a moment on the anti-trades — the upper winds which blow 

 from the equator towards the poles. The equator, from which they 

 start, is a circle about 24,900 miles in circumference j the poles are mere 

 points, and, therefore, the whole of the air that blows towards the poles 

 must turn back in any case before it reaches the pole, and must begin 

 to turn back before it has gone very far on its journey. And, as a fact, 

 a great part of it does turn back between 30° and 40° of latitude, which 

 I have already mentioned as being the limit of the trade-wiiuls. A part 

 of the remainder descends to the earth's surface, and sweeps the North- 

 ern Atlantic and the North Pacific as a southwest wind. 



On the chart which represents the average distribution of atmospheric 

 pressure in January, there are two somewhat interrupted zones of high 

 pressure over the ocean in these latitudes. These mark the regions in 

 which the anti-trades descend to the earth's surface, and from which 

 the trade-winds start. Over the ocean in all higher latitudes, both in 

 the northern and southern hemispheres, the barometer is low — for the 

 most part, indeed, much lower than over the equator; and the region 

 intervening between the zones of high pressure and the seat of lowest 

 l)ressure is that of ])redominant southwest, or at all events westerly 

 winds. Since our islands are situated on the border of this region of 

 low pressure, southwest are our prevailing winds. 



But now two questions arise : First, why are these winds westerly, 

 and not simply south winds ? And secoiul, how is it that the barometer 

 is so low over the North Atlantic and North Pacific oceans, and also in 

 the southern hemisphere in high latitudes, seeing that in these latitudes, 

 at least in winter, the sun's heat is so much less than at the tropics? 

 The chart represents the state of things in midwinter of the northern 



