228 



NA TURE 



{Jan. 3, 1889 



40° of latitude in either hemisphere. Were the earth's 

 surface uniformly land or uniformly water, there probably 

 would be a system of trade-winds all round the globe, 

 blowing from both hemispheres towards the equator ; but 

 even in that case they would not extend much, if at all, 

 beyondtheir present limits. In the first place, every great 

 mass of land sets up an independent system of air 

 currents, since the land is hotter than the ocean in the 

 summer, 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 out- 

 flow 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 

 distinctly in the monsoons of South-Eastern 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 north-east trade- 

 wind. And it is only in the winter that a north-east 

 wind blows, and this is then termed the north-east 

 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 sphere 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 ; 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-winds. 

 A part of the remainder descends to the earth's surface, 

 and sweeps the Northern Atlantic and the North Pacific 

 as a south-west wind. 



On the chart which represents the average distri- 

 bution 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 pressure is that of pre- 

 dominant south-west, or at all events westerly, winds. 

 Since our islands are situated on the border of this 

 region of low pressure, south-west are our prevailing 

 winds. 



But now two questions arise : first. Why are these winds 

 westerly, and not simply south winds.'' and second. How 

 is it that the barometer is so low over the North Atlantic 

 and North Pacific Oceans, and also in the southern hemi- 

 sphere 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 mid- 

 winter of the northern hemisphere, and yet everywhere to 

 the north of latitude 40° the deep blue tint indicates that 

 the pressure is lower than even in the southern tropic, 

 where the sun shines vertically overhead. Clearly this 

 low pressure must be due to some other cause than the 

 warmth of the air. 



The explanation of this remarkable distribution of the 

 atmospheric pressure, of the existence of two zones of 

 high pressure in latitudes 30° to 40°, and of very low 

 pressure in higher latitudes, except in so far as they are 

 modified by the alternations of 'and and water, was first 

 given by the American physicist, Prof. Ferrel. Its full 

 demonstration is to be obtained only from the consi- 



deration of somewhat recondite mechanical laws, but a 

 general idea of the causes operating may be gathered from 

 very simple considerations, which may be demonstrated 

 with a terrestrial globe. 



Starting with the well known fact that the earth re- 

 volves on its axis once in the twenty-four hours, let us see 

 what will be the consequence, if we suppose a mass of 

 any ponderable matter— that is, any substance having 

 weight, no matter whether light or heavy — to be suddenly 

 transferred from the equator to latitude 60°. 



As the circumference of the earth at the equator is 

 about 24,900 miles, any body whatever, apparently at 

 rest at the equator, is carried round the earth's axis at 

 the rate of 1036 miles an hour. But in latitude 60% 

 where the distance from the axis is only half as great 

 as at the equator, it is carried round at only half the 

 same rate, or 518 miles an hour; and at the pole it simply 

 turns round on its own axis. Supposing, then, a mass of 

 air to be suddenly transferred from the equator to latitude 

 60°, with the eastward movement that it had at the equa- 

 tor, it would be moving twice as fast to the east ai that 

 part of the earth, and, to any person standing on the 

 eai-th, would be blowing from the west with a force far 

 exceeding that of a hurric-me. It would be moving east- 

 wards 518 miles an hour faster than the earth. Indeed, 

 its movement would really be far greater than this. In 

 virtue of a mechanical principle known as the law of the 

 conservation of areas, which means that any body re- 

 volving round a central point, under the influence of a 

 force that pulls it towards that point, describes equal areas 

 in equal times, instead of only 518 miles, it would be 

 revolving round the earth's axis 1554 miles an hour faster 

 than that part of the earth. I need not, however, spe- 

 cially insist on this point, because, as a matter of fact, the 

 air which constitutes the anti-trades is not suddenly trans- 

 ferred, but takes a day or two to perform its journey, and 

 in the meantime by far the greater part of its eastward 

 movement is lost by friction against the trade-wind which 

 blows in the opposite direction underneath it. The point 

 on which we have to fix our attention is that, when the 

 anti-trades descend to earth, they still retain some of 

 this eastward movement, and blow, not as south, but as 

 south-west or west-south-west winds. 



On the other hand, the trade-wind, which blows towards 

 the equator, is coming from a latitude where the eastward 

 movement is less than at the equator, and its own move- 

 ment 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 north- 

 east. Similarly, to the south of the equator, the trade- 

 wind, instead of blowing from the south, comes from the 

 south-east. 



Thus, then, we have in both hemi-pheres 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 in- 

 fluence of centrifugal force, that causes the two zones of 

 high barometer in latitudes 30° to 40°, and the very low 

 pressure in higher latitudes. It is not difficult to under- 

 stand how this comes about. You are probably 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 equator, the difference 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 oft' at a tangent, and, so long as it is re- 

 tained in the sling, that tendency is resisted by the tension 

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



