298 HOW RAIN IS FORMED. 



heuiispbere, aud yet everywhere to tbe north of latitude 40o the deep 

 bine tiut 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 3(P 

 to 40O, and of very low pressure in higher latitudes, except iu so far 

 as they are moditied by the alternations of laud and water, was first 

 given by the American physicist, Professor Ferrel. Its full demonstra- 

 tion is to be obtained only from the consideration 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 revolves 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 subptance 

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

 ferred from the equator to latitude 60°, 



As the circumference of the earth at the equator is about 24,900 

 miles, anybody whatever, apparently at rest at the equator, is carried 

 round the earth's axis at the rate of J, 036 miles an hour. But in lati- 

 tude 00°, 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. Sup- 

 posing, then, a mass of air to be suddenly transferred from the equator 

 to latitude 60°, with the eastward movement that it had at the equator, 

 it would be moving twice as fast to the east as that part of the earth, 

 and, to any person standing on the earth, would be blowing from the 

 west with a force far exceeding that of a hurricane. It would be mov- 

 ing eastwards 518 miles an hour faster than the earth. Indeed, its 

 movement would really be far greater than this. In virtue of a me 

 chanical principle known as the law of the conservation of areas, which 

 means that anybody revolving round a central point, under the influ- 

 ence of a force that pulls it towards that point, describes equal areas in 

 equal times, instead of onl}' 518 miles, it would be revolving round the 

 earth's axis 1,554 miles an hour faster than that part of the earth. I 

 need not, however, specially 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, aud in the mea!i- 

 time by far the greater part of its eastward movement is lost by friction 

 against the trade-wind which blows in the opposite direction under- 

 iieath 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 their east- 

 ward movement, and blow, not as sonth, but as south-west or west- 

 southwest w'inds. 



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



