Jan. 3, 1889] 



NATURE 



229 



the earth, and the substance of the earth itself, has a 

 tendency to fly off at a tangent, in consequence of its 

 rotation on its axis, and this tendency is resisted and 

 overcome by gravity. Were the earth not revolving, 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 revolv- 

 ing faster than the earth, and tend to make the atmo- 

 sphere 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 this tend- 

 ency increases rapidly in high latitudes. Easterly winds, 

 on the other hand, tend to render the form of the atmo- 

 sphere more nearly spherical, and they, too, press 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. The result of these two 

 pressures in opposite directions is to produce the two 

 zones of high barometer in the latitudes in which we find 

 them — viz. between the easterly trade- winds and the 

 westerly winds, which are the anti-trades that have de- 

 scended 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 distribution of atmospheric pressure as 

 indicated by the barometer, is the result of the sun's 

 action in equatorial regions. It is this that gives the 

 motive power to the whole system, so far as we have as 

 yet traced it, and it is this that produces those great 

 inequalities of atmospheric pressure that I have so far 

 described. 



It remains now to see how storms are generated by 

 these westerly winds. In so far as they retain any south- 

 ing, they are still moving towards the pole in the northern 

 hemisphere— that is to say, they are advancing from all 

 sides towards a mere point. Some portion of them 

 must therefore be continually turning back as the circles 

 of latitude become smaller and smaller. But they are 

 now surface-winds, and in order so to return they must 

 rise and flow back as an upper current. This they do by 

 forming great eddies, or air-whirls, in the centre of which 

 the barometer is very low, and over which the air ascends, 

 and these great air-whirls are the storms of the temperate 

 zone and of our latitudes. It is the ascent and dynamic 

 cooling of the air in these great eddies that cause the 

 prolonged rainfall of wet stormy weather. How the 

 eddies originate, or, rather, what particular circumstance 

 causes them to originate in one place rather than another, 

 we can scarcely say, any more than we can say how each 

 eddy originates in a rapidly-flowing deep river. Some 

 very small inequality of pressure probably starts them, 

 but, when once formed, they often last for many days, 

 and travel some thousands of miles over the earth's 

 surface. 



Two such storms are represented on the charts of 

 February i and 2, 1883, one on the coast of Labrador, 

 the other to the south-west of the British Isles. The 

 first of these appears on the chart of January 28, in the 

 North Pacific, off the coast of British Columbia, ("n the 

 29th it had crossed the Rocky Mountains, and was tra- 

 versing the western part of the Hudson's Bay Territory. 

 On the 30th it had moved to the south-east, and lay just 

 to the west of the Great Lakes, and on the 31st between 

 Lake Superior and Hudson's Bay. On February i it 

 had reached the position on the roast of Labrador shown 

 in the chart, and on the 2nd had moved further to north- 

 east, and lay across Davis's Straits, and over the west 

 coast of Greenland. After this it again changed its 



course to south-east, and on February 4 passed to the 

 north of Scotland, towards Denmark, and eventually on 

 to Russia. 



The second storm had originated off the east coast of 

 the United States between January 28 and 29, and on the 

 following days crossed the Atlantic on a course somewhat 

 to north of east, till, on February 2, it lay over 

 England, 



These storms always move in some easterly direction, 

 generally between east and north-east, and often several 

 follow in rapid succession on nearly the same track. It 

 is this knowledge that renders it possible for the Meteoro- 

 logical Office to issue the daily forecasts that we see in 

 the newspapers. Were it possible to obtain telegraphic 

 reports from a few stations out in the North Atlantic, 

 these storm warnings could be issued with much more 

 certainty, and perhaps longer before the arrival of the 

 storm than at j.resent. In the case of such storms as 

 that which reached our islands on February 2, we often 

 have such warnings from America, but their tracks are 

 often more to the north-east, in the direction of Iceland, 

 in which case they are not felt on our coasts, and hence 

 the frequent failure of these American warnings. 



It is the region of low pressure in the North Atlantic 

 that is the especial field of these storms. As they pass- 

 across it, they produce considerable modifications in the 

 distribution of pressure, but some of its main features re- 

 main outstanding. Thus there is always a belt of high 

 barometer between the storm region and the trade-winds, 

 and in the winter there is almost always a region of high, 

 barometer over North America, and another over Europe 

 and-Asia, however much they may shift their places, and 

 be temporarily encroached on by the great storm eddies. 



These regions of high pressure are the places where 

 the winds descend, and, as I mentioned in the earlier part 

 of this lecture, these winds are dry, and generally accom- 

 pany fine weather. On the contrary, the eddies, where the 

 air ascends, are damp and stormy, and especially that 

 part of the eddy that is fed by the south-west winds that 

 have swept the Atlantic since their descent, and so have 

 become charged with vapour. 



And now we are prepared to understand why east, and 

 especially north-east winds are generally so dry. They 

 are air that has descended in the area of high barometer 

 that, especially in the winter and spring, lies over Europe 

 and Asia, and has subsequently swept the cold land- 

 surface, which does not furnish much vapour, and there- 

 fore they reach us as dry cold winds. To begin with, 

 the air comes from a considerable height in the atmo- 

 sphere, and in ascending to that height in some other 

 part of the world, it must have got rid of most of its 

 vapour in the way that has been already explained. In 

 descending to the earth's level it must, of course, have been 

 dynamically heated by the compression it has undergone, 

 but all or nearly all this heat has been got rid of by 

 radiation into free space on the cold plains and under the 

 clear frosty skies of Northern Asia and Northern Europe, 

 and it then blows outwards from this region of high 

 barometer over the land, towards the warmer region of 

 low barometer on the North Atlantic Ocean. 



Thus we see that, in all cases, rain is produced by the 

 cooling of the air, and that in nearly all, if not all, this 

 cooling is produced by the expansion of the air in ascend- 

 ing from lower to higher levels in the atmosphere, by what 

 is termed dynamic cooling. This last fact is not set forth 

 so emphatically as it should be in some popular text-books 

 on the subject, but it is an undoubted fact. It was 

 originally suggested by Espy some forty years ago, but 

 the truth is only now generally recognized, and it is one 

 of the results which we owe to the great advance in 

 physical science effected by Joule's discovery of the defi- 

 nite relation of equivalence between heat and mechanical 

 work. 



