Dec. 12, 1889] 



NATURE 



125 



pressure in about lat. 30' in both hemispheres, two prin- 

 cipal minima at the poles, and a minor depression on the 

 equator, together with strong west winds in middle and 

 high latitudes, and an excess of easterly winds in equa- 

 torial regions. The two tropical zones of high pres- 

 sure determine the polar limits of the trade winds, and 

 the whole system oscillates in latitude with the changing 

 declination of the sun. Further, as a consequence of the 

 fact that the great mass of the land is restricted to the 

 northern hemisphere, whereas the southern hemisphere 

 presents a comparatively uninterrupted sea surface, on 

 which the retarding friction is less than in the northern 

 hemisphere, the west winds of middle and high latitudes 

 are much stronger in the latter than in the former, and 

 by their lateral pressure cause a slight displacement of 

 the tropical zones of high pressure and the equatorial 

 zone of low pressure to the north of their normal positions 

 on a hypothetical uniform terrestrial surface. 



The great modification and extension of Hadley's 

 theory thus introduced by Prof. Ferrel depends mainly on 

 two points of the first importance. By all previous writers 

 it was assumed that a mass of air at rest relatively to the 

 earth's surface on the equator, if suddenly transferred to 

 some higher latitude — say, e.g., 60° — would have a relative 

 easterly movement in that latitude equal to the difference 

 of rotary velocities on the equator and on the 60th 

 parallel, or about 500 miles an hour, the difference being 

 proportional to that of the cosines of the latitudes. This, 

 however, would be true only in the case of rectilinear 

 motion. In reality, as Prof. Ferrel was the first to demon- 

 strate, the mass of air would obey the law of the preserva- 

 tion of areas, like all bodies revolving under the influence 

 of a central force, and its relative eastward velocity in 

 latitude 60° would be 1500 miles^an hour, being as the 

 difference of the squares of the cosines. If, on the other 

 hand, any mass of air at rest in latitude 60^ were suddenly 

 transferred to the equator, it would have a relative westerly 

 movement of 750 miles an hour, and any mass of matter 

 whatever moving along a meridian is either deflected — or 

 if, like a railway train or a river between high banks, it 

 is not free to yield to the deflecting force, presses — to the 

 right of its path in the northern, and to the left in the 

 southern, hemisphere. 



The second point first established by Prof. Ferrel is 

 that, in virtue of centrifugal force, this deflection or 

 pressure to the right in the northern, and to the left in 

 the southern, hemisphere is suffered in exactly the same 

 degree by bodies moving due east and due west, or along 

 a parallel of latitude, and therefore also in all intermediate 

 azimuths. 



From the first of these principles it will be readily seen 

 why the west winds of middle latitudes are so much 

 stronger than the easterly winds of the equatorial zone ; 

 and from the second, how these opposite winds, by their 

 mutual pressure, produce the tropical zones of high 

 barometer and the polar and equatorial regions of low 

 barometer. 



In subsequent chapters are discussed the modes in 

 which the general circulation of the globe affects the 

 climates of different latitudes by determining the distri- 

 bution of rainfall in wet and dry zones, and inequalities 

 of temperature through the agency of marine currents. 

 Also the causes that modify and disturb the regularity of 



the ideal system, the chief of which is the mutual inter- 

 action of expanses of land and sea. The general excel- 

 lence of these demonstrations is indisputable, but we 

 have marked one or two passages which appear to us to be 

 of doubtful validity, and which we recommend to the re- 

 consideration of the author when the time comes, as we 

 doubt not it will ere long, for the issue of a second edition 

 of his work. 



The first point to which we would take exception is 

 what seems to us the too great influence ascribed to 

 mountain-chains in deflecting the great atmospheric cur- 

 rents. That they deflect the surface winds, like other 

 irregularities of the surface, and in proportion to their 

 magnitude, is, of course, a matter of universal experience • 

 but, in the absence of other causes operating to produce 

 a diversion of the greater currents, their action in this 

 respect appears to us to be merely local. As an instance 

 we will take the case of the Western Ghats of India, an 

 escarpment from 3000 to 7000 feet in height, running 

 athwart the direction of the summer monsoon of the 

 Arabian Sea. The wind charts of the Arabian Sea, issued 

 by the Indian Meteorological Office, show no appreciable 

 deflection of the monsoon wind on the windward face of 

 this range ; and if the same cannot be asserted of the 

 corresponding wind in the north of the Bay of Bengal, 

 where it impinges on the coast range of Arakan, it is 

 evident that the deflection of this current to north, and 

 eventually to north-west, is caused by the indraught 

 towards the heated plains of Northern India. 



We believe that a similar explanation will be found to 

 hold good in all the more conspicuous cases cited by 

 Prof. Ferrel. Thus, at p. 183 he says: — 



" The air of the lower strata of the atmosphere in the 

 trade-wind zone of the North Atlantic, having a westerly 

 motion, and impinging against the high table-lands and 

 mountain-ranges of Mexico, is deflected around towards 

 the north over the south-eastern States, and up the Mis- 

 sissippi valley into the higher latitudes, where it com- 

 bines with the geneial easterly flow of these latitudes, 

 and adds to its strength. This completely breaks up the 

 continuity of the tropical calm belt and dry zone, so that, 

 instead of a dry region with scanty rainfall, such as is 

 found in North Africa, Arabia, Persia, Beloochistan, and 

 Cabul, we have on the same parallels in the southern 

 and eastern United States a region of abundant rainfall, 

 and all the way up the Mississippi valley and in the in- 

 terior of the continent there is much more rain than in 

 the interior of Asia." 



Taking this passage as it stands, or only together with 

 the immediate context, it might be understood to imply 

 that the author ascribes this great diversion of the winds 

 of the Gulf of Mexico, together with all the rainfall they 

 bring to the southern States of America, solely to the 

 influence of the comparatively low mountain-chain of 

 Central America. That such, however, is not his mean- 

 ing is evident from his subsequent remarks on p. 215, 

 where, in describing the monsoons of North America, 

 after noticing the high temperature of the land area in 

 summer, he says : — 



"On the southern and south-eastern coast, in connec- 

 tion with the deflection referred to [in the passage quoied 

 above], it causes the prevailing winds to be southerly and 

 south-easterly, instead of north easterly, as they would 

 otherwise be in these trade-wind latitudes." 



