214 PKOGRESS OF METEOROLOGY IN 1889. 



the fact that the great mass of the laud is restricted to the northern 

 hemisphere, whereas the southern hemisphere presents a comparatively 

 uninterrupte<l sea surface on which the retardiug 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 press- 

 ure and the equatorial zone of low pressure to the north of their nor 

 mal positions on a hypothetical uniform terrestrial surface. 



"The great modillcatiou and extension of Hadley's theory thus intro- 

 duced by Professor 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 trans- 

 ferred to some higher latitude — say, e. (j., 00° — would have a relative 

 easterly movement in that latitude equal to the difference of rotary ve- 

 locities on the equator and on the sixtieth parallel, or about 500 

 miles an hour, the difference being proj)ortional to that of the cosines 

 of the latitudes. This, however, would be true only in the case of rec- 

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

 strate, the mass of air would obey the law of the preservation of areas, 

 like all bodies revolving under the intluence of a central force, and its 

 relative eastward velocity in latitude G0° would be 1,500 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 (30° 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 Professor Ferrel is that, in vir- 

 tue of centrifugal force, this deflection or pressure to the right in the 

 northern and to the left in the southern hemisphere is suffered in ex- 

 actly 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 discus^sed the mo«les in which the general 

 circulation of the globe affects the climates of different latitudes by de- 

 termining the distribution of rain-fall in wet and dry zones and inequali- 

 ties 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 interaction of expanses of land and sea. 



" Professor Ferrel's book covers very much of the ground of which 

 modern meteorology usually takes cognizance, and in the thoroughness 



