OF THE earth's ROTATION ON WINDS. 249 



two directions combine with motion of the earth from west 

 to east_, and there results a north-east wind in one hemi- 

 sphere and a south-east wind in the other. Indeed, as 

 the diameter of the parallel circles continues diminishing 

 in proportion as we recede from the equator_, and as all 

 the points situated in the same meridian turn round the 

 axis of the earth in twenty-four hours, it follows that they 

 move with a velocity much greater as they are nearer to 

 the equinoctial line. But the masses of air which flow 

 from the north towards the equator have an acquired ve- 

 locity much less than that of the region towards which 

 they are directed. They therefore move more slowly than 

 do the points situated near the equator, and they oppose 

 to the elevated parts of the surface of the globe a resist- 

 ance analogous to that of a well-defined north-east wind. 

 For the same reason the trade-wind of the southern hemi- 

 sphere blows from the south-east.^' (See Kamtz's ^ Meteo- 



logy/ p- 38.) 



Sir J. Herschel, in his 'Meteorology,' thus explains 

 this point. After speaking of the return of air from the 

 north towards the equator, he says, '^ In this account of 

 the production of wind, however, no account is taken of 

 the earth's rotation on its axis, which modifies all the 

 phenomena, and gives their peculiar character to all the 

 great aerial currents which prevail over the globe." '^ To 

 form a right estimate of its importance, it is only neces- 

 sary to observe that, of all the winds which blow over the 

 whole earth, one-half at least, more probably two-thirds, 

 of the average momentum is nothing else than force given 

 out by the globe in its rotation, in the trade-currents, and 

 in the act of reabsorption or resumption by it from the anti- 

 trades. Since the earth revolves on an axis passing through 

 its poles from west to east, each point in its surface has a 

 rotatory velocity eastward proportional to the radius of its 

 cu'cle of latitude, and any body of air relatively quiescent 



