166 THE WEATHER, AND WEATHER PROPHETS. 



(30.) In the view we have taken of the production of 

 the trades, the immediate verticality of the sun acts as a 

 disturbing force. In its passage from solstice to solstice 

 it causes an annual fluctuation or oscillation to and fro 

 of this medial line, and of the northern and southern 

 limits of the wind-zones ; which, where those limits cross 

 the ocean, is but of moderate amount, because the 

 medium temperature of the intertropical seas varies but 

 little with the seasons. But where they cross extensive 

 tracts of land, their oscillations to and fro may become 

 very considerable, owing to the high temperature which 

 the land is capable of acquiring, Now in this case, so- 

 soon as the autumnal equinox is passed, the vertical sun 

 enters on the full breadth of this vast continental tract ; 

 and commences throwing up torrents of vapour and in- 

 tensely heated air, the latter being far in excess of what 

 it would be over an equal area of sea ; while at the same 

 time, owing to the sun's then rapid motion in declina- 

 tion, the limits of the wind-zones retreat southward, and 

 their regularity is disturbed and broken ; which cannot 

 but give rise to great temporary confusion and disturb- 

 ance in the winds themselves. As to the " atmospheric 

 wave" which recurs periodically at this season, it results 

 most probably from the operation of the more general of 

 the causes above mentioned, by which a large amount of 

 extraneous air and vapour is thrown into the atmosphere 

 of the North Pacific; causing the south-west wind of that 

 ocean to sweep with increased force up the western 

 slope of that vast range of lofty mountains which fringes 

 the North American continent; and to be thrown up 



