SCOTTISH GAEDENS 



dome or ridge corresponding with the apparent path_ 

 of the sun round the earth. From the top of this 

 ridge the heated air flows away towards the poles, 

 descending to the earth's surface again at about 

 30 N. and S. latitude. The circumference of the 

 earth at these latitudes being very much less than 

 at the equator, the surface velocity in diurnal rota- 

 tion is necessarily diminished in proportion. But 

 the descending air current retains, not only much 

 of the heat, but also much of the high eastward 

 velocity imparted to it in equatorial regions, the 

 result being a general movement of the atmosphere 

 in the northern temperate zone from s.w. to N.E. 

 Land areas, being far more extensive and numerous 

 in the northern hemisphere than they are in the 

 southern, interfere powerfully with this general drift 

 of atmosphere by causing local differences of tempera- 

 ture ; but it has a clear oceanic course of about 4000 

 miles in passing from the coast of Florida to the 

 Land's End. By virtue of its heat, this warm air 

 current is able to absorb the moisture which is con- 

 tinually being given off by evaporation from the 

 ocean surface, and to carry it eastward in the 

 invisible form of vapour. But when the air current 

 is chilled, whether in summer by meeting high land 

 which lifts it to a colder stratum, or in winter by 

 striking land which at that season is colder than 

 the sea, it loses the power of carrying the vapour, 

 which is suddenly condensed into the visible form 

 of rain or snow, mist or fog. Such is the chief 



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