SPECIFIC GRAVITIES AND OCEANIC CIRCULATION. 321 



inevitably follows that over the eastern parts of America the prevailing winds are 

 north-westerly, and over western Europe south-westerly. Hence, the prevailing winds 

 at this season blow the surface waters of the ocean from the American coasts and West 

 Indies across the Atlantic, then northward over its eastern side, and thereafter round the 

 north of Norway and along the north coasts of Siberia. This state of things results, as 

 regards Europe, in the setting in of a strong ocean current, bringing the waters of 

 warmer latitudes to its western shores ; and, on the other hand, as regards America, the 

 draining away from its shores of its warmer surface waters ; and as this water is necessarily 

 replaced by the upwelling from greater depths, it is therefore of a lower temperature 

 than that removed by the surface currents. 



Similarly, the wind system of the North Pacific is ruled by the low pressure in the 

 north of that ocean, resulting in north-westerly winds in the east of Asia and south- 

 westerly and southerly winds in the west of North America. The great dryness and 

 extremely low temperature of these north-westerly winds have the effect of lowering 

 to a still greater degree the temperature of the North Pacific Ocean. 



In the southern hemisphere, south of lat. 35° S., there occur no circumscribed regions 

 of low pressure, but, instead, a broad ring, in width about 30° of latitude, of very low 

 pressure, passes completely round the globe, falling to a mean pressure of about 29'000 

 inches near latitude 63° S. Over this broad space of low pressure the prevailing winds 

 are strong and approximately westerly or north-westerly all the year round. These 

 strong north-westers, which blow in upon the Southern Ocean by the surface currents 

 they originate and maintain, inconceivably enormous volumes of water from lower 

 latitudes, and these waters moreover of a comparatively high temperature and salinity. 

 These warm and specifically heavy waters, on advancing in their passage southward to 

 those parts of the Southern Ocean where the surface temperature and salinity are low 

 owing to the heavy rainfall, icebergs and melted snow, sink to greater depths and thus 

 become overlaid by the specifically light waters of the Antarctic region, as suggested by 

 Dr Murray.* A marked feature of the waters of the Southern Ocean is the interdigita- 

 tion of currents differing widely from each other both in temperature and salinity, the 

 colder of these currents having their origin, no doubt, in the numerous icebergs of these 

 regions. An important part played by these vast currents of warm and specifically 

 heavy water, is to mitigate very materially the cold of Antarctic regions, particularly 

 at great depths, and thus confine the ice-clad area to its present limits. 



At this time of the year restricted systems of low atmospheric pressure are not to be 

 seen over the ocean, but over the land of the Southern Hemisphere. Of such systems 

 there are these three : — In Australia, South Africa, and South America. The best defined 

 of these systems is Australia, where on all coasts winds blow from the sea upon the land, 

 under whose influence the surface currents of the ocean are directed towards the land. 



Prevailing Winds in July. — -In this season the geographical distribution of pressure is 



* " The Renewal of Antarctic Exploration." By John Murray, Ph.D., LL.D., of the Challenger Expedition (Geogr 

 Journ., vol. iii. p. 18, 1894.) 



