10 
THE VOYAGE OF H.M.S, CHALLENGER. 
the ocean from the American coast and West Indies across the Atlantic, then northwards 
over its eastern side, and thereafter round the north of Norway and along the north 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 replaced by the upwelling of the water from greater depths, 
it is therefore of a lower temperature than that removed by the surface current. 
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 
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 occurs no circumscribed 
region of low pressure, but a broad ring, in width about 30° of latitude, of very low 
pres.sure, passes completely round the globe, falling to a mean pressure of about 
29'000 inches near lat. G3° S. Over this broad space of low pressure the prevailing 
winds are strong, and approximately west or north-westerly all the year round. These 
strong north-westers pour south into the Southern Ocean, by the surface currents they 
generate and maintain, inconceivably enormous volumes of water from lower latitudes, 
and these waters, be it noted, of a comparatively high temperature and specific gravity. 
Tliese warm and specifically heavy waters in their passage southward by and by 
necessarily sink to greater depths, and are overlaid by the cold and specifically light 
waters from the melted snow and icebergs of the Antarctic region, as suggested by Dr 
Murray.' A striking feature of the waters of the Southern Ocean is the interdigitation of 
currents differing ver}' widely from each other both in temperature and specific gravity, 
the colder of these currents having their origin, doubtless, in the numerous icebergs 
of thc.sc regions. An important part played by these vast currents of warm and 
.s|>ecifically heavy water is to mitigate, most materially, the cold of the Antarctic region, 
particularly at great dejjths, and thus restrict the ice-clad area to its present limits. 
At this season, south of the equator, systems of low atmospheric pressure are not 
situated over the ocean but over the land. Of these systems there are three, — in 
Srmth America, South Africa, and Australia. The most pronounced of these is in 
.\ustralia, and the result is that, on all coasts, winds blow from the sea upon the land, 
.and at the same time the surface currents of the ocean arc directed towards the land. 
July . — In thi.s season the geographical distribution of pressure is exactly the reverse 
in Australia of what ol)Liins in Januar)'. Everywhere it increases on advancing from 
the coast into inland regions. The lowest pressure, about 30 000 inches, occurs near 
T^.F Rfisrwal of AnUrctic Ex]>loration, l*y .John Murray, Pli.I)., LL.D., of the Challenger Expedition (Geogr. 
Jrrrrm., Tol. ill. p. IS, 16i»l). 
